This was originally my own collection of a broadcast tape recorder, which I used to cultivate my personal taste and interests. However, recently my protégé has... @YofoaStudio I've been assigned this job. It's rare for this machine to operate under heavy load, but "Old Bull" is struggling. This morning, while working, I suddenly noticed that one of the VU meters wasn't responding, but the input and output levels were normal. So, I performed an emergency surgery:
[Troubleshooting] Since the input and output levels are normal, the signal path is also normal. The problem likely lies in either the VU measurement circuit or the meter itself being damaged.
By touching the head coil with an ohmmeter, I found that both heads were undamaged, so I removed the head drive board.
Initially, I didn't notice any problems when I disassembled it. However, while "feeling" the probe with my hand:
It's malfunctioning… specifically, D101, D102, D201, and D202 are making contact with the diodes. I checked the schematic to see if there was a reason:
Because I saw that it was a point contact diode, I thought it should be a germanium diode. The forward voltage should be quite low, so I measured it:
I found that the consistency wasn't good, so I decided to replace them all. At this point, I would also like to introduce my long-time collection:
Open a package and test each one individually for continuity voltage:
I selected 4 that were roughly equivalent, and then applied them.
Does this feel a bit out of place, using older state-owned components?
According to the manual, a reading of 0 VU should be obtained when measuring at 400 Hz and -10 dBV.
Since Vrms is slightly higher than expected, I've added a bit more headroom.
The main coordination is complete; now we're assembling the casings and will continue to fight for another thirty years!
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