Just received a Model D, s/n 78xx. ua726 oscillator board. It’s cosmetically stunning and the keyboard is fully refurbished, so it plays great. Sounds great too. (The synth, not the keyboard…)
Unfortunately, the headphone out is broken. It’s almost, but not completely silent. The little sound it makes is weirdly distorted and it dies away after a minute or so of playing.
Any ideas what may be the problem? I’m going to replace the electrolytics on the rectifier and psu board anyway and since the headphone circuit is on the psu board, it’ll get new ones too. But is there any other ‘usual suspect’ in this case?
I would start by cleaning the edge contacts and connector of that board. Verify that no wires are broken or desoldered coming from, and leading to that board from the edge connector (especially the phones volume pot and jack). Then replacing C19 and C21 on the board itself. If that doesn’t fix the issue, proceed to checking Q10-11-16 with a transistor checker and diodes CR1 and CR2 with a multimeter.
Some transistor checkers (like my 1974 Hy-Tronics) permit in circuit verification. But most, and also the multimeter way, need to have them off circuit to verify them reliably.
From experience, it is seldom a component failure, but rather a bad contact or broken wire somewhere. Although components do fail, from time to time. Especially electrolytic capacitors and tantalum ones, too, after all these years.
Best of luck to you, Jonas.
Keep us posted.
Well, I’m not sure what fixed it, but it’s fixed. For now at least…
I changed all electrolytes on the PSU (and rectifier board, as preventive measures), but that didn’t do the trick.
I cleaned the edge connectors of the filter and PSU board and that didn’t do the trick either. I then went on to suspect C15 (ceramic, PSU board), removed the filter board for the third or fourth time, inspected the suspect cap, didn’t see anything, wiggled it a tiny, tiny bit, gently, just to feel for a bad solder joint, felt solid, re-seated filter board, and lo and behold, now it works.
Have another issue though, but I’ll start a new topic for that.
Quite possibly. It worries me that I don’t know exactly what’s the problem, or what fixed it. But I guess it’s all part of the joy of vintage gear!
Very nice to have a working headphone out, nevertheless!
