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Old 11th Aug 2019, 12:06 am   #1
em536716
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Join Date: Dec 2017
Location: South East London
Posts: 301
Default HMV Thorn 1500 Low audio and no video!

Hi all,

Been a year since I posted last. I was given a HMV branded 21inch (or bigger perhaps!) thorn 1500 styled tv. I thought this would be a fun job to commit to since the first 1500 I restored for myself was mainly quite easy... I was wrong.

The set was presented to me with a frame fault, which turned out to be an open section on the mains dropper (HT feed to frame timebase). Then the odd fault of the brightness control. It would magically fade out all of a sudden and it was quite touchy. I tested the pot first and all seemed fine. Both out and in circuit, and let’s just say after lots of swearing and tired eyes over the diagram, I went back to the potentiometer, took it apart and found out all the carbon has disintegrated and now was magically testing open.

With the new pot installed I suddenly lost video, with only faint audio from my VCR. I’ve checked the video amp transistor voltages and all seems fine, other than a little low LT feed to it. I also checked VT 4/5/6, all seemed fine other than the emitter voltages which seemed way to high on all of them. I checked the resistors between emitters to ground and all seem fine. I also swapped out the original DC caps along along with the others.

Bob the bodger has been on here before me and it seems it’s had some work done to it before. I dislike PCB designs, well early ones, as they’re so prone to W falling apart!

I’ve verified that video amp is working by connecting a .1 cap between pin5 of V1 and collector then base of VT9/8/7, and I get a form of black bars on screen. This proves at lead that the video amp is amplifying!

Does anyone have any advice on what to do/check? The CRT seemed lovely and bright. I’d love to get it going!
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