TMB 272 - lazy video amp?

[TNC]

Have just about finished working on this set - now produces a stable and reasonably bright picture (for a CRM 191 that is!) but there remains an odd fault that I am having trouble sorting...after switching on from cold, the raster (and sound) appear within 30/45secs (about right I guess) but the video signal takes anything up to 4 minutes to appear!...but when it finally does it's quite OK!

If it's switched off and then on again whilst still warm the signal appears immediately.

I have checked the video signal valve line-up - fitted new ones in the tuner and checked all the 6F1s, replacing one which was low-emission. My thinking is that it's more likely to be a resistance rather than a capacitance fault but I would be grateful for any thoughts on how best to track down this sort of problem.

Thanks,

Trevor.

[Steve_P]

From cold, check all the anodes and cathodes. The one that is wrong at switch on but changes as the set warms up is probably the faulty one. Change the resistors here.

Cheers,

Steve P.

[TNC]

Thanks Steve for your advice - I finally managed to get back to the set and make the additional checks.
The resistances around V3-V10 checked OK with the exception R53 - V10's anode load of 6.8k which was reading around 9.3k. As it's a beefy one I have shunted it with a 25k and got near enough the correct value. This improved the voltage to V10 but the video was still taking a while to come through so in a "back to basics" moment I checked the HT - which at 195 seemd a bit low - so I powered up it to 220v from a separate source and that solved it!...so a 39p bridge rectifier from Maplins mounted at the rear (where the selenium recs. lived) has solved the problem.
I prodded around a bit more and found another 6.8k had gone high - R92 - but given its place in the circuit it doesn't seem to have any adverse effect so I have left it for now...oh and I replaced a couple of caps. I had missed!
The only thing left is the adj. of VR1 - the 5k AGC servo control - the service sheet gives some info but it's not clear enough to me what I should be measuring - any advice there would be much appreciated.
Other than that, the final result can be seen in the attached .jpgs - the tube is a bit weak and has been boosted by 15% - but its quite watchable - so thanks again for your comments.

Trevor.

[TNC]

Still tidying up this set and the last item is the frame linearity which is not particularly good with the adjustment at max (zero resistance on the adjustment pot). I seem to recall mention on his forum that the Newnes book shows a modified linearity circuit - I am wondering if someone would be kind enough to post that circuit so I can try it out?

Am I correct to assume that linearity adjustment is achieved by feedback?
Also, it seems to me that problems with linearity invariably involves the top half of the frame being "streached" - never the opposite ("compacted?") ... is this a correct observation...if so, can someone explain why this is?

Thanks in advance,

Trevor.

[FERNSEH]

Hi Trevor,
Have you replaced the AGC decoupling capactor that lives in the tuner? It causes all sorts of strange contrast control faults. I have to thank Murphy310 ( Trevor of Kilmarnock) for directing me to the problem.

DFWB.

[TNC]

Hi David - yes I have replaced the two caps. in the tuner and generally the set performs well but the linearity could/should be better - do you think the that AGC cap. could affect linearity?

I am interested to know how component values (or the circuit) varies in the
revised version and either to adopt it - or perhaps just play around with
component values in the current ciruit.

Thanks,

Trevor.

[FERNSEH]

Of course changing the tuner AGC decoupling capacitor will not effect the frame linearity. I was only suggesting that the capacitor is often overlooked when one is tracing vision AGC faults.
The picture is not compressed at the bottom of the picture so that confirms that the cathode bypass capacitor is OK. Compression at the top of the picture can be caused by the frame output valve being overbiased.

DFWB.

[beery]

Hi Trevor,
there is a diode which I think feeds the syncs into the frame oscillator. Replace it with a 1N4148 signal diode.
When this diode goes leaky it causes poor frame linearity.

The Newnes book shows a slightly different circuit to any TMB272 that I have seen. I suspect that there may be two types of output transformer, which different characteristics. All the TMB272's I have seen have an extra linearity slider pot underneath the chassis.
I tried converting my set to the Newnes circuit once and the results were terrible. In my set it was in fact the diode that was at fault, so after replacing it I converted the circuit back to how it was (I had to borrow another TMB272 to copy it) and it was fine.

Cheers
Andy