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!

 

[sweetbeats]

Ahhhh these are important details. The transformed line voltage is probably borderline too low and throwing the timing circuit/6V trail for a loop. The system is designed for 115VAC give or take at 60Hz, but if it is a 2:1 step-down transformer the system will “see” less than 115V because of the lower line frequency. It’s not a matter of oscillation from the 50Hz line frequency, because the AC is rectified by the regulated power supplies, but rather it’s a matter of too low rectified voltage. I’d have to find the formula, but whatever the AC voltage coming off the secondaries of the main transformer for the multiple power rails is, will rectify to a lower unfiltered DC voltage because of the slower line frequency, and it must be right at borderline too low for the +6V rail and be oscillating as the timing circuit stabilizes. Sometimes you can use 50Hz power on a 60Hz device and be 100% fine. Other times not.

 

[frnkfrkl]

Yeah, well, hey, that didn't last long. Tested it on the bench. Fine. Put it back in its spot and connected it to my computer (DAC) and I got about 30 seconds of functionality. It then just cut off all audio and I'm back to exactly the same fault as before. No blown fuses this time. I used this machine for years here in France with no issues. NOT the same step-down transformer that I have now, however. I'm showing 119vAC coming out of this step down. (Bronson++, 1000 watt). Dug out my old and inherently dangerous nude transformer and tried it. No luck there.

J171 CONNECTED: pins 1&2, 4.8 vDC on power up rising to 5.9 vDC after 6 seconds
J171 NOT CONNECTED: pins 1&2, 5.6 vDC on power up rising to 6.7 vDC after 3 seconds

I'm hearing the crackling again, J171 connected or not but more noticeable connected, corresponding to the 3 and 6 second ramping up of DC mentioned above. It was probably was there all the time but-ambient noise and old ears when I was listening before. VU meters dim when I put it into "play" and the motors start spinning.

I'm contemplating disconnecting power to the VU lights (basically taking 4 resistors out of the equation). If I'm grokking the circuit correctly, that would not cause an open circuit, it would just remove the lamps from the circuit. Unfortunately, some disassembly required. What do you think of that idea, before I go down that rabbit-hole and start taking shit apart again? And the most confounding: Why did it work and then NOT work? Again.

 

[frnkfrkl]

Disconnected the VU lamps. Now 6.3 vDC (more than before) on pin 2, J171, CONNECTED but still no sound.

 

[sweetbeats]

I don’t know if I can help you. Operating the machine that was built for 115V/60Hz power on something other than that may have had permanent consequences. One cannot play loose with these things. At this point, if it was me I’d pull the fuse for the +6V rail, and jumper the K-102 relay signal contacts to see if things at least work that way. Be prepared for power up and down thump.

 

[frnkfrkl]

Oh, wow. Just wow. I noticed the DX-4 Noise Reduction Unit lights were not lighting up in any mode. I had disabled the DX-4 with the buttons on the front of the unit because these early tapes I was transferring did not have any noise reduction on them. I had thus not noticed that the DX-4 was not making a connection (no lights anyway when disabled). What flows through the DX-4 module? Audio, record and playback. What happens if there's a fault in the connection on that circuit? I reasoned that no audio would get through. I checked the connector for the DX-4. One wire out of 4 was broken off. I traced it to ground. I resoldered all 4. Miracle (or science, depending). Audio is back! I'm betting that this was the problem all along. It was intermittent because, for a time, the wire was holding on by a thread. When I finally put it all back together, it finally snapped for good or got displaced to a point where no connection was possible at all. I think, maybe, this time, it's good to go. However I'm not yet proclaiming victory. I've learned my lesson.

 

[frnkfrkl]

Ok. One more thing. I got the crackling figured out. Bad connection on the connector for the VU meter lamps. If I poke at it and futz with it, I get the noise. Now, to fix that problem (the VU lamps no longer light up after I disconnected and reconnected the connector), I would have to pull the Mother Board again and that's just not happening. I'm going to button this thing up and too bad for the VU meter lights. I am going to reinstall it in its location for transferring tapes. If it survives the 10 ft trip between my bench and its final resting place, I'm done.