BRC 3K. Thorn in my head.

[Davewantsone]

I have a 19inch 3k BRC colour TV. I got it working a couple of years ago and have stored it in a warm dry place. I thought I would get it out and power it up.
It came on and worked OK for several hours.
The following day I switched it on and after the one second delay and a brief sound hiss I heard a crack and the receiver went off.
The cut out had blown together with the 59V fuse. I have spare known good PS and LTB boards. Thinking it may be the LTB I replaced this. Switching on again I got the same crack and fuse blow. I then replaced the PS board only to discover it did the same.
I now seem to have 4 duff boards or are they?
I have disconnected the tripler, the tube base. The 1000V and 400v supplies checked and double checked all the power transistors on all the boards and they read OK. All typical components have been checked.
I have isolated the frame sound (set white switch) and disconnected the 59v (wire going to pin A on LTB). The fuse still blows with the trip it happens so quick I am unable to measure what is the actual voltage on the 59 v rail. I think the 72v zener may be conducting which in turn will cause the crowbar thyristor to fire and blow the trip.
What am I missing do both PS now having the same fault, caused by the LTB?
Any suggestion would be appreciated.

[Peter.N.]

Degaussing thermistor?

Peter

[Davewantsone]

Thanks Peter for your reply. I cannot see how the degaussing circuit can blow the 59 v line fuse. A fault could cause the trip to go.
The degaussing circuit seems to work as on switch on the second before the set comes on you can hear the degaussing circuit working and decaying.

[Philips210]

Hi.

Looking at the circuit diagram, the 58-65V rail feeds the line output stage. This being protected by fuse F603 (2.5A). Also on the power unit, R628 (2 Ohm) connects to the 58-65V rail which then feeds the field output stage.
Maybe you can isolate the field output stage and re check the power/ line output stages. It's ages since I last worked on a BRC 3000/3500, probably nearly 30 years ago but I remember many a tussle with the power supply unit.

There's a good servicing article on the 3000/3500 series in Television magazine in the September 1978 issue. I can scan this and send it to you if required.


Regards
Symon.

[Davewantsone]

Thanks for your reply.
I have already isolated the 59v from the LTB and operated the set white switch which disconnects the supply from the frame-sound board. I have a scrap power board with a R2010 transistor which tests OK, I will fit that tomorrow into one of the duff power supply boards. Maybe the transistor in both cases is breaking down when 240Vdc is applied. Do you by any chance know what the output voltage would be without F603 fitted? I must remember to have the supplies to the LTB and frame disconnected on switch on!
Thanks for the offer on the mag. I already have it!

Whilst I am at it can anyone let me know suitable replacements for the R2010 and R2008 thank you.

[Philips210]

Originally Posted by Davewantsone

Do you by any chance know what the output voltage would be without F603 fitted?

Hi.

I'm afraid I don't know, I assume you mean measuring the output from the chopper transistor at the junction of L603 and F603. The feedback amp VT608 will not be providing feedback from the 58-65V rail so there will be no regulation. I assume the output will go high. Maybe another forum member who owns a 3000 or 3500 can confirm this.

Originally Posted by Davewantsone

Whilst I am at it can anyone let me know suitable replacements for the R2010 and R2008 thank you.

Although not replacements as such, you can use an R2010 in place of the R2008 but not the other way around.
I've had a look through my Thorn spares and came across a few R2008Bs but no R2010s unfortunately. I have a spare R2008B line output transistor if you need one.

Regards
Symon.

[Davewantsone]

Thanks Symon for the offer. I may take you up on it. I will see if the R2010 works tomorrow and let you know. At present as I see it the R2010 on both boards go S/C when 240 volts is applied this then flows through f603 and causes the 72 volt zener to conduct which feeds current to the crowbar trip thyristor which on activation causes the trip to blow. I think the fuse blows due to excesive HT. I have never had the fuse go and not the trip or vice versa. Its always together.
I still will have something wrong with the LTB which caused the faults in the first place.

[Peter.N.]

Originally Posted by Davewantsone

Thanks Peter for your reply. I cannot see how the degaussing circuit can blow the 59 v line fuse. A fault could cause the trip to go.

You are quite right, sorry, senior moment another one.

Peter

[Philips210]

Hi.

There's also a really good coverage of the BRC 3000 by Steve Pendlebury here:

http://www.radios-tv.co.uk/?q=taxonomy/term/15&page=1 and also here:

http://www.radios-tv.co.uk/?q=node/218

Regards
Symon.

