Now that the continuous clicking from the relay is fixed I can hear a different sound. While loading the output with about 6k ohm I can hear what sounds like a high voltage discharge around the high voltage transformer. The sound increases as you select the higher ranges. Is this high voltage discharge sound normal for these units? Also the 1200v range is disabled. Can someone point me where I can re-enable it? Does anyone know if this is a hardware or firmware disable?
I guess its similar to a TL866 programmer from EEVBlog #411. Going to order it tonight. If the firmware you posted is from a unit with the 1200v working i'm willing to give it a shot. I have zero experience with eprom stuff. I've done a few arduino projects but this is uncharted territory for me. I've scavenged a few eproms from my junk bin that I can experiment with. I have a NM27C128Q and NM27C256N that look pin compatible. Can these be substituted for the 27C64 that's in the Valhalla?
Well at this point I'm going to send an email to Valhalla Scientific and see if they can offer some advice.
It just occurred to me that the bodge might be a "make sure if it accidentally gets selected it doesn't stay selected" bodge, I'll have to look at it some more, with the benefit of the full schematic. I'll do that and re-post if I see something interesting.
Sure enough its a trace connecting the switch to pin 13 on IC8! I quickly wired a bodge wire and reassembled it. Well the HV selection activates but the output is not what it should be. The output voltages are roughly 10X less than displayed. The 120v selection matches the output but with the 1200v selected the output is far reduced even when adjusting to a voltage near the 120v range. So now I need to figure this out. I thought it could be something in the calibration settings. I quickly followed the calibration procedure but can't get the 1200v range to adjust.
Thanks forrestc. I should have been a little more scrutinous. I definitely lack some knowledge about how this thing actually works. I did find another cut trace and pin 5 of RLB. I just wasn't looking hard enough! So I reconnected that and powered up. SPARKS!!! Well just one but I definitely flinched! I don't remember hitting it but C12 seems to have made contact with an iron. It could have been me but this was a salvaged unit so who knows. I've patched it with some silicon adhesive for now but it will have to be replaced of course. I shop on Digikey and I'm having an issue finding a suitable replacement. Anyone have a suggestion?
Starting with the high voltage source at capacitor stack C203-C206 feeds into the high voltage transistors TR7-TR11. TR12 and TR6 create a Darlington correct? This seems to be the control point for the main voltage source. This is controlled from IC5 (ICL7650S) Chopper-Stabilized Operational Amplifier. This amp is being controlled by TR13-TR16 which in turn is controlled by the DAC. The main feedback line is tapped from D8 (on the schematic). The voltage selections switch on and off RLA, RLB, and RLC that provide feedback for the amp. Now the defeat bodge has pin 5 and 11 from RLB cut and linked to +5v. Pins 4 and 10 are linked to the base of a transistor inserted at pin 5 of IC112 and ground. So if the relay was triggered by any means the +5v would trigger the transistor pulling pin 5 of IC112 to ground. This also means that RLB was fully removed from the feedback path. So this bodge did not introduce anything into the feedback path and RLB was isolated. So they introduced there own feedback mechanistic using RLB's functionality so in the event it was forced on it would prevent the control hardware from generating a signal that would result in a high voltage output (pin 5 on IC112). I think that's right or someone can correct me.
I'll finish with a question. With these repairs done is it worth having it calibrated? WOULD anyone calibrate this unit with these repairs? Would they look inside or just do the manual calibration? What would that cost? I bought this because is was not functioning and thought it would be fun to fix it up. I'm not sure if this thing is still in demand somewhere or if it's simply too old for real use. Now that it's working I'm not totally sure what I should do with it. I have a few HV power supplies. I did learn something though.