Roland JC 77 reverb issue

[xtian]

On the bench, Roland JC 77 2x10 combo, solid state. Reverb broken, with strange symptoms, and can't figure it out. If I rap on the tank, I get a nice springy sound amplified thru the amp. So the reverb return seems to be working. I can BARELY hear any guitar thru the reverb, and only if I crank the volume and really hammer the guitar.

I put 1KHz sine wave at front panel input, 1v p-p. Put my scope probe at the reverb tank input (marked "4" on schem), and I turn the volume up until sine wave distorts, then back off until clean, and I have 5v p-p at reverb input. This indicates to me that the reverb send is working properly as well as the return! But I'm only seeing 0.1v (100mV) sine wave at the reverb return, and after the makeup gain stage, signal is still only 200mV, and very noisy.

So, is it the tank itself? Don't think so--I just received an Accutronics replacement this afternoon! Both old and new tanks measure to spec in terms of DCR.

Should my signal going into the tank be a lot stronger to make this work?

[teemuk]

Your schematic is actually for JC-120 head version, but luckily the preamp design (including reverb) is pretty much the same in JC-77.

Ok, 1V input is way too high to represent a nominal guitar signal. why not heed the reference voltages in the technical documentation instead...?

These say that when using 1 Khz sinusoidal input signal, when you input 21 mV to HIGH input, or 85 mV to LOW input, you should read 290-300 mV at the output of IC3, which is the point feeding the reverb driver. You should then measure about 1.4V signal from the reverb input terminal and about 60 millivolts from the output terminal. Does this match?

Note: For proper accuracy, refer to schematic in how to adjust the amp's dials for the measurements. There's a section that says "Signal Voltages (rms) Taken Under Conditions:" ....and so on.

I'm only seeing 0.1v (100mV) sine wave at the reverb return, and after the makeup gain stage, signal is still only 200mV, and very noisy.

The recovery gain stage should have gain factor of approximately 8x. Something is not right here.

[xtian]

Thanks, teemuk, but I get the same (or similar) results. I put 85mV (RMS) at input with all controls set as suggested at bottom of schematic, and I see 210mV at IC3 output. At reverb input, I have a distorted wave, and have to turn the volume control down--I get 2v RMS (~5v p-p) at reverb input, and again, nothing at reverb pan output.

What kind of volume drop should I expect thru the reverb tank? It's 8AB2D1A (DCR 1R input/215R output, or 10/2575R impedance).

[xtian]

BTW, the schematic came from here: http://music-electronics-forum.com/t17962/

and the poster (Steve L) wrote:

PLEASE NOTE: The person that scanned this apparantly thought it was a JC-120 schematic and labled it as such, however it is really a JC-77 amp.
You can tell by the single channel and the hi-treble pot.

[guitician]

Don't reverb units respond to transients? A steady sine wave may not work very well. I would just touch the tip of the plug and give it a pulse of 60hz.

[xtian]

Good point. But the ultimate test is to play guitar thru it, and it's not working.

[guitician]

Whats your AC output on IC1b? That voltage is fed to the driver(Q3,Q4) then the tank input. Id check those caps c14,15 for leakage too.

[xtian]

AC output of IC1b is about 15v p-p. Then after Q3/4, at reverb tank send, it's 5v p-p.

[guitician]

5 volts doesn't seem like much swing from a ~ +/- 17v driver. How did those caps look? C8 may as well be checked too.

[xtian]

Good call! I have not pulled out the caps yet, but C14 measures 27pF and C15 measures 25uF with my Chinese "test everything in circuit" tester box. I'll try replacing these.

[guitician]

Those diodes may cause that tester trouble in accuracy.

[xtian]

And of course, you're right. I clipped out the caps, and they read to spec. Now I'm replacing them with new ones, because I have 10uF/100v caps on hand. Observing polarity!

[xtian]

FYI, I already checked conductivity of wires connecting PCB to reverb tank, both signal and shield, for both in and out connections.

[xtian]

So how to I test Q3 and Q4? (I'm a vacuum tube guy.)

[guitician]

Shorts show up as low ohms to a resistance test, and opens wont show any charge up, which is a gradual increase in ohms until the meter goes out of range. But for a microfarad value test you will need a capacitance tester.