CRT SMPS Repair

[Ultron81]

Hi everyone,

My buddy recently picked up an old Sony PVM-1271Q monitor (dated 1987). When you press the power button, nothing happens, no power light, no tweet tweet, nothing.
 

I picked it up from him and started checking it out. All the power supply outputs were dead. I removed the power supply board, and started checking for obvious failures. I noticed 2 power resistors (R609 and R610 - in the schematics attached they have them listed as 2 resistors each, on my board there are only 2 total) had cracked solder joints. Fixed them up, still no dice. I replaced all the capacitors on the power supply board. Still no good.

I started poking around with my meter. DC rectifier is OK (I get 167VDC across the main filter cap). I get no B+ voltage on the secondary side. So I checked the drive transistor (Q605), and only get ~4VDC on the collector (with reference to the primary side ground). This seems to fluctuate a little, between 3 and 4V. I removed the TR and tested, seemed OK. I then checked all the other transistors on the board, and they all seem OK. I decided to start looking for schematics. The PVM-1271Q service manual online seems to have the page missing with the power supply board. So, I was able to find the PVM-1371QM (overseas version) online, and the circuit layout is the same, with mostly resistors and caps dealing with the rectified voltage as different values. I do have the values for the 1271Q though, so I can see if anything is bad.

I went through and checked all high wattage resistors, and they were OK, except one was out of spec (R602 was reading 9.4M instead of 8.2M, so I replaced it with 2 3.9M in series). I also checked all the secondary rectifier diodes, and they seem to check OK.

CRT SMPS are new to me. I've never messed with them before. I see there is feedback to IC601 from the H. Pulse. Looking in the upc1394c data sheet, this is to sync with the horiz. flyback to prevent noise in the picture. Is it possible to test the power supply board without this connected?

I checked some voltages. IC601 pin 8, I was getting around 5.29V. Pin 3 was 5.2V, Pin 4 was 2.10V, Pin 14 was 2.791V, and Pin 2 was 5.25V. Pin 6 was 0.05V.
D602 is supposed to be a 17V Zener - I was reading ~ 13V across it, and seemed to drop over time.

One thing I thought was strange - I noticed THP602 (PTC resistor) goes really high in resistance pretty quickly. At room temp, it read 280 ohms. I thought it was my problem, but after looking at the schematic tonight, I'm not sure if this is by design now. I see a "kick" drive, and it looks like during startup, the IC601 drives Q604, Q606, and Q602, which then drives Q605. Then, I'm guessing the THP602 goes high resistance, kills the supplies of Q604, 606, and 602, and then the IC drives Q603, which then drives Q605. Is this correct?

Strange thing, when I pulled the Kick oscillator board out (F2 board), voltage across the zener D602 was 17.2V, and IC601 pin 8 was 6.4V (like the schematics).

Hoping someone can point me in the right direction. Am I wrong in thinking its a problem on the primary side, or should I be looking on the secondary side?

I attached the block diagram and the schematics from the international model.

 




 

[tautech]

First identify IC601 and download at least 2 datasheets for it. Then post a link for them here or attach them.

Study datasheets inside out and back to front and if you're lucky a typical application circuit listed might closely match the one you're working on. < more common than you might think. 

Often the VCC supply to the SMPS controller IC is the issue.
For startup VCC voltage, there's often a bleed resistor chain from the HV DC to the IC VCC and the zener limits how high VCC can be. Unless the IC's UVLO VCC level is surpassed they just won't start.
Even dud leaky caps on the VCC can prevent VCC voltage from being high to beat the UVLO.

Open or drifted dropper resistors, bad caps and open diodes are the typical failure points for SMPS on the primary side.
Watch the big HV DC cap as it'll give you a hasty shock if you don't respect it and discharge it. Even then they can still creep up in voltage again after discharge.

Be careful and do the study. 

[coromonadalix]

T604 is the feedback transformer for the power supply ic601,  normally the kick is there to pulse start the psu and get self oscillating

The most important capacitors are near ic601, like c621 c615 c616 c624  if they are leaking or bad the ic could not work or even start ... maybe the one c655 on the kick board too, c625 is in the feedback or c612 on q605 drive circuit.

