STUPID AMP! - 1970 Harmony H550 Solid State HAIR PULLER Repair
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- เผยแพร่เมื่อ 2 ก.ค. 2018
- This amp is a dum dum and a stupid head. Looks like I picked the wrong week to quit sniffing glue. Nothing makes sense. Chaos reigns where're this amp doth go. His name who plays upon it is Death. The Horsemen are riding. God help us all! Run and hide...run and hide. Amen.
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Word of wisdom... Put a piece of heat shrink tubing on your meter probe only exposing the tip of it. This will help protect you from shorts in tight spots.
A tip for solid state class B amp testing: Remove the speaker or dummy load, add about 470 ohm resistors in series with
both power rails to limit the current. Then do your trouble shooting. That means faults won't cause bug currents that fry things.
Make sure to put the resistors after the power supply caps so they can't zap things.
Oh yes, that also makes it easy to measure the current drain too, which is useful for checking the bias pot is functioning. You need to overcome your unwarranted fear of solid state circuitry :)
Oh absolutely.
And stop changing caps because you think they 'migjt' be faulty!
@@DrWatts-bi1jv If their paper and oil or old electrolytics, they gotta go, anything over 30 years old, has to go, it's stupid to keep runing an amp with old parts like that.... its risky and not worth it.
I actually like certain early solid state (all transistor, no ICs) amps. There is a nutty, woody sound that I find very pleasing. And as always I love the TV Jones Telecaster
I've been binge watching your repair videos. Good job! Very entertaining!
Hi thanks for all videos. I started out in life in the 1970’s working for a company that made just transistorised test gear. One of things we made was a transitory tester, which along with testing the gain would test the reverse brake down of the transistor junction which with some failure modes could be quite low, like 20 plus volts across the junction. Sort of like a zener diode. All transistor had a brake down voltage but at that time if not now some would get a lot worse with time. Cheers Gary
Once again, Earning your keep the hard way! Persistence pays off! That is exactly why I like your channel!!! You always figure it out in the end!!! Good job!!!
Hi Mr. Guitologist, good on you for seeing this to the bitter end! Like valve testers, transistor testers don't show the full picture. Whereas it's easy to plug a suspect valve into a piece of known working equipment to see if it's good, it's much harder when testing a transistor. Thank you for posting this.
Nice work Brad, learned a lot today. Still going to review a few things but this was really informative. Thanks for all your hard work.
Hi Brad, Just a little service tip. Denatured alcohol and an old toothbrush works wonders cleaning those circuit boards.
Commenting near the beginning of the watching. But those big blue caps remind me of the huge caps that were on tape drives (computer, that is) back in the '70's and early '80's. There was a "hot potato" drill where techs would do a "heads up" and toss one to someone. They claimed that they would sometimes zap the heck out of the catcher. Oh for the simple times! (And actually, I never saw anyone shocked, as the ones they were tossing were usually the ones coming out of the drives, and even back then the techs understood that they had better discharge the capacitors before replacing them.)
Brad, your channel rocks! Thumbs up!
wow cant believe it I bought this amp back in 73 new for 50 bucks from a friend I ran two home made bottoms and two amp heads
one of them the harmony I never used the reverb or tremble don't remember how good they worked I was amazed how clean the harmony sounded and liked it good old 2n3055 outputs
take care enjoyed watching brings back memory's
Your patience paid you Brad. Congratulations! It's a kind of misadventure that can happen sometimes especially with germanium transistors (and sometimes with silicon ones too). A tester is not always reliable. If it test good, it's not a guarantee that it's good. But if it test bad, it's always bad. I thing that the main reason is because a tester send only a few Volts to the transistor witch is not "the true live" operating condition. I have seen transistors that start to leak somewhere above 10 Volts but where fine below this voltage. Personally when I am testing a transistor, I first use my multimeter. If it test good, then I use my old Heathkit IT-11 capacitor tester on "leakage" position starting a 3 V and gradually climbing until it start to leak between Collector and Emitter with Base open. If it start to leak anywhere before its rated CE voltage, I replace it.
