I'm having an issue with my Yamaha K1020 tape deck that has me stumped. Mind you, I have a decent amount of repair skills having worked on a lot of audio equipment, specifically tape machines, so its rare I ask for help, usually being the one who offers it.
The issue is with trying to adjust PB levels just after the head amp. I can see (and hear) evident changes in level, but the output VU meter doesn't respond, always reading the same level. The signal before the adjustment trimmers doesn't go anywhere else according to the schematic, so it makes no sense why it would behave like this.
I attached a snapshot of the schematic section in question and a copy of the SM to show what I mean. The output level trimmers are circled in red.
The issue is with trying to adjust PB levels just after the head amp. I can see (and hear) evident changes in level, but the output VU meter doesn't respond, always reading the same level. The signal before the adjustment trimmers doesn't go anywhere else according to the schematic, so it makes no sense why it would behave like this.
I attached a snapshot of the schematic section in question and a copy of the SM to show what I mean. The output level trimmers are circled in red.
Attachments
I figured out the linearity correction adjustments for the VU meter were very far out of adjustment, creating a "flat spot" in the meter's response. I went through the calibration procedure described in the SM and was able to correct the meter issue.
Well, I spoke too soon... when I adjusted the high/low meter linearity settings, I discovered noise on the L channel coming into the monitor circuit. It wouldn't allow the record level to go past +4 dB and added noise on top of it. Turned out R147 (22k) measured out of spec at 80k, so that was the reason why it was adjusted so out of whack to compensate for the problem with R147.
I may just create a new thread on modding this tape deck since I've come this far with it. I purchased this deck from a guy on ebay about 20 yrs ago for about $30, at the time tape decks were still not respected as much. Fast forward now, these things are selling for over $500 in decent condition. This Yamaha sounds excellent, even with notmal bias tapes. I'm also going to install a new rec/pb head and thought some of you may benefit from seeing the process.