Another reverb down, unbelievable

[loopyloo]

Just checking out another one of my amps (yes I have a few) and the reverb in it doesn't work.
The tank is good and the wiring from it to the amp board is good but I found a burnt resistor.
The amp works fine in every way except for the reverb so I have to assume it has something to do with this resistor. Trouble is, what size to use ? because there's no schematic available for it.

[hinekadon]

whats the amp brand model ? i think its the shunt resister 1k at a guess? can you check to see if one side is tip and the other is body on the phono jack regards don

[loopyloo]

whats the amp brand model ? i think its the shunt resister 1k at a guess? can you check to see if one side is tip and the other is body on the phono jack regards don

Looks like the resistor is not related after all.
I've got the board lifted and found a couple of other issues. Will post some more info and pics tomorrow.

[loopyloo]

This is what it looks like.
The only photo I could find on google.

[hinekadon]

looks like a cheense knockoff of a fender to me early 90s range about 50w see a few they werent to bad but suffered reliability problems ? could be that some one has stuffed ac into the output headphone socket at a guess not a uncomon fault may help you fault find regards don

[Uncle Fester]

Cracked solder joints most likely. Anything single sided from China and exposed to vibrations like that combo amp would have fatigued joints

[loopyloo]

The resistor reads 190k ohm out of circuit.
It runs from the headphone socket (plug tip) through a greencap to ground. So not related to the reverb.
I've never plugged headphones into it so never noticed there was a problem there.
The board has 2 other issues I'll fix : the fuse must have been pushed in with some force because the two clips were bent sideways causing one side on both of them to fatigue and break, The fuse was literally just sitting there under gravity, lovely !. Also I will replace two low quality 2200uf filter caps because one of them has shrunken wrap. Those parts should arrive in a day or two, then I'll get back to the reverb.
In the mean time, 200k ohms doesn't sound right for that resistor does it ?

[hinekadon]

the resister is a shunt should be about 1k from memory its to stop bangs from connections in and out of the socket

[loopyloo]

the resister is a shunt should be about 1k from memory its to stop bangs from connections in and out of the socket

Ah so that's what it's for.
Well I should have a 1k somewhere for it then.

[loopyloo]

Cracked solder joints most likely. Anything single sided from China and exposed to vibrations like that combo amp would have fatigued joints

Surprisingly the soldering actually looks quite good. I'll find out though when I pull out my loupe.

[Uncle Fester]

I wouldn' waste time looking as it is not always that obvious, just solder over all the ones that hold larger components first and especially the pots.
Tricky to see sometimes if there is a crack right where a solder pad joins to the track and is quite common with pots soldered directly on to single sided boards.

Odd the burnt resistor if it is in series with a cap. That may be to suppress wild oscillations but that is usually directly connected to the speaker output not the head phone jack. Trace the resistor to the amp output, there should be another resistor in series to lower the signal to headphone level/impedance and then there would be no way it could have burnt out.
Only other reason: the former user somehow managed to plug an amp output into the phone jack and cranked it up (thought it would power the speaker, LOL).

[hinekadon]

I wouldn' waste time looking as it is not always that obvious, just solder over all the ones that hold larger components first and especially the pots.
Tricky to see sometimes if there is a crack right where a solder pad joins to the track and is quite common with pots soldered directly on to single sided boards.

Odd the burnt resistor if it is in series with a cap. That may be to suppress wild oscillations but that is usually directly connected to the speaker output not the head phone jack. Trace the resistor to the amp output, there should be another resistor in series to lower the signal to headphone level/impedance and then there would be no way it could have burnt out.
Only other reason: the former user somehow managed to plug an amp output into the phone jack and cranked it up (thought it would power the speaker, LOL).

thats what i said (could be that some one has stuffed ac into the output headphone socket at a guess not a uncomon fault may help you fault find regards don)

[loopyloo]

Replaced components and resoldered some joints but reverb still out.
Back together now for more checking.

[hinekadon]

hi loopy i wonder do you have an oscilloscope the two op amps are the delay line so you need to check which one is not working the reverb pot controls the depth and also the delay time by the resistor and caps between the two op amps but first check 15 +_v rails that supply them don

[Reschs]

Quickest way to determine which side of the spring is at fault is to simply touch it.
With all settings correct, you should hear it, if the output side is working.

[loopyloo]

Quickest way to determine which side of the spring is at fault is to simply touch it.
With all settings correct, you should hear it, if the output side is working.

Yeah that was the first thing I did before checking the connections.

[loopyloo]

hi loopy i wonder do you have an oscilloscope the two op amps are the delay line so you need to check which one is not working the reverb pot controls the depth and also the delay time by the resistor and caps between the two op amps but first check 15 +_v rails that supply them don

Will be checking with my oscilloscope tomorrow.
I suspect one of the op amps might be gone so I ordered a few today and they will be here tomorrow.
Also I haven't checked the reverb pot yet.

[hinekadon]

those op amps are the same as 471 but dual in one case not that that matters but to keep in mind , unlikely to be the pot unless its had a good bash in which case it will fall apart

[loopyloo]

It's the pot.
10k log read 8.5k and no connection to the wiper.
Replacement fixed the reverb but there is another problem. The more the reverb is added, the more 50hz (actually I think it's 100) is introduced into the signal. It's quiet with reverb all the way down but quite noisy when full up. Also changing any other pot like volume and such makes no change to the loudness of the noise, like it's directly injecting into the final stage of the amp.
Thoughts ?

[hinekadon]

audio loop ,something has lost its ground or the center tap of the 15vdc rails