Guild 98RT: Trem circtuit not working

[Jonny toetags]

So I've managed to wire this thing properly and fix the pilot lamp (thanks to the help in my other thread) and properly reposition the filter caps (they were glued inside the cab).
Now it's time to tackle the initial problem (and why I got the amp so cheap): the defective tremolo.
From what I have read, this amp is very similar to the early Ampeg Reverberockets and what I should be looking at are caps 1-5
Should I just replace them all? Cap 5 has no markings on it at all. I'm guessing electrolytic, but value?
This is what I'm working with:

[gui_tarzan]

Maybe these will help.

[audiopete]

According to the schematic, you need the footswitch to engage trem. You can use a shorted 1/4 plug. Have you done that?

Is the trem tube good?

Voltages around the trem tube test good?

Resistors test to spec?

Intensity pot test good?

Film caps are not the first thing I would look at, but there's always the possibility. The original cathode bypass electrolytic 22uF would be my first suspect (#1 in your photo - someone has put in a 40uF/20V cap at some point.). Use Sprague 25uF/25V there and please solder it and the black wire to its proper place in the circuitboard. With any old amp, you need to do a cap job and replace all electrolytics and eliminate issues like that. Any cap with a positive symbol (+) is an electrolytic.

[Jonny toetags]

One thing I noticed is that when I play with the footswitch, The trem engages for a split second.
I bypassed the actual switch and I get the same temporary split second trem when I touch the wires together.

[Wes]

I second audiopete on the 25v/25uF bypass cap replacement.

I have seen this issue in many amps- tremolo initiates, but the oscillation stops a second or two later.
Most of the time it has been because of a faulty cap or too large a value cap in the loop of the oscillator.

In your case- C12, C14, C15 which are all .05. Try substituting one at a time with a good, new .047 and
see if that helps. Maybe even try a .022.

[audiopete]

Could be a weak tube too, but if you don't do the tests suggested and answer the questions, no one will be able to help you. It's all just guessing at this point.

If the tube tests good, voltages are there, resistors and pot are up to spec and you have a fresh bypass cap installed, then I would look at the film caps in the feedback loop.

[drewl]

Its best to just replace all the .05uf caps as well as the cathode caps in theses old amps to get the tremolo running strong as it should.
Even if only one or two are leaking now, the others are probably weak and may fail later.

I have an ancient Ampeg G12 on the bench now with weak intermittent trem.
Aside from the oscillator caps I think I need to replace the LDR too.

[Jonny toetags]

I was waiting on some parts.
-All electrolytics on the board have been replaced and put to spec (put a 22uF in the trem circuit)
-Tubes tested 'ok', but I still ordered a few 6SN and 6SL's in case/for stock.
-Resistors all tested close to value.
-0.47's came in...going to switch those .05's out next.
-Also started re-flowing/checking all the solder joints (it IS old and a few spots looked suspicious). I have the dreaded 'random hiss and crackle' once the amp warms up....so I'm going to go through all the solder joints.

I will report back once i get those caps in.

[Jonny toetags]

Alright...here's an update:
-First thing I did was replace the 3 caps in red (see image). After doing so, I noticed that the trem worked, but was weak, even at full strength .
When engaging the trem on the footswitch, the initial press still had that nice strong trem for a second.
-After pocking around, I noticed the cap in yellow looked cracked. After switching it out, the trem woke up!



BUT...now my reverb seems a bit distant and noisy. It never ends with these old things eh?
The only other thing I did was re-flow some suspicious looking joints and I changed out the two 1K resistors on the power sockets (they looked black and dry)
Has anyone had any experience with tube sockets causing excess hiss and crackle? I'm debating changing them mostly because they really look haggard and seem real cheap and loose.

[audiopete]

You're cracked cap was C29 - coupling cap from phase inverter to power tube - if it was leaking it would affect trem and bias on that tube, so good job there. Those mustard caps are usually very solid and reliable.

[Jonny toetags]

You're cracked cap was C29 - coupling cap from phase inverter to power tube - if it was leaking it would affect trem and bias on that tube, so good job there. Those mustard caps are usually very solid and reliable.

Yeah...that's why I normally wouldn't have looked at them.
Luckily, I pulled a mustard with the same value off the switch when i wired the grounded plug.
Still looks stock

Now I have to figure out what the hell happened to the reverb. It was dead quiet and full sounding until last night

[52 Bill]

Now I have to figure out what the hell happened to the reverb. It was dead quiet and full sounding until last night

Try cleaning the tube sockets for the reverb tubes as well as the RCA plugs for the reverb send and return cables.

[Enzo]

Those mustard caps are usually very solid and reliable.

Yeah...that's why I normally wouldn't have looked at them.

And THAT right there is why I shudder every time someone asks "What are the common failures in my XYZ?"

NEVER think up reasons not to check something.

We troubleshoot by isolating the problem, not by looking it up on a list of most likely failures.

[Jonny toetags]

Try cleaning the tube sockets for the reverb tubes as well as the RCA plugs for the reverb send and return cables.

I had already cleaned them all up

[Jonny toetags]

I did put new preamp tubes in. I might put back the originals.
Speaking of which, seeing as the lettering on the originals is gone and I keep reading different things on schematics: what is the proper tube layout?
I have read 2x6SL7 2x6SN7
3x6SL7 1x6SN7
1x6SL7 3x6SN7