The sync seems to be ok but a scope will show problems if they are small or distorted. Poor LF response is often the cause but could be in the RF/IF alignment, detector or video stages. If the sync pulses look ok on the scope I would be looking at the video waveform after the sync take off which leaves a few components. May be worth looking at the decoupling on the first anode of the CRT and focus as well.
After a bit of time on the set this morning, I have found the fault. Usually we are used to Hunt's mouldseal capacitors being leaky. But this one, a 0.25uF, turned out to be intermttently open circuit.... or nearly so, about 3nF to be precise. Enough to pass higher frequencies though.
I found it by using a 'scope to have a general look at the waveforms around the video stage and going to the sync separator. All seemed perfectly normal until I examined the actual video applied directly to the CRT. If you consider the below circuit, the video waveform was fine at the point indicated, but after R52/C45 and off to the CRT, it was clearly different. See the before and after snaps of the 'scope trace. The low frequency response should be certainly be better than the last snap, taken at the CRT cathode.
So, my suspicion fell upon C45. Bridging this component with a 220nF capacitor removed the fault from the screen, and resulted in a normal video waveform.
Here below is C45 (centre of pic), rather a pig to get to, naturally. Lastly, a screen shot after it was replaced.
To understand the black art of electronics is to understand witchcraft. Andrew.
Again referring to the above diagram, on the extreme left you can just see the cathode follower triode section of V6, the PCF80 video output. There are two Hunts capacitors that are physically in good condition, so I thought I would leave them in place and not disturb them.
They are C43 that couples video from the pentode section to the triode grid, and C45 that couples the final output to the CRT cathode. Now, it is quite possible that these capacitors are leaky, but it probably does not matter here as they are each shunted by resistors.
What would be your opinion?
Well, my opinion is that I should have replaced the damn things earlier.... but then I would not have had the pleasure of finding a fault. C43 is still in place though, at least this one is easy to reach should it decide to play up.
To understand the black art of electronics is to understand witchcraft. Andrew.
Good design, the CRT is driven from a cathode follower to reduce the effects of stray capacitances. Interesting to note the sync separator is supplied from a tap on the cathode resistor. C45 gone low capacity resulting the low frequency component of the video signal is attenuated hence the spearing. The black on white box at the top of Test Card C is there to show up any shortcomings in the LF response.
There was still a lot of these sets in regular use well into the sixties. Stablemates to the T21 was the popular TS17 and T346. Super reliable sets despite being derided by folks who worked in the posh shops selling Bush and Murphy. Rental companies used Sobell sets, that's how good the sets were. Remember Rentaset?
As a matter of interest, that similar PCF80 cathode follower is used on several sets. Certainly on my Decca DR303 and the two RBM chassis (TV125). However, one problem that I have encountered is arcing in the PCF80 triode section during the warm-up period.
My theory is that as the HT is solid state rectified in those chassis, it is something like 320VDC immediately after switch on before the valves have heated. This is all very well, but as the triode section of the PCF80 heats fairly quickly, a high HT level exists between the triode grid and it's cathode. It is at this moment that I have noticed on the three seperate set's at one time or another, sparking inside the PCF80 triode section. This can't do the poor old PCF80 any good.
To understand the black art of electronics is to understand witchcraft. Andrew.
So, my suspicion fell upon C45. Bridging this component with a 220nF capacitor removed the fault from the screen, and resulted in a normal video waveform.
Here below is C45 (centre of pic), rather a pig to get to, naturally. Lastly, a screen shot after it was replaced.
What a pig indeed - I can only see the lead at one end of that cap. "Delicate surgery, eh James? Lovely stuff".
I'm amazed at the huge difference between the two scope traces. I'm equally amazed at the frequency gratings, That has to be better than new, surely? Though admittedly, the sig will be direct from the Aurora, with no pesky factory chimneys in the way, or bad connections between aerial and receiver. ??
I'm equally amazed at the frequency gratings, That has to be better than new, surely?
Me too if the truth was told. There has been no need to make any adjustments to the RF or IF whatsoever. It must have been aligned well at the factory and remains so to this day. Even the local oscillator in the tuner needed no adjustment, perfect tuning on CH1, 3 and 11 plus the fine tuner control does not even need the slight alteration between switching channels as can often be the case.
To understand the black art of electronics is to understand witchcraft. Andrew.
One thing I have attended to is the dropper resistor R123. Although working ok, it was loose in it's broken fixing and flopped about a bit. The wires had crumbled insulation due to the heat and the RS polo resistor was just about hanging on by a thread. This 14 ohm resistor had been fitted during a previous repair long ago to bypass an open circuit 16 ohm section on the dropper.
I do not have a replacement dropper of the correct type, so I decided to fit an alternative resistor to replace the RS polo. I had a 16 ohm that I affixed to the chassis nearby, bottom right in the picture below. I also fitted a long bolt through the main R123 dropper to secure it better to the chassis and replaced the dodgy wiring.
R122 (middle in pic) is a twin 50 ohm that supplies the PY32 for HT. This also has a spare terminal at the top to aid physical connection of the thermistor in series for the heaters. (R123 is on the upper left).
To understand the black art of electronics is to understand witchcraft. Andrew.
The above post marks the end of electrical repair/restoration to this wonderful set. This evening, I re-fitted the line stage screening can to the chassis and then cleaned the cabinet. After some use of Rustins scratch cover to mask the inevitable odd scratch, the cabinet is in remarkable condition with it's original finish and polished up very well indeed.
I intend to remove the CRT surround mask and glass implosion screen soon for cleaning too. Then upon completion, the chassis complete with CRT can be re-installed.
Upon my re-fitting the CRT to the chassis previously, I did take the liberty to temporarily put the chassis back in the cabinet to ensure that I had the CRT correctly aligned and levelled up so that it fitted the mask properly. Some minor adjustment was needed, but it now is all correct and the chassis can go straight back in.
To understand the black art of electronics is to understand witchcraft. Andrew.
I'm amazed at the huge difference between the two scope traces.
It's not that surprising really. If you take into account that R55 and 56 on the above circuit will offer a fairly low impedance to the video output and with C45 being the very low value that it was, the low frequency roll-off would be considerable.
To understand the black art of electronics is to understand witchcraft. Andrew.
I have now cleaned the front glass and re-installed the set into it's cabinet. There is not much else to say really except that I am truly delighted with the results. This is a well designed and made set capable of extremely good pictures.
To understand the black art of electronics is to understand witchcraft. Andrew.
Okay, so this chassis was quite a while before the GEC influence.
I once read an entry in Wiki that said something along the lines "The internet is a wonderful source of accurate information...... [Abraham Lincoln]". Obviously a urine extraction, but I saw the point.
I like the way you said "Its Wikipedia but should be reasonably accurate.". . . .
