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Old 22nd Oct 2020, 4:49 pm   #21
Kyle__B
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Default Re: Blaupunkt Orbis transistor portable

Well I just said I found them so it's alright. It's insulation that I'm worried about now.
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Old 22nd Oct 2020, 5:43 pm   #22
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Default Re: Blaupunkt Orbis transistor portable

I'd get them and see how small they are - it might be possible to get them into an EHT cap with a long stem. I've used good quality heat shrink on EHT leads before with some success, but I think you'll need to use trial and error. If it works, you could possibly use a resin to 'pot' the diode in a suitable enclosure.
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Old 22nd Oct 2020, 7:32 pm   #23
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Default Re: Blaupunkt Orbis transistor portable

Quote:
Originally Posted by Kyle__B View Post
Well I just said I found them so it's alright. It's insulation that I'm worried about now.
Ah sorry, I was taking a long while to reply to Welsh Anorak. I didn't refresh the page so I didn't read the good news

The insulation strength of normal heat shrink might be insufficient, but before you insulate, make sure to avoid any sharp ends, so the solder connections should look like round blobs. This to avoid corona effects and insulation breakdown.
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Old 23rd Oct 2020, 6:54 am   #24
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Default Re: Blaupunkt Orbis transistor portable

Turns out the price the German site wants for postage is a small fortune. I did however find that one of the big electrics sellers here does a 16kV fast diode, unless anyone has any objections I can put two in series and it'll do up to 32kV?
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Old 23rd Oct 2020, 2:01 pm   #25
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Default Re: Blaupunkt Orbis transistor portable

That should work just fine.
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Old 27th Oct 2020, 6:49 am   #26
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Default Re: Blaupunkt Orbis transistor portable

I got some 2CL77 diodes in the post today (20kV, 5mA, 100ns) as well as some heat shrink rated for 24kV/mm.

Put it in with three layers of the heat shrink and gave it a go, got a picture. So good news is you were right about what was wrong.

Bad news is after I put the back on and sat down to watch it, it lasted about ten minutes before the picture started breaking down with an arcing noise. Looking in the back, saw orange flashes coming from where I'd tucked the cable, looked like it was going to the transistor heatsink/shield around the edge of the chassis.

Weirdly when I took the back off, no sign of injury anywhere to the heatshrink, or the diode. No burn marks, nothing. Of course, my multimeters can't test diodes with such a big rating on them.

I'm going to go back over everything again and make sure that it hasn't fried any components with arcing, test the voltages on Ux etc...

Hopefully 20kV was just not a high enough rating, and that if I put 2x 16kV series in resin it will be alright.
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Old 27th Oct 2020, 2:17 pm   #27
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Default Re: Blaupunkt Orbis transistor portable

Now I've gone and confused myself about how to choose something to pot these diodes in.

I'm looking at this stuff "HellermannTyton 771-B-SF" and the sheet says it's got a dialectric strength of >20 kV/mm, but the product description describes it as being for electrical insulation of low voltages, but only mechanical/moisture protection at medium voltages.

Not sure what to believe here. Is the figure of >20 kV/mm unrealistic for real life use, and I should find something else?
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Old 28th Oct 2020, 3:26 am   #28
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Default Re: Blaupunkt Orbis transistor portable

I was curious to see if the problem with the last diode was caused by air under the heat shrink causing corona. Orange would be a mad colour for it...

I was curious to see if hot glue filling those air gaps was a possible solution, so I tried it out for a couple of minutes. Originally I was going to let it run until failure again, but it was making a hissing sound and letting off some strange smelling gas that gives me a headache. Windows open at 3AM in october is not my kind of fun.

So yes, if any of you know of a suitable resin or epoxy for potting high voltage electrics, I'd really like to know.
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Old 28th Oct 2020, 5:26 am   #29
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Default Re: Blaupunkt Orbis transistor portable

To correct my last post - noise and bad smell wasn't from using hot glue to fill air gaps under the heat shrink. Testing it in the dark revealed a big purple corona glowing as the wire crossed over the heatsink. Moving it away solved the problem, and it ran fine for half an hour until I turned it off.

I'm going to say that this bodge is fine, but I'll keep an eye on it. The 16kV diodes are fairly cheap so I'll see how long they last in daily use.
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Old 28th Oct 2020, 11:15 am   #30
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Default Re: Blaupunkt Orbis transistor portable

I'd make a cylindrical former to pot the diode(s) in. Make the joints as tidy and rounded as you can and centalise them in the former. There are quite a few resins you could use from the likes of Dow Corning, but some can be very expensive. Ordinary epoxy has quite a good insulation factor, especially when applied in a fairly thick layer.
Once cured, I'd make a plastic bracket to attach it to so it's well clear of any metalwork.
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Old 28th Oct 2020, 11:34 am   #31
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Default Re: Blaupunkt Orbis transistor portable

I've had it on all night watching stuff, no thermonuclear detonation (or even warming up) of the hot glue bodged section, so that's good!

