Hiya, Love the walk thru Stuart..That's unlucky for the owner..! Last thing you expect to go on a not too old amp, is the mains transformer..Thanks for sharing..Ed..uk..😀
Nice one Young Stuart!! Had a brand new Twin Reverb in 78 and had the optional Fender factory orange JBL 120F speakers in it!! (God I loved driving that JCB fork lift moving it about!!)
The process of elimination is always the shortest route for any issue where ampification is concerned. Well done, Stuart. I look forward to the episode featuring the 68' Fender. Cheers!
Another great video - thanks - The sub board with the nasty surface mount components is actually to take place of the traditional fender neon and light dependant resistor or "roach" for the tremolo circuit this making the amp ROHS compliant as the "roach" contains Cadmium which is not allowed under ROHS rules for Europe. The board on the right next to the power transformer ( with gets ridiculously hot by the way which is probably why they fail ) is to provide that sub board with power - the best thing to to with these amps is to fit a low speed high volume 240v fan or two directly below the power valves to keep the chassis cool, being careful to place them in such a way as to not induce hum into the circuit and then replace the sub board and power supply with a £15 roach off of Ebay - they fit directly onto the board and even have a marked place where it fits. These mods will very much improve the reliability of this amp and make the tremolo as it should plus keep the bias stable as when these amps get hot the output transformer tends to wander in value.
@Chen Ma , the traditional Fender tremolo "roach" contains a cadmium photo cell and a flashing neon light; the cadmium cell varies its resistance and turns on and off according to the rate of the light flashing, and that's how the tremolo effect is done.
I had this done. Swapped out the circuit for a roach. The place for it is there waiting. I also had the 240V tap connected. It's there too unbelievably. It's just risking the PT to have it running at 230 when I have 240 at the wall all the time.
Also worth noting that the mounting hardware for both of those pots on the underside of the chassis should be tightened securely. One is obviously for bias and I assume the other is a hum balance circuit for the tube heaters. If either one loosens and loses ground you could have *major* problems (noise and hum, or blown tubes and blown transformers). I'd relocate the thermistor somewhere off-board as well. I noticed the screen resistor's hanging off the output tube sockets by an inch, with full- length leads. Is that how it was factory-built? The resistor leads could actually be subject to sufficient vibration from the speakers that they eventually break.
The last thing I'd be looking for as well Stuart. Probably now no more expensive than buying a set of 6L6's .. I'd take the opportunity to replace it with one that has a 240 V input winding as despite what the narrative continues to be on the 230V , I've yet to find a 13 amp outlet within the UK with less than 240 V showing on the DVM.
Thanks for the video Stuart, always fun to watch a fellow amp repair person in action. May I ask where you get your repair jobs from, is it mainly private owners or do you work for backline companies, bands etc? Thanks
At 5:04, that thermistor will run hot enough to break down its own solder joints within just a few years (especially because lead-free) and probably de-bond the foil from the board. Not to mention that the heat from the thermistor will bake the heck out of the BIAS 😖potentiometer only an inch away from it and probably cause it to change in value or fail prematurely. PS, Stuart, if the thermistor was "shorted", that short-circuited device is in series with the transformer and would merely act like a piece of wire; it would not cause your current limiter bulb to glow so excessively, nor would it cause the amplifier to become inoperable ---- although the amp might intermittently eat fuses for no apparent reason because the fuse value was likely calculated with the extra series resistance of a cold thermistor. (Inrush current at turn-on would be higher without the thermistor resistance in circuit, and so you'd likely need a slightly higher-value fuse).
@@stuartukguitarampguy5830 That's probably the winding for 240 V AC input, the one that should be used in this amp instead of the 230 V input.... after all, you guys use brown as the color for "hot" on the power cord. I suppose you could have a little bit of fun at customers expense by wiring amplifiers to use that Brown wire for the AC input and then you could say to the customer "it'll sound like the infamous Van Halen brown sound"! 😲
@@halvach1998 , it wasn't touching the chassis when Stuart was doing the measurements, and the way it was bundled up with the other wires, it likely never did. Regardless, the transformer was definitely shorted internally.
I'm needing to replace the tube sockets in a fender 1978 bassman . Looking thru ur past videos didn't see anything on tht subject . I cldve missed it do u recall any about tube sockets ? Thx
Hi I very rarely need to replace a socket. I find that cleaning with DeOxit and re tensioning the sockets with a sharp thin spike does the trick. If you rally HAVE to replace the socket just make a careful diagram of where each wore goes (take a couple of pics too) and then it's fairly logical to replace it.
