Dear all, swapping the diode D6 did it; everything works fine. Thanks to all of you who helped, without you I would have been clueless!

Things that I fixed:
- 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 measured 'closed'
- Replaced Zener diode D6 to restore +15V supply

Thanks again, everyone!

Quote from: g1 on November 13, 2018, 01:52:09 PM
At the base of that transistor is D6, 15V zener.  Have you checked it and it's connections?

Thanks for this reminder, the diode is blown and shorts to ground! I'll look for a replacement.

Quote from: g1 on November 10, 2018, 01:55:56 PM
Maybe Rens can verify what C50 connects to.

It's definitely there, and connects to point 6 (so to the + terminal of C144 through a jump wire, and the transistor).

I tried to fit a 2SD726 and it didn't work. Still no +15V at its emitter. +43V is present though at the collector. Maybe it's broken and I should try another one (I've got two).

I have one similar-ish NPN transistor that's labeled D726 in the power section of an old bass amp. If this is the same as a 2SD726 that I found on alltransistors, the values are:
Vcb: 100V
Vce: 80V
Ic max: 4A
Pc: 60W
datasheet: https://alltransistors.com/pdfview.php?doc=2sd726.pdf&dire=_inchange_semiconductor
Pinout is the same as 2SC1624.

Vcb is lower than 120, is this within a safe margin to try it, or is it too low?

Hey Phil, by saying "can't trace +15V back to C144" I meant I did not measure +15V at C144 which it should originate from. The positive terminal of C144 is jumped to point 6 on the PCB, which leads to the emitter of 2SC-1624Y. (see page 2 of the schematic, it's jump 6(point 6) to X, a point on the PCB that's directly connected to C144 + terminal). The jump is directly connected to point 6 (so the transistor emitter and C50) and IC1 leg 3.

Help me understand: the +15V rail is provided by the ps section C144, right? the +15V supply is then amplified by 2SC-1624Y which uses the +43V for this? (I don't know any semiconductor theory, sorry for that). So if the transistor isn't working, there should be +15V but not enough for all the current in the circuit that uses +15V? Because I only measured +0,6V at the points that should read +15V (including point 6 and the + leg of C144).

Could you help me pick an easily available transistor substitute for 2SC-1624? I'm a bit overwhelmed by all the options I find on alltransistors.com and the fact that lots of them are obsolete. Any help in picking the right Zener diode after that step would be greatly appreciated too.
I have an old SS bass amp at home that has some TO-220 transistors, I will check if any of those can be reused.

g1, part of the schematic with this transistor on it is attached to this post.

Thanks Phil, I will look for a replacement with those values.

The original transistor is more or less obsolete. Original Toshiba is available on ebay from China, but relatively expensive and of course a long shipping time. What would be suitable alternatives for the 2SC-1624Y?

I found a lot of possibilities on alltransistors.com but because I don't understand what this transistor does exactly, I don't know which values should be exactly the same as the original. Comparison of alternatives with the values I took from the datasheet: https://alltransistors.com/crsearch.php?mat=Si&struct=NPN&pc=15&ucb=120&uce=120&ueb=5&ic=1&tj=150&ft=10&cc=60&hfe=70&caps=TO220&r1=0&r2=0&r1r2=0.

Can anyone help me with this?

I tested the 2SC-1624Y: no signal passes between either of the legs. I guess this means it's fried, although the connections are not 'open' but closed (don't know if this is a common fail, you usually read about blown transistors that pass signal both ways).

Will look for replacement/substitute.

Interesting question, following...
Looking at the schematic I don't see why the external speaker would be in series (pic attached). It looks like it's parallel, or am I wrong? It also says 8 ohm minimum next to the ext.spk. outs. I also have a late 70s version and this is from its schematic. Maybe Roland changed the ext speaker wiring over time?

Follow-up: would it be possible to hook up 2x 8 ohm (bass) cabinets to this amp? I wonder if the internal speakers are fine with the bass frequencies, as some guitar speakers aren't.

Reverb: Pin 3 of IC1 gets 0,6V. Traced that supply back to R152 (through a jump wire); no +15V there either. R152 leads to IC4; leg 13 and 14 should provide +15V but give +0,6V.
That said, I can't trace back +15V to C144 in the supply section (if it's called that way ); the + side of C146 gives 0,6V, - side 0. -15V and -43V and +43v are still present. On the negative terminal of C144/cathode of D14 I measure 0V.
The supply of +15V might be the issue, also disabling the other effects; as far as I can see only the effects paths use +/-15V.

Chorus: Yes, the return path seems fine. R137 hums only when effects are engaged.

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.

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?

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  )

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.

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.