Thorn/BRC 2000

[ed731pdh]

Fantastic. Just needs a "Dawn Of Man" piano key VCR to go with it. :D

[ppppenguin]

Having scored a success with my 33 and FM3 http://golbornevintageradio.co.uk/forum/...p?tid=6462&pid=70501#pid70501 I thought I'd take a quick lookat the 2000. It's been rather neglected for a few weeks. I've changed most of the electrolytics and the dubious looking 1000V mixed dielectric caps on the convergence board.

So I plugged in all the boards except line timebase and EHT, placed fingers in ears and switched on. Was rewarded by a fairly bright flash from somewhere I couldn't identify but might have been around the scan coils plus a fairly quiet pffft noise. No magic smoke that I could see.

Pulled out all except PSU, regulator and IF. Got another flash and pfft. Bit odd. There's nothing that's running at high voltage or power in the IF and I'm pretty sure the flash wasn't from IF, PSU or regulator.

Went back to PSU and regulator only which I've had powered up before. All calm but not much at the regulator pass transistor (VT2, VT3) collectors. Time for some proper faultfinding. Start with getting the main 55V regulated supply to work. With luck the pair of 30V supplies will also come up at the same time.

[ppppenguin]

Absence of 55V was easy enough to find. There was a hairline crack in the track right by pin 1 of the regulator board where the unregulated supply comes in. It could have been a deliberate cut but more likely a stress fracture due to the edge connector being pulled off carelessly. So now I have 55V and 30V supplies.

Time to put back the IF board and see what happens.

[ppppenguin]

IF plugged in, PM5519 to aerial socket, scope on Y output of IF board. Power on, tune around, bingo! We have a signal.

Video board next but dinner's ready so another time.

[ppppenguin]

Plugged in the video card and got some video at the CRT cathodes. Then a couple of problems. There's something intermittent in the PSU/regulator so the main rails are coming and going. What's definitely not fun is that the UHF modulator in my PM5519 has gone intermittent too.

[ppppenguin]

That's better. The intermittent in the set was some dry joints on WW resistors in the PSU regulator. The PM5519 justs seems to be behaving itself now.

Plugged in all panels except LTB and EHT. As well as video at the CRT cathodes there's now sound and field scan looking OK.

Next step, line timebase. I have 2 of these, the original one to the set which looks like it's been got at, with a couple of parts missing and some scrappy soldering suggesting that the LOPTs have been removed at some point. The other board looks much better so I'll give it some power gently and see what happens. Won't be before tomorrow now.

I need to look at the 55V rail adjustment which is very touchy. The pot has too wide a range and is a bit scratchy despite a dose of Servisol. I may well add a resistor in series with the top of the pot and change the one in series with the bottom. The 73V unreg supply is rather high, around 85V. Mains voltage is close enough to 240V and the set is on the 240V tap. May well drop a bit when the LTB and EHT are loading the transformer fully.

[Geoffrey]

"Dawn Of Man" piano key VCR.

I think that I have one of those in the loft, a Ferguson with wired remote control.  Sorting through some boxes this afternoon I found a 2000 tripler, unused in it's delivery box!

Other finds were manuals for the Thorn 850, 900 and 980 (I was a dealer), a Mullard 1964/5 wall chart and copies of the Rank publication Service Skill.  The July 1980 edition No.105 features Miss United Kingdom with Mike Long, Rank Central Southern Area Field Technical Advisor, those were the days.

Best wishes,

Geoffrey.

[ppppenguin]

Tried the better of the 2 LTB boards. Nothing. Dead. No power from regulator and it drags down one of the 30V rails via the diode on the junction block. I'll probably pull out that diode as well as L6 on the LTB and run it up on an external PSU. Then I can decide whether it's the regulator or the LTB itself. Taking this sort of cautious approach to minimise risk of killing any other parts. Probably not necessary but I don't have lead lined undies

The photo is all the caps I have replaced. About 65 of them which is almost all the small aluminium electrolytics, all the bead tantalums and a few high voltage mixed dielectric. The 1n 1kV caps on the convergence board were replaced with some 1n class Y which fit quite nicely. The 47n 1.25kV was trickier. I settled on a 1.6kV part in the RS catalogue. It doesn't fit nicely but it will do. All of these caps are related to the 1st anode supplies. Repalced the smaller value tants with polyester - they fit very neatly. In all other cases the new electrolytic is smaller than the old and fits easily.

What is people's experience of the tripler? I remember them back in the 1970s as being a in a white case with white silicone potting. The one on this set may well be a replacement. It has a clear plastic case with clear silicone.

[ppppenguin]

Running up the LOP board on an external supply suggested a short circuit or something else badly wrong. So I disconnected the power feed to the output stage and things came to life. There's drive to the output transistors so I connected power to the output stage via a 100R resistor. Obviously taking too much current. One of the R1039 output transistors is duff, with a base-emitter short. What's a modern replacement? Need to replace both? All I have here are some BU508A which are in a tab package, BUX80 which may be OK at 400V rating and BU102 which I'm pretty sure aren't up to the job. I can't find any data on the R1039 but at a guess it's taking no more than 250V. Other parameters will be critical too. I suspect BU105 would be a good replacement but I don't have any.

Here's the reasoning. The supply to the LOP stage is nominally 53V. If 53V will do a forward stroke in approx 52us then for 12us approx flyback it will take 53V * 52/12 = 230V. I have a feeling I've missed a factor of 2 somewhere so that could be 460V overall. Split between 2 devices in series so around 230V per device. Not very demanding in modern terms but it's not just voltage rating. Current gain and switching time matter a lot. In theory I could dig out my Telequipment transistor curve tracer which hasn't been plugged in for over 20 years but I suspect that would give me more grief.

The other LOP card (the original one in the set which has been somewhat got at) has the early square LOP transistors, type D1418. These at least both show as OK on cold test with a DMM. So I think it's time for a close inspection of the board, fix any obvious damage and run it up.

Oh, and just for fun, the PM5519 has got a great looking hum problem. The circle has lumps out of it that are moving at the difference between it's field frequency and mains. That's for another day, it's working well enough for now.

[Murphyv310]

I've BU208A's and other older types here. I'll have a wee look after tea Jeffrey.