Pre-war Television Fault

[Panrock]

Thanks David.

Hi Christopher. Yes, it did still show leaky when stripped back. The test was rudimentary, with an Avo. Using the voltage from my megger would likely have completely butchered it.

Steve

[FERNSEH]

The attachment shows the post-war production EMI capacitors.

36355L .5MF 58mm X 19mm diameter.
38219EC .22MF 33mm X 15mm diameter.
38211EA .1MF 33mm X 15mm diameter.
38210EA .1MF 33mm X 12mm diameter.
36729E .047MF 29mm X 12mm diameter.
36700F 0.047MF 37mm X 10mm diameter.
38211DW .022MF 25mm X 10mm diameter.
38211DV .015MF 15mm X 10mm diameter.

Pre-war capacitor
36355J .5MF 58mm X 19mm diameter.

DFWB.

[Panrock]

Thanks! Some considerable work has clearly gone into that. I'll let you know how I get on.

Steve

[Panrock]

Here's what I did.

I decided on reproducing FERNSEH's "36700F 0.047MF 37mm X 10mm diameter" capacitors, since all those needing replacement are of this capacity, and the 10mm diameter was easy to obtain by using the sheath of some SO-213U RF cable I have lying around - of the same diameter. The first picture shows the sheath sections being removed to serve as the capacitor formers. Once cut, the sections pulled off the cable easily.

The second picture shows the various parts almost ready for assembly. The labels were printed on white paper with a suitable coloured background added, since I found my printer wouldn't print onto brown paper! Attention was paid to finding a suitable font for the text. The labels were cut and sprayed on their backs with Scotch paper adhesive.

The new capacitor ends were taken care of by small blanking grommet/caps, which fit tightly into the SO-213U-derived formers. Small holes were drilled into these to allow the capacitors' wires to pass through.

The third picture shows the 'EMI' capacitors assembled.

Steve

[Patrick Dixon]

Nice job.

[Panrock]

Thanks for the kind comment.

The RF cable used to make the tubes/formers was RG213/U. Can't think where the SO prefix I stated came from... This morning I discovered the labels, secured with Scotch spray adhesive, were beginning to unwrap! This was put right by adding thin lines of more conventional adhesive under the edges and pressing down. The 'foil' ends of the internal capacitors have been aligned with the left side of the label text.

Early next week I'll replace C47 and R25, and all the now-suspect capacitors with these new items and move on to deal with any remaining faults. I'll continue to post progress here.

Steve.

[Panrock]

All the dubious capacitors have been now changed with the new editions, and each has received a coat of varnish to look the part and merge in.

It wasn't possible to operate the television section when shutting down the EHT mains transformer, since this transformer also supplies the heaters of the television section valves. So, instead I have removed the EHT rectifier valve, an HVR2, to render the under-chassis EHT bleeder chain of resistors safe. I've also added a temporary cardboard safety shield over the transformer bottom itself, to draw attention to its presence and prevent touching. Because there is now no EHT load on the winding, I am running the set during test at a reduced mains voltage.

Some popping is still being heard on the sound, but only very occasionally. A dry joint was found at the grid connection of the sound output valve V5, a KT41, but his hasn't fixed it. There was one 'pop' heard about 15 minutes into this morning's test, followed by a slight increase in volume and then pop another half an hour later. I've noticed it also occurs with the volume control at minimum. So it could be a heat-related 'creak' from the sound output valve KT41 or maybe the sound output transformer - normally it's the primary winding that causes crackling. It probably isn't on the power rail because of the (slight) gain change. This sort of fault, which can't be relied upon to occur for long periods and is over in a fraction of a second when it does, is annoying!

Soak testing is not only time-consuming - it's boring. So I'm using the time to post here. I am not including the picture tube with the soak test. This is better conserved. So the video output is displayed on a scope. The frame speed is now correct too, and all otherwise looks good.

Steve

[Panrock]

I'm now hopeful I've got to the bottom of the 'popping' associated with a condition of excessive minimum sound volume. There's been no more sign of it today after many hours of soak testing (in radio mode).

I was poking around amongst the components around the cathode end of V4 when I touched the rivet at the positive pole of electrolytic capacitor C50 with a small C connected to chassis. There was an almighty pop to end all pops and the minimum volume returned to near zero as it was supposed to.

Could this intermittent fault have been caused by oxidisation under the rivet? This is an original pre-war electrolytic that has been re-formed. A quick squirt of the rivet with WD-40 appears to have cured it. I know WD-40 is not approved of in some quarters here but I have had good results with it long-term, in previous cases of rivet oxidisation.

