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Crumbtrail TV Forums: TV Equipment: TV Repair Forum:
Help me diagnose and fix a TV-like power supply..

 

 


JonB
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Jan 5, 2017, 4:16 PM

Post #1 of 15 (298 views)
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Help me diagnose and fix a TV-like power supply.. Can't Post

hi

Not sure if this is the correct place to ask, but I am trying to repair a computer with a power supply fault. It's an Amstrad PCW 8256, and it uses a TV power supply to provide power to the computer and CRT tube. The symptom is that it is dead, and I have checked fuses and that 240v is appearing on the board. The bridge rectifier is OK. At the heart of the thing is a power regulator chip, the STK7308, which was common in TVs once upon a time. I have tried to find some technical specs on the thing but so far nothing detailed has come to light (although there are several single sheet specs).

There is a schematic for the power supply in the service manual, here: http://www.cpcwiki.eu/manuals/Amstrad PCW 8256-8512 Service Manual.en.rar

I am seeing 0v on all pins except for 14 & 15, where I am seing 315v (!) relative to pin 1.

Is there anyone here with TV repair experience who can offer any advice?



jts1957
Veteran


Jan 5, 2017, 5:09 PM

Post #2 of 15 (292 views)
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Re: [JonB] Help me diagnose and fix a TV-like power supply.. [In reply to] Can't Post

I'd make it relative to the negative lead of C5005.
C5005 is main filter across the bridge. 315V is raw B-Plus and is connected to pins 14/15, so is normal.
Check Resistance between pins 14/15 & pin 11.
Check R5006 & R5007 (You may want to lift any one of the 4 leads involved to obtain a steady reading)


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Location: Far, Far Away


JonB
New User

Jan 5, 2017, 7:09 PM

Post #3 of 15 (285 views)
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Re: [jts1957] Help me diagnose and fix a TV-like power supply.. [In reply to] Can't Post

Hi jts1957

Thanks for your advice!

R5006 & R5007 are correct. I measured 180k on each (well, 183k and 181k) to be precise. I might have to revisit that, one of them (R5006) took a long time to settle down, whereas measuring it on another (working machine) gave 180k immediately. Does that suggest anything to you?

All the other resistors in the high voltage part of the circuit (feeding TS001) measure correctly, and match the working machine. But the input of TS001 is dead.

C5005 looks OK, no bulging or other unpleasantness visible. Someone (on another forum) suggested changing C5006. Does that make sense to you?

STK pins 11-14: 0.4 ohms.
STK pins 11-15: 0.4 ohms.

These measurements were taken with the STK removed from the circuit. I think it is blown. I fitted another one from a working machine and it didn't work either. Then, when I returned it to the working machine, the "working machine" became another "dead machine". So I guess the board in the first machine has something wrong with it that is killing the STKs. On the board, the readings for 11-14 and 11-15 are all over the place, in the 15 - 20 Meg Ohm range.

I'm confused by your statement that 315v is normal for pins 14/15. The schematic is showing 7.4v. Am I missing something?

This IC needs some voltages or resistances applied to "program" it, right? Do you know how it is setup for a specific output voltage?

Regards
JonB


(This post was edited by JonB on Jan 5, 2017, 7:17 PM)


JonB
New User

Jan 5, 2017, 7:22 PM

Post #4 of 15 (280 views)
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Re: [jts1957] Help me diagnose and fix a TV-like power supply.. [In reply to] Can't Post

Oh, also, I wanted to ask you something.

The board includes the HT circuit and flyback transformer for the tube. I have discharged it and disconnected it but of course the HT flylead is still there and loose. If I want to test it like this, how should I secure the high tension flylead? I thought I might remove the flyback transformer.. would that make it safe from the HT perspective or should I reconnect the tube?


jts1957
Veteran


Jan 5, 2017, 7:40 PM

Post #5 of 15 (278 views)
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Re: [JonB] Help me diagnose and fix a TV-like power supply.. [In reply to] Can't Post

Didn't realize you had more than one unit.

http://imgur.com/a/URVjf

Any electrolytic can dry out. C5006, C5007 & C5012 are prime suspects.

STK IS BAD.

Edit:
Float anode lead in mid-air with at least 6" separating from anything conductive. Non conductive work bench is presumed.


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Location: Far, Far Away

(This post was edited by jts1957 on Jan 5, 2017, 7:44 PM)


JonB
New User

Jan 5, 2017, 11:09 PM

Post #6 of 15 (269 views)
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Re: [jts1957] Help me diagnose and fix a TV-like power supply.. [In reply to] Can't Post

Thanks for the image. No voltage drop across the transformer then.

