1500 Chassis Volts Measurement

[Duke_Nukem]

Can someone do me a favour and measure the heater volts on each section of a BRC1500's heater chain ? Also what is their HT6 (book says 26V) and also the emitter volts of VT8 (the fuse that drives the video output fuse) (Book says ~5V).

TTFN,
Jon

[molochkartovski]

V1 pin4 101.1v pin5 57.2v
V2 pin4 78.4v pin5 68.8v
V3 pin4 68.8v pin5 57.2v
V4 pin4 96.7v pin5 107.7v
V5 pin4 78.6v pin5 96.4v

HT6 24.9v

VT8 emitter -5.9v

hope that's helpfull to you, mine worked perfectly (apart from teletext lines) after I'd changed all those horid little black electrolitics and a few coupling and decoupling caps.

Richard

[Duke_Nukem]

Thanks Richard,

Mine is pretty much working but HT6 is excessive (the only reason it is 27V is cos I slapped a zener across it) as is VT8 emitter (7V); all resistors involved are new and I'm sure the dropper is running even hotter than it should which, unlikely as it seems, only leaves the valve heater chain !?

TTFN,
Jon

[gdTV]

Have you checked W7 and C83 for leakage? If they are suspect then I would think C58 will be feeling the stress (warm) and some hum bars may well be present too.

[Duke_Nukem]

Have you checked W7 and C83 for leakage?

Not specifically. The sympoms are excessive HT6 (which would actually reduce if those bits were leaking) and after a few minutes very slight smoke from R111 (dropper section in series with heater chain). It's a brand new dropper resistor so initially I was happy to put down the extremely slight smoke as being due to grubby fingerprints, especially as during this time the volts acrross it were stable, however it is persisting.

In all other respects, despite the rubbish aerial and snowey picture it is well locked, linear, very stable and no hum bars (just a shame I can't tune in a good picture and good sound simultaneously ).

Pin 5 of V4 is 8 volts lower than Richard's yet VT8 emitter is over 7 volts.

I may be worried over nothing but I just can't shake the feeling that something just 'aint right.

Also PL509 heater was dropping only 14V against Richard's 18V and was VERY slow warming up. It was labelled faulty (spelt M-A-Z-D-A) so I replaced it with a model of perfection (spelt M-U-L-L-A-R-D) which has clawed an extra couple of volts back.

TTFN,
Jon

[Dual Standard]

Originally Posted by Duke_Nukem

Also PL509 heater was dropping only 14V against Richard's 18V and was VERY slow warming up.

Jon,
I assume this is a typo but just incase.
The 1500 chassis should have a PL504 not a PL509, though I can't see why this would cause the symptoms you describe.

Mike

[Duke_Nukem]

(PL509)...I assume this is a typo but just incase.

Just a typo

Having trawled through many boxes full of the darn things whilst trying to find another PL504 I got PL509 on the brain.

TTFN,
Jon

[gdTV]

Originally Posted by Duke_Nukem

Not specifically. The sympoms are excessive HT6 (which would actually reduce if those bits were leaking) and after a few minutes very slight smoke from R111 (dropper section in series with heater chain).

Yes, but leaving the high HT6 to one side, there is nothing else that should really stress the dropper to that extent except for something more than half waves making it through W7. Well maybe a leak in C77, but that would have the PL504 lighting up pretty quick!

The Thorn manual quotes the voltage at the end of the heater chain (CRT pin1) as 43 volts, but quote this value is measured with a moving iron or hot wire instrument. If this were to be high then R136 would probably be chief suspect I guess as R79 is new. Again, this should not make the dropper run hotter though.

As there is not much in the way of regulation (except Z4 across the tapped of 12v Tuner Supply) then the final voltage must be determined by the loading presented to the output of R78.

So it seems that some loading may be missing and that the dropper 'steaming' is just a distraction. I do remember these doing this sometimes after a dropper change, and having to run sets for a long time before they could go back to the customer without the fear of slight smoke causing panic!

What would the voltage rise to without the Zener added? Is R29 ok? It seems a little odd I agree.

Graham.

[gdTV]

As a postscript to the above....

I'd definitely have a close look at Z4 and R29 (3.9K). May be worth substituting Z4 with a 12v Zener to prove a point. Z4, although actually tied across the 12v tapped supply from HT6 via R67, is referred to as the 'HT6 Regulator' in the parts list. R29 is referred to as 'part potential divider for HT6' (earthy end).

Graham.

[Duke_Nukem]

Hi Graham,

The 12V zener was checked early on so as not to risk anything knackering the tuner (though I suspect the zener is more likely there for stability).

All the parts in the supply to the tranny sections have all been checked / replaced - the 317R section was initially o/c (though I trapped this whilst still at low volts on the variac) so just in case this had been what retired the set (and not the fact that it looked like it had spent a considerable time at the bottom of the atlantic) I checked everything. 12V zener is fine.

The fault is definately excess volts at junction R136/R79 (volts across R30 is naff-all) and hence excessive volts at end of heater chain which if anything suggests too little heater chain current. I've re-checked resistor values and they are correct.

I fitted the zener whilst I was already sure a tranny or two had popped and not removed it when signs were it was still, er, "zenering". Removing it shows HT6 starts at 28 but drops down to 26V -> ok now ... so doesn't add up

... that the dropper 'steaming' is just a distraction.

I think you are correct. So I just ran the set for five minutes, ... then lost vision and picture, just left with a raster ... another flamin' three-legged fuse popped (TR6) , though I was continuously monitoring HT6, which was blameless.

Sound and vision now back (DOH! should have used the opertunity to check R18) and ... frame timebase now playing up. I sometimes think that some sets just don't want to be fixed.

TTFN,
Jon

[gdTV]

Sounds like it's pretty much ok then - you'd be bored if it was too easy

Re: field timebase -
Not sure what the problem you have is, but there was one production change (schedule D onward I think) which may be worth noting.

R48 (22K) becomes 33K and R49 (33k) becomes 22k (so effectively you can swap them). Both resistors are in the anode feed to the Sync Separator with their centre point feeding the frame osc. Reason given for change is to improve frame sync.

Of course, it may be already done in your set, but just a thought....

Graham.