Trying to fix a Gerd Schulte Compact Phasing A

[Rockhorst]

Posted this on DIYSB, thought I might as well post it here too:

I've got an old German phaser unit that isn't working. When I turn it on it does pass a guitar signal but no phasing happens. The copper tracks have some corrosion on them but I didn't find any continuity issues. The LFO light bulbs don't light up but are still working (tested them out of the circuit). When in circuit, voltoge over them is very low.

Here's the schematic:


My main suspect is upper one of the two 2n1613 transistors, which is connected to the bulbs.

DMM reads: C: 26V, B: 1.3V and  E: 0.8V.

The other 2n1613 shows: C: -7.4V, B: -7.8V and E: -8.6V. This transistor feeds the on/off indicator bulb which is working fine. The minus sign is slightly confusing (measurements taken relative to the same ground point).

I haven't checked any caps or the opamps yet, as the bulbs not lighting up seems to indicate that the problem isn't in the audio section.

Any and all help and suggestions welcome. If there's any measurements that need to be done, let me know.

[jubal81]

i agree. Sounds like the LFO isn't working or the bulbs are burned out. I'd check the net at the bulb to see if there's oscillation.

[Rockhorst]

The bulbs themselves are good, I tested them. The glow wires still look quite new, actually. The voltage on each bulb is very low (0.4V per bulb, should be about 7V) and I don't see any oscillation. I'm a noob when it comes to debugging LFO's, so if anyone has got some pointers or a good starting point...

[Rockhorst]

Bumping this for an update and new problem...turns out that to start the LFO, you need to connect pins 1 and 2 (bottom left of the schematic) of the connector with an external switch. Best phasing/vibe sound I've ever heard. So everything was fine.

A few days ago I was testing the switch pedal had made and the phaser dropped on te floor by accident. About a half meter drop, but I managed to slow it down with my foot. I reckoned the 'only' damage was a small ding in one of my guitars (hmpf). But now when using the phaser it seems rather thin instead of the lushness I recall. I was thinking that this was psycho-acoustic...my brain fooling my ears, but here's the weird part: when I disengage the phaser (disconnecting pins 1 and 2) for about half a second you get the last phasing period and it sounds extremely deep, lush and beautiful. Only when switching from on to off and only very shortly. After that, the magic is gone again...bulbs still working fine so the LFO seems ok...yet it sounds uninspired.

I completely clueless, although I have a theory that it might be a transistor opening up or something. Anybody?

[Rockhorst]

I did a quick test: the bulbs, and hence the LFO, do turn off the moment I hit the switch. Where does that extra swirl come from?