Roland JC120

[Valvehead]

I have a jc120 here, the model with the bright switches. it was worked on before= 1 output transistor blew and burnt 1 of the speakers. Has a new chip in there. It passes diode test/ resistance and matches the other on that side. when i first started testing i was clipped to the Red/ blk speaker wires and those have 30vdc . no speakers are hooked up . At first the new chip got pretty hot, around 160 and climbing so i shut it off .

testing the or/brn speaker wires i have 46mV dc.

Heres what is odd= I went back and tested the red/blk still have 40vdc , but the chip wouldnt get hot ! stays the same as the other 3. Its a 2SD845

Re-tested the newer one and it still passes the resistance/ diode test .Im assuming its ok ? Where should i look earlier in the circuit ?

The amp doesnt light up my bulb limiter

jc.pdf

[glebert]

Have you verified the schematic you are using matches the amp you are working on? There are lots of variants of JC120s, some only lasted a year or two, but there are several that had brite switches. The shematic you included looks very modern relative to the JC120's I've worked on (all with brite switches) and that schematic doesn't have 2SD845 output devices.

I would put my money on one of two problems 1.) Fake 2SD845, it doesn't look like any "legitimate" suppliers have this part, so if it came from Asia via Ebay or whatever I wouldn't be surprised if it failed 2.) While it looked like there was one bad transistor, in reality there are more components that are bad, like driver transistors or the complementary PNP devices has issues. If possible I will replace any failed device and whatever devices are adjacent to it just for good measure and matching of pairs.

Too bad it took out a speaker, when I looked for JC120 speakers they are expensive to get replacements, I don't think you can get them reconed, and they do play an important part in the "JC120 sound."

[DrGonz78]

Is there a serial number on the amp? With JC120’s they have a range of serial numbers on some of their schematics.

[Valvehead]

heres serial and rear . is there a substitute for that chip so i can get one from mouser , etc... ?

[DrGonz78]

Well regarding serial numbers that looks like it would fall into 5th edition. G1’s post number 5 on the thread linked and it is the second pdf in the post.

[DrGonz78]

I guess just check the schematic. Still could be incorrect but sometimes this locates a correct schematic.

[glebert]

Based on that serial number and the brite switches I would guess that was February of 1986. The first two digits are the months after July of 1972 and it rolled over in Nov. of 1980. The schematic you posted was from September of 2000, so I would guess that is why there are differences at least in parts used.

I would look at the Part 1 section of this service manual, it shows 2SD845 devices and is probably closer. SM_JC-120_11052.pdf

I don't know about sub parts, perhaps someone else can chime in.

[Valvehead]

Thanks . I will print that and do some testing . Is it OK to unplug each output chip without discharging caps ? I remember that was a no no on the Marshall mode 4

[Valvehead]

Man ...i cant read that scem...no matter how much i zoom. super blurry. Can someone tell me exactly where those 4 chips are ? to the left of the speaker pairs? I do see two +37v/-37v rails . It also says 21.5 v right to the speakers?? Is that correct? I thought we want almost 0 mv to speakers?

[Helmholtz]

It also says 21.5 v right to the speakers?? Is that correct? I thought we want almost 0 mv to speakers?

21.5 V should be output voltage (AC) for full power (around 58W per side).

[g1]

Man ...i cant read that scem...no matter how much i zoom. super blurry. Can someone tell me exactly where those 4 chips are ? to the left of the speaker pairs?

I don't know if there are other differences in the "82" schematic, but the power amp section at least seems the same, and is much more legible.
see attached.

[Valvehead]

Thank you !