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…

 

[frnkfrkl]

As for the misguidance, no harm, no foul. It just got me to a point where I have to do things right and completely. Which I should have aimed for from the start. A Band-Aid on a wheezing chest wound won't cut it.

And, yeah, that's a really shitty place to put a circuit board. The bane of this machine is circuit board accessibility and cheap-ass Molex connectors. My 80-8 is a much more serviceable and robust machine.

I'm much farther out in the weeds with this thing that my intended use for it warrants but I'm stubborn, for one, and I really don't have much choice. I'll probably stumble across Dr. Livingston at some point.

OK. Off to work. Perhaps radio silence for a while, I'm going to need to go slowly. I need to do some more reading to better grok things first. And some time to procrastinate...

 

[frnkfrkl]

Ah. IN CIRCUIT, C801, nominally a 1000uF cap tests at 74,000uF. That can't be good...

 

[sweetbeats]

You can’t test caps in circuit.

 

[frnkfrkl]

I knew that once upon a time. Tests OK (973uF) out of circuit.

The reference for the diodes is obsolete. Would any 1a, 200v diode do the trick or do I have to get more precise? By definition, it is a "silicon rectifier diode". I have some 4006s here. Overkill on specs but ok to use? As long as I'm here, I'm just going recap the whole thing, electrolytics anyway. Sourcing the caps won't be an issue, the diodes have got me a bit confused. Lots of different types and styles and values.

For the caps, i presume Vishay, F&T, Nichicon or Panasonic would be a safe bet, correct? High temp values (105°) just to be on the safe side?

When you say the bridge rectifier, do you mean D801-D804? I should probably do those too, along with D805 and D806. R801-804 all test within spec.

The question I have essentially concerns sourcing the appropriate diodes. Visually, they look different than, say, 4006s. I just don't want to fuck this up.

Aside. This is the first time I've recapped anything. Orientation is printed on the PCB so no mystery there, at least.

 

[sweetbeats]

Recapping typically only refers to the electrolytic caps; typically no need to replace any of the non-electrolytic caps unless there’s a known fault involving one.

I’m not familiar with F&T caps. I use Nichicon, Panasonic, Vishay/Sprague, Cornell Dibiloer or Mallorey. 105C yes.

On the diodes if it was me I would just use any 4007 parts because that’s what I have on-hand here. Glass passivated is better if you can find them, but not necessary.

Yes the “bridge” is made up of discrete parts D801~804. I think. Post a pic at some point if you can of those parts. I don’t know if you can get to that, but the schematic is not clear if they are discrete or an integrated bridge. I’m assuming they are discrete because discrete part identifiers are listed (4 diodes for the bridge, instead of, for instance, just D801 for the whole bridge which would indicate an integrated assembly).

Does that help?

 

[frnkfrkl]

Here are a few photos. D801-D804 are discrete. They apparently are used for rectification. I have 4006s here. OK?
D805 is the same reference. 4006, again?
D806, however, is different. A switching diode, 40V. 4006 ok?

I also have, for whatever reason, several Zener diodes, 10V, 1watt, DO41. I assume that's not what I want for any of these.

All diodes in circuit look physically different than 4006s and look more similar to my Zener diodes. But, never judge a book by its cover, right?

 

[frnkfrkl]

One more question that i should probably know the answer to: Can you test diodes in circuit? There are 4 banks of 4 diodes on the PCB. Two of the banks test as one would expect, 590 and zero. The other two banks test 590 and maybe 240 the other direction. Those, however are connected to caps so I'm guessing that they're not testing correctly. I'm really not spotting anything yet on this board that is an obvious glaring fault...

 

[frnkfrkl]

Well, all the caps are out and, guess what? They all test within spec (+/- 20% or better). Sigh. Everything in here looks OK.

 

[sweetbeats]

Well, all the caps are out and, guess what? They all test within spec (+/- 20% or better). Sigh. Everything in here looks OK.

There are more things to test besides capacitance. ESR is another. But, look…those caps are over 40 years old. And these are not caps that were manufactured like some of the high-quality computer-grade caps that are found in the power supplies of like my Ampex and 3M machines that are approaching 60 years of age and still test within spec, capacitance and ESR. Those old caps are likely in need of replacement. New caps will bring related performance specs to better than new, the caps aren’t expensive, etc. You should not be groaning about this. You pulled them. Replace them. Market it with that recapped power supply. Its a marketable value-added feature.

 

[sweetbeats]

One more question that i should probably know the answer to: Can you test diodes in circuit? There are 4 banks of 4 diodes on the PCB. Two of the banks test as one would expect, 590 and zero. The other two banks test 590 and maybe 240 the other direction. Those, however are connected to caps so I'm guessing that they're not testing correctly. I'm really not spotting anything yet on this board that is an obvious glaring fault...

I’m not sure what your numbers mean…590, 240, etc. Depending on the tester, diodes should read a given voltage in the forward direction and zero in the other.

Regardless, it’s best to test diodes out of circuit.

I would be going hog wild on all your diodes.

I recommended shotgunning the +6V bridge diodes because you don’t have a scope or know how to use one. As far as we know, the +6V rail is the only rail with which we are seeing issues in the systems powered by that rail. I’d leave the rest alone. Remember…everything you do runs a risk of further damage. That’s just the nature of this work. So if it isn’t broken, or if we don’t have indication it’s broken and the components of the system are typically reliable (like with diodes, for instance…they have no component with the potential to dry up or leak like an electrolytic cap), as long as a diode was properly spec’ed in the first place, and Teac does a relatively good job with this, there’s no reason to go crazy.

 

[sweetbeats]

Here are a few photos. D801-D804 are discrete. They apparently are used for rectification. I have 4006s here. OK?
D805 is the same reference. 4006, again?
D806, however, is different. A switching diode, 40V. 4006 ok?

I also have, for whatever reason, several Zener diodes, 10V, 1watt, DO41. I assume that's not what I want for any of these.

All diodes in circuit look physically different than 4006s and look more similar to my Zener diodes. But, never judge a book by its cover, right?

Thanks.

The 1N4006 parts are indeed overkill for D801~805, but there’s no harm in it. Use what you have for those parts. D806 is different.

D806 provides for that reverse-flow of the timing circuit at power down immediately breaking the circuit to ground for the muting relays’ coils and breaking the signal path…ensuring this happens first so the impulse voltage can’t propagate to the signal path and thump the outputs. It has to be a really fast diode and that’s why they put a different part there. A standard “reverse recovery time” for a diode might be something 2 micro seconds. That’s the spec for the 1N4006 parts. The original part installed for D806 has a reverse recovery time of 1.5 nano seconds. It’s rated for 40V forward voltage. I couldn’t find anything to match those specs exactly, but I didn’t do an exhaustive search. What I did find that’s super cheap and readily available is a 1N4148. It’s reverse recovery time is 4 nanoseconds. It’s rated for 300V forward voltage. I think that’s what you can use there. The difference of 2.5 nanoseconds should make a difference. Even “ultra fast” rated diodes have a reverse recovery time even 5x slower than the 1N4148.