[xavier60]

Whatever Q605 should be called, there should be the same B+ voltage on its Collector, via the primary winding.
That Kick circuit has me boxed. The only purpose I can see is as Under Line voltage Lockout, but why so elaborate using a PUT?

[T3sl4co1l]

I wonder what the "REMOTE" function does.  Maybe it (and the "kicker" F2) is to implement hiccup-mode UVLO/current limiting?  R604 kinda implies enough supply current to maybe keep it running continuously, even if it's not getting adequate aux from D607.

Yeah, T603 pins 2 and 5 should have low resistance, if not it's fuxx0red.

Did you... by the way... check for cold solder joints yet?

Tim

[shakalnokturn]

You shouldn't be getting 4V on Q605's collector.
Either you had the wrong reference point when measuring that or there is a very bad solder, cut track or as said above T603 is open.

[Ultron81]

Thanks for the replies everyone.

First identify IC601 and download at least 2 datasheets for it. Then post a link for them here or attach them.

Study datasheets inside out and back to front and if you're lucky a typical application circuit listed might closely match the one you're working on. < more common than you might think. 

Often the VCC supply to the SMPS controller IC is the issue.
For startup VCC voltage, there's often a bleed resistor chain from the HV DC to the IC VCC and the zener limits how high VCC can be. Unless the IC's UVLO VCC level is surpassed they just won't start.
Even dud leaky caps on the VCC can prevent VCC voltage from being high to beat the UVLO.

Open or drifted dropper resistors, bad caps and open diodes are the typical failure points for SMPS on the primary side.
Watch the big HV DC cap as it'll give you a hasty shock if you don't respect it and discharge it. Even then they can still creep up in voltage again after discharge.

Be careful and do the study. 

I attached the only data sheet I can find of upc1394C (IC601). I went through and tested most diodes out of circuit, and they were OK. I'm not sure if I got to all of them though. And I agree with the supply on the IC, the data sheet says a minimum of 6.1V, I'm only getting 5.2V.

T604 is the feedback transformer for the power supply ic601,  normally the kick is there to pulse start the psu and get self oscillating

The most important capacitors are near ic601, like c621 c615 c616 c624  if they are leaking or bad the ic could not work or even start ... maybe the one c655 on the kick board too, c625 is in the feedback or c612 on q605 drive circuit.

Thanks. I've replaced all electrolytic caps on the board, EXCEPT the one on the kick board. I will do that today. Figured with the age of the TV, they all should be swapped out.

I wonder what the "REMOTE" function does.  Maybe it (and the "kicker" F2) is to implement hiccup-mode UVLO/current limiting?  R604 kinda implies enough supply current to maybe keep it running continuously, even if it's not getting adequate aux from D607.

Yeah, T603 pins 2 and 5 should have low resistance, if not it's fuxx0red.

Did you... by the way... check for cold solder joints yet?

Tim

I attached the datasheet for the IC, the "REMOTE" pins 3 and 4 seem to work together. Remote control of the power. Pin 3 seems like an output, when it's high, the output is off, when it is low, the output is on. I measured pins 2 and 5 on T603, and its less than an ohm. I reflowed and added solder to all the transformer and transistor pins, and obviously all the parts removed (electrolytic caps, high wattage resistors, diodes). I've done a visual inspection, but I'll do another once-over.

You shouldn't be getting 4V on Q605's collector.
Either you had the wrong reference point when measuring that or there is a very bad solder, cut track or as said above T603 is open.

I thought the exact same thing. Why am I getting 4V on the collector? The tracks look OK, and ohm out OK (they are pretty large). I removed Q605 for testing and put it back in, so it has fresh solder. I added solder to all the T603 pins. It's got me confused. I used the negative of the large filter cap (C611) as reference. The only thing I can think of is Q605 is partially on, just never shutting off like it should?

I do have an LCR meter. I also have an o-scope, but I don't have an isolation transformer, so I'm afraid to start probing with the o-scope on the primary side.

 

[shakalnokturn]

Have you checked R608?
You should have continuity from bridge positive to Q605 collector, and from bridge negative to Q605 emitter.

[T3sl4co1l]

Broken trace somewhere??

Tim

[Ultron81]

Broken trace somewhere??

Tim

Yes!

