Roland JC 77 reverb issue

[guitician]

You could connect a small 8 ohm radio speaker to the reverb send and listen to it. It should sound like the guitar if all is well with that drive circuit.

[guitician]

At reverb input, I have a distorted wave

I missed this about the distorted waveform. How is it distorted? That should tell you a lot about what is going on with the driver circuit, which should be sending a clean waveform to the tank.

[xtian]

I like how you think. I have a battery powered "Smokey" guitar amp (a 1/4-watt amp). I ran the reverb send out to the Smokey amp. I can hear my guitar, but quietly, and it distorts early as I raise the volume on the Roland.

I think this supports the idea that one of the transistors Q3/4 has failed. Need more help testing. I can touch the pins, but can't get to the solder pads w/o removing the PCB. None of the pins show shorts to each other. All show high resistance of over 1M, except center pin to one outside pin which reads about 2.1k, which looks like it's the value of R10 and R11 on the schem.

Other ideas on how to test the transistors?

[Chuck H]

...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).

It might help to understand how the reverb works. The signal loss through the tank is infinite. only reverb escapes. And not much at that. The voltage at the input isn't indicative to a loss of signal, it's a current issue. 5V into a 14.7R load (10R input impedance with a 4.7R series resistor) is 1.7 watts. That's plenty of drive to the reverb tank. The interesting thing here is that there would be more than half a watt dissipated through that 4.7R resistor (R17). The amount of reverb from a properly working tank output is so small that if you're measuring for signals you would expect from an instrument or amplifier stage output you could mistake it for meter drift. Probably less than 7mV. You mentioned .1V above but perhaps that is a default reading from your test gear because there's no earthly way for a reverb tank to put out 100mV.

That test was also done improperly with 1V of input to the amp when the test spec indicates 85mV low gain/21mV high gain. Your next test was with 85mV, but was that into the low gain input or the high gain input? Strange and confusing results can happen when you overdrive input stages. So I'm going to reiterate that it's 85mV into the low gain input only and 21mV into the high gain input. If you had 85mV going into the high gain input that could still skew test results.

If you have the cursory few mV of reverb at the output of the reverb tank then 200mV at the output may be correct. Strangely, the schematic doesn't specify the part for the reverb amplifier. Not that I could do much with that. I have very little experience with SS amps.

That you can hear a small amount of reverb with the amp going balls out is telling. Since this is a combo I think it's possible the speaker is straight up shaking the tank and that is what is making the small reverb sound you hear. This still indicates the output is working. So... What of the input? You didn't measure any shorts at the tank, and even tried a new one. You have drive voltage at the tank input. But I think that voltage is too high. No way Roland decided to drive the tank with over a watt and jeopardize R17 as under spec'd. So...

I was originally concerned that the reverb tank you had and the new one you bought might not be the correct spec. But Accutronics seems to think it is according to some info I found on line.

So there's drive to the tank, the output circuit is working and two tanks are testing fine so that only leaves R17. If this resistor is open it might well cause the problems you're having. It would also explain what seems to be excessive drive since all the reverb tank drive power would now be shunted through R14 at 39k, raising the circuit impedance considerably.

Check R14 to see if it's open, if there is a cracked or cold solder joint or any damage to the circuit pad or traces.

[guitician]

I just check for the ~ .6 to .7 volt diode drop of the junctions N-P-N, P-N-P. My meter has a diode check function that I use. There can't be any voltages present in the circuit, and if there is low ohms looped across the junction you may not get a drop displayed.

Did you get that 5v P-P with the tank connected?

[xtian]

Top trace is signal exiting IC3b, bottom trace is signal as sent to tank.

That test was also done improperly with 1V of input to the amp when the test spec indicates 85mV low gain/21mV high gain. Your next test was with 85mV, but was that into the low gain input or the high gain input? Strange and confusing results can happen when you overdrive input stages. So I'm going to reiterate that it's 85mV into the low gain input only and 21mV into the high gain input. If you had 85mV going into the high gain input that could still skew test results.

With 1v p-p at input, and by varying the Volume control, I am able to get a clean sine wave throughout the system. Am I still setting myself up for trouble?

Anyway, earlier test was with 85mV into Low input.

Thanks for the details on the signal coming out of the tank.

[xtian]

Chuck, R14 (spec'd as 39K) reads only 2K!

R17 reads as spec'd, 4.9R.

[xtian]

False alarm. R14 is indeed 39K.

[guitician]

That attachment isn't working for some reason.

[xtian]

Attached again as JPG (last one was PNG).

[guitician]

Now is that loaded by the tank? It looks like it's overdriven into clipping. But I would say that both transistors(Q3&Q4) are working since it's tracing a sine wave. Try turning the volume down so the top trace is at .3v.

I remember the last reverb I worked on, and I could hear the springs rattle when the master volume was down all the way and played some strong chords.

[Chuck H]

Make sure R17 is indeed connected to the reverb transducer on one end and ground on the other.?.

[teemuk]

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.

Duly noted. Note though, that I'm looking at a real JC-77 schematic for reference and it shows 8-ohm speakers hard wired to the output. This one on the other hand shows four speaker jacks, two per each channel, as in JC-120 head version. As far as I know, JC-77 also don't have a "sub in" jack but JC-120 head does have one.

JC-120 head runs from bipolar 40VDC supply rails, JC-77 from bipolar 33VDC rails. I think power transistors are also different type.

Other than that, I can see where the confusion comes from: JC-120 head is also a single-channel amp and has the hi-treble pot. In fact, it's pretty much identical to JC-77 in its preamp design and for that part it suffices as a technical reference to fix that reverb thing.

[xtian]

Thx, teemuk. Can you post the real 77 schem?

Chuck, R17 is firmly connected to ground on one end, but shows 1R resistance between the other end and the reverb send. R14, which shares contact with R17 also shows the same 1R resistance to the reverb send connection point. I can't get under the board to see why.

I spoke with the owner last night, and he instructed me to give up on the reverb. So I can't charge for my time, and I can't give you all the satisfaction of the repair. Bummer.

Thanks very much for chasing the rabbit down this hole!

[Chuck H]

At the junction of R14 and R17 there SHOULD be the same reading (1R through the transducer I suppose) to the reverb send.

If you can let this go and the customer doesn't care then more power to you. I'd hop up and down and try not to give the amp back before I fixed it. Charging less for my time if only to see it done. But that's my problem