BUSH TV12A in a local second hand furniture shop

[Atlantic 52]

Hi Guys,
I managed to buy at a good price a Bush TV12A of 1949 last April from a local second hand furniture dealer in Aldershot. He cleared a house in Camberley including the loft contents and this set was in the loft. It came with its oil filled magnifier and he found that in the garden shed! It was lucky he knew they should go together. I stripped it out and gave it a jolly good clean as it was all very dirty. The very matt case (uncracked) came up well after a wash and T-cut polish. Anyhow, I have finally got round to working on it thanks to this winter weather and gone through the set carefully. It is amazingly unmolested and a fine candidate for restoration. I found an O/C electrolytic mains smoother and a good number of other capacitors have been replaced by me. The set is now lively in its 'noises' and the line timebase seems good. The problem is the EHT is extremely low. I have measured the LOPT and EHT Tx cold and the windings are close to the manual figures. The line drive to the PL38 is good as is the HT rail. I did measure the top cap of the PL38 (I know you shouldnt) and it's just over 200VDC and close to that on AC. When scoped (yes, I know I shouldnt) the waveform is very peaky and seems to be the inverse of the normal line signal except it does ring a lot but the peak to peak value is unreadable on my scope, its off the scale. The EY51 doesnt glow and I did try replacing it. All the resistors & caps in the HT and EHT side have been checked or substituted to see if it helps but it doesnt. The EY51 heater winding measures 1 ohm, does that seem right? I have found that if I look at the screen with all the room lights off I can just about see a ghostly raster and a central ion burn. Its very weak and 'borealis' like with shape moving about under the brilliance control settings. Is the EHT generator or the LOPT a likely failure? All the DC conditions around the PY31 and PL38 seem okay. The line and frame scan coils are also within cold ohms test range in the manual. It gives a nice strong line whistle and horizontal hold adjustment changes pitch as you'd expect. The frame timebase seems to be running okay. I am thinking the LOPT is at fault. I have not tried a replacement PL38 as I don't have one neither do I have a PY31 to try but the weak screen illumination I do see does vary with width control adjustment. When I tested the PL38 for emission it read okay. The scoped waveforms suggest its working, the amplification is there anyhow. Any ideas please?

[Lloyd 1985]

Hi,
Nice find! Have you still got the Visconol capacitor in circuit? If so disconnect it and try again, it might be dragging down the EHT.

Regards,
Lloyd

[MALC SCOTT]

Try adjusting the ion trap on the crt neck. Replace every wax capacitor, Malc.

[McMurdo]

tv12a should have a straight gun.
Try visconol as per Lloyd

[Atlantic 52]

Thanks guys, I did try disconnecting the Visconol but it made no difference. I even tried disconnecting the EHT cap, no change..

[McMurdo]

Any problem in the RF stage can lead to a black screen especially if the tube's a bit lazy. Is the audio coming through ok? What are you getting for the eht volts?

[DangerMan]

Can you see whether the dim hazy picture is roughly the right size? If it is, the EHT is likely to be reasonable. If the EHT is low, it will be too big and vice-versa.

Next check might be the crt A1 voltage.
The A1 voltage is derived from the EHT by a bleeder chain of high value resistors which are different in resistance depending on the exact vintage of the set (they were changed in later ones). 10 meg and 27 meg spring to mind, but I might be a bit mistaken on the exact values.
There is also a wax decoupling capacitor for the A1 at the bottom end of the paxolin lopt panel. All these are suspect... the resistors (or at least one) almost invariably go high or open circuit and the wax capacitor goes leaky.

Your CRT is an early metal based no-ion-trap one so I guess the resistors were 10 meg ones of the big "carbon rod" type.
Might be worth checking that its heater voltage is correct at around 6.3v AC although my experience is that these don't suffer with the partially shorted heater problem that the later ones do (in which case that voltage might measure roughly half of what it should).

[Heatercathodeshort]

The line output transformer struggles very hard with the TV12. I think it is suffering from long term damp and will require a long slow stay in the airing cupboard or snuggled up close to a central heating boiler. It will not respond to being 'force' heated.
They only produce around 5kv when working correctly but that is enough to produce a good well focused picture. Good luck with it. Much nicer from a technical aspect than the common as muck TV22. John.

[FERNSEH]

The line output stage resembles the circuit in the TV1 and TV2, although the TV12 has the refinement of having an efficiency diode, described by Bush as a "ring rectifier".
It goes without saying there will be moisture in the line output transformer.
A method to drive any moisture in the winding has often been discussed in this forum. A current of say 30mA is passed through the EHT winding, this will warm it up. First, measure the resistance of the EHT winding to the anode of the PL38. After the resistance has been determined the applied voltage can be calculated by simple ohms law for a current of 30mA. If you want to play safe start with 20mA.
The construction quality of these sets is exceptional, the only let down is the rather flawed line output stage, as John says it struggles to supply 5KV. The picture will not be as bright as the TV22.

DFWB.

