Tascam 34 - meters doing strange things

[frnkfrkl]

One more thing: J171, pin 3 to chassis ground, resistance measure, MACHINE OFF, NO POWER = 6.0 MOhm. Powered up = .OL. So something is opening that circuit at power-up.

 

[sweetbeats]

Grounded J171, pin 3 to chassis ground. No change.

I'm still a bit worried about the crackling sound on power-up (well, the no audio problem is also an issue, of course). It seems to correspond to the flickering VUs. Once the machine has stabilised, the lights are at full brightness and the crackling sound stops. It also seems to correspond to what I assume would be the delay time before the relays are supposed to kick in (or out). There must be some correlation between the crackling and strange VU meter behaviour and the non-tripping relays because, if I'm understanding this correctly, they are on the same rail.

Don’t strap pin 3 J171 to chassis ground…strap it to the power rail’s 0V reference on pin 2.

The power mute timing circuit on the power supply PCB doesn’t “share the same rail” as the +6VDC lamp power rail per se, but, yes, there is association. As I stated earlier the timing circuit uses the lamp rail as a low voltage reference for system power up and power down. The explanation was posted earlier from the Theory of Operation section in the manual. On power up C802 charges through R801 (which sets the time duration for charge) and when the cap approaches full charge, at which point it blocks DC, Q801 switches on providing a path to ground for the mute relays, the coils of which always see +24V on the other side. The timing circuit just closes the path to ground for the other side of the coils completing the circuit and allowing the coils to energize and close allowing audio signal to pass. And then on power down the timing circuit immediately opens that path to ground de-energizing the relay coils, interrupting audio signal flow. So the timing circuit isn’t powered by the +6V rails per se, but uses it as a reference.

You should essentially never see AC voltage in any considerable amount on the +6V rail. Did you end up recapping the power supply? If it was me I’d be getting that sorted out first; the +6V supply. You indicated AC volts measured 2.4V between pins 1 & 2 of J171. That’s not right. I’d be putting my scope on the outputs of the bridge rectifier for the +6V rail, as well as the node at the positive side of C801, the main filter cap…seeing what’s happening there to sort out, initially, if the bridge is bad or the main filter cap has fallen out of spec and is allowing the rail to oscillate and as a result the substantial AC voltage. There should be very little ripple there, at C801. It’s not a regulated DC supply, so some ripple is okay, but it should be in the millivolts, not 2.4V.

 

[sweetbeats]

One more thing: J171, pin 3 to chassis ground, resistance measure, MACHINE OFF, NO POWER = 6.0 MOhm. Powered up = .OL. So something is opening that circuit at power-up.

I would actually consider 6MOhm open circuit. That is very high resistance and the fact you have something measurable when the system is powered down is likely only because of some stray path to ground when components are not in their powered state. The bottom line is, after a few seconds, there should be a low resistance path to ground measurable at J171 pin 3. Where I remain confused is the fact you manually grounded pin 3 and there was still no change. But something is not right with the +6V supply, and always always always start with the headwaters…the power supply has to be working right before you do anything else. You could also shotgun the two diodes in the timing circuit (D805 & D806? My copy of the schematic is not very good quality). I know you were talking about doing that earlier and I steered you away, but I’m changing my opinion on that a bit. But first we need to sort out the lamp power rail.

I think this is exactly what I would do…

I’d remove D805 and R803 isolate the power rail from the timing circuit, and then scope the outputs of the bridge diodes and look for the correct half rectified wave, and also scope the positive side of C801 and look for clean DC (very little AC ripple). Depending on what I found I’d replace whatever as indicated. With the power rail working correctly I’d jumper J171 pins 2 & 3 to manually ground the muting relays and see if signal was passing. Depending on the outcome of that I’d then re-group and identify next steps.