BRC 2000 restoration.

[jayceebee]

ISTR a 2N3055 was a good substitute.

[MALC SCOTT]

Hi, i have tried a 2n3055 in place of the 2s034 and still no results. When the faulty (leaky all round) 2s034 is re-fitted the set works but no ht regulation due to faulty transistor. All other components on the regulator pcb inc silicon have been tested/replaced.
Any ideas ?? Cheers, Malc.

[HamishBoxer]

Stated wayback that the VT7 fails.

David

[ronbryan]

What is the base voltage when you replace the 2S034 with the 2N3055? Could the overload trip VT8/VT9 be operating and robbing the VT7 base of drive? Otherwise work back and measure voltages around VT6 base and emitter.

Ron

[MALC SCOTT]

With VT7 (2S034) replaced it appears that the trip circuit has operated. The voltages at VT8/VT9 show the voltages indicated in the service manual as under trip condition. How do i disconect the trip cct?? Can anyone explain how the trip cct operates? Cheers, Malc.

[ronbryan]

The manufacturer's explanation of circuit operation is shown here under the "Trip Circuit" paragraph on page 1.

https://www.vintage-radio.net/forum/...9&d=1304934025

Basically the trip is a two transistor bistable (VT8/VT9) that monitors the voltage across R27 (i.e. the current through it), compares it with a zener voltage W4 and trips when the current through R27 exceeds a certain threshold. The trip then removes the base drive from VT6 and VT7, reducing the output voltage to 28V.

A power off and 30 second wait is apparently needed to reset the trip.

Ron

[MALC SCOTT]

Hi Ron, many thanks for your explaination. The trip condition is present at swith on. How would you diss the trip in order to take voltage checks? Cheers, Malc.

[ronbryan]

The problem is: do you have a genuine overload, or is the trip circuit faulty. If the circuit is in overload, damage could presumably be done while you take your measurements with the trip disabled.

To answer your question, the trip could be disabled by removing VT9. That would stop VT6/VT7 base drive being removed whether there is an overload or not.

If you read more than 3V across R27 (i.e. 56V -53V = 3V according to the schematic in the link above) with the trip disabled, then it would appear that a circuit on the load side of R27 is taking too much current. If the voltage across R27 is less than 3V, then the trip circuit has a fault.

P.S. It would be a good idea to do a cold resistance check on R27 to ensure it is still 3.3 ohms.

Ron

[MALC SCOTT]

I removed VT9 and the set runs ok. No nasty hum bar and both the set 55 volt and eht pots adjust as they should. The output voltages are now correct from the reg psu. I suspect the fault lies around the trip circuit itself. Both trip transistors read ok. Cheers, Malc.

[ronbryan]

Well done Malc.

Has the (probably 8.2V) zener diode W4 gone short?

Ron

[MALC SCOTT]

An update to let everyone know the progress. After an overhaul of the convergence pcb i now have a very good converged raster. The video drives/ grey scale tracking/ crt grid bias etc were adjusted as per the manual. The result is now i have a very nice bright, sharp mono picture. After several hours running i checked the crt emissions with my BK 467 crt tester and all three guns read 1.6 all round which is just about as good as it gets. The focus pot assy is u/s (wiper burnt out). I have bodged it for the time being but i cant leave it in this condition.
The next job is the lack of chroma to sort out. I need to be undisturbed when i start to scope the chroma pcb. Should have a full day to myself next week.
Well over 100 components have been replaced during this mamoth restoration. Nice to see test card F at the end of the tunnel! Cheers, Malc

[ppppenguin]

Well done so far. You've won most of the battles.

I don't think the decoder should be a great problem. It's a very straightforward design, all discrete, not a chip in sight. Just a matter of feeding the set with colour bars and tracing diligently with a scope. There were 2 versions of the decoder, one with the massive DL1 delay, the other with the lighter DL1A.

If you haven't done so already, IMO it's worth disconnecting the tint control. Not needed on a PAL set and it caused colour drift on mine years ago. Heaven knows why they included it at all.

[Arnold69]

Congratulations, I've been looking for such a set for ages, but am content instead with two 19 inchers, a Marconiphone 3701 and a Ferguson. I thought I'd add my thoughts. All the advice so far is very sound. When I got mine (admittedly advertised as 'working'), this is what I did, and recommend from my experience. I turned them on - my house has a 30mA RCD however. Check the mains tap is correct - the manufacturer lists this as a cause of poor perfomance, and if its way out your tube voltage will be wrong. My EHT regulation had gone - picture ballooning with brightness. I got a EHT probe and measured 15 to 28kV! Fixed that first, suggest this is one of the first checks. Tube grid voltage all over the place due to faulty TR on frame board. Careful the system switches work, they can take out line o/p stage. Beware undocumented mods where system switching inhibited, can cause confusion. As said elsewhere all electrolytics must be treated with suspicion. My sound even went due to one failing! Got set to point where it was apparent tube u/s. Got it rebuilt by RACS - very impressed now. Look forward to hearing how you get on. Arnold69

[Arnold69]

Apologies, in my last reply, I hadn't realised all the correspondence on subsequent pages. All I can say is congratulations you're nearly there!

[Tazman1966]

Morning Malc

I spotted this thread while looking for something else (as you do) and wondered if you ever got the decoder working?

Look forward to an update.

[MALC SCOTT]

Hi, i am still looking for a chroma pcb for my 2000. The one which is fitted has had all of the silicon replaced and all electrolytics. It is difficult to work on this pcb in situ. I also need a focus control. Cheers, Malc.

[Welsh Anorak]

Hi Malc
I've got some boards in, er, interesting condition! I'll see if there's a decent chroma PCB and there's a bare chassis which might still have the focus control on it. PM me your details again and I can get them sent to you.
Glyn

[MALC SCOTT]

Cheers Glyn, PM sent

[ntscuser]

How does the power consumption of these sets compare the valve-driven equivalent?

[Welsh Anorak]

Hi
Interesting question. Pretty similar to a GEC hybrid, less than a set using a PD500. They did run warm, though for a good room heater you needed a G6! Just over 400W, I thank you...
Glyn