[Atlantic 52]

Hello Chaps,
Thanks go to all for your inputs, much appreciated. I have spent all day on the set and found that the very low EHT was down, in the main to two problems. The first when I found that the 2nd in line 250uF mains smoother (C26 in the Bush manual) was O/C. I found that C28 was O/C days ago due to excessive hum, this one caught me out as I put my capacitance meter across C26 without realizing I was actually measuring the C28 further up the HT rail, silly mistake! Anyhow, that done the LOP stage was a lot more lively due I guess to 50 volts of AC ripple NOT going into it! With that replaced the EY51 heater could then be seen glowing nicely. I then found that the EHT Visconol was draining volts away in a big way. With that replaced I had a raster! lots of frame trouble and an obvious Ion burn but it was working! Now for the frame, I found lots of resistors in the frame stages that were 30-40% high and replaced them. The frame form cap of 0.5uF C5 was faulty even though it measured okay, obviously leaky with volts. I then found that the frame hold pot of 50K ohm was open circuit. A lot of messing about finding a suitable replacement and fitting it, not easy getting at the wiring! After all that I got a good test card once I had fiddled about a bit. I found that the focus was at one end and the 50Uf C27 across the focus coil was faulty. I have replaced a LOT of capacitors and resistors in the set now. I have only replaced the EY51 just to be sure when I was fault finding, no doubt the old one is okay. So, the lovely set is now running well and on soak test to see how it holds up over time. Thanks to all in helping steer me toward my goal of getting this nice set running again after Lord knows how long. - George

[McMurdo]

Looking good! The reservoir cap you found o/c tested ok in mine too with a meter but after half an hour on test it vented hot wax all over the underneath of the circuit with a loud hiss.

You've probably noticed you wont get a round circle on the test card if the border squares are at the edge of the screen as the tube has the earlier aspect ratio of 5:4

Ion burn of course is part of its patina and an inherent flaw.

[Lloyd 1985]

Fantastic result! Picture looks nice and bright. I wish I could get mine going as well as this!

Regards,
Lloyd.

[Atlantic 52]

Thanks, yes its been very rewarding getting it going again and obtaining great pictures. I have run it for 2 hours then rested it for 2 hours. It was then on for 3 hours tonight. I have had two instances of RF gain changes resulting in the need to adjust the contrast. I'll investigate when it shows up more often. It can take hours to show itself. Apart from that its been perfectly stable needing no hold adjustment at all. I have added a couple of pictures so you can see how good it is and see the final tally of replaced parts.

[stevehertz]

Personally when working on these sets I just change all the waxies and all the electrolytics. Job done, end of, no coming back to bite you! But a lovely find and congrats on a fine looking set and a lovely picture. I had a TV12 and also the wooden cased, rare TV11. I've still got a 'mint' 22A waiting to be restored.

[McMurdo]

the EF50 valve bases are a prime suspect for random changes of contrast

[Heatercathodeshort]

I'll certainly back Kevin up here. Remove each EF50 [with care] and clean each pin with 1800 grade abrasive paper. Do not overlook the valve spigot as this connects with a spring 'ring' under chassis. Also clean the valve base where the retainer holds it tight in it's holder. [You wish!]
Finally coat each pin with a tiny smear of MS4 silicon grease.
On no account spray switch cleaner onto the valve holder itself. These are made of Paxolin and can become conductive causing all sorts of alignment problems and odd happenings. It's worth taking some time on this task. Best of luck with it. John.

[Atlantic 52]

Many thanks for all the feedback chaps, much appreciated. I'll investigate once it plays up more regularly, right now it runs for hours without any issues. The only thing I have found since getting it going again is that I can hear a low level hissing like corona discharge. I can smell ozone but its not bad. I have looked carefully around the LOP & EHT Tx area in total darkness but can't see any blue arcing anywhere. Could it be within the canned LOP? The picture isnt affected in any way by this, no vertical sparkle line or the like and it doesnt seem to change in volume over hours of use. It is in a hot room near the woodburning stove at the moment and about 23 degrees C with low humidity. Even when the set is run a lot cooler it doesnt change what I hear or smell. Any ideas?
Many thanks,
George

[Freya]

Its most likely to have EHT leaking on the Paxolin board the rectifier is mounted on. Mine did this and it took some careful cleaning with IPA to remove the deposits causing the tracking.
To prove this on yours, wire the ends from the rectifier straight to the output wire bypassing the tag board. You may need to do all three.

If the corona has gone one this has been done then its the board at fault. Mine was sucking almost a third of a kV through the board.

[Atlantic 52]

Many thanks for this help Stephen, I sure makes sense of what could be happening on my set. I'll have a go at it soon and let you know the outcome.
- George

[Lloyd 1985]

Mine makes hissing noises too, especially on low brightness settings! I took a photo of the chassis running in a darkened room, shutter set to 30 seconds, f 5.6, ISO 1600. The result was quite interesting! Spot the little blue sparkly things...

Regards,
Lloyd.