Sony PVM-20M4E Blowing HOT and other ICs

[Retrorepair]

Hi all,

I have another Sony challenge on my bench and think I'm a bit out of my depth on this one.

It was sold to me as needing a new flyback which I at first dismissed, but not totally sure now.

Basically Q601 (STR-M6524) on the PSU board will, after about half hour or so of use, blow a fairly large hole in itself. Q602 (STR-S3115) will at the same time short it's input and output and Q501 on the main board (2SD1878, the HOT) will short all three legs. Odd behaviour.

I'm struggling to work out which is the main cause or at least closest to it. The base and emitter do seem very short even with the HOT out (pretty much dead short) but they connect almost directly to a small transformer through two inductors on each leg.

Does anyone have any ideas where to start looking beyond this? I've replaced them all but like I say, after half hour of use, there's a loud bang and Q601 has a large hole in it again (actually somehow tripped my RCD too).

[Refugee]

Have a very good look at the power supply decoupling capacitor on the HT rail feeding the driver transformer on the 2SD1878.
It was a stock fault on CRT computer screens in the days when they were worth repairing.
There may be one or two caps that are getting a bit old that might do it but the one on that driver transformer was a common one.

[Maarten]

Assuming the HOT blows first, it could also be bad soldering on the driver transformer.

On the other hand, a HOT blowing shouldn't generally cause trouble in the power supply. Maybe feed it through a series lamp (between the +B rails and the line output stage) and monitor the output of the power supply as well.

[Retrorepair]

Well it worried me that the base and emitter of the HOT were a dead short, pulled the HOT and low and behold, those pins on the board are still short.

These lead back to a transformer and only pass through an inductor on each pin. I pulled this transformer to test out of circuit and indeed the secondary pins are 0 ohm across the pair. One of these pins is also tied to ground so I'm assuming I shouldn't be seeing a short there as it looks to be the HOT's supply voltage.

This also throws up questions though, mostly, why after replacing all these components does it turn on and function perfectly for about half hour before deciding to go bang? Also like you Maarten, I'm confused why the HOT shorts along with the PSU ICs shorting. They don't seem to have a direct relationship.

The HOT by the way after removing from the circuit reads a short between collector and emitter and 93 ohms between base and either collector or emitter.

[Retrorepair]

This is a picture of the transformer, it's T500 on the board and according to the service manual, it's part code is 1-426-668-11

Not sure why Sony only ever has their part code attached to these, surely they don't make all their own transformers?

[Cobaltblue]

Even if they didn't make it (although they probably did) it would have been made to their design its unlikely to be an off the shelf part.

Cheers

Mike T

[Retrorepair]

To be honest, I've a feeling maybe the secondary should be short, at least very nearly. Just took a look at another, much older consumer Sony chassis and the secondary on the transformer near the hot there is pretty much short too. So back to square one.

[agardiner]

Although nothing's impossible, if the set is functioning correctly when you replace the various parts, then I doubt it is flyback transformer related. Far more likely to be de-coupling caps or a problem with the line drive waveform causing overheating.

This was exactly the kind of fault that used to bug me back in the day of CRT tv repairs and usually resulted in a rebuild of the line driver stage to correct.

[Refugee]

Firstly those driver transformers are pretty well primary HT volts to one volt so the secondary will read short on a normal DMM.
The primary will be driver transistor collector on one end and HT with a decoupling capacitor on the other. Replace the decoupling capacitor in sight and check any others going back to the main SMPSU.

[Chris55000]

Hi!

Faults like this that cause chain– reactions of destruction are demoralising, especially when they destroy devices like this repeatedly!

Begin with the power supply:–

Note there is no high–capacitance mains h.t. smoothing in this design, three 0.47 μ 630 V capacitors in parallel, C612, C613 and C638 are all that is used in this design!

Replace the two resistors R1602 and R1603 from the mains h.t.+ line to the source pin of the STR–M6524, the device will be repeatedly destroyed if either of these go high or o/c!

Next replace the following electrolytics in the power supply:–

C616, C618 and C619 and also make sure none of the secondary side reservoir capacitors C621/3/5/7 & C629 have gone rotten, replacing as necessary!

