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.