Well done spot on diagnosis. Transisters from 70s gear give multitudes of problems and ive replaced loads .
I had a freebie amp a few weeks ago with a little nick in the power cord ,and the plug cut off. I put on a new plug and taped up the little nick, but it wouldn't power on. I disassembled and reassembled the damn thing 3 times over 4 days, testing and retesting everything I could to figure out what the deal was. During the final assembly (after giving up on it) , I discovered by accident that the whole problem was just the little seemingly insignificant nick in the damn power cord! Every time I picked the cord up to test it for continuity, it was making the connection, but every time I plugged it into an outlet, gravity was pulling the bad connection apart!! After completely replacing the cord, I did an autopsy on the nicked section and was able to see the deceptive intermittent break :)
GREAT persistence Brad, my hat's off to you man.!!!
Hi Brad! I've always just used a digital multimeter on diode test to test transistors. It's at least a half decent go - no go test. So with an NPN transistor, red lead to base & black lead to emitter you should measure say 0.6V dc. Next part of the test - leave red test lead on the base & move the black test lead to the collector, & you should read say 0.59V dc (the now forward biased B-C junction).
This second measurement is reading the normally reverse-biased base-collector junction. Most of the time, the base-emitter measurement will be a millivolt or 3 higher than the base-collector reading. This can help identify collector or emitter. Just reverse test lead wire colours for PNP transistors.
As other guys here have said, the little tester you built has shortcomings. A 'curve tracer' may help, or one of those component testers that show a loop, or a straight or angled/bent line on a cro screen.
Great work repairing this distinctly-less-than-classic-yet-oh-so-interesting amp head! As for the confusion regarding the status of the transistors you replaced, I would suspect the component tester; I recently saw a very informative video by a gentleman named John, who maintains the JohnAudioTech TH-cam page, and who reviewed a component tester very similar, if not identical, to your own tester. He found it gave VERY inconsistant readings, and concluded that, while it did a fair job IDENTIFYING components, it was next to worthless for testing them!
I always look for the burnt leads where someone else has soldered something. That orange wire near those rmc caps has a definite scorch mark!
It`s beautiful, I love vintage. I have a Peavy classic VT series think its from the 70`s too excellent reverb and for some reason the power toggle switch is off when in center and on either left or right maybe that's polarity idk but anyway that Harmony was totally worth your time and patience ✌🎸🔊
You always need to look for leakage and for HFE. When you tested the first one it showed an HFE of 33 which is very low given that the transistor specs at an HFE of 90 min. This tells us something was not right with it.
é realmente ótima esta construção de amplificadores obrigado um abraço
Thou shalt over come and thou did, a pain but you did. Great video bro.
The RMC ceramics have been pretty good all around, they used a better epoxy coating that stood the test of time. Lots and lots of RMC .02 & .01 1kv were at a local surplus store, along with a box of NOS 1968 CTS 1 meg linear pots. I grabbed what $40 bought, Money well spent!
I love watching your more difficult repairs!
I need to buy a lot of ceramics from somewhere. I'm running out.
The Douglas Self books on solid-state amplifiers is a really good reference for explaining how these things work.
Thank you, I have a few ACC 260s, "The Doors" amps, they require a nice overhaul. One of them has a wild harmonic runaway after the amp warms up, starts screaming notes above 700hz, like a feedback loop. Another is shorting to chassis with some nasty voltage.
So far I'm interested... seems like another great vid.
I have a Harmony Amplifier re-badged and sold through Sears, model LA15, re-capped it and changed the power cord and it sounds real nice. I also have a JC Penney amplifier sold under their Penncrest house brand, not sure who actually manufactured it, but I also changed the all the capacitors and power cord and it sounds great, in fact is one of my favorite amplifiers. I am currently looking for a Montgomery Ward amplifier to complete my Trifecta, of 3 Department stores amplifiers.
