Sony TC-K620 Cassette Deck Power Supply Problem

Hi,

I have a Sony TC-K620 tape deck with an odd power supply problem. I could use a scan of the schematic or service manual if anyone can send it to me.

The problem is, it does not shut off completely when switched off. The cassette well backlight glows dimly and the capstan motor runs slowly when "off". This model has a DPDT push switch (1 terminal unused) wired to the motherboard; the transformer primary and all secondaries are not directly switched. The switch itself is fine, so I suspect a leaky transistor/diode/cap/etc. somewhere on the motherboard is at fault. In the "off" position, I read about 2.6 volts on the 12 volt lines, 1.6 volts on 5 volts, -11.8 volts on -28 volts, etc.; in other words, the power supply section is not shutting off completely as I think it should. The 12 volt supply powers the capstan motor (and I suspect the cassette backlight also), and the 12 volt regulator section appears to be controlled by the 5 volt supply -- everything in the power supply area is so interconnected, I can't even figure out what the primary suspect(s) should be.

Difficulty of looking at the underside of the motherboard is hindering investigation, since even with the back panel off, I can only swing the board about 3 inches away from the bottom panel due to the hardwired connection to the display daughterboard, and the multitude of cables going to the area behind the record level pot.

TIA for any help.

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Alan "A.J." Franzman
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Alan 'A.J.' Franzman
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The TC K590 has a distributed power 'switch' using trannies with integral B-E resistors of types DTC114 (controlling the 12V regulator paired pass trannies) Q816 on this pcb

2SA1348 2SA1345 2SC3402

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N Cook

Sent the s/m to the verizon acct. Hope it got through.

Mark Z.

Reply to
Mark D. Zacharias

I have received the manual OK. Thank you!

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Alan "A.J." Franzman
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Alan 'A.J.' Franzman

For anyone else having this problem (there must be others!):

After printing out the relevant part of the schematic and studying it for a few minutes, the first component I looked at on the board turned out to be the culprit -- R216, a pullup in the power control circuit, is a 22k instead of

2.2k as called for in the schematic, apparently due to a factory assembly error. Paralleling a 2.4k resistor with it kills the capstan motor and backlight and brings every supply voltage down to near zero but one (the -28V only goes down to about -3.5, but that's a lot better than it did before. I'll do some more checking to see if there's anything else amiss.)

I'm off to do the fix now, thanks again for the help!

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Alan "A.J." Franzman
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Alan 'A.J.' Franzman

On yesterday's inquiry from another TC-K620 owner with the same problem, re-investigation reveals that I mis-reported the part number; it's R816, not R216, which was a 22k (red-red-orange-gold) instead of 2.2k (red-red-red-gold). This is near the back corner of the mainboard, left-hand side if you're facing the front of the unit.

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Alan "A.J." Franzman
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Alan 'A.J.' Franzman

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