How To Repair Audio Power Amplifiers Without Blowing Them Up! This sort of repair can be tricky
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- เผยแพร่เมื่อ 5 ก.พ. 2025
- Repairing PA Audio Amplifiers can be quite tricky, as all the stages of the amplifier are directly connected to each other. You often find some obvious fault such as shorted output transistors, but if you replace them, check to see the short has gone, and power the thing up, don't be surprised if it just goes BANG and destroys all the expensive replacement parts you just fitted. This sort of repair needs a more methodical approach
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Thank you
Richard
*They did have TIP50 transistors at the electronics store in Las Palmas so we can continue with this one after the weekend* 🙂
Looking forward to the next video. I am basically studying, learning, and implimenting various approaches and techniques from online "mentors" like yourself as I develop and implement them into my own style and approach.
Notably yourself, Jordan Pier, Trevor from Trevors Bench, Vintage Audio Addict and XRay Tony. Without the content provided this will become a lost art as the lion's share of modern equipment is designed in the vain of planned obsolescence. Thank you for sharing these videos.
@@Poppinwheeeeellllllieeeeezsame here. I love my sont strdb940 amp. I'm on my 3rd because they're getting on a bit. I'd like to learn how to repair them. I'm field service engineer by trade so I'd to learn how to do boards.
"If this video wore you out"... I was thinking, "What? It's over already?" 🤣 Good on, thanks for making technical repair accessible!
Then you are well suited to this sort of repair 😃
Richard, R21 looks burnt, and there is also a larger electrolytic cap with the board stained around it, (leaky?). I am sure you already noticed these issues, but you want to make it an educational video, showing the fault-finding process. Owever, I would have placed the visual inspection for burnt components right in the beginning. Good luck with the rest of the repair. I always find your videos very helpful for a beginner, and sometimes even an old hand like me find some new things to learn. 👍👍👍
Actually I was working with the naked eye, you guys get the advantage of the magnified image on the recording, so I didn't spot the R21 but I have taken note of the various suggestions here and we can check them all out on the followup. Hopefully I get the TIP50 soon.
Thanks for the diode relay explanation. Wonderful info 👍
On the contrary Richard, I found this video very informative. Yes it takes some time to check components, but how else can you be sure you have found all of the problems?
Hi Richard, the coil wrapped around the resistor is to suppress any RF transmission, back in the day when fet amps were all the rage (I built quite a few) it was found that these amps when driven could transmit on the MW band, hence the need for that suppression.....
Isn't it the usual complement to Zobel network, purpose of which is to ensure stability of the NFB loop with capacitive loads? Sure looks like it.
Heya, I thing people who have patience and like to work with small "stuff/components" and have intress in electronics really love to see and do this kind of work. wel I know I do. and I see a follow up coming soon yes yes
Perfect application for a Huntron Tracker. I'd also suggest putting the new transistors on the component tester, measure the gain of each, and use the 10 that have the same or closest gain.
To excpensive the Huntron. Take a Hameg oszilloscope or use a normal 8-12V ~ transformer and connect it at the x-y connectors from an oszilloscope. Diy schematics can you find in the Internet.
@@Lightrunner.exactly, a hameg HZ65 could be used with every scope that supports X-Y mode and otherwise build one yourself. Googling for octopus component tester will give you plenty of schematics to start from.
Update.... because a huntron tracker is a super rare measuring device .
@@Lightrunner.I’m very surprised Richard hasn’t busted out his Hameg and scope yet. He’s got all the gear 😊
@@Mark_C1 I do have one, and also the home made one I built on a video some time ago
Thanks, Richard, it's great how you are doing the analysis and explaining why. Great teaching 👍. This is the best way to learn electronic repair. Thanks and be blessed in Jesus Christ name amen
Hi Richard🤗🤗
R 21 looks sad like seven days rain 😭.
It's not enough to just replace the power amplifier transistors. In analog audio amplifiers, most preamplifier transistors are DC coupled. It is important to record characteristics for control purposes(curve tracing). The same applies to the remaining power amplifier transistors.
Intresting power amp 👍👍👍👍. Thx Richard
Can you elaborate on the "DC Coupling" (just got into audio amps recently). I am trying to wrap my brain around this. Thank you for your patience! Thank you!!
@@TallyTechandTroubleshooting In my understanding it means the output stage of one section is connected directly to the input stage of the next section... an 'AC' coupled amp is coupled using capacitors (that block the DC), hence 'coupling capacitor' - and means if there is any unexpected DC level due to failure (short typically) of one stage, it's hard to kill the connected stage (cascade failure).
With DC coupled amps, failure in one area may cascade through multiple stages by the time any circuit protection kicks in.
I've repaired a Peavey CS800 which had a habit of killing quite a few parts when an output transistor goes short, usually ending up taking more output transistors with it too (not helped due to the DC speaker protection being a crowbar triac short to ground meaning the next half-cycle overloads the quasi-complementary opposite bank of transistors if a large enough signal present - no rail fuses!). That amp also had 5 pairs of transistors in parallel (10 in total per channel), expensive sods too (MJ15024), and skimping by using ebay or aliexpress specials wouldn't work as the fake ones generally failed short under enough load (and you can guess it, then takes out more when they do).
