OB-XA Pot Board Voltage Wrong In Service Manual?

[meener777]

I am working on an OB-Xa and its been recapped so I am doing a full calibration. First thing to check is all the voltages. The service manual says "two voltages are developed on the pot board, +5.6V and -5.0V". When I checked the 5.6V (supposed to be +/- 350mV) I got 6.1V, everything else in the PSU checks out 100%. So I pulled the pot board, checked the caps, they are good. Pulled the resistors and they were good too. Eventually I pulled all these components, the two resistors, two caps, and the voltage regulator and I put it all on a bread board. Still got 6.1 Volts. Replaced all these components, still 6.1V. I also simulated this circuit in LTSpice and I got 6.1V.

Does anyone happen to know what to make of all this?

The unit is working fine except for the LED on the Osc 2 detune doesn't extinguish properly at 12 o'clock, its more like 10 o'clock, so far I'm not sure why that is happening and I was hoping fixing this voltage would take care of the LED issue. For what its worth Im pretty new at circuits and learning as I go on this restoration project so basic stuff that might be obvious to someone else might not be for me.

[meener777]

So it looks to me like this is an error in the schematic and its possible at least some units (mine?) might have shipped with an out of spec pot board power supply. I wouldn't be surprised to see if this got fixed on later models (mine is Nov 81) because anyone who checked the 5.6V supply as the service manual says to do would see 6.1V. But if that happened it seems like there would be an update to the schematic which there doesn't seem to be on later versions of the OB-XA service manual. Anyway I wonder what the story is.

Here is the same circuit from the OB8 service manual a few years later and it looks like they put it into spec by lowering resistor R1 from 100 Ohms to 74 Ohms. This can be found on the pot board schematic.



I dont have a 74 Omh handy but I simulated it and it looks like it would come in a tad high at 5.84V (the target is 5.6 +/- 350mV). A "perfect" value would be 53 Ohms (that gave 5.5994 V). I went with 47 Ohms which I had handy and that gave me 5.532V which is in spec.

My LED dead zone for osc 2 detune does appear to be more centered now, although its still a little tiny bit to the left.

[meener777]

FWIW it looks like getting this voltage in spec did not fix my LED dead zone issue on the OSC 2 Detune knob. It still moves around sometimes, especially hard left almost to 10 o'clock. If anyone knows what might cause that (or if its normal?) I'd love to get that fixed up.