Cossor 917 woes

[Murphyv310]

Hi.
I've another set here to restore. A careful examination on the set gave me no reason to think I'd land up in trouble, just the usual wax cap issues and out of tolerance resistors.
Well my hopes were dashed today and it reminded me of a previous 917. The dreaded B8G valve bases used in this TV. one base lost its tag with just a light touch. I only had one suitable base that was a used one with one with oddly one tag in half and the same pin!
So after drilling out the old one and replacing it plus checking the components around it it's now replaced. Then after taking the photos I see another in the same boat, the octal bases are also of poor quality and very flimsy. I had this issue with the other 917 I did and that one I believe is bereft of a CRT as it went down to air a few years ago. I might be able to get the set if the owner wishes to part with it for some of those things that you exchange for goods.
If I get stuck I do have a radio that I could cannibalise but that's a last resort. I do need to change the resistor on the new base as I've fitted a 12k instead of a 10k, definitely need to get an eye test again, some noticeable change since October 2020.

[ppppenguin]

Not familiar with the set but those valveholders look really horrible.

Good luck!

[djrm]

Seeing that woven insulation and the body-tip-spot resistors takes me back a bit.

[Murphyv310]

Hi.
I find that the majority of body, tip, spot resistors are wildly out these days. Perhaps not so noticeable in a radio but they cause many issues in TV receivers.

[boater sam]

I think  are expecting too much Trevor. After all the guarantee has just expired on the set and the makers have to make money by buying ever cheaper components.
I should however be inclined to complain to Cossor in the hope that they will send you some new bases, FOC. 

[Murphyv310]

Hi Sam.
I'll post the letter this morning once the snow stops.

[Murphyv310]

Hi.
I've found some NOS valve bases on ebay so that's one issue solved.
Did a little more this afternoon but got too cold later so gave up for the day. Normally I'd put the heater on in the morning to warm the shed but last months bill was way too much so I've been restricting it's use. I've a small gas heater too but it causes dampness so it's only used for short periods along with the dehumidifier.
Hopefully it may be a little warmer tomorrow.

[Murphyv310]

Hi.
I've been working on the set over the last few days. Slow progress as these are not the easiest to work on but all of the leaky wax coated paper capacitors have now been replaced in the timebase sections, PSU and audio stages. The RF section is yet to be done but tomorrow I'll gradually power up the set and see what transpires.
I normally do a blanket change of capacitors these days and to be honest this is a very awkward set to work on and doing them one by one and a test is far too time consuming, I've never had an issue working this way. Resistors are in the main within tolerance with the exception of half a dozen, still OK for a test though.
Pictures over the next few days.

[Murphyv310]

Hi.
Between yesterday and today the smoothing and other HT electrolytic decouplers have been reformed.
Now have both line and frame timebases running on 100v HT.
I am running the heaters from the sets mains transformer and the HT from the Radpack via the variac.
Two of the controls had lost their wipers, one was replaced with a high grade Russian control and the other by a good one from the spares box that went O/C five minutes later, hence the 15k resistor across it. /next will be the RF deck then a test with the CRT in place.
There is no EHT rectifier heaters and both they are certainly not O/C and the TX is fine too and a spark at the EHT Anode, Will need to investigate the reason, A silicon EHT Diode could be used if this a transformer issue.

Once the RF deck is done I'll do the resistors that are out.
It's an awkward set though but we are making progress.

[Geordie McBoyne]

For my Beau Decca TV reconstruction project an RF deck from a Cossor 916/7 is used.
Valve types apart the design of the RF and IF circuits is very similar to the Plessey signal circuits.
The Cossor unit has 6AM6 pentodes and the Plessey has the Mazda 6F13.
The Cossor RF unit is a perfect fit on the special chassis made for the Decca TV.
In terms of performance the Cossor RF unit is excellent. However, from the time to time the mixer-oscillator stops working. I've a suspicion that the valveholder is causing the problem.

Geordie McBoyne.