I could take a reading from the working one.. should have thought of that!

Yes, STK is bad. But something killed it, and the other one I put in there, so I'm reluctant to replace it until I can work out what is causing it.

Is there a way to test capacitors?

Re: the anode, I honestly don't like the sound of that. Be just my luck to lean over it and ZZZZAP! Crazy

Think I'll reconnect it to the tube.


jts1957
Veteran


Jan 5, 2017, 11:32 PM

Post #7 of 15 (268 views)
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Re: [JonB] Help me diagnose and fix a TV-like power supply.. [In reply to] Can't Post


Quote
Is there a way to test capacitors?


There are only three and a quick check at Digikey suggests cost is less than $1 each.


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Location: Far, Far Away


JonB
New User

Jan 6, 2017, 10:42 AM

Post #8 of 15 (259 views)
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Re: [jts1957] Help me diagnose and fix a TV-like power supply.. [In reply to] Can't Post

OK, they are on order, along with a bunch of STKs.

The capacitors were cheap as you said, but the handling charge meant I paid 6.35 for them. Still.. Smile


JonB
New User

Jan 7, 2017, 10:58 AM

Post #9 of 15 (248 views)
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Re: [JonB] Help me diagnose and fix a TV-like power supply.. [In reply to] Can't Post

What about Q5001? Is that a likely candidate?


jts1957
Veteran


Jan 7, 2017, 9:17 PM

Post #10 of 15 (245 views)
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Re: [JonB] Help me diagnose and fix a TV-like power supply.. [In reply to] Can't Post

http://www.learningaboutelectronics.com/...to-test-a-transistor

Accurate results are best obtained with the transistor out-of-circuit (at least two of its three legs 'floating'). Even the transistor inside the STK can be checked fully, you tested C-E and found shorted.


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Location: Far, Far Away


JonB
New User

Jan 12, 2017, 3:08 PM

Post #11 of 15 (203 views)
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Re: [jts1957] Help me diagnose and fix a TV-like power supply.. [In reply to] Can't Post

Right, I replaced Q5001 and those capacitors. New STK fitted. But dare I fire it up? Not too keen on killing another STK (although I have two spares).


(This post was edited by JonB on Jan 12, 2017, 3:08 PM)


jts1957
Veteran


Jan 12, 2017, 5:47 PM

Post #12 of 15 (194 views)
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Re: [JonB] Help me diagnose and fix a TV-like power supply.. [In reply to] Can't Post

You're kidding, right?

Measure for shorts across C5017 - C5018 & C5019.

Take a variable AC supply and set it to 60 - 70 percent of your Mains and have at it.
Otherwise hook it to full mains and it will either work or produce 'magic smoke.'


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Location: Far, Far Away


JonB
New User

Jan 17, 2017, 4:01 PM

Post #13 of 15 (159 views)
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Re: [jts1957] Help me diagnose and fix a TV-like power supply.. [In reply to] Can't Post

Well, I powered it up.

Nothing. No hum, buzz, smoke, heat, nada. Probably zapped another STK, too... Frown


jts1957
Veteran


Jan 18, 2017, 5:45 AM

Post #14 of 15 (148 views)
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Re: [JonB] Help me diagnose and fix a TV-like power supply.. [In reply to] Can't Post

Did you retest the 180K resistors out-of circuit?


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Location: Far, Far Away


JonB
New User

Feb 14, 2017, 8:48 AM

Post #15 of 15 (42 views)
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Re: [jts1957] Help me diagnose and fix a TV-like power supply.. [In reply to] Can't Post

In the end, I decided to rip out all the power circuit components and install a modern 5v/12v power supply.

As soon as I remembered that the monitor is run entirely off 12v and had taken another look at the schematic I hatched a plan to get round my lack of knowledge. The monitor might just as well be an entirely separate PCB from the power perspective; it really does only have 12v going to it. The schematic posted is for the older PCW8256 machine which uses 5v in the monitor's video circuit, but the later PCW9512 that I'm trying to fix is slightly different in that it derives the 5v supply for the video input circuit from the 12v supply. So I removed most of the monitor's power supply circuit, freeing up space for the new power supply, fed in 12v to the monitor and 5v to the computer's CPU board and it came to life. Hooray!

..with the added benefit of having a new power supply that I hope won't be going pop any time soon.

jts1957, I would like to thank you in particular for all your great advice.

Regards
JonB

 
 
 


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