At about 6:40 you measure the resistance of the mains transformer primary to be 17.5 ohms and then say that's about what you would expect. I calculate E^2 / R to be about 3,000 watts. Seems very, very high to me. Did you really mean that 17ohms sounds reasonable for a working mains transformer, or that such a low resistance sounds correct for a transformer that is blowing fuses?
Resistance and impedance are not the same; the *inductive* impedance of the transformer is much higher than "17 ohms" at 50 Hz or 60 Hz. Most of the resistance that Stuart measures here in the primary circuit of the transformer (including the fuse, power cord and on/off switch) is actually due to the thermistor, which functions as a resistor when it's cold (but decreases in resistance as it heats up), versus the relatively low resistance of the transformer itself. If you were to measure the resistance of the primary of the power transformer right at the transformer itself ---- so that the thermistor isn't part of the measurement ---- it probably would be around 5 or 6 ohms at most! The impedance of course will be much higher than that when the transformer has 50 Hz power going through it. And since you're familiar with the basics of ohm's law, If you want to do a little math you can reverse engineer the approximate impedance of that transformer by calculating it from the known power supply voltage and the wattage rating stated on the back panel iof the amp, or using the amperage rating of the fuse (although the fuse is probably rated at least 30% higher than the usual steady state current draw when the amplifier is being operated). It will be both fun and a useful learning exercise if you do the math and post the results here!
@@stuartukguitarampguy5830, Correct primary resistance would probably be less than 10 ohms; but that's DC resistance and not impedance at 50 Hz !. See my other post to Jim here. Your measurement of transformer resistance included the thermister, the power cord, and the on off switch as well. The thing about series resistance is: it adds up! 🤣
@@goodun2974 Ah yes of course. I had forgotten about that series thermistor when I did the reading! I wondered hwy it was so much lower when I measured straight acros the primary winding. Makes sense now.
@Stuart ukguitarampguy I'm wondering if you can see if a mains transformer can be tested to see if is good with a multimeter. You went through a process to determine it was faulty that included putting power through it. Just wondering if you can do it with a meter.
On the board, all of the traces that went no where / had no traces and connected to nothing from the primary side of the transformer…. Is it possible the that the transformer had multiple primary winding sets for different ( international) voltages ?… so this amp was built to be set up for different supply volts?… 120v or 240v ?…. Just a guess
Perhaps in Europe you need power transformers with a *250 volt* primary winding in order to lower the voltages to the tubes and capacitors. Here in the States Id like to see transformers spec'd for 125 or 130 volts AC input.
Excessive current draw from a circuit fault; excessive heat (faulty circuit, or device undersized/under-rated for the electrical requirements of the circuit, or being left on overnight); or perhaps faulty manufacturing.
It's always because it draws too much current, the very thin wire in the primary heats up and then just eventually pops like a fuse. What CAUSES the over current can be quite a few things of course,. Fuses should protect against this but sometimes don't.
@@stuartukguitarampguy5830 , modern transformers may contain a *thermal fuse* inside, tucked up against or perhaps into/underneath the windings, to electrically open and prevent the transformer from catching fire if it overheats. Not the cause of a shorted transformer, however. Sometimes its possible to open up the transformer to bypass, or preferably replace the blown thermal fuse, pushing it up against the windings.
At 2:00, those are IC capacitors, aka Illinois Capacitors, not "IEC" capacitors. Cheap, crappy caps, not made in Illinois either, and known-to-fail when they age, but probable too new to be bad, not yet anyway.
@@stuartukguitarampguy5830 I didn't see any balancing resistors across those two first stage, series-wired capacitors.. Fender originally used 220K or 270K, something like that, across each cap to equalize the voltages so that one cap wouldn't die prematurely if it didn't spec out exactly the same as it's mate. If the resistors aren't already there, perhaps in between the caps and covered up by the glue, then they really should be added. Not that it makes any difference to this particular amplifier with a bad transformer! [ Edit I watch that portion of the video again and realized that the balancing resistors are physically located between the ends of the two capacitors, and since you can sort of see the foil traces running at an angle under the board, those tiny little things are indeed the balancing resistors. Not only are they perhaps a little bit too small for the job, they're located where you would normally expect to find the dropping resistors from one power supply filter stage to another. Hence my confusion.]
