Sony TC-K620 Cassette Deck Power Supply Problem

[Alan 'A.J.' Franzman]

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

Email: a.j.franzman [ A T ] verizon [ D O T ] net

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[N Cook]

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.

--
--------------------

Alan "A.J." Franzman

Email: a.j.franzman [ A T ] verizon [ D O T ] net

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

 

[Mark D. Zacharias]

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

Mark Z.

 

[Alan 'A.J.' Franzman]

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

I have received the manual OK. Thank you!


--
--------------------

Alan "A.J." Franzman

Email: a.j.franzman [ A T ] verizon [ D O T ] net

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[Alan 'A.J.' Franzman]

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

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!

--
--------------------

Alan "A.J." Franzman

Email: a.j.franzman [ A T ] verizon [ D O T ] net

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