Amp problem help sought - Amps Discussions on theFretBoard (2024)

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• ICBM Frets: 72906

  April 2016

  Hard to say, but yes it does sound like it's in the power amp. Fading over 3-4 seconds might imply something upstream of the filter caps, which is about the discharge time probably - but not necessarily. Does the power light fade out too?

  Given a likely second hand value of no more than about £100, you're right to be wary of spending too much money finding out...

  "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

  "Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein

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• mart Frets: 5205

  April 2016

  The power light stays on, only the sound fades.

  Given its value, it's going to have to be either a cheap and easy fix, or else it'll be binned and replaced.

  See Also

  Volodymyr Zelensky accuses Russia and China of undermining peace summitBass not playing properly

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• jpfamps Frets: 2734

  April 2016

  Another common issue is power resistors in the power supply becoming up soldered.

  As someone who repairs amps for a living, I'd have a quick look at it but it would likely to be a "marginal" repair from an economic point of view.

  It's certainly worth having a look inside though.

  www.jpfamps.com

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• DJH83004 Frets: 196

  April 2016 edited April 2016

  Gear for Music in York are putting a lot of these amps on EBay at the moment as spares or repair, not sure if this is one, but generally they have transit damage (from China). Definitely worth having a look to see if any components have become dislodged. I have fixed a couple in the past where the heatsinks have come dislodged and ripped out the output chip, I appreciate this is not the same fault but certainly worth a look.

  www.classicvalveamps.co.uk Nottinghamshire

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• mart Frets: 5205

  April 2016

  Thanks for all the help, guys.

  This amp has been owned and used for several years, so I don't expect such serious transit damage - although perhaps some damage from rough handling.

  I'll try and get the amp chassis out later and see if there's anything obviously loose or poorly soldered. @DJH83004 are there any useful tricks or tips in getting this thing opened up, or is it straightforward?

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• DJH83004 Frets: 196

  April 2016

  @mart From memory, the chassis just slides out of the top section, after removing the upper chassis screws, but I can't remember if you can disconnect the speaker connections from the amp or you may have to remove the grille and speaker to get access to the speaker connections.

  www.classicvalveamps.co.uk Nottinghamshire

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• Danny1969 Frets: 10498

  April 2016

  If it's a power amp fault then the signal volume should still be present on the line out socket and possibly the headphone jack, depending on how it's wired. If it fades there too then it could be global PSU. We've assuming it's a linear supply but some of this cheap stuff is built using switch mode as the parts are much cheaper (small transfomer, small caps)

  www.2020studios.co.uk

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• mart Frets: 5205

  April 2016

  Thanks. I have already removed the speaker, as that was the first thing I checked, before I realized that it was working at full volume for the first few seconds after switching on.

  The line out seemed to be fine, but the headphone output produced no volume. I guess that this is because the line out is coming before the power amp, whereas the headphone out is an attenuated version of the power amp out. Or maybe there's a separate problem with the headphone socket.

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• sgosden Frets: 1995

  April 2016

  I had similar issue with my Marshall,

  was some dry solder on resistor in power amp section, which then screwed upthe bias in the valves.

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• mart Frets: 5205

  April 2016 edited April 2016

  I've taken the chassis out, and can't see any obvious problems - no dry joints or loose components, or anything obviously awry.

  I tried the phones out again just now, and it worked for the first few seconds, and then faded away, just like the speaker. So maybe it is a PSU issue? If so, presumably some multimeter readings ought to tell me? Here's the PSU, in case that helps:

  http://i.imgur.com/hh64yG7.jpg

  As always, any help gratefully received!

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• DJH83004 Frets: 196

  April 2016

  I think from your tests so far you have determined that the line-out is functioning, which suggests the pre amp section is good and therefore that section of the PSU feeding it is OK.

  The headphone output sounds like it is derived from the power amp module as both it and the speaker output shutdown after a short period.

  My thoughts are heading towards a faulty output chip, it will be no doubt be a TDA... variant, which have internal shut down circuitry, to provide over-current and thermal protection. If you identify the chip, check the datasheet for the power supply pins, measure the voltage on the pins which will eliminate the PSU and you are pretty much left with a duff chip, (and I have changed dozens of them over the years, I'm afraid).

  www.classicvalveamps.co.uk Nottinghamshire

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• ICBM Frets: 72906

  April 2016

  DJH is probably right. There is another possibility though - what's in the pic is just the transformer, not the whole power supply, but it's slightly unusual for this type/cost of amp in that there are separate high and low voltage windings and thus separate power supplies for the power amp and preamp.

  The line out - and the 'power' LED - will be driven from the preamp supply, the speaker and the headphone output from the power amp supply. So a fault in the power amp supply - anywhere from the orange/black/orange winding to the output IC or transistors - could still be the problem. Metering the voltages when it's failed should show which.

  "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

  "Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein

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• DJH83004 Frets: 196

  April 2016

  @ICBM - good spot, both centre tapped so suggesting dual rails, maybe one rail on the power amp side is down or low and the chip is shutting down? Definitely worth identifying the output IC and checking the +/- supplies on the pins or tracks - but careful not to short anything as they are very close together :-)

  www.classicvalveamps.co.uk Nottinghamshire

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• mart Frets: 5205

  April 2016

  I suspect @ICBM was right, although not quite as expected ...

  I took the whole thing apart, and unplugged all the internal connectors but couldn't see anything wrong. Plugged it in to check if the power light faded along with the sound and ... it didn't fade, not the light, not the sound ... the amp was working perfectly.

  I've now reassembled it, knocked it about a bit to check for loose connections and no, the thing is working perfectly.

  So I guess it must have been a slightly dodgy link in one of the power connectors.

  Many thanks for all the suggestions. Having a team of amp techs on hand was very reassuring.

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• ICBM Frets: 72906

  April 2016

  Ha . That's surprisingly common - it's the reason I dislike those jumper connectors.

  I don't like repairs like this though - because you can't be *absolutely* certain it isn't something else you've accidentally disturbed in the process of checking it, or necessarily which one of the connectors it is (although in this case it would have to be the one from the 27-0-27V winding to the board), so you can't be certain the problem won't come back… usually at the worst possible moment for the customer, which does not make you popular .

  My usual approach - unless there is some reason why doing this is going to make any future work very difficult, and even then I usually do since it reduces the chance of needing to have it apart again anyway - is to cut the connector off and solder the wires directly to the PCB.

  It isn't always the contact between the connector and the pin, either - it's often the crimping between the connector and the wire, or sometimes where the pin is soldered to the board underneath.

  "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

  "Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein

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• mart Frets: 5205

  April 2016

  I know what you mean. I was actually a little disappointed when it started to work because it was "fixed", but I hadn't actually fixed anything. So who knows if it'll stay working.

  Fortunately for me, since it's for a friend and not a paid job, if it breaks again they won't have any grounds to be cross at me. I'll just tell them to take it to a proper amp tech and ask for a quote.

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• DJH83004 Frets: 196

  April 2016

  Well done! It is a bit frustrating when you can't pin the problem down 100% Lets hope it was a plug / socket issue rather than say a badly soldered component or pin on the PCB that is now reconnecting - time will tell

  www.classicvalveamps.co.uk Nottinghamshire

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