@@stuartukguitarampguy5830 , ps, If it seems strange that my comment about the resistors occurred *after* you mentioned that you thought the balancing resistors were already in place on the board, it's because I edited my original posting specifically to mention not seeing the resistors, and then my edit disappeared, so I wrote it separately again. TH-cam is flaky this way sometimes.
@@goodun2974 TBH I really can;t remember as I did the amp 6 months ago and only just got around the editing the vid. I haven;t heard back from the customer so I presume all okay for now. In general I don't do work other than fix the fault, unless it's something glaringly obvious. E.g. I don;t change filter caps which aren;t bulging, add resistors which aren;t there originally etc etc. The only reason for this is cost. Most of my customers are not well off and (e.g. £75 to fix a fault is fine but £150 to do that, change caps, add resistors etc etc is usually not). My view is that an amp is not a critical piece of gear (99% of my customers are not giging musicians). If it goes wrong again at some point, they can always bring it back.
@@stuartukguitarampguy5830 , I point these things out as I see them so that anybody else who owns or works on one of these amplifiers might get a preview of the kinds of things that are likely to fail, and therefore have a chance at addressing them, if budget and interest allow.
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Hiya, Love the walk thru Stuart..That's unlucky for the owner..! Last thing you expect to go on a not too old amp, is the mains transformer..Thanks for sharing..Ed..uk..😀
Cheers Ed
Nice one Young Stuart!! Had a brand new Twin Reverb in 78 and had the optional Fender factory orange JBL 120F speakers in it!! (God I loved driving that JCB fork lift moving it about!!)
Yes they are crazy heavy!
The process of elimination is always the shortest route for any issue where ampification is concerned. Well done, Stuart. I look forward to the episode featuring the 68' Fender. Cheers!
Thanks Alex.
Good job Stuart. I love watching you figure out what's wrong with these amps and seeing you repair them.
Thanks Zack.
Another great video - thanks - The sub board with the nasty surface mount components is actually to take place of the traditional fender neon and light dependant resistor or "roach" for the tremolo circuit this making the amp ROHS compliant as the "roach" contains Cadmium which is not allowed under ROHS rules for Europe. The board on the right next to the power transformer ( with gets ridiculously hot by the way which is probably why they fail ) is to provide that sub board with power - the best thing to to with these amps is to fit a low speed high volume 240v fan or two directly below the power valves to keep the chassis cool, being careful to place them in such a way as to not induce hum into the circuit and then replace the sub board and power supply with a £15 roach off of Ebay - they fit directly onto the board and even have a marked place where it fits. These mods will very much improve the reliability of this amp and make the tremolo as it should plus keep the bias stable as when these amps get hot the output transformer tends to wander in value.
Really useful info thanks Jools.
@Chen Ma , the traditional Fender tremolo "roach" contains a cadmium photo cell and a flashing neon light; the cadmium cell varies its resistance and turns on and off according to the rate of the light flashing, and that's how the tremolo effect is done.
I had this done. Swapped out the circuit for a roach. The place for it is there waiting. I also had the 240V tap connected. It's there too unbelievably. It's just risking the PT to have it running at 230 when I have 240 at the wall all the time.
Also worth noting that the mounting hardware for both of those pots on the underside of the chassis should be tightened securely. One is obviously for bias and I assume the other is a hum balance circuit for the tube heaters. If either one loosens and loses ground you could have *major* problems (noise and hum, or blown tubes and blown transformers). I'd relocate the thermistor somewhere off-board as well. I noticed the screen resistor's hanging off the output tube sockets by an inch, with full- length leads. Is that how it was factory-built? The resistor leads could actually be subject to sufficient vibration from the speakers that they eventually break.
Hi start, that board is an electronic tremelo, I thought this may help ..i've not long repaired one of these myself..great video again.
Yes apparently that's right. I didn't get involved in that part of the amp though.
The last thing I'd be looking for as well Stuart. Probably now no more expensive than buying a set of 6L6's .. I'd take the opportunity to replace it with one that has a 240 V input winding as despite what the narrative continues to be on the 230V , I've yet to find a 13 amp outlet within the UK with less than 240 V showing on the DVM.
YEs the way they 'guessed' at 230V is crazy!
Great video Stuart I will be needing to see you soon need you expertise 👍🏻
Thanks Tony!
Great video Stuart, very helpful
Cheers Zac
Thanks for the video Stuart, always fun to watch a fellow amp repair person in action.
May I ask where you get your repair jobs from, is it mainly private owners or do you work for backline companies, bands etc?
