Pre-war Television Fault

[Panrock]

When delivering the set, the customer mentioned that the minimum sound volume, particularly on radio, was not as low as it should be. So I have critically reset the operating point of the on/off toggle switch triggered by the movement of the volume pot - and on my Avo monitored how low the resistance of the wiper on the pot gets before the switch actually triggers. The reading looks very good... but we shall see.

For audio content, I shall be tuning into Jib-on-the-Web on Medium Wave through my SSTran pantry transmitter. This reaches the workshop with a strong signal on 327 metres.

For video content, my Aurora is waiting with Test Card C.

Ready to switch on!

Steve

[Panrock]

At last I had some time to return to this job. Today, I ran the chassis in radio mode first and noted that the minimum volume level, while only modest, is definitely unacceptable. This will be investigated next.

Then I ran the chassis for the following three-quarters of an hour in television mode. The minimum sound volume was now fine. Not much to report on the picture side... the original fault isn't immediately apparent... but I will be testing much more than this.

I welcome ideas for reasons for the excess minimum volume - on the radio only.

Steve

[greg_simons]

Great picture there, wouldn't mind a squint at that beast close up.
Greg.

[Panrock]

Hi Greg, drop me a PM and I'm sure something can be arranged. Steve

[stevehertz]

Great picture.

[Catkins]

Originally Posted by Panrock

I welcome ideas for reasons for the excess minimum volume - on the radio only.

Steve

At minimum volume the grid of V4 (MHD4) should be very close to earth potential, and so you should not get any sound from radio or television What is the resistance to earth on the grid, and what happens when the grid is directly shorted to earth? If you still get sound it must be getting in somewhere else as well?

A increase in volume might be due to higher HT on radio only due to less load. Does the HT voltages change hugely between radio and television?

[Techman]

I seem to remember that this strange fault has come up before a few years ago and it was down to faulty HT decoupling/smoothing, not the reservoir - perhaps there's an extra electrolytic that's switched in and out depending on mode and it's lost its capacitance. Unfortunately, I can't remember the exact details now.

[Catkins]

Originally Posted by Catkins

A increase in volume might be due to higher HT on radio only due to less load. Does the HT voltages change hugely between radio and television?

I perhaps should have mentioned for those not familiar with the circuit, HT is obtained from a single transformer and applied to all "live" valves. In radio mode only the first 5 signal valves are live, where the heater voltages for those valves are provided by that same transformer. In television mode there are an additional 9 live valves, where the heater voltages for those are supplied by a second transformer which also supplies the EHT (extra HT) for the CRT anode.

[Catkins]

Originally Posted by Techman

I seem to remember that this strange fault has come up before a few years ago and it was down to faulty HT decoupling/smoothing, not the reservoir - perhaps there's an extra electrolytic that's switched in and out depending on mode and it's lost its capacitance. Unfortunately, I can't remember the exact details now.

Exactly. Without knowing the results my guess is the sound is getting in from somewhere else (which is usually caused by poor HT decoupling/smoothing) and is worse on radio only mode due to higher HT which is due to less load.

[Panrock]

Well, shorting the grid of V4 to deck makes no difference! However, shorting either live tag of the volume control to deck - does!

This set uses long lengths of screened lead, piping the audio signal from one side of the chassis to the other. It looks like this (original) lead may not be in the best of 'screening' health.

The residual volume can also vary; sometimes with a 'pop'. To me this says "valve".

Steve

[Panrock]

This sound leak wasn't helped by being intermittent. However, since cleaning around the cramped contacts underneath the base of the second sound IF transformer, it seems to have gone. Fingers crossed. I am not really much wiser as to the precise cause...

More sound testing to follow, then I return to checking the television operation.

Steve

[Panrock]

The minimum volume syndrome and the audio 'pops' are definitely cured. Right now, I'm soak testing the television...

Steve

[stevehertz]

Well done Steve.

[Panrock]

Thank you for your support, Steve.

Some serious soak testing today. All seems fine so far.

As a point of interest, the glass envelope temperature of the sound output bottle (KT41) has been measured at 130 deg C (with an ambient of 22 deg C). The sound is fine and the operating voltages spot-on, so I assume this sort of temperature is 'par for the course'. Even so, I propose to leave off the ventilated screening cover provided for this valve. Everything is so crammed in! In 1938, packing television as well as radio in a cabinet this size must have been a struggle.

Steve

[Panrock]

Originally Posted by Panrock

All seems fine so far.

Oh no it isn't! At 100 minutes in, there was a click and the raster collapsed, leaving a brighter section at the bottom, distorted and running at a flickering speed.

This is likely to be the original 'presenting fault' showing up at last. What can it be?

The trouble with this sort of fault is that, not only does it only put in an appearance at odd intervals; when it does one's every instinct is to do a quick shut down to prevent more damage being caused!

Assuming it's gone when I power up again, I guess I shall have to leave some instrumentation like a meter and a scope pre-connected for when it happens again.

Steve

[stevehertz]

As obvious as it seems Steve, ask yourself what 'situation' (not the exact fault) would cause the "raster to collapse" as it does? Then place your meters to monitor those voltages that would most likely cause this. Again, obviously, it is something breaking down or even an intermittent connection somewhere. Best of luck.

[Panrock]

Well, the 'good' news is that we've now got a 'proper' fault - ie. it's always now there.

The frame timebase seems to have gone into a 'digital' mode... it flips between two states - and at the wrong speed.

Your guesses? More anon.

Steve

[stevehertz]

I'd change components in the frame circuit one by one; a process of elimination. Not very analytical I know, but that's me!

[FERNSEH]

Possible the grid resistor of the blocking oscillator has gone open circuit or very high value. Slider connection in the frame hold control OC.

DFWB.

[Panrock]

I haven't got back to it today. Thanks for the ideas. I suspect the transformer. But we'll see.