You're certainly more brave than I ever was at this. When I saw pieces of that age with a phenolic board and ancient transistsors I'd call the customer and tell 'em there was no charge if they come pick it up right away.
Love your stuff, man!
Brad, I'm glad you and the amp can live in Harmony again.
Love your channel. My suggestion would be to find an old service manual from the 70's for a high end receiver such as Sansui AU series. They have explanations of the discrete output stage component by component.
They are a balancing act between the positive and negative rails to keep the DC at the speaker rail at zero.
It really is like a diff amp with DC feedback from the speaker output driving progressively higher voltage and current stages that ultimately have enough current and voltage to drive the power transistors, but there are things like bias, current mirrors and such in there that require some knowledge of too really understand the config.
BTW, never have a load on the speaker output when troubleshooting a transistor stage. Esp when initially bringing it up I a variac. If you get DC offset, you won't provide a load for the transistors to try and transfer the rail voltage into.
Good luck.
You are correct on the date. A small band that practiced close to where I lived had that same amp. I remember it being so very loud. That was the big deal at that time.
It was an expensive amp in its day! The head and cab would have set you back about $550.
The Guitologist The high price sound right, they had the first Gibson SGs I ever saw. I took lessons from their mother and she had an old Les Paul that was made just before they stopped making them. I didn't realize what she was playing for years later. I'm still in love with her legs on up. Trying to look up her dress is why I stink at guitar.
SOLID STATE STAND FOR THE SOLID HAMMER NEEDED FOR THE JOB!!! cheers bbrad hope u have a great day ;-p
This may be of no use - but I have seen another YTer use FLIR to look at circuits. I think the logic is that a hot diode is a good indicator of current (maybe intentional, maybe not). ...and of course, I could have it all wrong... :)
Thanks for sharing these things. Always great to watch and learn.
I have a thermal camera. In this case, I didn't want to leave it on long enough for stuff to start going up in smoke.
excellent point.
Saying 'We're good to go" is the problem, lost count of your vids where you say this and something happens
All in all, another good video:)
What a great expression of patience... I wanted to throw it out the window for you.... lol!
My brother.. Before moving to Florida from Pennsylvania (about 1988) gave me A Harmony 550 H The cabinet had 2 15 inch full range Utah speakers.. The head didn't work at all... My Father owned his own electronics company. He paid one of his "Techs" to fix it.. He could only make 2 channels work.. first one.. and second one.. no effect channel. ONE WAS "Bright" ONE WAS NOT. For me being 15 it DID sound good! Was LOUD!
Sounds can good to me, nice work. The axe skills help too.
Great Vid !!! Don't Trust your component tester , with Tranys ! Yep when troubleshooting low volt power rails just lift components until you find the bastard ! I've had a Solid State few hair pullers. The worst is Cascade failure ! Had a NAD SS amp , replaced one bad component then a bunch failed .
U have lovely hands,Sir Brad.
Oh that was a fun one :) I have been there for sure. I think maybe those testers do not due a full test, being they run on 9v maybe they just don't push the component to fail voltages, even some multi meters have fooled me in the past.
Either way. .... its a cool looking unit. Would be cool to use it for a conversion.
wow, such pretty finger nails. they must take a long time to keep like that.
Just a suggestion: faulty solid state amps are notorious for baking speaker coils, either due to parasitic oscillations or massive DC offsets, normally to one of the supply lines. To protect your speaker (assuming 8 Ohms coil) when testing an unknown amp best to connect a 16 Ohm resistor in series with the speaker and then connect a 12 Ohm resistor in parallel with the speaker and 16 Ohm resistor so that the amp sees an 8 Ohm load.
Great Tip!