Thanks
Hi. I'd say about 50% are word of mouth or repeat business and the other 50% are from my website. Let me know if you need help.
At 5:04, that thermistor will run hot enough to break down its own solder joints within just a few years (especially because lead-free) and probably de-bond the foil from the board. Not to mention that the heat from the thermistor will bake the heck out of the BIAS 😖potentiometer only an inch away from it and probably cause it to change in value or fail prematurely. PS, Stuart, if the thermistor was "shorted", that short-circuited device is in series with the transformer and would merely act like a piece of wire; it would not cause your current limiter bulb to glow so excessively, nor would it cause the amplifier to become inoperable ---- although the amp might intermittently eat fuses for no apparent reason because the fuse value was likely calculated with the extra series resistance of a cold thermistor. (Inrush current at turn-on would be higher without the thermistor resistance in circuit, and so you'd likely need a slightly higher-value fuse).
Thanks. Yes I realised after doing the edit that I was wrong about the thermistor. Useful info as always thanks.
What's the loose brown wire you can see near the bias board?
Hi Tom, that's the famour ... er... loose brown wire thingy.... (tbh I can't remember)
@@stuartukguitarampguy5830 That's probably the winding for 240 V AC input, the one that should be used in this amp instead of the 230 V input.... after all, you guys use brown as the color for "hot" on the power cord. I suppose you could have a little bit of fun at customers expense by wiring amplifiers to use that Brown wire for the AC input and then you could say to the customer "it'll sound like the infamous Van Halen brown sound"! 😲
I saw that brown waving about !
If that was touching the chassis & blowing the fuse, perhaps the Transformer is OK
@@halvach1998 , it wasn't touching the chassis when Stuart was doing the measurements, and the way it was bundled up with the other wires, it likely never did. Regardless, the transformer was definitely shorted internally.
Sint the red stripe in case it is set for US power (110Vac)?
I'm needing to replace the tube sockets in a fender 1978 bassman . Looking thru ur past videos didn't see anything on tht subject . I cldve missed it do u recall any about tube sockets ? Thx
Hi I very rarely need to replace a socket. I find that cleaning with DeOxit and re tensioning the sockets with a sharp thin spike does the trick.
If you rally HAVE to replace the socket just make a careful diagram of where each wore goes (take a couple of pics too) and then it's fairly logical to replace it.
Good o utube , I didnt get a notification. Yeah tht makes sense .Thank u sir .
At about 6:40 you measure the resistance of the mains transformer primary to be 17.5 ohms and then say that's about what you would expect. I calculate E^2 / R to be about 3,000 watts. Seems very, very high to me. Did you really mean that 17ohms sounds reasonable for a working mains transformer, or that such a low resistance sounds correct for a transformer that is blowing fuses?
Resistance and impedance are not the same; the *inductive* impedance of the transformer is much higher than "17 ohms" at 50 Hz or 60 Hz. Most of the resistance that Stuart measures here in the primary circuit of the transformer (including the fuse, power cord and on/off switch) is actually due to the thermistor, which functions as a resistor when it's cold (but decreases in resistance as it heats up), versus the relatively low resistance of the transformer itself. If you were to measure the resistance of the primary of the power transformer right at the transformer itself ---- so that the thermistor isn't part of the measurement ---- it probably would be around 5 or 6 ohms at most! The impedance of course will be much higher than that when the transformer has 50 Hz power going through it. And since you're familiar with the basics of ohm's law, If you want to do a little math you can reverse engineer the approximate impedance of that transformer by calculating it from the known power supply voltage and the wattage rating stated on the back panel iof the amp, or using the amperage rating of the fuse (although the fuse is probably rated at least 30% higher than the usual steady state current draw when the amplifier is being operated). It will be both fun and a useful learning exercise if you do the math and post the results here!
Hi Jim, Tbh I had no idea what the 'correct' primary resistance for that transformer would be. I was waffling...
@@stuartukguitarampguy5830, Correct primary resistance would probably be less than 10 ohms; but that's DC resistance and not impedance at 50 Hz !. See my other post to Jim here. Your measurement of transformer resistance included the thermister, the power cord, and the on off switch as well. The thing about series resistance is: it adds up! 🤣
@@goodun2974 Ah yes of course. I had forgotten about that series thermistor when I did the reading! I wondered hwy it was so much lower when I measured straight acros the primary winding. Makes sense now.
I'm wondering if you got a mains transformer for this and got it working. Is there a way to check the mains transformer with a meter?
Hi Micharl I didn't quite understand your question.
