Just measure a known good diode. Other diodes and junctions should be roughly in the same ballpark.
But it's more good/bad than just somewhat different. Meters will vary in how they measure.
That is surely 2SC1096, a Japanese market standard.
https://www.futurlec.com/Transistors/2SC1096pr.shtml
https://alltransistors.com/adv/pdfdatasheet_jmnic/2sc1096.pdf
I bet any medium-power NPN will work here. Pin-out may be the biggest headache.
Ah! That's very helpful.
But I went and ordered a complete power supply board that was pulled from a salvaged Akai, because it was available, as cheap as a couple of transistors including shipping - and will be easier for me to install.
When I do install it and power up, do you think there is a risk that something on the preamp board will now blow the power board again? The original fault was caused by me touching something with a screwdriver, not a failed component.
How can we be sure? Use a 10ohm @ 1/8W in all power lines excepting ground. If one or more smokes, thus.... something is still wrong.
This operation was successful - swapping out the power supply board. Something on the board had blown 😎 I was very lucky that there was an exact match on ebay from a salvaged deck. It was as cheap as a couple transistors, after shipping. Tested all transistors on the preamp board before powering up.
Now I'm back to where the story started, and need to adjust levels - without blowing up the deck again. 😆
Thoughts, anyone, on rigging a 24v phantom power mic input on this? I have a Schoeps that seems fine with 24v phantom, and this deck has 24v going to the preamp board. What could possibly go wrong!
You could take pride in replacing all the electrolitic capacitors with new better ones and especially the input transistor with a lower noise one like 2sc2240, ksc1845...
"Phantom" pretty much demands a balanced input. This AKAI is a cheep un-balanced input. Your best path is to pick up a rock-shop mike preamp:
Ummmm... the market has got insane. I have trialed this one and it did not suck. I paid half of what they want now.
Art Tube MP Professional Mic Preamp/Processor
$109.99 SKU #484020000000000
https://www.musiciansfriend.com/pro-audio/art-tube-mp-professional-mic-preamp-processor
Thought about doing that if something had blown on the preamp board, but I think I will practice "if it ain't broke, don't fix it" for now. It would have to make a dramatic difference to be worth it, and it's one of those things people never, ever demo an A/B test of. 🙄 The deck needs a couple other small things, like one of the input pots.
You're right, the world has gone crazy, or more specifically, boomers have gone crazy with their excess pandemic disposable income.
I could use an interface, but I think there are actually cheaper units that just do phantom power. I thought it would be very neat to be able to just plug mics into it without needing another piece of kit. Will get it running in good shape first.
You will definitelly hear the difference after changing the first transistor with this one.Those caps have more than 50 years man...A guy I know did it and he said it does help.I also have a 4000ds with defective op-amps...i asked a lot of questions about it.
Hmm, okay. Which is the first transistor? Could you point me to it on the layout here?
One thing it definitely seems to need is the 220pf caps on the head unit. A little off is a lot in those small caps. Makes a sound when you switch modes, and playback levels are out of whack when switching between them.
Out of curiosity, why not all the other transistors? I didn't count them, but there's about 16 on the preamp board or so - others are also in the audio signal chain...
There are 9 electrolytic caps on each of the four segments of the board - no chance I'm going to replace all of those without lifting a trace on the board, so I will have to let it be. It would be easier to rebuild the whole board on a piece of perf board.
I did go ahead and make this modification of the four C458 transistors to new KSC1845 transistors. Nice upgrade, I can hear the difference. Worthwhile for $1.36. Signal-to-noise on the machine outperforms the original manufacturer's spec by a significant margin, even with very old, crappy tape.
I was also able to obtain a 'fake' 7.5ips capstan sleeve by matching the exact dimensions to an existing part, and the higher speed really opens up the high end. A good compromise between quality and tape consumption.
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If it sparked it is close to a capacitor. A limiting resistor likely died. If it is a dead diode they usually short out when blown. Measure all with a meter set to ohms and look for 20 or less ohms across the diode.