I had the same thing happen with that exact LCR meter. I had a difficult repair where all of the components tested good. The MOSFET's tested good with the component tester, but all of the symptoms were telling me the FET was bad. Replaced the FET's, and it came to life. There was 1000V on the FET's, so maybe it was a case if components testing good at low voltage, but not working properly at high voltage.
I had one of those H510s you showed in that catalog about 1997 or so. Got it at a pawn shop for $20. It had cool reverb but even cranked to 10 it didn't have enough volume so I sold it. I've always wondered why it lacked volume, it was ridiculous.
Nice ali chassis and lots of parts. This *might* be like a MusicMan with all those tropical fish and Allen Bradleys. If it didn't work fast I'd canibalise it for parts though!
I don’t have those fancy transistor testers and the way i test them is by building a circuit around em, something like a first gain stage from a big muff for example should work with just about any bjt or a mosfet, and obviously you would have to account as to wether it is npn or pnp.
Hey Brad, I have been working on a 2000 Mexican J-Bass with a badly bowed neck. The customer told me that the original neck was bowed excessively and when he tried to loosen the truss rod, it broke, the truss rod. So he ordered a replacement from an individual on e-bay. The new neck ,according to the serial # is also from a 2000 Mexican J-Bass. When it arrived it came in a NEW FENDER NECK box, should of been a clue that this neck had issues as well. He mounted it on the guitar and when he tuned to standard E A D G ,surprise, surprise the neck bowed loke the original neck. He brought it to me and I measured about .050" relief at the 7th & 9th frets when tuned to standar E. I placed blocks on either end of the fretboard and placed my longest leveling bar on the blocks. I then placed a clamp at the middle of the neck and cranked about .025" bow at either end with the truss rod under no tension. I then placed a couple washers on the truss rod after lubricating both the rod & nut . After tightening the truss rod and removing the clamp I had a slight bit of back bow on the neck. However once I tuned the bass up the bow returned. We were wondering if Fender may have had a bad batch of necks on 2000 Mexican Basses. Any info on this issue would be appreciated. Thanks for your time and consideration in this matter, keep up the great content, Jeff 'Gonzo the Guitar Guy ' Kellogg
Is the truss rod slipping? Did you try retightening? Maybe threads stripped?
The Guitologist the threads seem fine , I think there was a bad batch of necks that year as both necks, the original and the one bought off e-bay are from the same year by serial numbers. Was wondering if you had heard of this issue before. Or maybe a bad batch of truss rods ?
The circuit uses a cap bootstrap design with string diode bias circuit and them little transistors with the diodes going to there collector pins are short circuit or v limiting, if there is a short on the output they clamp the drive volts down.
With a supply line of 38 plus and minus volts this amp would be about 60 to 100w...
Electronics tech here..
You can remove the short circuit transisor parts and the amp work just fine.
Electronics engineer here...
I hear your frustration as had many headache amps to..
The amp uses long-tail pair..Next bootstrap vas stage..a simple bias string diode..
Another viewer commented my thoughts - ECG is a replacement part. So Q3 and Q4 have likely been replaced before.
Looking at their specs, diode junctions looked ok in your test, but Hfe seemed really low. Datasheet claims a minimum of 90 for ECG129... that was my first thought. Yea, they tested as functioning, but low gain. Make some wonder if there's a bigger issue.
Pain in the ass, indeed... good work finally getting that sucker functioning! Let's hope it ain't a bandaid.
I wouldn't have suspected that would have caused a short of base to collector, which is what seems to have been occurring at operating voltage.
The Guitologist yea that doesn't quite add up.
I have worked very few SS amps, but most are 60s/70s early stuff. Most of it was poorly designed, seemingly taking a tube circuit and subbing transistors at lower voltage. LOL they don't work the same, so the stuff is full of issues.
Do the bird now...(?)
I fixed a solid state Standel from 1969..(on my channel).. I'm not a SS tech natively, but I took it on. I found the bad parts in a Kustom, Dangit I can find it in a Standel. It was worth the effort. What doesn't kill you makes you stronger... just remember that. Transistors are a whole other ball of wax.
