Tascam 34 - meters doing strange things

[sweetbeats]

Awesome. Okay. So maybe you had a failed bridge diode, or even though the filter cap was measuring in spec as far as capacitance it was not performing correctly dynamically because some other aspect was out of spec.

Is the snap crackle pop at all outputs or just the headphone output?

 

[frnkfrkl]

The Rice Krispies is audible IN THE ROOM. That is to say, it's not specifically in the audio circuit, it's something that can be heard with no audio, headphones or otherwise, hooked up. The machine is physically making the noise. It is faint, but present. It is accompanied by flickering VU meter lights until, after 3-5 seconds, everything stabilises. My Spidey Sense tells me a capacitor is malfunctioning somewhere in the startup circuit downstream from the power supply board and sputtering until it finally charges.

There is a fairly loud hum in the headphone amp circuit. If I deselect the channels (1-4) feeding the headphones, the hum goes away. It is a separate issue from the noise at startup. Which brings me to:

There are 33 electrolytic caps on the MONITOR AMP PCB. It is readily accessible and I was wondering about shotgunning those while the panels are still off the machine. They are also 40 years old and there's a ton of them. Couldn't hurt and it's an easy job. That would probably clean up a lot of stray and, as yet undetected, leaky wonkiness in the audio chain.

Anyway, these are my musings. Please comment and correct where necessary. This is more than I ever wanted to do but, in for a dime, in for a dollar...

 

[sweetbeats]

Dude…okay…thanks for clarifying…so there is mechanical snapple crackle pop unrelated to the audio outputs…! That’s a bit freaky.

So where are we at? You recapped the power supply PCB and replaced the bridge rectifier diodes for the +6V supply, and did you also replace D805 and D806? And you still have flickering VU meter lamps and mechanically audible bad electrical noises on power up, but you are now able to pass audio to the outputs? Is that right? And there is hum in the headphone output. Is there hum in the line outputs 1-4?

There are elements related to the muting circuit on the monitor amp board that I don’t understand…have studied that block to understand it. Recapping certainly wouldn’t hurt.

FWIW your theory about what is happening, cap malfunctioning and sputtering until it charges, that’s not it…that’s not how they work. They can either charge or not charge (complete failure) and if they can charge the issue is whether they can charge within spec, and depending on their application in a given circuit, if they are out of spec that can cause other switching things not to work right. It’s less common for electrolytic caps to just completely fail, but they can leak, they can dry out, making it so they can’t do their job right or well.

 

[frnkfrkl]

I'll have to connect it to something to see if there's hum in the main outputs. That setup is on the other side of the room, of course.

Yeah, that Krispiness and the accompanying light show is a bit freaky. I can't really tell where in the machine it's coming from. I get the impression it's near the VU meters but I might be led astray by the hypnotic lights and just associating the two. They are related but I can't localise the noise, really. At least there's no smell and no smoke for now.

I'm going to recap the MON AMP PCB and then see where I'm at. 40 year-old electrolytics, it's time and can't hurt.

 

[sweetbeats]

But can you confirm audio passes to the outputs now after the work in the power supply PCB? Otherwise, if not, it seems to me nothing has changed.

 

[frnkfrkl]

Will do tomorrow. Things have changed however. Before, no VUs, no audio at phones ( nor outputs). Now, VUs functioning and audio in phones. I need to verify audio at outputs but this is progress.

 

[sweetbeats]

Oh okay. That’s all I needed to know. If you have audio in the headphones and activity on the meters you should have audio at the line outs. I just wasn’t clear on that.

 

[frnkfrkl]

Just for giggles, I powered up the machine WITHOUT the Mon Amp PCB installed. Still have the noise, still have the flickering VU meters so that problem in NOT on the Mon Amp PCB. Waiting on the caps for the PCB anyway, perhaps I'll get a cleaner audio circuit once recapped.

 

[sweetbeats]

Word of advice: be careful about powering up with certain assemblies removed. I think it’s not likely an issue in the Series 30 tape machines, but I’ve had bad outcomes in other machines, namely the 388, powering the unit with one or more assemblies removed. We would all like to think the system as a whole was designed in such a way as to allow for safe operation with this or that removed. But that’s not always the case and you have to be way more knowledgeable than you or me combined to know that in advance. I learned this the hard way when working on a 388…can’t recall what I had removed from the card bay…so many years ago…but I know it was on the right side (so not the amp or dbx cards…signal amp cards are, IME, generally safe to have one or more removed under power), maybe the reel servo card…anyway, I did that so I could access…yes it was the reel servo card, because I needed access to one of the bias amp cards, and the way power flows through the 388, I think having the reel servo card out interrupted correct power interconnections and something reverse biased and there was smoke and stink. I would just be careful on the 34/34B with the monitor amp assembly because it’s not just signal amplification but also part of the muting circuit…loops in and back out…

I’m getting more and more confounded by your situation…would be so much easier if I had the machine in front of me. Again, I can’t see what you see, so I don’t always know what to ask, and you don’t necessarily recognize things or know what to share or ask. This is what makes it hard.

Just to verify, if you pull the fuse that powers the +6V rail, and then power the unit, you do NOT get any of that audible physical electrical sputtering noise, correct?

 

[frnkfrkl]

Well, on your advice I’m not going to power up the machine until I get the pcb recapped and installed. The only thing I see hooked up to the VU meter lights is that 6v rail and I took care of that on the power supply pcb. I’m wondering if there’s not some wonkiness with the lights themselves or a bad connection there. The crackling sound corresponds to the flickering so that would make at least some sense.

 

[RFR]

Might have some solder joints needing to be reflowed?

 

[frnkfrkl]

Who knows. I’ve got to find out where those joints might be and look. I’ve completely reflowed the mother board so the only other connection is at the VU meters themselves. They get jostled a bit on disassembly so I’ll have to take the front off to verify that there’s nothing amiss there. They’re all wired together from one 2-pin connector so if one is wonky it’ll manifest itself across all 4 VU meters. First the recap and then I’ll attack that problem. The weird thing is that it stabilises after about 3-5 seconds which would tend to debunk my hypothesis.

 

[frnkfrkl]

Ok, MON AMP PCB recapped and installed. Audio works great, cleaned up a lot of noise and buzz. Transport fine, all lights work, all functionality restored. The VU lights still flicker on power up but stabilise after 3 or so seconds. I'm thinking it has potentially something to do with the outboard power supply I'm using. This is a 110v, 60Hz machine and I'm using a transformer to drop from 230v, 50Hz here to the 110v required voltage but still 50Hz. Maybe that 50Hz oscillation frequency is throwing a small wrench in the works. I wouldn't think it would matter much, after rectification and smoothing but who knows? I don't seem to be hearing the crackling. I'm not going to worry about it. If something else breaks, I'll deal with it then. I'm calling this done and a success. It was a long haul and wouldn't have been possible without your input. Thanks for everything. If it suddenly catches fire somewhere down the road, I'll be back. For now I guess I'l be signing off here and getting back to transferring tapes. Cheers!