70's Roland JC120 (CH1 Sawtooth)

[dubulup]

Good Morning all (who has interest). Love the forum and it (you badasses) has helped me a lot...help the musicians in my small town. I'm a tube hi-fi guy, but love to meet the community of musicians and help them when I can.

I found the below schematic, but SS is not a strong skill of mine...although I have been learning.

This Roland is in amazing condition, however has a saw tooth waveform on channel 1 with no input. Channel 2 and effect board is perfect (from what I can tell). I chased this wave form to E1 on the board which has a a small hunting DC voltage unlike channel-2. As an unskilled SS tech, I swapped output transistors to see if the problem swapped to channel 2 and it did not.

I also cannot locate the E1, C1, B1 pins in the schematic...making me think there is a page of the schematic I'm missing?!? Maybe I'm just not verse in all these Q's, haha!

Just a note, the channel 1 board does amplify a signal from the input, and the tone stack operates properly...just this sawtooth waveform, I haven't come across. Now, that I'm typing this I see a few zener diodes on the board that I can't spot in the schematic, so maybe there is a different schematic I need to find.

I'll be drawing out the output stage from examples I can find to refresh myself in collectors, bases, and emitters.

Thanks in advance for reading...and I really appreciate any thoughts, this helps organize my thoughts and track my journey.

Cheers!

[dubulup]

2000 words

[g1]

Now, that I'm typing this I see a few zener diodes on the board that I can't spot in the schematic, so maybe there is a different schematic I need to find.

What is the Serial # ?

[dubulup]

good question, the owner had taken the backplate off...but I now have that SN: 718143; as suspected outside the range of the diagram I linked.

[Delta362]

Your serial number looks like it uses the 3rd version of the service manual - Nope

Edit: Looking at the photos, you need the big service manual below as that matches your board.

[Delta362]

Here are a couple other editions of the service manual

[Delta362]

And a big one

[Delta362]

Looking closer at your photo, it seems the big SM is the one you want. I see the zeners in there. E1 goes to R52. See page 7 for the schematic.

Channel 1 and 2 seem to be about the same with some minor variations in component values. Can you compare the two board voltages? I do see that there is a sawtooth waveform on the Chorus OSC (shown on page 1).

[dubulup]

Big thanks for providing these docs, I'll download and pour through them.

[dubulup]

Channel 1 and 2 seem to be about the same with some minor variations in component values. Can you compare the two board voltages? I do see that there is a sawtooth waveform on the Chorus OSC (shown on page 1).

DC voltage all seem similar for the similar pins that have wires on them...see board 2 photo. The only difference I measured was on CH1 E1's DC reading was hunting +/-0.030Vdc (so 60mV swing) while CH2 E1 was solid 0.005Vdc.

[g1]

Your scope says the sawtooth is 120Hz, the power supply ripple frequency. It looks like the standard ripple waveform.
Do you hear hum or measure DC at the Ch.1 output?

[dubulup]

I do see that there is a sawtooth waveform on the Chorus OSC (shown on page 1).

I don't believe this is the same type of waveform I'm getting. Mine doesn't change with volume and is constant with zero effects board.

This "feels" like one of these parts I'm not familiar with has a failed cap...and something has shorted or open to ground. There are not many voltages or notes on the schematic but at least this is a starting point to take meaningful measurements beyond the pins.

[dubulup]

Your scope says the sawtooth is 120Hz, the power supply ripple frequency. It looks like the standard ripple waveform.
Do you hear hum or measure DC at the Ch.1 output?

that's what I thought as well, but since CH2 is not showing the same symptoms (PS is common, is my assumption) and I don't measure this large of ripple on the PS board, I haven't dug in there yet. I hear hum and measure no DC on the output.

The power transistors should be TR's on the schematic correct? I still can't seem to locate where E1, C1, B1, etc are in the service manual...please help a blind man, ha!

[Delta362]

I did not see E1, C1, or B1 on the schematic, but I was able to follow the board trace to see which component it went to. The other end of the wire looks like it goes to the output transistor Q12 (remotely mounted). You'll probably find the same for the C1 and B1 leads. Q12/Q13, Q31/Q32 are your output transistors for the boards.

Note, transistors are Q, not T in this schematic.

[dubulup]

I do see that there is a sawtooth waveform on the Chorus OSC (shown on page 1).

I don't believe this is the same type of waveform I'm getting. Mine doesn't change with volume and is constant with zero effects board.

I was wrong. even with all effects turned off...it seems something has failed on the effects board because when I remove basically the only difference from CH1(assuming FB to chorus Q40-B) CH2...CH1 behaves properly; EDIT: scratch that...still humming when connected to a load. Back to the drawing board.

Thanks for setting me straight TR vs Q (Q's I look for little 3 prong devices...TR I look for heat sinks)

EDIT: something interesting is the power switch position does matter. UP-ON position the saw tooth amplitude is cut in half, DOWN-ON...not really sure why there are two ON positions yet. I don't see any type off "half" power circuitry.