Got a link? I'll watch that later.
Standel, there's a wonderful thing we never see here in the UK. Always makes me think of that crazy set up John Cipollina had.
th-cam.com/video/dx_-IjQLNkM/w-d-xo.html Part 1
th-cam.com/video/t3GVXBpD3j4/w-d-xo.html Part 2
th-cam.com/video/AVSt-_HDY_s/w-d-xo.html Part 3 Reverb issue.
This Standel is a great amp.... Got it cheap since it was broken.
Hey, that power amp has overload protection! A great deal SS amps these days still don't. The design seems conventional. Schematic is unreadable, but I'd say all transistors in this thing are silicon, just some in TO18 cases for more dissipation OR because that's what was available cheap ;-)
Also you might want to tap the screws lightly with a hammer before unscrewing (through the driver of course), helps them loosen if they have some corrosion.
At the start of the vid ,the amp looks so clean +nicly!I think you are antipathic against "nontube
AMPS! ROCKN ROLLY BRAD!
Those metal can transistors appear to be bc109c they are also silicon they just use a metal can. The bigger metal cans can house silicon or germanium but im guessing they are silicon also. Great video though love watching your vids
i was impressed by how clean the amp was to be that old.
Sounds a lot nicer than a modern day solid state amp.
oh nooooi the magic smoke!! put it back in
They used to color the tops of transistors, for matching purposes. They would test batches of them and sort them according to their current and voltage characteristics. Then when they were used in an amp, they just had to use all of the same color to get the best matches.
I think in this case they were probably separating the FET from the other transistor.
All so there noise spec
Looks like an old Tektronix curve tracer would be a useful tool if you work on solid state stuff. It will expose a transistor to actual operating conditions.
Agreed. Tektronix 575.
Yeah that tester is more of a component identifier. You have to test transistors at voltage and feq, witch sucks because you need a high end tool.
After this repair, I feel like a high end tool.
Haha
Good knowledge though! Those transistors have to be tested under load it seems. Like you said that tester is more of just an identifier.
for solid state stuff current checking and a good curve tracer will set you right. You can't just look at voltage alone. Also leaky transistors can test as being higher gain than they actually are so those cheapo meters can't always detect it.
I've recently started watching. I've seen a few videos and was wondering. Have you ever, or do you ever, install board connectors when you get into a situation like this one, to make the boards somewhat modular and easier to remove and work on? Or do you try to keep amps as vintage and jankey as possible when you get them??? I saw one video where the guts were a skeletal build that I would have built a board for but that's just me. TY!
Solid state stuff is a real house of cards balancing act. It takes nothing to throw the bias off and completely skew the readings you get. Then it becomes a real needle in a haystack. Fair play to you staying with it, many techs would just write it off. Do you have the jack darr book? Some good logic in there re solid state gear. Interesting story somewhat unrelated. I recently picked up an old marshall 8008 valvestate transistor power amp for a good price and for nostalgia sake to drive a pair of old cabs I have. The original/previous owners can never have played with a clean tone as soon as I whacked it up I noticed distortion on one channel. Turned out to be a wrong resistor, from the factory, in the bias string of one channel. Miles off. It was just biased right on the edge of cutoff. You'd never know with a saturated tone. 25 years plus later it's sorted lol. It's regarded as one of the better pieces of equipment of that range. The heat sinks are massive which is where most things skimp. It gets a bad wrap with people saying it's not loud enough but they generally forget that it was designed to be driven by the then current valve preamps they made with several volts swing on the outputs of them, consequently most of the modern processors don't drive significant enough levels for these amps to get there. It's a shame that solid state stuff gets such a bad wrap, when it's done right it can be a joy. Since the cardiac arrests I've only used an upgraded Sessionette combo which has held its own against some serious gear. I can't carry nor justify owning the big valve stuff. My plexis and matamp went, and I've got an old ac30 that I'm clinging to like Jack in titanic lol. Great video mate I hope it inspires more cool quirky amps to be saved. Hope the weather is good there, North Wales is currently sweltering.
