Roland JC-120: no effects working, any tips?

[rens]

Hi all,

I hope it's ok I ask this here because of all the other Jazz Chorus questions I found on this and other forums already. However, none of them describe the issues I'm facing with my newly acquired JC-120. It's probably a late 70s/early 80s amp, serial 176578. Toggle switch for the effects and bright switches. The BBD IC is a MN3004.

I was told the effects did not work, and they were right. Chorus, reverb and tremolo are all not working. Channel 2 works fine though, clean sound and even (a tiny bit of) the distortion. When I engage the chorus, a very soft 'pop' can be heard, indicating that something is switching. However, no chorus sound. Vibrato switch does not make a sound. Both speakers and power amps work fine.

A solder joint in the reverb tank was loose so I fixed that. Still no reverb. I suspect some connection in a part of the circuit that all effects share is faulty. Does anyone know a certain region or connection in the schematic that could be the source of this problem? Schematic (I think it's the right one) is attached.

I will start tapping solder joints soon, but I would like to ask first if there is something obvious I should check. On first sight, I don't see components that might be burnt or broken. I don't own a scope but I have a multimeter and I can solder. However, I don't understand the schematic well enough to see where a possible fault could be. Hope someone can help!

Cheers,
Rens

Edit/solution:
- disconnected legs 3 and 4 of IC5 (op-amp) that were connected during a previous faulty fix by someone; this restored the -15V supply.
- Replaced R195 in the power supply section for good measure, as its resistance was too low and might have blown something.
- Replaced transistor 2SC1624Y with a 2SD726 as the former turned out to be broken
- Replaced Zener diode D6 to restore +15V supply

Everything worked after this.

[Cpt. FixIt]

You don't really need to understand the schematic in full detail, take a look at the signal flow first to narrow down possible causes for the problem.
Just to make sure: Both (input) channels are passing signal/working ok, just no effects? This suggests that either:

a) the whole effects section doesn't get any signal
or
b) the signal from the FXS does not make its way back

Is your multimeter capable of measuring AC millivolts? Do you have anything you could use as a makeshift signal generator? You could even use a CD/MP3 player with sinewave files...

The FX section is located on the lower left of the schematic, starting with the reverb around IC1, next to the footswitch connectors. The signal is fed through R91(47k). if you trace that back you see it is directly taken from the VOL pot VR4. You say the channel passes signal, so we can rule out the reverb input. At the end of the reverb section, signal is taken from the wiper of the REV pot and fed back to the gate of Q4 via R22.
The chorus section(the delay part to be precise, as the LFO does not matter at the moment) input is R121, output C97 behind Q25, and finally routed back into the main signal path before the "main ins".
The switching is done slam-to-ground style, so there is no relay or sophisticated FET switches(which is a good thing IMO).

So, what would I do? Considering that reverb and chorus get their signal from different points in the schem and also feed their output to different stages, I can only come up with 2 possible reasons:
Either both happen to be shot or there is a problem with their supply, which is the only thing they really share. At least, if the 15V were not present, neither of both would pass signal.
So I would check if +- !5V are ok at C144/C145.

[rens]

Hi Captain,
Thanks a lot for your elaborate reply; it really helped me understand the circuit better.

Unfortunately my multimeter can't measure AC mV, so measuring the mV values that are on the schematic won't work (for now), I fear.
As the pcb is still mounted I could not measure C144 and 145, but I did measure R195 (2,7K 0,5W), that should supply C145 with -15V (right?) It measures +45V on one side and 0 on the other side that should pass the -15V to C145. I guess that means that it's fried? The + and - 43V supplies work fine.

throughout the chorus circuit I do measure +15V correctly (although it reads little over 14V, which shound be fine?) but at R109  don't measure -15V. Could that be the problem? As I don't see that many -15V points in the parts of the circuit that are connected with the clean signal path, would it be logical that the clean signal path is fine but the fx signals don't work because -15V is missing?

[Cpt. FixIt]

OK, it looks like you're on the right track. It is absolutely normal to have some deviation from the voltages in the schematic, 14pointsomething volts are fine.
No -15V would explain why your FX section does not pass signal. Now you should find out where those -15V get lost. It could be R195, but a short further down the line or a broken trace/solder joint are also quite common.
The next logical step in this game of divide and conquer would be to measure the voltage drop across said R195. Take the measurement directly at its legs. Zero volts would confirm a broken resistor. 43 volts would suggest a short, with C145 and D14 being the closest suspects.

[rens]

Voltage before resistor (between resistor and ground) measures 43V, after resistor with ground 0V. Voltage measured parallel to the resistor is 43V.
Tomorrow I will disconnect the PCB to measure the big caps there.

[Cpt. FixIt]

Sorry for not being clear enough: voltage drop across a resistor is measured in parallel. So R195 looks good, you can confirm that by measuring its resistance, just make sure all the caps are discharged before you do that.
Quite likely something is shorting your -15V to ground. I will take a look at the schem and possible culprits again during lunch break.

Edit: After a quick look I'll take a wild guess here. From experience I know that zener diodes very often go short when they fail. Additionally, there is no other 'easy' pathway to ground(ok, IC5 maybe), so D14 is the main suspect now.
The original part 05Z-15A is not available anymore, just put in any suitable 15V zener diode, it doesn't make any difference. Be careful not to put it in the wrong way.
If you pull the board, take a close look at its solder side and reflow everything that looks questionable. Considering the age of your amp, you could also swap the electrolytics as a preventative measure while you're at it.

