Troubleshooting SS amp...30vdc on OUTPUT!

[Prattacaster]

No voltage on those pins.

[Jazz P Bass]

I think you need a scope & a sine wave generator for thia part (or a good ear)
TP 1-3 concerns setting the input waveform for symmetry.
Just for grins & giggles can you set the center voltage as indicated in the SM.
I am curious what that does to the bias draw.
If it does not help then put it back.
FYI the SM I have is for the G50.
That may explain the lower center voltage requirement.

[Jazz P Bass]

[QUOTE=Prattacaster;215698]G100 Service Manual.pdf
This is the manual I am using, it is a pretty close match. This one says center voltage 45vdc but with a B+ of 93vdc. I have a B+ of 122 so I need to be half that like R.G. mentioned.
QUOTE]
That B+ of 122Vdc is really high.
The schematic indicates a 2200uf/100V capacitor.
This caps voltage rating is being exceeded.
I see there are taps on the transformer for Mains Input of 130/117 & 110.
I would try a lower input tap to bring the B+ down.

[g1]

That B+ does seem too high, like JPB said you may need to use a different tap for the PT primary. What is your wall voltage?
As far as the bias goes, I'm a little iffy on replacing the diode pack with 3 diodes, can you try the original or is it shot? Those special diodes can be fairly critical and may not always give normal diode drops.

[Jazz P Bass]

The 130 tap should do the trick.
Unless it is on the 100 tap instead of the 117 tap

[Prattacaster]

It is already using the 130v primary tap, or rather it is using the grey primary lead. What if I used a full wave recto instead of a bridge, I'd rather not do all that work but I agree that the input cap is underrated. What if I hooked another cap in series with the existing cap?
The bias voltage is still going to run high, this needs to be address. How long can I leave the amp at idle safely with the bias running 10x what the manual calls for?

[J M Fahey]

The main overvoltage problem is that , beyond plain voltage specs themselves, *second breakdown* failure possibility becomes *much* higher.
Transistors can easily die driving regular speakers in "normal" conditions.
Re-check transformer windings and taps, you can't have *that* high +B, specially if you are using the 130V tap, check where "the other" wire is connected to.

[Jazz P Bass]

I know they are not doing much, but are the two 120K resistors in good shape (on the BR, at D2)
I would check the Vac of each winding, regardless of the color.
It wouldn't be the first time a tap was wound wrong.

[Prattacaster]

ok, ill do all that, I dont like seeing the voltage that high either, I just thought it was the difference between design and wall voltages of today, but there could definetely be other issues, I'll check it out.

Even if I get the voltages down the bias is still going to set high, With the bulb limiter the b+ was 88vdc and the bias was still around 10x higher than spec.

[J M Fahey]

It wouldn't be the first time a tap was wound wrong.

Maybe, why not?
Or even simpler, the schematic and the actual amp may not be of the same version, and colors do not match.
Anyway, 94V DC mean 66 or 67 VAC on the secondary; I would leave secondary wires unconnected, with only the multimeter hooked to them, and try any reasonable combination of primary taps which gives me around 66VAC secondary.

[Prattacaster]

I just saw that the input filter cap is 160v rated, so no problem there, I will figure out which secondary to use to give me the proper voltage.

HOW ABOUT THE BIAS! It is 10x the schemo spec. Do I need to add/takeaway another diode in the diode chain? Change a resistor?

[Jazz P Bass]

Are we working off of the correct schematic?
Is your G100 a head like the SM picture?
Here is what I have.

[R.G.]

HOW ABOUT THE BIAS! It is 10x the schemo spec. Do I need to add/takeaway another diode in the diode chain? Change a resistor?

Almost all class AB SS amps bias by applying a bias voltage between the bases of the top and bottom half driver transistors. For instance, in the
schematic here the bias components are the 220R pot marked "idling current", the 56 ohm resistor in parallel with it, and the TS51 diode in series with those parts.

If you short the 56 ohm resistor and the TS51 diode with a wire from the top end of the 56R resistor to the cathode of the diode, the bias is forced to zero volts, and the amp is then running slightly in class C. The idling current will be very, very small, perhaps unmeasurably so. The amp will still work, but will be prone to a slight buzzy or "ghost note" distortion on soft passages; the softer the sound, the worse the distortion. This is classical crossover distortion.

If shorting those components does not dramatically reduce idling current, something else is wrong with the amp.

If shorting those components does drop the idling current, try shorting only the 56 ohm resistor and measure the idling current. If it's still too large, replace the TS51 diode with a 1A rectifier diode, like one of the 1N400x series. A diode with too large a voltage across it will cause too high an idle current. So will an idle current pot with a high resistance or open wiper. Given the "chain of disasters" failure this amp has had, both of those are possible.

I don't like this style of bias adjustment because of the high currents through that idling current pot. I much prefer a "Vbe Multiplier". I can tell you how to proceed with that if you want.

[Prattacaster]

Dude great info, I replaced the bias diode with 3 series 1n4004's. So after I troubleshoot via jumpering I might find that using 2 1n4004's would lower the bias? I also replaced the bias trim pot so I know to pot itself is not the issue. Big thanks!

[Prattacaster]

I have the G100 series schemo that JP Bass has, and then the manual that I attached earlier in this thread, that manual schemo is closer, but not perfect, It didnt have the 12 ohm resistor to ground off the recording out, but we've been over that one. Everything else is right on.