Roland Jazz Chorus JC77 chorus Circuit.

[ziggy007]

Gday all,
Quite possibly a dumb question, but I'll ask anyway. (my pride is diluted by some nice red wine )
I'm troubleshooting the above amp which has no chorus. Output dies when Chorus, either fixed or manual is selected. Reverb works fine so i know everything on the output side is ok.
The chorus cct itself seems to be working ok as there is a controllable (via rate and depth pots) small voltage bleeding thru (about 40mv)

Im trying to under stand the theory behind : a) the fet switching. Namely Q21. and the two BJT's Q18 and Q19. Solid state is undoubtedly my achillies heel but with Covid I'm seeing heaps more and I am too proud to say no!!

thanks
Gav

[Enzo]

OK, you do know there are two power amps, one for each speaker? So there is no "the" output.

So does sound come out both speakers?

The way it works is with chorus off, the dry signal should come out both speakers. With chorus ON, the dry continues in one speaker while the chorused sound comes out the other. Notice teh signal path is constant to the upper speaker and amp, while Q17 and Q21 decide whether dry or chorus go to the lower amp and speaker. SInce only one of those should be on at a time, Q19 inverts teh control signal so that happens.

[ziggy007]

Thx Enzo. Yes I do.
The signal on the speaker output 36 disappears when chorus is engaged. But turning up reverb allows that output to work as it bypasses those fets.
I was trying last night to understand how that a Q18 and 19 were biased on. Mainly 18.

It all briefly worked for a second once, but when I celebrated and re installed the board (loosely) it disappeared again. No chopsticking or removal can reproduce. Back to the bench in an bit to nut it out.

[g1]

I was trying last night to understand how that a Q18 and 19 were biased on. Mainly 18.

Q18 emitter is tied to -15V, base is also but through resistor. When switch is open, both B & E sit at -15V, so transistor off. When switch is closed, base is pulled up toward ground, making it more positive than the emitter, turning Q18 on.

[ziggy007]

Thx g1.

I was right. Indeed it was dumb question!! So obvious when I now read the diagram correctly with your help. thanks mate!

Ive dug in deeper and a bit more methodically. With 1khz 21mv input signal, There was no output from the Ic6 getting past R133, Lifting the lower leg on the 100k R132, isolating it from the 6.5v fed from R137/R138 voltage divider now allows the Chorus (& Amp) to work correctly. This is my next point of confusion!!!
Am i right to assume that voltage is there to bias Q27? Now with it removed, everything works!
All the components in that path check on spec.

[g1]

Yes, that voltage divider in the lower left corner (R137/R138) is just creating a reference voltage, primarily for IC5 inputs. They are also using it for TR27 base. Is R127 and it's connections good?

[ziggy007]

Yes, that voltage divider in the lower left corner (R137/R138) is just creating a reference voltage, primarily for IC5 inputs. They are also using it for TR27 base. Is R127 and it's connections good?

Yes. R127 is spot on and all grounds/ connections are sweet.
I must say i dont understand why having that Reference Dc avail thru R132 would physically stop the AC signal out of the IC from passing thru those resistors to at least the base of Q27.

[g1]

Have you checked if the AC signal levels before and after that area are proper with the 'fix'? Is it possible Q27 is bad and when you do the fix it is just turning it off and there is some signal getting through but not the correct amount?
Maybe post DC voltages for E, B, & C of Q27 with R132 connected, then with it lifted

[ziggy007]

Everything checks ok, but due to the intermittency of this I just went thru and re soldered every connection in that Reconstruction filter area post BBD stage output and now everything is stable. Obviously a micro crack or similar in there somewhere.
Thx for the pointers mate and a Happy new year to you.
Cheers Gav