[rens]

I took out D14 and tested it: it looks like it still works. R195 measured 1,57K (instead of 2,7K) and by the looks of it it has gotten quite hot some time. But seeing the glass fiber jacket it's supposed to be able to get hot. Would this be a problem? Would a preventive swap of both components be a logical step?

Speaking of IC5, someone apparently tried to rule out shorts between its legs in the past, there are some scratch marks between the solder joints of its legs. Can't find any other signs of someone fiddling with the board or dodgy joints.
I'll swap the caps, good call.

[Cpt. FixIt]

If you measured 43V across R195 it surely must have got hot. 43V/2700Ohms x 43V are about 0,66 Watts, already over its rated 1/2Watts. Even at 20% tolerance, 2160 Ohms are the lowest acceptable value for it - replace it!
Oh, and don't power up the amp without the zener in place...
How did you test the diode? Some failures occur at higher voltages, so replace it, too.
Zero volts at the junction D14/R195 means a direct short to ground. If we look at all the -15V connections in the schematic, there's not that much left: R52/R55 - 390k. R109 - 1k. R111, R114 - also 390k. Q32 - highly unlikely a pathway to ground. R173, R176, R177, TR8, C121 and R181 are also quite unlikely.
If the zener diode really works, I would look at IC5 next.

[rens]

Leg 3 and 4 of IC5 are connected through a solder joint which means a direct short between -15V and ground according to the schematic. I will re-solder it tomorrow, that should be it!

It looks like this IC has been replaced and was installed incorrectly. Could the short have damaged it (and would it be wise to replace it)? I would have to replace it with an equivalent (reading NE5532 as an equivalent now) as my local parts shop only has an SMD IC in stock.

Tested the diode with the diode testing mode on my multimeter after I took it out. Forward bias was little over 0.8V, reverse bias OL so I guess it works fine.
I'll replace the resistor and caps.

[Cpt. FixIt]

Congratulations, I also think you found the original problem. :dbtu:
I would just re-solder IC5 and see what happens. If it is dead and you have to desolder it, it is a good idea to put in a socket. A 5532 should work in that spot, but you never really know until you try.
You still have a local electronic parts guy? Be grateful, treat him nice and make sure to send some business his way - I find it really annoying having to wait several days for a few components that cost next to nothing plus several bucks shipping...

[rens]

Ok, I put everything back and switched it on. Small 'pop' sound through speaker, hope it wasn't anything blowing up.

Channel 2 still works fine and I think the distortion now does something. The return from the reverb works, but the send does not (only an echo from rattling the reverb tank can be heard, not the input signal); I'll have to look into that.

But now: chorus and tremolo. For both effects, when I engage them, the sound just pans to the left speaker when first both speakers were working. I have read something about this but I've read so many JC-120 threads lately I wouln't know where. Seems like a problem with the oscillator that just keeps hanging on one side. Will look further into it after work.

(Unfortunately it turned out my local parts guy just stopped his business 2 months ago, condemning me to mailorder parts  )

[Cpt. FixIt]

Looks like you just fixed someone elses incompetent fix.
So obviously you now have two different faults, one on the reverb driver side and one on the stereo chorus/vibrato output.
As there are no dangerous voltages in the reverb section, you could try touching pin 10(IC1) to induce some hum, and check the input of the reverb tank for continuity(point 7 and 8 in the schematic). There's not that many possibilities here, signal is taken from the volume pot, goes into driver TA-7200P and directly to reverb tank input.

Now if you turn the chorus/vibrato switch to the center position(off), both speakers work, right?
I will take a closer look at the schem after work. Should not be too hard to weed out your remaining problems.
You did a great job so far, just keep going, that amp is definitely worth it.

[rens]

I will get back to it asap, but first thanks for all your help and patience so far! I'm learning a lot to add to my limited knowledge on electronics.

Both speakers work when chorus/vibrato are off, but with either effect only the left speaker works (and passes clean signal).
Question on the reverb: should point 7 be connected to 13 and 8 to 15?

[Cpt. FixIt]

Question on the reverb: should point 7 be connected to 13 and 8 to 15?

No, not at all. Point 7/8 are the input of the reverb tank, basically a speaker with springs attached instead of a cone.
Point 13/15 are the output, sort of a dynamic microphone with the other ends of the springs attached instead of a membrane. So you should measure some resistance between 7 and 8, if they are open, the tank itself is the problem.

In your particular JC, the left speaker always puts out the clean signal, if you engage chorus/vibrato, it gets switched into the right speaker's signalpath only. So your signal gets lost somewhere in the chorus/vibrato section.
It could be the BBDs clocking oscillator, as a BBD will not pass signal without clocking(P1 and P2 in the schematic), but there's quite a few other possibilities.
Touch one of the leads of R137 with chorus or vibrato engaged and see if you get some hum from the right speaker.
Oh, and there is some sort of noise gate in the chorus/vibrato circuit, which checks for signal at the 'dry' speaker, to the very right in the schematic, around Q34 and Q35. If the connection at the end of R188("*2 1CH 20") is missing, it won't open.

[rens]

Reverb: 7-8 measures around 700 ohms when connected, but I guess it has to be disconnected to know for sure?
Pin 10 does hum, but I could not hear if the hum went into the tank. Tapping the tank with something does give an echo to the signal, but not the guitar signal.

Chorus: R137 does not give significant hum when touched, some faint crackling can be heard though when touching it.

The connection from R188 to point 20 of the channel 1 PCB looks fine and measures fine.