So I tested for continuity like shakalnokturn said, and I noticed I had about 500k resistance from the + side regulator line to the collector of Q605.
I couldn't see any breaks. Started testing continuity from one component at a time, and found the break on Pin 10 of the transformer. I had to remove the solder from the pin to see the break. The positive rail goes from the regulator to pin 10 of the transformer, which isn't connected to a coil, then theres a jumper between pin 10 and 5. One of the first things I did was check between Pin 10 and the positive rail, but I must have been putting pressure on the pin, causing it to touch the broken track underneath. Never bothered checking it again. 


Put in a jumper wire, and now I have my output voltages. Thanks for the help!!!

Is it normal to hear some buzzing coming from the transformer?

[tautech]

Is it normal to hear some buzzing coming from the transformer?

Not really.

It can be a symptom that the PSU is working too hard with too much load on the secondary.....like dud caps on the LV side.
You say you replaced all the caps.....on the PSU board ?

[Ultron81]

Is it normal to hear some buzzing coming from the transformer?

Not really.

It can be a symptom that the PSU is working too hard with too much load on the secondary.....like dud caps on the LV side.
You say you replaced all the caps.....on the PSU board ?

Yes, I replaced all the electrolytic caps, primary and secondary side on the power supply board. The PS board is still out, so there is no load on the supplies. I will put it back into the monitor and check the voltages. I'm wondering if its working harder with no sync from the Horizontal pulse.

[tautech]

Is it normal to hear some buzzing coming from the transformer?

Not really.

It can be a symptom that the PSU is working too hard with too much load on the secondary.....like dud caps on the LV side.
You say you replaced all the caps.....on the PSU board ?

Yes, I replaced all the electrolytic caps, primary and secondary side on the power supply board. The PS board is still out, so there is no load on the supplies. I will put it back into the monitor and check the voltages. I'm wondering if its working harder with no sync from the Horizontal pulse.

Ah OK, then it most likely needs a load. 

[shakalnokturn]

Considering there was a broken track under the transformer one could assume it may have had some physical abuse, a loose or cracked core is a possible cause of audible noise.

[vk6zgo]

Those old Sony SMPS are usually fairly easy to fix.
"Scattergun" replacement of parts is often the quickest, if not the most elegant approach.

Unfortunately, you are not in the fortunate position I was, where, working for a TV Network, I had Sony parts division "on speed dial" &  access to replacement parts with a very short delivery time.

The biggest nuisance from my perspective, was the multitude of different SMPs designs they used.
My theory was, that when a new Engineer first turned up, "all bright eyed & bushy tailed",  the Senior EEs sent the annoying little sod off to design yet another "switchmode" supply.

By way of contrast, Sanyo used the same basic circuit for around 15 years.

From my experience, the Sony SMPS are happy to run unloaded, unlike Philips ones of the same era, so you can use the same technique of dropping loads off to eliminate them as causing the problem, which was commonly used with linear power supplies.

[Ultron81]

Those old Sony SMPS are usually fairly easy to fix.
"Scattergun" replacement of parts is often the quickest, if not the most elegant approach.

Unfortunately, you are not in the fortunate position I was, where, working for a TV Network, I had Sony parts division "on speed dial" &  access to replacement parts with a very short delivery time.

The biggest nuisance from my perspective, was the multitude of different SMPs designs they used.
My theory was, that when a new Engineer first turned up, "all bright eyed & bushy tailed",  the Senior EEs sent the annoying little sod off to design yet another "switchmode" supply.

By way of contrast, Sanyo used the same basic circuit for around 15 years.

From my experience, the Sony SMPS are happy to run unloaded, unlike Philips ones of the same era, so you can use the same technique of dropping loads off to eliminate them as causing the problem, which was commonly used with linear power supplies.

I agree with you, I have opened up 3 of these Sony PVMs, and they all have different SMPS designs! Since the online copy of this model's service manual is missing a bunch of pages, trying to find a manual with the SMPS was not working out. I had to purchase a copy of the european model's service manual to get the schematics.

 What was nice about the other 2 I worked on was the SMPS board was mounted on the side, and easy to get to. On this one, the SMPS board is mounted on the bottom, so to get to it, you have to remove the frame from the CRT support, which means removing the anode wire, and disconnecting about 10 cables to pull the frame free that holds the 4 boards.

Anyway, I reinstalled the power supply board, and it seems to be working, other than some geometry adjustments needed. Thank you all for your help!