North Wales don't know the meaning of "sweltering". Try about 95F at 50%+ relative humidity. Louisville can feel like a farmer's ass crack in the summer.
The Guitologist well we can't compete with that. We get pretty high humidity on the estuary here though. You can grow penicillin on a funky old guitar case overnight.
Good old American determination at it’s best!
You need to find a Sencore Cricket transistor tester, I have the pocket cricket. It has tests for gain and a very sensitive leakage test both forward and reverse for all junctions of the transistor, those quick testers miss a lot they are good for quick I.d. of parts though. You got lucky that it didn't pop the outputs too. Had similar problems over the 40yrs. I have been servicing electronics, The board layout and build style was common back then, parts not id'ed, no voltages listed, oh yeah, one slip of the probe shorting something on a solid state direct coupled amp can ruin your day, not to mention all the semiconductors and usually a bunch of resistors too.
Metal can transistors does not mean germanium. I still buy metal can variants of both silicon bipolar and j-fet transistors, in current production today.
I just assumed because of the package and age. Thanks.
Like the good ol workhorse 2N2222A. Love them.
Yep. Looking at 2:35 the parts list shows 6 types of transistor, all of which are Silicon, not germanium, with one FET and the usual output stage suspects of the good ol' 2N3055's
When it's semiconductors the only really sure way is substitution - resistors, caps and chokes all test easily and you can rely on them but the semis can break down or play up at working voltages yet show up fine in testers - even really good ones.
You can make a real simple diode junction tester with a transformer with low voltage 2ndary and a resistor. Yes, it's just that simple.
You X/Y your scope (as i recall across the resistor, you'll have to Google it) and can view the quality of the E-B, B-C, and E-C junctions. It's also great for zeners up to the limits of your transformer 2ndary.
Between it and your DVM diode checker you can ferret out most transistor issues.
I made one way back in the 70's and am still utilizing it successfully. I'll bet it would have caught that bad driver transistor.
good job for sticking w it man that was a nail biter you deserve 3 attaboys lol ..keep up the good work im learning alot, thank you for your service..
Thanx Brad... that wuz brilliant. The jinx... oh the jinx. Where will it strike next?
cheers
Always wise to monitor the idle current and DC offset on a solid state amp of unknown condition while ramping up in variac.
Typically you'll need to get part about 35vac or so for the bias tracking to kick in.
Definitely good idea to first just diode check output xistors to be certain not sorted before even ramping up on the Variac.
It was always a challenge working on these amps with no silk screening in the board😬
Please make some videos if you are able. Always a lot of good tips in the comments.
You should test components with I-V curve tracers. Most faulty transistors may seem OK at a certain Q point for the cheap tester, but it may not be ok with another Q point.
All those feedback loops can make things fun. I'm guessing your tester doesn't check for leakage. Taking voltage measurements is the only way. (War story alert!) I had a stereo amp where a diode in that string of diodes opened up (but only sometimes. If you probed it, it would make connection.) When those diodes open up, the bias resistors turn on the top transistors sending the output positive. The feedback loop still had control of the bottom transistors, and it turned them on to try and bring the voltage back to zero. Net result? A short across the power supply and a blown fuse. After going though a box, I got smart and used the "dim bulb" trick (yes on a solid state amp) to have power without burning anything up. With the low power, all the voltages were wrong, but I could still figure what was turning on and why. It's a repair that sticks in my memory, even if it was 40 years ago.... That was a painful repair, but you came out the winner. Good job.
This is the reason its vital to place the bias pot in the right part of the bias spreader circuit so if it goes open the bias voltage drops, not rises - even a crackly bias pot can ruin your day if this precaution isn't done. The standard
Vbe multiplier is great as when the transistor switches on it clamps the bias to minimum, a safe state.