[Hybrid tellies]

If the 60v HT line fuse is blowing its either due to excessive HT volts causing damage to the line Output stage such as a blown Line Output transistor. If the 60v line is correct then I would start by checking for line output faults such as blown line output transistor LOPT etc.
If the HT line is high then I seem to remember that this was caused by faults in the feedback amp stage of the PSU.
Sorry its a bit vague but its a long time ago since I worked for Radio Rentals.

[Davewantsone]

Thanks for the info.
I have found out that a suitable replacement transistor is a 2sd350a.
With other commitments i'll have to leave the TV till the week end.

[Hybrid tellies]

Just another thought. If the 60v line goes high this should operate the crowbar circuit which then fires out the main cut out. So I would think the blown 60v line fuse is down to a load or short circuit on the line output stage or possibly a short circuit in the frame stage.

[pichacker]

I seem to recall you could get a partial idea where the fault was lying by listening to the set at switch on. An immediate hrumph followed by the trip popping was normally a shorted psu transistor/crobar/reccy..

A short delay with degaussing action followed by shutdown was either a line stage / frame fault or the crowbar firing due to detecting excessive voltage (or leaky)

This assumes that the cap providing the soft start was not o/c.

[Davewantsone]

Originally Posted by Hybrid tellies

Just another thought. If the 60v line goes high this should operate the crowbar circuit which then fires out the main cut out. So I would think the blown 60v line fuse is down to a load or short circuit on the line output stage or possibly a short circuit in the frame stage.

Well thats what I posted earlier. the voltage rises above 70v causes the crowbar trip which blows the 2.5A fuse and the cutout trip. It is as if both power supply power boards have faulty R2010 which break down and go S/C when the collector gets 240V. I have isolated the LTB board and operated the set white switch to disable the frame board. I have one R2010 from a duff power board which I will fit this weekend. Something went wrong on the LTB to cause the fault in the first place. I had a good spare LTB which I do not know is now any good. I will try the transistor tomorrow.
But it is like said earlier the set has a 1 second delay whilst the degauser operates and the 30v line stabilises. I would imagine the R2010 should operate the crowbar trip and the fuse straight away rather than wait 1 second if it was breaking down.

[Philips210]

Hi.

I was looking through some of my old Television magazines and found in the March 1980 issue, in the Service Bureau section, mention of using a dummy load for the 60V rail. This involves disconnecting the 60V supply to the line output and field output stages and connecting a 1kW electric bar heater instead. This will draw about 1A from the 60V rail.
This seems like a good way to run and prove the power unit under simulated load conditions.

I don't recall mention of using a variac for the 3000/3500 SMPS as there could be problems with start up due to a low 30V rail.

Regards
Symon.

[Davewantsone]

Well I got round to putting the R2010 from an old duff board. Powered the set up and nothing! This R2010 must be going open circuit on load. I have sent off for a couple of 2SD350A which I was told may work in place of the R2010. Has anyone tried a BU508A? Its a different shape but who cares if it works. I read somewhere these R2010 are rated at 10A 500V 100W. I find that incredible seeing this set must have been designed in around 1968.

[Philips210]

Hi.

Maybe a BU536 could be a suitable substitute for the R2010, it has an IC maximum rating of 8A. It's also TO3 construction so should be a simple substitution assuming the base and emitter lead outs are the same as the R2010.

I did just notice that you can use a BU208A in place of the R2008 according to my ECA reference guides.

Have you thoroughly checked the rest of the silicon in the power unit and the condition of all the electrolytics?

Regards
Symon

[Davewantsone]

Well I have just ordered 4 off BU536 for 1.79 inc postage off eBay. So hopefully when they arrive I can continue. Thanks for the info.

[Hybrid tellies]

From memory the crowbar circuit senses the 60v line and fires the thyristor which is connected across the 240v un stabilised DC line. So if it fires it will operate the cutout I don't think it blows the 60v 2.5A fuse. I will have to dig my 3000/3500 circuit manual to confirm this.
Personally I have never heard of the R2010 chopper transistor being substituted for a BU208A.

[Davewantsone]

Well I am still waiting for these transistors.
I decided to do some component replacements. As I had stated earlier. With the 60V supply removed from the LTB. When I switch on I would get static noise from the sound for about a second before the trip and F603 would blow.
I replaced W602 and C605 as well as C617, 2u2 and C619, 140uF. connected back up and no fuse blow and static noise through the speaker. So one of these little devils must be breaking down at 60V. Connected the 60V back to the LTB and the fuse blows again but not the trip!
So I am now going to have to look at the LTB. No doubt something else that breaks down at high voltage is the culprit.
Is it OK to run the LTB with the tripler disconnected?