Tascam 34 - meters doing strange things

  • Thread starter Thread starter rob aylestone
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Well, every time I try to upload a photo, it doesn't seem to work. But, if it did, this is the mess. I've got the card loose but I need to try and wiggle it around some wire bundles.
 

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Yeah, if I don't want to unwrap those bundles, I think the transformer, fuse holder and board will come out as a package. There are some wires soldered to the control board as well so it'll be a balancing act unless I want to start desoldering those, too. And I don't, really. Not for one transistor. So. That's a tomorrow project. I looked at Mouser and Digikey but neither of them cross-reference to 2CS1645B. NOS, yes, in the States (I'm in France), $17.95 a pop plus shipping, plus customs....for a transistor. $0.76 AliExpress but who knows what reference or what I'd actually receive. This whole project has waited 30 years (transferring tapes to computer) so tomorrow I'll just see if I can get THAT whole mess out of there and just stare at it until I can find a transistor. No big rush...keep calm and carry on, as the Brits say.
 
I wasn’t saying looking on the Mouser or Digikey website for cross-reference. Get old-school and call them…or open a live chat.

Generally it’s possible to navigate PCB assemblies out around hard soldered wires…it’s not uncommon for the power supply to have to dismount the main transformer, power switch, fuse PCB…

Not sure what the shipping would be, but, no, never spend money like $17.95 on a small current TO-92 transistor…that’s why I commented about shopping diligently…quick eBay search bring up these options:

https://ebay.us/m/egZpmU

https://ebay.us/m/BUTrU3

Yes, who knows what the shipping would be for the first one, and it is certainly a reasonable concern to question the authenticity of the component in the second one, but counterfeit parts are much more a concern or common with integrated circuits. I’d be comfortable purchasing either of the above.
 
And yeah okay I think if it was me I’d dismount the main power transformer, fuse PCB, and that sensor PCB mounted to the back of the takeup reel motor…take lots of pics, put hardware into different labeled baggies…those steps might help you make some gains with extracting the power supply PCB.
 
And the back board (control). It's on a bracket with just two screws so it'll come out with the rest. I ordered the transistor(s) from the first link. I bought 5 because, well, ya never know. Not too painful, shipping, etc.

One further question: should I be looking at D805, rectifier diode and D806, switching diode? D805 should be fine because I'm seeing 6Vdc at pin 1. I've got a 4006 around here somewhere which, I imagine would be overkill. D806, I'm not so confident because it is inline with the transistor and the bridge rectifier and something caused the transistor to fail and blow the fuse. Old age, maybe but while I'm there...I could probably do a complete recap, as well and I probably won't though the caps would be more easily sourced. Hmmm....it's just, how far down this rabbit-hole do I want to go?
 
Personally I would not suspect or have concern with D805 and D806. I mean, you can test them, but this is a relatively low voltage low current circuit, and all those diodes do is ensure proper current flow at power up and power down in order for the transistor “on” state to be delayed at power up, and go “off” first at power down. We know, based on your tests, the transistor Q801 is bad. It may have simply failed from age, and went dead-short emitter to ground. Current if I understand the circuit correctly current is t supposed to pass to ground through Q801…the emitter is a reference to ground so the transistor knows when to go “on” depending on the voltage level at the base. But if it failed and current went to ground through Q801 when it failed, I think that would pop the fuse…and after failing Q801 might be open circuit to ground which is why it’s not blowing the fuse, but also not passing enough current to the collector to energize the muting relay coils. You could always test this and clip the emitter pin and jumper the base to the collector, turn it on and see if you have audio…beware you will likely get a power thump in the outputs at power up, so turn down the volume level of your monitoring system. I realize you’ve got it all pulled apart now and that can be scary trying to ensure nothing is touching anything it shouldn’t be touching. I’m sorry I didn’t think of this earlier, but it is something you could do to verify the rest of the circuit is working…verify the transistor is the culprit.

If you do this, again, make sure you allow adequate time for the large caps on the power supply PCB to drain before working on anything.

Regarding the rabbit hole, if it was me, I’d probably recap the power supply while you have it out. If you do this, at minimum, use 105C temp rated caps.
 
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