@@TallyTechandTroubleshooting hi, first step you must learn how resistors, capacitors, transistors, diodes and the other semiconductors (BJT,JFET,MFET,IGBT..and so on,) work. If you know this look forward. Every semiconductor component has a different characteristic, e.g. Blocking voltage, flow voltage, saturation range, upper, lower limit frequency. In order to allow these components to work with each other, the operating point must be set with discrete components on these characteristic curves. In the video of Richard with his analog amplifier (usually a type A/B amplifier) (there are different amplifier types, A,B,A/B,C,,D,E,F,G,S,T) are The driver/preamp transistors (e.g. tip31) are connected directly to the power stage transistors via resistors. If a power amplifier transistor gets a short circuit, the preamplifier transistor works with incorrect voltages and is in most cases overloaded or its characteristic curve changes. With a digital voltmeter in the diode test or ohmic test area, this damaged transistor can still have "normal" values. To see the characteristic curve of a semiconductor, x-y testers / curve tracers are used (the old Hameg oscilloscopes had one built in, or the curve tracer from the Huntron company, or build one yourself). If only the output stage transistor is replaced, the defective preamp transistor causes the new output stage transistor to be displaced in a working area, which in most cases results in immediate destruction of the same. this is only briefly explained. I hope this helped a little. I must use Google to translate my thoughts otherwise my head would explode😵💫, its long time ago when i wrote so many english😭😭🤣(pssst... do not tell this the others)
@@TallyTechandTroubleshooting Exactly as other replies. In A DC coupled or directly coupled circuit (amplifiers are a very good example of this) the output of each stage of amplification (usually transistors collectors or emitters) are directly connected to the base of the next stage. There may be some resistors in between but both the AC signal (music) and the DC voltage level is passed to the next stage. And so on. Typically the amplifier will have four or five stages of amplification, each one driving more current into the next stage, and the final one driving the speaker at high voltage and current. Because of this, if for example you have a failure in one of the first stages of the amplifier (the input end of it) this can put a DC level into the next stage, and so on, until at the business end of the amp (output stage) you can have huge currents flowing and damage the components. Alternatively, if you have a failure in the output stage , for example shorted output transistor, this can feed an excessively high DC voltage 'backwards' into the previous stage (driver transistors) which can cause a failure there, which in turn puts too much voltage or draws too much current from the previous stage (pre-drivers) causing a failure and so on in a kind of cascade all the way back through the amplifier to the input if you are really unlucky. Identifying the shorted devices in the output (transistors etc) and replacing them then powering up will likely just destroy them again as you didn't find the other faults. In a design where there are capacitors between each stage of the amplifier, only AC signals (music) can pass through and DC levels are blocked so this sort of failure is much less likely. I hope that helps
@@Lightrunner. Yes knowing all that stuff helps massively of course but if you have a good channel PCB to compare with the bad one, all you are really looking for is differences between the two rather than having to know exactly how all the components work and what you would expect to see on a meter. A bit like those 'spot the difference' pictures you find in puzzle books.
Hi Richard, I have noticed what looks like a burnt resister at R21 worth checking it.
If you want to care of your amplifier to make sure things doesnt go bang, i would say replacing the thermal compound between the transistors and the heatsink is a good idea. Also just a clean up of dust is good.
This one uses those little grey pads rather than thermal compound. They dont degrade or dry out as far as I know
Is that a crack in the transistor at 17:00 ? - it did test OK out of circuit. The burnt looking resistor R21 is noticeable at 27:50 when the 2 boards are side by side.
Thanks WIll check on next part
If you want to prevent the mounting pins coming loose while heating them you can use a small locking surgical clamp. Mounted between the PCB and the point were the heat is applied it prevents the spreading of the heat to the solder pad.
I use those a lot for all kinds of solder work were you don’t want the heat to be spread to other parts.
Haemostats?
@@AnnaVannieuwenhuyse I had to lookup the name but yes, that is what I mean.
Hi Richard, R21 is burnt
@@WayneD-sn1en it has been mentioned, will check on the followup. You guys are looking at this magnified onthe video, I'm working with naked eye
Really interesting to watch
Hi. Looks like R26 has been previously replaced, is it the right value?
Hi you might want a look at R21 next to the pot P3 it looks burnt could be high or open circuit 😮
Thanks WIll check on next part
What's the point of the 0-ohm resistors?
thanks. hope you will show how you test it?
That's what the video shows.
@@AnnaVannieuwenhuysetesting after repair. Powering it up and setting quiescent current (biasing)
@@cozycactus Yeah there will be a followup
As a few other have mentioned, R21 doesn't look happy (on the faulty board).
Thanks WIll check on next part
r21 on left of screen how it's reading? I would change r21
I wouldn't touch a car power amp with a barge pole nowadays, as they are much more trouble than they are worth....cheers.
I found one on the curb out for trash, took it home, opened it up and found one tranny and one resistor burnt. way too much trouble to fix, but good for spare parts and modifications
The speaker relays are probably worth changing now you have easy access to the solderside of the pcb..
Yes. A lot of arcing residues over there
I can see that some electrolytic caps are blue and some are black. Could be previously repaired.
r21 looked a bit rough on one board
a few others have mentioned that one. You guys have the advantage of seeing this in magnification on the video, I'm using nakedeye. Will check
R21 is toast - and the joint around there and the pot looked dodgy.
Thanks WIll check on next part
Your new devices are from Onsemi, which is a spinoff from Motorola, so they would be the same parts
Better safe than smokey.
Thats a damn big amp.. 3.2kW ? LOL.. what dummy load you got to test that ?
I can understand why the customer wanted it repaired... and the due caution - looking for deeper damage. Whats the probable cause of failure? short circuit of a speaker line?
What protection is on those boards? D.C. only ?
cant believe how many youtube electronics guys get stuff from aliexpress/ebay !!!
It requirs special temperament to repair electronics !
If you can't remember, wouldn't it be prudent to look at the video that you made 11 months ago?
Well I was probably going to look at the other channel pair (identical to this unit) that are still in the amplifier chassis.
❤