As for the theory explaining this fault, I confess I'm not sure about it and welcome your views! As I see it, the bad connection to C50 and lack of decoupling could have meant stray IF pick-up was developing a signal across R22. The grid of (detector/avc/1st AF) valve V4 then acted as a detector and the resultant audio signal was amplified by the valve. The strange thing is that placing a short on the valve's grid cap to deck during the fault condition made no difference.

If all continues to plan, tomorrow I'll test in television mode with the tube in place and then get started with reassembly.

Yours in hope,

Steve

[high_vacuum_house]

It could certainly be a bad connection.

I had the pilot light in my Tektronix oscilloscope not light up. I was certain it was the bulb but after messing around cleaning the bulb and connections would still not light, I found that the bulb holder assembly which bolts onto the aluminium faceplate which was the lamps current return, had a very high resistance between them. There was no corrosion and was solidly bolted to the front panel but there was still 6.3V between them!!

Removal, cleaning and replacement was a complete cure !! There must have been some almost invisible oxidisation somewhere here.

Christopher Capener

[stevehertz]

Well found Steve. That's why I always change old caps on sight, electrolytics and waxies. Yank em out like bad teeth, not worth bothering with, it's just buying into future functional problems (hours and hours of faultfinding rework) for the sake of under chassis originality.

I would be tempted to further ensure that revived rivet connection using a small sharp chisel or screwdriver to force metal to metal contact at an edge in a few places.

[Panrock]

Originally Posted by stevehertz

I would be tempted to further ensure that revived rivet connection using a small sharp chisel or screwdriver to force metal to metal contact at an edge in a few places.

Useful bit of advice there... thanks.

I have replied to your other point by starting a new thread in the General Vintage Discussions section.

Steve

[Panrock]

Following successful soak tests in both radio and television modes, it's time for the tube to go back in the cabinet.

There's always a little anxiety at this point in case there are any specks of dust trapped between the armoured screen glass and the tube face.

Here, the cabinet is seen on its side. The lead ending in the B7 plug is for the timebase connections. The tube mounting arrangements have had to be modified somewhat, for the 5FP4 tube.

Steve

[Panrock]

The main chassis is offered back in. The HT Rectifier valve at near-right has been pulled out for easier handling.

This chassis was very difficult and heavy to handle and position. Note the gardening gauntlets to protect my hands from being sliced and crushed by the chassis edges!

It's important to keep the chassis square-on during this operation. Otherwise it can jam. The wood surfaces beneath have been lightly greased to make them more slippery.

And at all times, I keep an eye out for things already in the cabinet, like the tube assembly, which could be possibly damaged.

Steve

[stevehertz]

Nice work Steve.

[high_vacuum_house]

Originally Posted by Panrock

This chassis was very difficult and heavy to handle and position. Note the gardening gauntlets to protect my hands from being sliced and crushed by the chassis edges!

It makes you wander how the ladies on the HMV production line did the final assembly, especially tasks like fitting the chassis and manhandling the completed sets. In the “setmakers” book the pre-war television factory photographs show there is very little lifting aids or PPE available unlike today!

The RGD console television I acquired last year was made from a pre-war design and it took 3 of us to get it in the car!!
Christopher Capener

[Station X]

There was no Health and Safety when this set was built. Workers were responsible for their own safety and if they weren't careful they injured themselves.

[Rich Woods]

Originally Posted by high_vacuum_house

It makes you wander how the ladies on the HMV production line did the final assembly, especially tasks like fitting the chassis and manhandling the completed sets. In the “setmakers” book the pre-war television factory photographs show there is very little lifting aids or PPE available unlike today!

The RGD console television I acquired last year was made from a pre-war design and it took 3 of us to get it in the car!!
Christopher Capener

Carefully I would imagine!

[Panrock]

Phew... it's all back in the cabinet now. The left picture shows the rear view and the right shows the set in operation, with a cartoon. On radio, the good control of minimum volume remains.

It's almost 'there'. On Test Card the picture looks slightly shifted to the left, so the back will have to come off (hopefully) for one last time.

Steve

[Panrock]

I should mention that the response of my digital camera plays tricks and the actual picture looks gentler to the eye!

This set is fitted with Dr Holden's circuit for switch-off burn suppression.

Steve

[Lloyd 1985]

That looks fantastic!

Regards,
Lloyd