But I think I've found out exactly why it is that the new 20kV diode so quickly followed it's predeccessor to the grave, but this Ersatz 32kV one hasn't. Voltage must be too high.

I realized that must be going on when I was trying to get the contrast and brightness adjusted clearly. I was watching a program where the overall brightness changes a lot between different shots, and the contrast knob was giving exaggerated effects - anything below 50% grey was black. When a closeup of a very bright face happened, the pre-failure symptom from the start of the thread reappeared.

Turning the contrast control all the way off gives a reasonable picture, and providing the brightness control isn't too high, stops any bad behavior.

There's a single voltage adjustment on the horizontal board, with instructions to make U3 to 21V. With no HT anode attached it was giving 24V, so I think I'll start there.

U3 also powers the (dead at the moment) audio board, so plenty of speculation there.
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Old 28th Oct 2020, 4:15 pm   #32
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Default Re: Blaupunkt Orbis transistor portable

As i thought, U3 still 24V with a raster. Ran out of adjustment at 21.8V, but I'm going to assume that will drop once I fix the audio.

Bringing the voltage down has improved the contrast knob a good amount, and it can get really bright before the jitter starts, I have to turn the brightness knob up so high that the whole picture washes out before it happens.

24 vs 21 doesn't sound like much but it's a bit more than 114.28%, 114.28% of 18kV is 20.5kV, No great mystery why it died then. As it is 21.8 is still 3.8% more volts than it really should be, and probably explains why it's not 100% cured.

Beyond that, picture is sharp and clear, seems to like my modulator more than most. Quite bad blooming when brightness is very high though, and the picture is off centre enough that going through menus on Netflix has to be done blind.

Next job will probably be sound. Got some TBA120s coming in the post just in case.
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Old 30th Oct 2020, 10:39 am   #33
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Default Re: Blaupunkt Orbis transistor portable

It's quite possible that the TBA 120 had been damaged by EHT arcing before you got the TV. You need to borrow an EHT meter to see what's going on as the regulation is carried out in the line output stage unlike most transistor based stages. A changed value capacitor can cause excessive EHT without many other symptoms.
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Old 30th Oct 2020, 8:20 pm   #34
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Default Re: Blaupunkt Orbis transistor portable

I'd be surprised if it got hit by arcing, it's just about as far from the anode cap as you can get, on the opposite side of the telly, on the furthest of four plugin boards. The chip's legs were all pitch black with soot when I got it though - not something I've ever seen before.

Getting hold of an EHT meter might be hard here. I did think about replacing all the capacitors at once, (there's only about 5 of them on that particular board), but not all the values seem to be made anymore.
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Old 31st Oct 2020, 12:12 am   #35
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Default Re: Blaupunkt Orbis transistor portable

Yes - the capacitor values are specific to the TV and also they need to be of a construction to withstand the frequency and voltage of a high-energy circuit.
For an IC to be affected by EHT arcing it doesn't need to be near the source as the earth lines and LT can carry spikes around the chassis.
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Old 31st Oct 2020, 5:40 pm   #36
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Default Re: Blaupunkt Orbis transistor portable

The black stuff can be silver sulfide or other corrosion products. Sometimes the insides of the IC will corrode as well, but you could even try to clean it up on the outside with some ammonia (first remove it from the PCB).

When replacing a TBA120, please note that there are a handful incompatible versions, identified by a suffix letter and/or suffix numbers or roman numerals.
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Old 31st Oct 2020, 7:13 pm   #37
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Default Re: Blaupunkt Orbis transistor portable

Oh I cleaned the chip legs up two weeks ago, it's just dead.
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Old 1st Nov 2020, 12:17 am   #38
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Default Re: Blaupunkt Orbis transistor portable

Maarten's right - I've spent ages on a no sound/uncontrollable sound fault only to find the suffix of the replacement IC was wrong.
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Old 6th Nov 2020, 2:23 pm   #39
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Default Re: Blaupunkt Orbis transistor portable

Yes I bought tba120s to replace tba120s. They finally came in the post today, turns out it's not the fault - the speaker was just blown. So that was a waste of a few quid.
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Old 7th Nov 2020, 10:00 pm   #40
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Default Re: Blaupunkt Orbis transistor portable

Finding a replacement speaker is a bit of a challenge. Magnetically shielded, 3x5 inches, 16 ohms, probably about 4 watts.

I can find plenty who're 8 ohms, but not sure if the impedance mismatch might stress the amplifier too much.
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