Sony CMT-M373NT (HCD-M373) repair, stuck in standby

[Chungalin]

No, this time there're no fireworks at the end of the post.

I'm running out of ideas with this one. It's a compact CD, MD, Tuner and Tape. When plugged to mains, main transformer relay clicks, all voltages appear, display backlight is lit (nothing shown), and after 5-6 seconds relay clicks again as it goes into Standby, constant red led, "--:--" on display (not blinking) and then it doesn't react to anything. Owner says it happened from a day to another, no previous problems.

All voltages at CN101 are present while the relay is active (A+10V, M+8V, M+10V, M+7V, MD+5V, D+5V). There's also ±27V for amp IC803 (STK403-030). I've tried disconnecting CD FFC (CN103) and MD FFC (CN102). Same result, just that it stays for a bit longer until it goes into standby. Tried making a bridge between Power tactswitch contacts, nothing. When pressing power button, KEY1 line goes from 3,4V to 0V, seems normal behaviour.

Not very reasonable to me, but could be an amp protection. Tested pin 100 of system control IC101 (PROTECT_IN) and reads 3,4V all the time, normal voltage according to schematics!

What else?

[Chungalin]

Re: Sony CMT-M373NT (HCD-M373) repair, stuck in standby

I tried to remove R200 in order to induce a protection, just to see if that makes a difference, but no, all the same. It’s very weird.

I’m about to dismiss this one.

[kaboom]

Re: Sony CMT-M373NT (HCD-M373) repair, stuck in standby

CPU clock failing?

Is there anything that enables/drops the clock during turn-on vs standby?

Can you upload the schematic or service manual?

[Chungalin]

Re: Sony CMT-M373NT (HCD-M373) repair, stuck in standby

CPU clock failing?

I’ve checked both crystals from IC101 (System Control) and perhaps there’s something here, thanks for pointing this. While I can clearly see X100 wave (32.768kHz), I cannot see anything from X101 (16MHz), just 1.5V and 1.3V at each side. According to SM, I should see a 2.8V wide pulse train. The "problem" is that it’s not a normal crystal (like X100) but a ceramic resonator with 3 leads.

If I remove it, the set won’t even start, so it’s doing something, although I can’t see it.

The most similar thing I have at hand now is a 4MHz ceramic resonator from a remote control. I’ve replaced 16MHz resonator with one of these and the set starts, but it lasts more than one minute to complete the starting cycle (about 7 seconds with the original 16MHz resonator) and after that it stays in standby like before, doesn’t react to from buttons.

Although I can’t read X101, I’m not sure now if this is the right direction. Perhaps my Hameg CRT scope is too old to show several MHz signals...

I’ve attached SM, downloaded from ElectroTanya.

[kaboom]

Re: Sony CMT-M373NT (HCD-M373) repair, stuck in standby

Found an error in the schematic.

One side of the 10.8V winding from the main transformer was unconnected. I have added this line in blue. Image attached.

Have you checked & verified all supply voltages, as indicated on the schematics? Almost sounds like a missing rail.

Many years ago, had a VCR that did something similar. Would go into "play," fully threaded, but tape never moved, no capstan motor or servo. FF or REW would work each time you tried them, but would STOP after 15 seconds.

An open/corroded jumper killed one of the +5V supplies.

Check all voltages on power, regulator, amp and main boards.

[Chungalin]

Re: Sony CMT-M373NT (HCD-M373) repair, stuck in standby

I've already double checked all voltages at CN101 (10V, 8V, 10V, 7V, 5V, 5V, 7,5V unreg), CN106 (3V, -26V), CN103 (5V, 7V, 10V, 10V), CN805 (-26V, 26V, 18V). 5V for CD are switched off by Q110. Checked +4V regulators (IC355, IC356). I think there're no more.

The set is quite well conservated, no moisture or oxidation. I've repaired other sets with problems like you say, but in this case I see everything very shiny. It's not very repair-friendly, as it has 3 boards interconnected directly without intermediate cables of FFC, leaving little room to insert probes.

It's time to give up, I have other repairs to do. Thanks for your help.