@Stuart ukguitarampguy I'm wondering if you can see if a mains transformer can be tested to see if is good with a multimeter. You went through a process to determine it was faulty that included putting power through it. Just wondering if you can do it with a meter.
You can check the continuity of each of the windings to make sure none of them are open circuit.
On the board, all of the traces that went no where / had no traces and connected to nothing from the primary side of the transformer…. Is it possible the that the transformer had multiple primary winding sets for different ( international) voltages ?… so this amp was built to be set up for different supply volts?… 120v or 240v ?…. Just a guess
Yes that's certainly posible.
Someone should tell Fender the UK has 240V. They even have a 240V tap but can't be bothered connecting it.
Perhaps in Europe you need power transformers with a *250 volt* primary winding in order to lower the voltages to the tubes and capacitors. Here in the States Id like to see transformers spec'd for 125 or 130 volts AC input.
Hi Stuart, what's the reason for that? It's a new amp, i wonder why the secondary winding failed.
Hi Mark. It was the primary. Something drew too much current but I never found out what.
@@stuartukguitarampguy5830 thanks stuart
What makes a power transformer go out?
Excessive current draw from a circuit fault; excessive heat (faulty circuit, or device undersized/under-rated for the electrical requirements of the circuit, or being left on overnight); or perhaps faulty manufacturing.
It's always because it draws too much current, the very thin wire in the primary heats up and then just eventually pops like a fuse.
What CAUSES the over current can be quite a few things of course,. Fuses should protect against this but sometimes don't.
@@stuartukguitarampguy5830 , modern transformers may contain a *thermal fuse* inside, tucked up against or perhaps into/underneath the windings, to electrically open and prevent the transformer from catching fire if it overheats. Not the cause of a shorted transformer, however. Sometimes its possible to open up the transformer to bypass, or preferably replace the blown thermal fuse, pushing it up against the windings.
@@goodun2974 Ok thanks.
At 2:00, those are IC capacitors, aka Illinois Capacitors, not "IEC" capacitors. Cheap, crappy caps, not made in Illinois either, and known-to-fail when they age, but probable too new to be bad, not yet anyway.
I think the resistors were there. Yes those caps are rubbish. It's all down to what the customer wants to spend though.
@@stuartukguitarampguy5830 I didn't see any balancing resistors across those two first stage, series-wired capacitors.. Fender originally used 220K or 270K, something like that, across each cap to equalize the voltages so that one cap wouldn't die prematurely if it didn't spec out exactly the same as it's mate. If the resistors aren't already there, perhaps in between the caps and covered up by the glue, then they really should be added. Not that it makes any difference to this particular amplifier with a bad transformer! [ Edit I watch that portion of the video again and realized that the balancing resistors are physically located between the ends of the two capacitors, and since you can sort of see the foil traces running at an angle under the board, those tiny little things are indeed the balancing resistors. Not only are they perhaps a little bit too small for the job, they're located where you would normally expect to find the dropping resistors from one power supply filter stage to another. Hence my confusion.]
@@stuartukguitarampguy5830 , ps, If it seems strange that my comment about the resistors occurred *after* you mentioned that you thought the balancing resistors were already in place on the board, it's because I edited my original posting specifically to mention not seeing the resistors, and then my edit disappeared, so I wrote it separately again. TH-cam is flaky this way sometimes.
@@goodun2974 TBH I really can;t remember as I did the amp 6 months ago and only just got around the editing the vid. I haven;t heard back from the customer so I presume all okay for now. In general I don't do work other than fix the fault, unless it's something glaringly obvious. E.g. I don;t change filter caps which aren;t bulging, add resistors which aren;t there originally etc etc. The only reason for this is cost. Most of my customers are not well off and (e.g. £75 to fix a fault is fine but £150 to do that, change caps, add resistors etc etc is usually not). My view is that an amp is not a critical piece of gear (99% of my customers are not giging musicians). If it goes wrong again at some point, they can always bring it back.
@@stuartukguitarampguy5830 , I point these things out as I see them so that anybody else who owns or works on one of these amplifiers might get a preview of the kinds of things that are likely to fail, and therefore have a chance at addressing them, if budget and interest allow.
Having worked on the real deal this makes me mad they continue to call this a Twin Reverb
Yes that drives me buts. Marshall do the same thing. Use a classic amp from the past, reissue it, and it's not much like the original.
300 pounds. 😱
I took this video as a demonstration of how fking awful modern Fender RI's are!