Next SS amp I get, I'm just going to rebuild it with all new parts. It'll be quicker than trying to diagnose the problem. And I won't feel so stupid at the end of it. :D
It is tempting to just do shotgun repair but it leaves ya wondering what the fault was in the first place. Throwing parts at the repair can introduce even more complications. My technique to SS repair has been to use light bulb limiter so as to hopefully not cook components/fuses and still be able to keep the amp running. If there is no output transformer then no load on the output until the amp is stable.
Hi Brad. If you could please answer this question. Can anything be done to slow the tremolo rate on a Princeton or Deluxe reverb? It just seems to never go slowly enough for my taste. Thanks for another good video.
Have to modify the transistor tester or make a transistor tester on the breadboard that test transistors at high voltages like +12, +24, +35, etc because most transistor testers are checking the transistor at +5vdc and +9vdc i think.
41:20
"power supply is good, the rectifier is good, I'm getting positive and negative voltage as I should"
Hey that rhymed xD
He's a poet and didn't even know it!
If you are going to branch out into solid-state repair, I suggest you invest in an old Tektronix 575 curve tracer. It's a tube-based instrument; can be calibrated with a meter, a manual, and an eyeball, and allows you to test transistors at (or beyond) real operating conditions. Might be worth looking into. I have a component tester similar to yours; didn't realize how misleading (and time-wasting) these could be! Curious, did you try your component tester on your new replacement transistors? If so, what was the hFE?
Whenever I find a NTE or ECG transistor I usually suspect it first. I only use them in extreme cases of not finding a suitable cross.
Hey Man ... you may or you may already have a Huntron Tracker .... great for testing parts still in circuit.... quick for primary quick tests....
Question re: Guitar books mentioned in another video.
I visited a local record store and came across 'The Electric Guitar' edited by André Millard was this the one you were talking about? It will not appeal to many as there are only a few pictures, but for those interested it has a wealth of information.
Hey Brad I'm a devoted watcher of your channel and I'm a learning luthier myself and I'm wanting to get into the amp side of it and I love the electronics part of guitar building and the different tones you can get out of the electronics. I'm wanting to know your advice on the diy Tweed tube amp kits you can get off of eBay.would that be a sufficient jump off point to start learning how to build and work on tube amps and how they work?if I could borrow some of your time to reply to my comment I'd be very grateful brother.i find tube amps very interesting and love how you can get different tones out of certain components.looking forward to hearing from you.thanks so much for what you do for us and love your humor in things.cheers
Who ever laid out the design of this PCB failed. Standing up components on end and then sticking on heat sinks is trouble. Another 10 pounds of electronic crap on a 5 pound PCB. Good fix Brad.!!
And they neglected to clip the leads short, which is asking for shorts and trouble if the board flexes!
The (nice) close-ups of all these long leads on the solder-side of the pcb made me anxiously grasp for my diagonal cutters.
After you fixed it did you pull a leg & check the reading of the smoking resistor ? I stay away from SS, but I like SS phase inverter circuits.
Brad, that Q4 PNP did test bad. Your tester reported hFE = 33 which is too low for the current it's being tested at. You'd be looking at something greater than 50 for a functional part.
My understanding of hFE is it's a measurement of the base to collector gain. Can you elaborate on your math there a little? I'm genuinely interested. Trying to learn more about transistor stuff.
Base to collector current gain is beta, and its value is somewhat different to hFE. I don't know the intricate details about those but you can find more by googling "beta verses hFE". Here's my understanding:
hFE is DC current gain. Collector current divided by base current. Varies with current, temperature, Vce, universal constants, phase of the moon, all sorts.
Beta is AC current gain. Using circuits like the current mirror, you can get very high values of beta like 2000. A common way to increase beta is to add a capacitor across the emitter resistor in NPN circuits. There's a similar technique in valves too, right? Beta can also be made to be much more stable across temperature than hFE with feedback.
