Sony KV-M1420U / BE-2A help

[1980s_john]

Hello,
I have one of these 1990 portable colour TVs with a nice Triniton tube. It has been working fine for years, but recently it fails to switch on, just see a single flash of the power LED then nothing.

I checked for faulty capacitors and found C821 100uF 16V had failed (it is next to a 12V regulator IC802 heatsink), replacing this made no difference. I have checked fuse PS801 and it is good.

I suspected the HOT Q802, as BE measured short - removing the transistor it tested OK, the short (2 ohms) remains in the circuit between B and E. I looked at the only circuit diagrams I could find (a 6 page file called sony_chassis-be-2a.djvu) but I couldn't see Q802, has anyone got a circuit diagram that shows this part of the circuit please?

Any other components to check please, I am not confident in powering up this set and measuring when live.

Regards,
John

[HamishBoxer]

Make sure no dry joints on the loptx.

[John123]

Found a few similar faults from related models. Could be another o/c IC protector type fuse (PS801 already ruled-out). Worse-case scenario, the regulator?

Sony KVM1400U - Dead with slight squeak at sw on
Check cct prot by loptx & STR54041

Sony KVM1400U - St/by only LED flahes when sw pressed
PS801 N15 cct prot

Sony KVM16TU - Dead
Replace STR54041 with the expensive Sony one - not a copy

[1980s_john]

Originally Posted by John123

Found a few similar faults from related models. Could be another o/c IC protector type fuse (PS801 already ruled-out). Worse-case scenario, the regulator?

Sony KVM1400U - Dead with slight squeak at sw on
Check cct prot by loptx & STR54041

Sony KVM1400U - St/by only LED flahes when sw pressed
PS801 N15 cct prot

Sony KVM16TU - Dead
Replace STR54041 with the expensive Sony one - not a copy

Thanks, I found the Sony KVM1400K service manual which has a complete BE-2A schematic including Q802 - this shows that the Base is driven from a transformer via a 3.3 ohm resistor, which corresponds to what I can measure (3 ohm, not 2, between B and E) so Q802 is going back in. I'll check a few diodes and resistors in the power supply, prior to looking at the STR54041.

Regards,
John

[1980s_john]

Hello,
Not found any more faulty components, I put Q502 HOT back in, and touched up the LOPT pins with some fresh solder. Powered on and green power LED (nb red means standby, green means on) shone for 2s together with some normal raster/HV noises then set died again. I guess it is some form of protection coming on. I suspect IC802 (L78M12CV regulator) as this feeds C821 which I replaced, or the STR54041 (as per above stock faults, called a power supply IC but is only just a few transistors and diodes), or a capacitor which fails at working voltage.

Any further advice appreciated, I wish there was a way of isolating the B (160V) SMPSU from the rest of the circuit to allow it to be tested independently, but it appears to have a feedback input from the LOPT - not sure if this is over- or under-voltage.

Regards,
John

[John123]

It's a shame the chopper IC has to be of Sony spec, as that's gonna be a nightmare to find nowadays. Typical Sony always doing things just differently enough to be mildly annoying!

If it is that, then your best bet would be to rob one out of a scrap BE2 set, or make a post in the Wanted section. I've had success in the past here with obsolete IC's, as the nature of our hobby means we tend to keep things 'just in case'!

Lots more BE2 faults here.

[ben]

That N15 circuit protector was a very common cause of no startup/standby, have you replaced it?

[gallowfields]

also check for dry joints on the line driver tx and also the frame i/c that would cause it to trip off
brian

[Maarten]

If it starts up, I'd not suspect the STR54041 at first.

[1980s_john]

Hello,
Thanks for everyone's comments. I first replaced IC802 (L78M12CV regulator), this made no change. I then replaced STR54041 SMPS chip, the TV now works to a fashion. When switched on from cold (ie off for more than 4 hours), the power LED shines green for 10s then goes out. If the set is then powered off then on the LED stays off for 5s, then turns green then the set comes up apparently in full working order (ie good picture and sound). This is OK for for me, but the set clearly has an issue (or two) somewhere in the power / LOPT circuitry.

BTW I had to buy two STR54041 (which was made by Sanken, same as the original not a Sony special, but is no longer in production). the first only had 10mm long legs so was too low down to fit the heatsink properly. The second had 15mm long legs, better but not perfect as they were bent differently to the original part. I tried straightening then rebending the legs, but didn't do a good job so now the chip is slightly angled against the headsink so only touches it at the top. So the chip probably won't last very long but I'll know better if it fails again.

Regards,
John

[gallowfields]

could you not extend the leads from the str so that it fits flush against the heatsink, remember as well to put some heat sink compound on it .
brian

[ben]

Or drill a hole lower down the heatsink

[1980s_john]

Originally Posted by ben

Or drill a hole lower down the heatsink

Not an easy option as the heatsink doesn't have a hole or use a bolt to mount the IC, rather it uses a small stud (raised disc) that aligns with the hole in the IC and then a spring clip pushes the two together.
Regards,
John

[Michael Maurice]

So file the stud off and fit the chip lightly lower down.

[Welsh Anorak]

Exactly what we used to do back in the day when these sets were getting old and the price of the originlal was prohibitive. Then we used used heatsink compound and a clip that Philips used on several sets to hold it in place. Of course now time's on your side drilling a hole would be the more elegant solution.