Guyatone Flip MKII GA-2000 Amp - screen resistor values?

[beedoola]

Hopefully my title has been changed/edited.

So I got this amp, replaced the bad screen resistors and got new 6l6s. Amp works but there is some hum. Goes away with the master volume off. Does this mean the caps on the board with the filter caps is good? If it was a bad cap on that board, would the sound still be audible if the master was off?

I took some pics of these electro caps..

[beedoola]

none of the electros seem to be bulging but 3 of the smaller ones have black/ what looks like burn marks..

[Jazz P Bass]

Short of having a cap tester, the best way to verify the health of a cap is to test how well it is removing ripple.
Set your meter to read volts ac & measure them.

[beedoola]

I have a capacitance feature on my Fluke 117... I removed the caps and the 47uf read a little high - 53uf. The 1000uf is at 994uf.

What is remove ripple?

[Enzo]

WHen AC voltage is rectified, the result is bumpy DC voltage. The filter cap is added to smooth the bumps out and leave smooth steady DC. Any remaining litle DC bumps that are left, we refer to as "ripple." It is basically a small amount of AC voltage riding on the DC voltage. Reasonably good meters can be set to AC volts and measure this ripple on the DC voltage.


Your meter can measure the capacitance of a cap, but it probably cannot measure the leakage or ESR. And a cap that works at the couple of volts from your meter, might work terrible at 300 volts.

[beedoola]

WHen AC voltage is rectified, the result is bumpy DC voltage. The filter cap is added to smooth the bumps out and leave smooth steady DC. Any remaining litle DC bumps that are left, we refer to as "ripple." It is basically a small amount of AC voltage riding on the DC voltage. Reasonably good meters can be set to AC volts and measure this ripple on the DC voltage.


Your meter can measure the capacitance of a cap, but it probably cannot measure the leakage or ESR. And a cap that works at the couple of volts from your meter, might work terrible at 300 volts.

does the blackness on those caps I posted an indication of them being bad?

For measuring the AC. do I just test the positive side of the caps with the negative probe to ground?

[beedoola]

I'm getting 4.2 AC on one side of the Electro caps, the positive of one of the big filter caps. Is that enough voltage to be suspect?

[Jazz P Bass]

If the capacitor in question is the voltage going to the center tap of the output transformer (A+) then that is probably o/k.
That node always runs with a few volts ac on it.
4 Vac is a little high.
If you have another cap of the same voltage rating you could clip it in parallel to see if that helps the problem.
It will lower the ripple but what you want to see is if the problem is helped.

[beedoola]

If the capacitor in question is the voltage going to the center tap of the output transformer (A+) then that is probably o/k.
That node always runs with a few volts ac on it.
4 Vac is a little high.
If you have another cap of the same voltage rating you could clip it in parallel to see if that helps the problem.
It will lower the ripple but what you want to see is if the problem is helped.

I'm not sure, I think it is the cap highlighted below. I email Tokyo Sound and they emailed me the schematic:

GA-2000?.pdf

[Jazz P Bass]

If that is the cap, that is the A+.
Check the other ones.

[beedoola]

All the other caps show under 1v AC - like 0.127 or less.

If I pull the preamp tube and fire up the amp, the noise isn't present, volume up or not.

I pulled a 12AX7 from my Ampeg V4 to test in case in was the preamp tube that came with the amp but the same issue occurs with both preamp tubes.

[Jazz P Bass]

It looks like the power supply is good, cap wise.
The next step is to look for leaky coupling caps.
They are the small value, nonelectrolytic caps.
The reason they exist is to block the plate Vdc from the next grid, as they pass signal Vac.
So you can check that with a DVM.
One side will be plate voltage & the other should be zero volts. (ideally & depending on the circuit)
A leaky coupling cap can play all sorts of havock with the next stage.

[beedoola]

I replaced the 3 .1uf 630v caps around the 12Ax7 and that didn't help. I probed around the .001uf 630v cap and that made the noise lessen a little, so I thought, so I replaced that but it didn't help.

The control pots PCB was super dirty and some shitty soldering at certain points. I sucked those up, cleaned the board with acetone. I also found a cracked resistor and replaced some of the poor solder joints. That fixed the weird sound I was hearing.

However, I have a new issue. Now, even with the master volume off, there is a hum - sounds like ground hum, what you would hear if you have a cable into the amp input with the volume ON yet no instrument plugged in. If I pull the 12AX7, the sound isn't there. With the Master volume off, there sound is there.

Could this mean its still coming from the control PCB, even if the volume is off?

[beedoola]

So I thought I fixed the issue; the sound coming from the master volume went away, but then I got the reverse - with the master volume off I'd still hear some noise. The noises were the same but it was still noise.

So I cleaned the tube sockets with Deoxit Gold and re-tensioned them. After that the amp worked great. I was having trouble with the two Channel LEDs - I had to replace them and the replacements won't go out completely when their respective channel is switched - I get some bleed through. Plus I had to buy a new switch for the Bass mode selector.

I get the switch, install it and the first problem is back! - the sound coming from the control PCB. When I turn the master volume off you don't hear any noise.

I pulled the two ICs to see if they were an issue or something at the Input, that doesn't appear to be the case.

Any suggestions on how to proceed?

[beedoola]

bump, if anyone can help, this is bugging me terribly.

If I pull the last cap before the Volume control and run my guitar straight there - bypassing everything on the control PCB, straight into the 12AX7 and 6l6s, the sound/hum/noise is gone, so its somewhere on the control board.