Bush TV12 Poor Contrast
 

Hello, ever since I've owned my TV12B I've suffered poor contrast. I put it down to the tube; it already has ion burn and I just left it at that.

I'd already aligned the RF chassis when it was first restored using my AVO generator and Thandar digital RF frequency meter (for accuracy) so I'm confident that's not the problem.

But the other day I fired it up and realised that the brightness control actually makes the raster very bright indeed, (going to a white-out) leading me to think the tube cathode emission is ok. The rear contrast control has to be all the way up otherwise both sound and vision disappear (the control is not faulty and is the correct value).

I've done various swap-overs of valves in both main and rf decks using my one spare EF50 and changing valves between the signal and scan/audio just to see if that improves matters; it doesn't. Nothing suggested in the original service sheet improves matters either.

Can anyone think of anything to try? During original restoration a few years ago (involving recapping and cold checking all resistors) nothing in the RF deck seemed amiss; resistors etc were all spot-on. Might a crt fault still lead to poor performance despite the bright white-out i can get? I wouldn't normally say so but someone may know otherwise!!

Cheers
kevin

Re: Bush TV12 Poor Contrast
 

It seems like low gain in the combined sound and vision stages.

Was the contrast control decoupling electrolytic 25uF (C31 Bush manual) changed?

Have you checked the screen grid volts on the early EF50 stages, V11, V12, V13?

Are the receiver section valve base pins making good contact?

I doesn't seem likely to be an AGC fault, as AGC isn't implemented, at least on the circuit that I have.

Ron

Re: Bush TV12 Poor Contrast
 

Early TRF TV's used a pot in the cathode of one of the RF valves to control the gain which was the only way the contrast could be set, so as Ron says if the gain is low the contrast will be low. Try adding a aerial pre amp if you can get hold of one, as a test.

Peter

Re: Bush TV12 Poor Contrast
 

My thoughts here is that the alignment is not correct. I have never experienced alignment drift on the TV12 TRF series. The TRF unit is built to a high standard and the example I have just serviced has not been touched since 1949 and is still spot on!
It's just a thought but it is possible that you have not peaked one of the vision RF cans correctly. It would be very time wasting to look for a fault that may not actually be present. Regards, John.

Re: Bush TV12 Poor Contrast
 

C31 I changed first time round; well actually restuffed as the original is tubular cardboard and date stamped 1949 which I just had to keep.

The reason I did a realignment is that when I first fired it up after goodness knows how many years, I didn't get any image or sound at all, just a raster. As a rough test I slightly tweaked / peaked each coil in turn until a discernable image appeared, albeit blurred, thus confirming it was at least alive. That's when I decided to do it according to the book. Being the 'B' version, the RF deck isn't aligned as per my Bush 'A' manual, however I have the Radio & TV Servicing volume for it and that's the one I followed.

John I suspect you may be right about the alignment, I do remember the book being a little vague about 'double' and 'triple' peaks, not helped by a few contradictions in the text which left me scratching my head a few times.

I will have another go and report back.

Re: Bush TV12 Poor Contrast
 

Hello again Kevin,
Just another possibility here. Two versions of the TRF unit were produced. One for Sutton Coldfield channel 4 and of course the original Alexandra Palace model, channel 1. Is it posssible you are attempting to align a channel 4 version to channel 1? The AURORA provides a very strong clean signal that will 'blast' through most incorrectly tuned circuits. It was just that you mentioned that by tuning the cores a weak signal began to appear. It is possible to tune the TRF unit to channel 2 the old Holme Moss with little modification but it won't stretch to channel 4. Worth a thought. Regards, John.

Re: Bush TV12 Poor Contrast
 

The TV11 and 12 had remarkably high gain, bandwidth, and stability, the fact that the contrast only needs to be backed off a little for both sound and vision to dissapear points to the first two R.F.stages V11 and V12. With the contrast control set for maximum gain measure the cathode voltages of the two valves mentioned, they should be on the order of 1.5 Volts and check that the anode voltages are about 190V, the screen voltages should be exactly the same as their respective anode voltages.
If all seems in order check the alignment again following the alignment procedure exactly as per Bush instructions. The Aurora as mentioned should drive the set crazy with severe vision on sound and visa versa with the contrast set to maximum.Finaly check the cathode and screen/anode decoupling caps. The valve holders I have been told can cause similar problems when they become corroded. Low cathode voltages indicate a low emission valve but only with the contrast control set for maximum gain.
Good luck.
Victor.

