Old UNITRA monitor blowing mains fuse

[Buzby123]

I bought this about 40 years ago. There was a pile of them on a garage forecourt in Birkenhead, between the newspapers and the screenwash. Bought two, £20 each, but I don't what happened to the other.

It was used with an Apple II back in the day, then relegated to the attic.

Today I brought down, turned it on, and got a raster. Unfortunately the raster faded, then the fuse blew. Replaced fuse, it blew immediatley.

The construction is easy to work on, but I'd like some guidance. I've got some more fuses arriving in a couple of days, so if there is any testing I can do before they arrive I'd be grateful for ideas.

Cheers,

Buzby

[Buzby123]

I found this monitor is based on the VELA T206 television, and there is a Youtube video showing inside. At 5:15 the presenter is handling two components, one of which looks like the one in my monitor. I can't understand the language, can anybody translate for me ?

https://www.youtube.com/watch?v=wyutJe-s5K0

My monitor has a white CRT, so according to this website ( https://oldcomputer.info/8bit/apdisply/index.htm ) it's a prototype of the TWM-315. Maybe that explains why its cabinet is not branded.

[its ur aerial]

I am not familiar with this actual model, but , the first action would be to check any mains input filter caps, the Diodes in the Bridge Rec and assuming it has a Switch mode power supply ( I am not certain it has from the pic`s) the the Chopper transistor.

Ken G6HZG.

[mark_in_manc]

That's interesting. I watched the video - I wonder where the semiconductors came from. I remember reading a story (probably linked from this site) about the DDRs ruinous attempt to become self-sufficient in semiconductor manufacture - and if it could not be done in the DDR, then it is unlikely to have been done elsewhere in the orbit of the USSR. That suggests imports, which suggests a drain on limited supplies of hard currency.

I travel to Poland from time to time with a Polish friend, and have an interest in things over there. I worked out that another consumer product which we don't talk about here, at the end of the 80s cost Poles about 10x the cost to UK consumers - partly because exports were subsidised to generate hard currency, and partly because my calculation normalises take-home pay which was very low in the east. So a 20 quid telly in today's (UK) money might be £60 for us or over £600 to them. Ouch.

[McMurdo]

Looks like a linear power supply with a remote power transistor? I would guess it's dead as the reservoir capacitor or another might've needed reforming after so long. Probably something in the PSU has gone short, these typically give out around 12VDC regulated (or thereabouts) to the main board. I'd have a prod about on the regulator board to start.

[Buzby123]

Originally Posted by its ur aerial

... check any mains input filter caps, ...

It looks like you may have found the culprit. I cut the filter cap out, which is the same as the red one in the video, and found it's a dead short across the black wires. The reverse side of it doesn't look too happy either.

The transformer has two primary windings in series, 27 ohms each, so that looks OK, and there's no short from primary to secondary.

New fuses should arrive tomorrow, then I try it again.

[cmjones01]

Originally Posted by Buzby123

I found this monitor is based on the VELA T206 television, and there is a Youtube video showing inside. At 5:15 the presenter is handling two components, one of which looks like the one in my monitor. I can't understand the language, can anybody translate for me ?

https://www.youtube.com/watch?v=wyutJe-s5K0

At last! A use for my Polish language skills in vintage electronics!

The presenter says "before using the equipment, even if it hasn't been used much before, it's worth changing these anti-interference filters for new ones". They look like mains interference suppression capacitors to me.

Chris

[Maarten]

Likely paper capacitors that might behave like RIFAs when powered on - or cause a short circuit.

Originally Posted by mark_in_manc

That's interesting. I watched the video - I wonder where the semiconductors came from. I remember reading a story (probably linked from this site) about the DDRs ruinous attempt to become self-sufficient in semiconductor manufacture - and if it could not be done in the DDR, then it is unlikely to have been done elsewhere in the orbit of the USSR. That suggests imports, which suggests a drain on limited supplies of hard currency.

Most cosumer grade semicondutors were made locally in the country the set was made. Both Poland and the DDR had an extensive semiconductor industry by the time this set was made, as had Czechoslovakia and Yugoslavia. More complicated chips might have been sourced from the USSR, however the DDR was able to produce every chip needed for home computers. Not sure about Poland, but they should have been able to produce a TV set or monitor entirely from parts from their local manufacturers.

[Buzby123]

Put new fuse in, left red capacitor out, switched on.

Raster appeared bright, then faded to dim. It might be OK, but my video source ( a modded ZX81 ) has now decided to stop working !. Now looking in dark corners of the attic for something else that generates video.

Regarding where the bits for this monitor were made, the CRT, transformer, and EHT have 'Made in Poland' on them. The speaker is labelled 'TONSIL'. The transistors and an IC have 'CEMI' written on them, and a lot of diodes are those little rectangular blocks with a pip at one end. I'm sure I saw those in a Russian radio years ago.

[cmjones01]

Originally Posted by Buzby123

Regarding where the bits for this monitor were made, the CRT, transformer, and EHT have 'Made in Poland' on them. The speaker is labelled 'TONSIL'. The transistors and an IC have 'CEMI' written on them, and a lot of diodes are those little rectangular blocks with a pip at one end. I'm sure I saw those in a Russian radio years ago.

TONSIL and CEMI are familiar names in the Polish electronics industry, as was. Both were part of the enormous Unitra conglomerate. I think TONSIL (which translates roughly as "sound strength") still exists, making speakers and other specialist wound components. CEMI has long since gone but used to manufacture a lot of semiconductors from transistors all the way up to microprocessors. Their TTL logic is still quite easy to find on the local marketplace sites, and in fact I used a CEMI 7438 to repair the disc interface in my BBC Micro a few years ago. Their old buildings were just across the road from a place I used to have an office in a district of Warsaw called S?u?ewiec (once an industrial zone, now a vast area of shiny glass corporate offices employing about 80,000 people) and had been repurposed into business units. Only now are they being demolished and replaced, I think with the inevitable apartment blocks.

Chris