1981 Roland Jazz Chorus 120 CH1 Problems - Solved

[phatt]

Ouch,, If all actives have been replaced then a failed Feedback resistor R66 (47k) can cause that problem. If it reads ok  then check for broken tracks which connect to the nodes either side of R66.
Phil.

[substatica]

Ouch,, If all actives have been replaced then a failed Feedback resistor R66 (47k) can cause that problem. If it reads ok  then check for broken tracks which connect to the nodes either side of R66.
Phil.

R66 tests fine and its connections to adjacent components are good. Attaching a photo of the board as it stands in case there's something obvious that I'm missing.

[substatica]

One of the things I've noticed is that when I test D1 on the bad CH1 board I get 1.8 M voltage drop the wrong way when using the diode test, the correct way is .56, off the board I get OL for the wrong way. On the good CH2 board I get OL the wrong way and .56 the right way. I've substituted a 1N4148 for the IS2473 on the bad board. I've checked the traces around D1 and they all seem fine. Getting a reverse drop on a good diode mean anything to anyone? Apologies if I'm using the wrong wording for the situation.

[phatt]

You need to test diodes out of circuit otherwise other components are in series with the diode giving flawed results.

Well I'm fast running out of possibilities.
You asked earlier if you could use a 2SC1624 for Q13 instead of 2SC1626.

Well now that I look at the actual picture Q13 is not the right package for a 2SC1626. The closest I can find that resembles Q13 in your picture is 2SC1628 which has a different pinout.

The schematic is hard to read,, the 6 could be 8.
So I'm Wondering what is that part? Also what transistor have you used to replace Q13?

The way they screen print transistor placement  is usually the flat edge as the front face and I notice Q13 would be back to front,,, Which makes me think it's not right.

I've simulated the circuit with some fault conditions trying to recreate the voltages you posted and found that if Q13 was wired up wrong then it would give similar fault readings.

If all passive parts are ok then it can only be the 7 transistors so at least one has to be faulty.
Phil.

[substatica]

I did test D1 out of the circuit and it's fine. Just in the circuit it's readings differ from the good channel's D1.

As for Q13, the diagram calls for 2SC1628 with EBC pinout, I had asked earlier in the thread (not noticing the pinout) if I could use a 2SC1624, which is BCE pinout. I had hoped a couple days back that this was my issue, however after testing the original and swapping back to it the problem persists. Its orientation matches that of the good channel Q13. Attached in an image of the good channel, CH2.

[substatica]

For now I've ordered some KSC3503 DSTUFS-ND to replace Q13's 2SC1628Y. Different pinout but should be able to make it fit.

[substatica]

Replaced Q13 with the KSC3503, had to put it sideways due to the different pinout -- and CH1 came back to life! So I guess Q13 was bad after all, even though it tested alright. Thanks for all the help everybody, especially phatt!

https://youtu.be/vbaU8HK2h1I

It's got some noise oscillating, but I'll sort that later -- it's got chorus!

[phatt]

Glad to hear you got it sorted out. :dbtu: :dbtu:
Phil.