Re: Bush TV12 Poor Contrast
 

Cheers Victor, I am certainly using the aurora but have never experienced either sound on vision or vision on sound.

Mine is the TV12B which is Sutton Coldfield ch4 61.75Mhz, which is how my aurora dip switches are set. The RF deck has the proper 'Sutton Coldfield' badge screwed around the aerial inlet, has 'TV12B' on a metal badge riveted over the 'TV12A' on the fibreboard back and was bought full of dust from a local house clearance sale. The components all tie-in with the diagram for the birmingham version.

Last night I went for broke and 'straight-tuned' the coils (ie peaking them all for max vision...not stagger tuning them as it actually requires) and was rewarded with a more contrasty pic..though still not particularly great unless the brilliance was so far up that I got flyback lines.

I shan't be able to align it properly now until I have more time at work since all my equipment bar the basics are up there these days. At the moment I am unplugging the RF deck and taking it in to work for the quieter moments...easter being the ideal time!

This weekend I changed a couple of the 470R screen feeds because one was at 520R and the other at 590R; although I didnt expect the new ones to make any difference and they didn't.

I certainly can't describe the RF performance as 'remarkably high'!

I actually have a TV11 as well stored somewhere so I might rout that out and it'll give me something to compare with.

Re: Bush TV12 Poor Contrast
 

Inspired by the helpful comments, I've tweaked a bit more and have got a better pic now with just a small amount of sound-on-vision; I need my sig gen to fine tune this out.
At least this proves the crt will show watcheable pictures!

The picture interference limiter is now doing something; ie if I turn it too far, the pic progressively turns the highlights then the entire picture negative; I'm assuming this is correct.

Re: Bush TV12 Poor Contrast
 

Great news! Looks a nice receiver. I find the TRF TV12 series much more interesting than it's superhet sister. Must admit, I tend to align TRF receivers without a generator relying on the signal itself. Works very well.
The limiter appears to be working correctly. The idea is to limit or eradicate anything over peak white. Ignition interference was a big problem in the days of fringe reception.
This produced large white blobs on the screen whenever an unsuppressed vehicle passed. The limiter was advanced until peak whites were just beginning to be clipped then backed off a small amount. This was the critical operating point and would reduce the size of the 'blobs' considerably.
You may have seen this link filmed in 1952 in the Watford area I believe. This gives some idea, in a light hearted manner of the problems encountered.
http://www.youtube.com/watch?v=0auRh...eature=related
Regards, John.

Re: Bush TV12 Poor Contrast
 

"never experienced sound on vision or vision on sound" obviously not about in the '50s then.:) Many of those that were used Johns approach and aligned the RF strip on the test card, in fact there was great competition as to who could get the sharpest 3mhz bars. Doing this really showed which recievers were capable of giving the best picture.

Peter

Re: Bush TV12 Poor Contrast
 

I meant not on this particular example!:)

I'm getting it now...a bit..but its hard to adjust it on a DVD due to the ever-shifting content. You think you've got it then a bass drum kicks in and you get the water ripples in sympathy.

Roll on the next bank holiday 8-)

Re: Bush TV12 Poor Contrast
 

I was getting this on a few sets but removed it by reducing the audio level on the aurora ever so slightly.there's an adjustment on the unit

Chris

Re: Bush TV12 Poor Contrast
 

Yes I was too!

Re: Bush TV12 Poor Contrast
 

You really need to do this on a test card with 400c/s tone. It is very difficult to align on a moving picture. [and very frustrating] The tone makes adjusting the rejectors easy. The Sutton Coldfield model is single sideband so it should not be too difficult to fettle.
Hello Peter! All I can say is 'Happy days!' John.