The datasheet for the NTE129 says the hFE should be greater than 50 at 0.1mA and greater than 90 at 10mA. The tester will be using a current within that range so we can assume 33 is too low.
+The Guitologist, +Prehistoricman, I don't know if the term "in general", or possibly "as a rule" applies in this for-instance, but, it's been my experience, that PNP Silicon transistors become "failure evident" in amplifier, or pre-amplifier circuits, when the forward gain is off (lower) by more than 10%. It doesn't take much for PNP Silicons to transition to "no-go" status, because their percentage of gain directly correlates to their ability to generate amplification.
Don't feel bad, as I've banged my head against the wall over the years, often swapping out ALL transistors in one section, for much the same reason - that you are going nutz. I've been a Motorola Two-Way Comm Dealer for over forty years, and forced to eat "the flat-rate shit sandwich", on more than one post-1972 MoTrac (when the audio circuitry ditched most of the germaniums, because of the tin-whiskering situation). When all else fails, blast the hell out of it with 'freeze mist'; it might not help to discover the problem, but it sure feels good to torture that very circuit by freezing the living hell out of it!
Good thread guys.
Prehistoricman, the high gain with a current mirror is voltage gain, not current - beta isn't any different, but the very high dynamic impedance of a current source/sink load makes for lots of voltage gain (which is transconductance x load impedance). The Early effect is the ultimate limit to voltage gain, and can be pretty much eliminated with a cascode stage (very like the much higher gain of a tetrode or pentode over a triode).
Do you think when your testing the Transistor its testing at lower voltage that's why it test good.
You’re all pumping up my ego with these comments about S.S. Audio repairs. I worked in an audio repair shop from 1980 to 1993. Might explain why I have less of my hair than I would like.
If you’ve ever repaired an SAE 2400 Power Amp, you’ll never forget it😩
Brad, What a PITA! It sounds like time to upgrade the Chinesium transistor tester?
Dang, your nails are really clean !!
Quentin James . 🤗🤗😆
Thanks! You should see my pecker!
Er, no thanks.....
8=====>
Awe!! you let the magic out.
Maybe there is a specific wat Certain transistors need to be tested that you just arent aware of? Can that be true for other types of components or can most be tested inplace or on your type of tester?
Where that trem sends half the signal to ground would it be possible to route that signal to another output. I could see some interesting possibilities for that if one could!
I have a question if you had a tube you could not or just wanted to go to solid state could u use a breadboard with electronics and plug in to a tube socket either directly or by wires
I was wondering, while you were testing, if this was going to be one of those things I see on some other vintage electronics channels where parts can test OK because the tester or meter isn't testing (or metering?) at operating current. I've seen it a lot with capacitors in old radios; they test fine on small meters but put it in one of the old ones that tests it at the operating voltage, and it's leaky as hell.
Dazatronyx Pedals THX is my favorite I would like to see you review it :-)
(@25:36) the schematic has voltage call outs (presumably DC w/respect to circuit ground) on either side of R8; -3.3v on one side and -1 volt on the other. That’s 2.3 volts over 470 ohms, or roughly 4.9 mA, with a dissipation of roughly 11 mW; well within the 1/4 Watt rating. Maybe there’s a significant AC voltage here? Meter time! 😎
Great fix Brad, like i have said im no electronics person at all, but like watching your videos, there interesting and fascinating, my fix for this amp would bee called a Hammer , haha, Cheers
HULK SMASH!!! I came real close. :D
Those metal case preamp transistors most likely silicone not germanium. I recall distinctly those being mostly NPN replaceable by Motorola type 2N2222's.
If the caps make it a go/no go, test them first, save yourself the time?
Have you ever heard of "Stone" amps? I have a SS one that was given to me I was going to use it for like a talk box amp , I'm not sure if it's worth trying to fix.
I do remember seeing one or two of those. Not sure where.