Decca 1000 restoration

[Derek]

Good to see several of you with these sets, I have restored a few over the years and now have 3 variants of the Decca system all working. The varying focus is definitely a common problem in my experience, on two of my sets the focus loses its sharpness after between 20 minutes to 1 hour of operation and just gradually deteriorates thereafter. Would be interested to know if any of you has found any common cause for this, but, as already has been pointed out, the focus is interlinked with so many other interdependent things. One thing I can say is that the remaining set (a 1000) keeps focus fine indefinitely, however the HT is running (unusually?) high at around 395V, not sure if this is a coincidence or not.

[murphyv310]

Originally Posted by Derek

One thing I can say is that the remaining set (a 1000) keeps focus fine indefinitely, however the HT is running (unusually?) high at around 395V, not sure if this is a coincidence or not.

I think you have nailed a problem that we may well forget. When I first started to work on my 1000 the HT was about 345v, slightly low with the low HT the focus did tend to drift. After replacing coupling and decoupling caps and hi value resistors in the psu my HT rose to 375v the result is focus that is stable over 2 hours of use. All those valves that are overrun with u/s coupling caps are loading up the HT and no doubt affecting the Decca's focus circuit, one to watch.
Trevor

[Derek]

Thanks for the feedback Trevor. Just one other general thought, after using these for quite a few years now, I reckon the weakest points in these sets are A) The line output transformer and B) The EHT blocking oscillator transformer. It must be said however this is after hours and perhaps even years of use. (Have not personally ever found problems with the CRTs or the EHT rectifier can for instance) It's my opinion that if we can find people to rewind these (Ed Dinning has done it for me before) or even learn how to do it ourselves then I see no reason why these sets shouldn't last forever! Wonder if any other Decca projection owners agree with me here or have had different experiences?

[Ghostuser]

My EHT unit was only running at half the required frequency, giving appalling regulation. I changed the frequency determining Rs and Cs, but to no effect, so suspected the 'Open Frame' blocking oscillator transformer, which, interestingly, is specifically mentioned as a potential probelm in the Mullard service sheet. Thanks to help from Mike Barker, I obtained a spare chassis with the Mullard 'Tin Can' transformer on board and this now works spot on frequency, producing good regulation. I swapped the EHT ringing choke can as well, as I had an unused spare, but that was probably unnecessary.
What I would like now is a good spare CRT. Gerry at Crowthorne tubes was unable to help. Anybody know of a source?
Regarding HT voltage and current drain, much of the focus current depends on the RF and IF valves. Maybe when the Cyldon mod went in, this was changed significantly. Anyway, I now intend to check emission of the related valves. Also, the HT drain seems pretty high for a GZ32. Perhaps a GZ34 would be a better idea?

Regards
Nigel

[Ed_Dinning]

Hi Nigel, the addition of a tuner may have upset matters and would certainly cause more drift as the copper wire in the focus coil will get hotter and increase in resistance further, and get hotter .........

Try the effect of a shunt resistor across the coil to reduce the current back to its normal value and hopefully put the control in the middle of its range again.
As a starter I would choose a resistor to bypass about 10mA.

Ed

[Derek]

Originally Posted by ghostuser

What I would like now is a good spare CRT. Gerry at Crowthorne tubes was unable to help. Anybody know of a source?

Was once told that "Kenzen" (Birmingham) could supply these as new, not sure if it still applies. My prediction about EHT blocking oscillator transformers failing turned out to be quite prophetic, yet another one has failed! Anyone else out there know who does rewinds? Definitely going to take Ed Dinning's advice and invest in a coil winder myself I think!

[murphyv310]

Hi.
Its nice to see all the replies to my work on the Decca.
One thing that I see cropping up is focus problems.
My set seems OK now that the EHT unit has been repaired, though there has been reference to the 13 channel Clydon mod.
When these sets were modified 2 EF41 valves were removed and the Clydon had an ECC84 & ECF82 these no doubt drew a similar current from the PSU so any slight variations in focus current should have been well within the focus pot's range..
I really think that the EHT unit should be re-checked and coupling caps changed if at all leaky. Watch that HT if low it could be a faulty valve or HT rect, Why not measure the total HT current if too high check out Decouplers, Couplers and for valves drawing grid current also you may have leaky smoothers.
Trevor

[murphyv310]

Hi.
Just to let you know I got some cream plain blackout blind PVC today.
The result is excellent even with the cream colour. there is no change in brightness at different veiwing angles.
Now to get some white material.
Trevor