Randall RG100 ES solid state questions

[Randall]

I'm looking at it with a scope and various tones from a generator. No effect at all. The bass has the most effect, the treble some, but none from the mid.

[Enzo]

Seriously, try it with music.

[Randall]

OK, I tried it with music, nothing. I suspect that .2 cap on the mid wiper, but I don't think it is my overall problem.

Looking at the input signal, am I correct in thinking that calls for a 6mV 1K sine? Isn't that quite low? My HP generator doesn't go near that low.

[DrGonz78]

I suspect that .2 cap on the mid wiper, but I don't think it is my overall problem.

Also, like they always say here it is not always a bad component but could be a bum connection within the circuit path that is the culprit. I would definitely inspect solder connections and traces in the suspect area of the circuit.

[Enzo]

6mv at the input is just their test condition, and with that at the input, the other signal levels should occur. Of course guitars out out more signal than that, but an amp needs to work on the quiet passages as much as the loud ones.

[g1]

They don't mention how the controls should be set to get those numbers, but I suspect the 6mV input is so the gain and volumes can be set to full.
If your HP can't put out such a low signal, run it into a pot and take the wiper to the amp input.

[Randall]

Could I get some help please with the signal flow in the PA section here? I am delivering a clean 1.6v p-p sine through the FX loop to the base of Q8, but after that, don't see the path to the drivers Q11 and Q12, or the outputs. As it stands, I see the same 9 v p-p sine on the bases and emitters of Q 11 and 12, as well as the bases of Q13 and Q15 outputs. I see the same wave on the cases of Q14 and Q16, but I suppose that is because they are all on the same output rail. The collector of driver Q12 and the bases of two outputs Q14 and Q16 have the same 1 v p-p messy looking wave.

I also do not understand what the trimmer is for or what to adjust it to, or the two IN914 diodes. All voltages seem to be reasonable.

[g1]

Signal should appear at Q8 collector (& Q10 base) then Q10 collector. From there on to Q11 & Q12 bases.
The trim pot and diodes are part of the bias string. Whether it needs adjustment or not depends on the voltages (DC idle) across R55, R56, R57, & R60.

[g1]

And that was just an answer to your question with no regard for the amp fault.
Yesterday you had 72 Watts of output. Unless you had to insert a massive signal to get that, your problem should be in the preamp.

[The Dude]

Yep, I thought we determined it was a preamp problem? Now I'm confused as to where we are here.

[Randall]

I as confused as anyone. I got 72 watts by inputting at the FX in jack with my HP signal generator turned up a fair bit. So I went looking in the preamp, where I found a couple of things, the mid control and a coupling cap. Corrected those. Then I realized because my signal generator is very hot compared to the test points on the schem, I am putting out much higher signals at the test points, including over four times the 250mV at the FX return, which is supposed to produce 20v signal at the speaker jack, and with my higher signals, and I am only getting 9 volts p-p. So, I'm back to the PA section.

It doesn't seem to me the outputs are amplifying. And if they test good, per Enzo, why aren't they?

"Signal should appear at Q8 collector (& Q10 base) then Q10 collector."

I don't have that on Q8 collector, just less than 1 volt sawtooth with a tiny bit of signal riding it. I also get 42vdc rather than the 38vdc on the drawing.

[The Dude]

I haven't re-read the entire thread. Did you try running the effects send out into another amp to see if you get normal preamp output?

[Randall]

From the Randall FX out with full open controls, it drives a GK 400RB to 150 watts into 8 ohms. 1K sine wave, hitting the Randall input pretty hard, but the sine is clean.

As I said, I have a more than 3 x bigger clean sine from the pre out hitting the Randall PA section at Q8 then the schem is looking for, and I still have next to nothing at speaker outs, so I'm focusing on the PA. I'm thinking I should find a suitable replacement to change out the diff. pair at Q8 and Q9 since I have nothing at the collector of Q8 but some noise?

Sorry for the vacillation, but I think this amp had more than one fault.

[g1]

Assuming that the signal injected at FX return is getting to Q8 base, and all the DC voltages around Q8 and Q9 are correct, then I don't know what else you could do aside from trying diff. pair replacement.
Can you post your EBC voltages for Q8 and Q9 ?

[Randall]

Right even with an 8 volt p-p 1K sine at the FX return jack (enormous, I realize), the same seen at Q8 base, I get about 70 watts at the speaker, but it doesn't show up at the Collector of Q8 or the base of Q10. SO how it it making those watts?

Q8 E=-0.87v, B=-0.27v, C=41.6v

Q9 E= -0.87v, B=-0.27v, C= 42.4v

Both the TIP98 on the schematic and the MPS8098's that are actually in there I find to be obsolete. I found a suggestion from Mr Fahey in another forum to replace them with Fairchild BC546. It has equal to or greater EBC voltage ratings, but less collector current rating. Both originals are 300mA, but the BC546 is 100mA. Should this really matter in this application with preamp signals?

Also, how important is it that the speaker jacks are isolated from chassis ground and only see ground thru that big .18 ohm resistor? As they are now, it appears they are only isolated by chassis paint, and it is failed, as if I remove the resistor connection, I get 2 - 4 ohms to chassis. Should these be plastic iso jacks?