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.

 

[frnkfrkl]

Wow. OK. I did read the theory of operation but it didn't all stick to my brain. Remember, I am NOT a tech. I'll need to read this a few more times to get it to sink in.

I did a sort of Kung-Fu transistor swap as well, that is to say I did not completely remove the board. I managed to get just enough room to solder the new one in place. Fun fact, the old one tested fine when out of the machine so that was a waste of time and effort.

I will need to disassemble the machine completely this time. I will need to buy and learn how to use an oscilloscope. I will need to source obsolete parts (actually, maybe not, diodes, caps and resistors should be available). The real hassle is that the machine needs to be reassembled to test what I've done and, if it didn't work, I need to tear it all apart again. This is above my pay grade but I don't have a choice because there's no tech anywhere near where I am. Sorry, just thinking out loud.

Could you recommend an oscilloscope? Total noob with one of those. I will read, disassemble, test what I can and advise at that point.

And, yeah, I had my doubts about pin 3 to chassis ground. I'll do pin 3 to 2 and see what's up.

Thanks again, I'm learning at your expense. You're generous with your time and I appreciate it.

One more question: When testing the power supply itself, is it necessary that it be connected to the things to which it is supplying power? That would seem physically impossible with all the wiring harnesses.

 

[sweetbeats]

…Remember, I am NOT a tech. I'll need to read this a few more times to get it to sink in.

I am not either. I consider myself an advanced hobbyist. I do my best not to get others in trouble. And FWIW, on your case, I’ve probably ready that segment of the Theory of Operation 4 times, and throughout our correspondence been in the schematics at least a dozen times. Notice it wasn’t until posts today I had my head screwed on more correctly regarding how the power mute circuit works. Aren’t electronics fun…

did a sort of Kung-Fu transistor swap as well, that is to say I did not completely remove the board. I managed to get just enough room to solder the new one in place. Fun fact, the old one tested fine when out of the machine so that was a waste of time and effort.

Well I was surprised the replacement of Q801 didn’t do it. And I’m sorry it didn’t and for the misguidance, but I’m learning along with you, and also, as is typically the case, there are details that become evident to me as we go that are important to know, because I’m not there with you, and I can’t see what you see and hear what you hear, but you don’t know what might be important and I don’t always know what questions to ask. And so it goes.

I will need to disassemble the machine completely this time. I will need to buy and learn how to use an oscilloscope. I will need to source obsolete parts (actually, maybe not, diodes, caps and resistors should be available). The real hassle is that the machine needs to be reassembled to test what I've done and, if it didn't work, I need to tear it all apart again. This is above my pay grade but I don't have a choice because there's no tech anywhere near where I am.

You don’t have to get a scope. I’m just telling you what I’d be doing because I have one and kind of know how to use it. Your unregulated DC rail is throwing way too much AC ripple. According to the data you’ve provided we know that’s true. It’s not a complicated circuit. You could always isolate the supply (lift D805 and R803) shotgun the filter cap C801 and see if that helps. And if it doesn’t, shotgun the diodes that make up the bridge rectifier. None of these parts are weird or obsolete; readily available and not expensive. And yes, it is a PITA sometimes depending on how the device is designed and built to resolve faults; can be a real challenge to have to tear-down and execute repairs, reassemble, test, tear-down…it’s what it is. This is how it goes. It’s one reason I value equipment that has a more professional field-serviceable element to the design…stuff with chassis designs that allow easy access to internal assemblies…stuff that’s modular or designed with plug-in assemblies vs hard-wired PCB assemblies that are buried inside and require chassis disassembly to access. And stuff that has higher quality PCB base material (glass fiber vs phenolic resin) because every time we tear down, do any solder work, manipulate an assembly, we risk damage, and the glass fiber stuff is more robust. So I feel your consternation. The best fun is when I make a stupid error and have to tear something all down again that I thought was done…that’s totally on me…or when I think it’s done and there is some unforeseen surprise that dictates otherwise. I’ve experienced that enough that it doesn’t upset me anymore. I’ve had to do that twice now on my comprehensive Roland JUNO-106 resto-mod.

Could you recommend an oscilloscope? Total noob with one of those. I will read, disassemble, test what I can and advise at that point.

I think you should hold off on getting into those weeds and maybe consider what I suggested above first.

One more question: When testing the power supply itself, is it necessary that it be connected to the things to which it is supplying power? That would seem physically impossible with all the wiring harnesses

It depends on what the problem is and what we are testing/trying to resolve. I’m being specific with you and telling you when I want something tested under load (connected) or unloaded (isolated/disconnected) if it’s not already contextually clear. It can depend because you can have a supply (or any type of circuit, really), that behaves differently depending on whether it is loaded or unloaded. I had a power supply that seemed faulty, so I isolated it and tested and it was great. That was confounding until I tested while placing it under load and discovered that one of the main filter caps had fallen out of spec, and was barely the appropriate value when new, so it could no longer keep up with the time constant of the waveform, but it didn’t misbehave until there was current flowing.



I hate to say it, but sometimes what one has to do, if there are things that need tested with all systems connected, and there’s no way to access those things physically when the device under test is fully functioning/connected internally, is to solder in flying leads (wires soldered to nodes or test points that extend outside the device under test so you can access these points when the device is assembled). Aren’t electronics fun…