Next check the primary side snubber components for dry joints/open circuits :–

R603, R627 and R630 – these run very hot in service, the snubber capacitors C614 and C635 for open–circuits or dry joints, and the primary side flyback tuning capacitor C615 for open–circuits or dry–joints.

The opto–coupler PH601 (Sharp PC111) will also destroy IC601 if it goes too low emission – the LED does – replace this with a new one!

Finally make sure you're not using a really cheap STR–etc., ICs, some of these have been faked, and also remove the power supply transformer T603 from the PCB and resolder the wire ends where they connect to the pins!

The h.t. controller IC602 has no components associated with it likely to cause repeated failure, a short on the h.t. output line, which I will come to next, is the main thing that destroys this IC!

Line–scan oscillator stage:–

Replace C531, C537, C541, C549, C550 and C552, these are all critical for correct drive and stability of the oscillator combination IC, the μPC1377 IC502!

Make sure the timing components C538/C539 and R553 are all the correct values also!

Line Driver stage:–

Replace C529, check R520, R532 and R543 (Fusible resistors sometimes go high in value) and resolder the wire ends of the Line Driver Transformer T500 where they connect to the pins. S/C turns in these aren't very common but Ed Dinning may be able to help if you suspect this!

Line Output Stage:–

The first thing I was thinking about when I read your tale was a "fake" or poor quality Line Output Transistor, try one from another supplier to what you previously used if possible!

Make sure the flyback tuning capacitors (C525, C526 & C527 in parallel) are all ok and not dry jointed, and make absolutely CERTAIN there are no burn–ups/dry joints in the scan coil plug/socket and the associated wiring, and also at the scan coils connection board itself!

Sony line flyback transformers are fairly reliable and in the years that I worked on monitors I never had to replace one!

Finally it is worth replacing the two electrolytic capacitors C523 & C592 associated with the beam–current limiter protection IC, IC500!

There's a great deal to go through I admit, but with a monitor as complex as this one is, you can't afford to take short cuts or miss things, this is why I've written it out in so much detail!

One of those little Chinese colour screen LCR–T1 Component Testers (about £20), etc., will be adequate for checking out all the non–electrolytic timing or tuning capacitors I enumerated above!

Finally make sure the h.t. voltage and over–voltage protection adjustments as detailed in the s.m. are accurately carried out as directed in the book once you've got it working!

Chris Williams

[Maarten]

Also, if the drive of the HOT isn't perfect (short in de drive transformer for example), it will get hotter and hotter before it fails. That can also occur if the +B is too high for example. If the HOT is at normal operating temperature when it fails, the fault is most likely intermittent or not directly related to the drive or load of the HOT.

[agardiner]

What a fantastic write up Chris has provided. If working through that doesn't fix it I would give up!

[HamishBoxer]

Went through my mind fake transistors, Sony do like Sony bits.

[Maarten]

Sony often used custom or selected types, or use existing types to their full specs.

[Retrorepair]

Wow thank you Chris! That's such a good write up I'm going to print it and keep it with my service manual!

You can't beat experience when working on these beasties and my experience in consumer and arcade CRT monitors doesn't cut it with this thing. As you say Chris, very complicated!

I'll report back with my findings and make sure this time I use a different vendor for the transistor!

[Retrorepair]

Ok, so all of the components on the PSU board (G) seem to be ok, R1602 and R1603 are reading 205k not 220k out of circuit, which could be an issue but R606, a 3w 150k is reading a dead short out of circuit! That can't be helping ��

[Retrorepair]

My mistake, thats a 0.150k. Should still read 150 ohm though? It's a wire wound resistor if that makes a difference.

[Retrorepair]

Worse yet, everything points to the "k" on the resistor being a red herring. What I thought was a 0 is actually an ohm symbol so between that and the manual it looks like it's 0.15 ohm and given the fact my DMM hasn't been calibrated in a few years, I'd say it's not impossible that's ok

[Retrotechie]

K will be the tolerance, 10%
I believe the resistor should read on its marking 0.15 ohm (with an ohm symbol).
Sounds like it is ok.

[Retrorepair]

Yeah I agree. I'm inclined to believe the PSU is ok, those 220k (actually 205k) resistors may POSSIBLY be just enough out of spec so I'll replace them anyway but my money is now on the A board being at fault and I'll be going through Chris's check list next.

I want to be as thorough as I can as this is costing 50 quid a try