Cossor 1039M MkII

[ColinTheAmpMan1]

Hi,
I have purchased one of these fascinating little scopes and it seems to work after a fashion (no, it's not the one on eBay for the silly price of £269.95). This is the little thing with a 2¾" CRT and the MkII uses a germanium diode for blanking, rather than the SD61 diode in the earlier version. There are several waxies and Hunts caps in there which I am planning to replace, having sourced suitable components from Just Radios in Canada. There are some faults, such as the X-shift control won't centre the trace, one time-base range is incorrect and some of the voltages are off the proper values. I think a lot of these are due to leaking or otherwise dead caps, which I hope will be be corrected with new components.

I have replaced the dodgy mains-connector with a proper, earthed IEC item, which just about fits, if you use an appropriately-sized connector on the mains lead. This is much safer than the original.

I have also purchased three new 6AM6 valves, should I need to replace any. Since the originals are Cossor and the NOS that I have bought are Brimar, I would like to use the original Cossor items, if possible.

The questions that I have are:
1) C22, the 0.5uF, 500V component in the voltage doubler is a Dubilier component. Should this be replaced, or, being a good quality item, is it likely to be OK?
2) C19/C20 is, according to the circuit diagram, a 32uF + 32uF, 350V electrolytic, but this scope has a Daly item fitted which is 32uF + 50uF + 16uF. It has the 32uF and 16uF sections paralleled. I am guessing that this is a replacement, but I might be wrong. Any thoughts? I am planning on replacing it with a 32uF + 32uF anyway.
3) The Y-shift control works the wrong way, in that turning it clockwise makes the trace move down and vice versa. Is this correct? A positive voltage on the Y-plate does move the trace toward the top of the screen.
4) The Brilliance control tends to affect the vertical (and horizontal) position of the trace by a small amount. I can understand this might be normal, but does anyone know if I am right ?
5) This is a compact little blighter and I think some components could be tricky to replace. Has anyone any experience of doing what I am planning, or am I a pioneer?
Any advice gratefully received.
Colin.

[WME_bill]

Cossor 1039 MkII.
Interesting little scope. The MkII with the flat faced screen Cossor 24D/ Mullard DH7-91 is a big improvement on the earlier MkI with 23D/ECR30/VCR139. It has a much greater tube sensitivity and better sync with a separate valve.

I usually reckon not to replace capacitors on test equipment unless they are clearly faulty, as often the disturbance of replacing parts makes other faults appear. Components on Test gear are often of a higher industrial quality than consumer grade used in radios.
Watch out for dry joints. I had several on my 1953 MkI version. Requires careful searching to check all the voltages.

1.C22 voltage doubler capacitor. It is paper, so unless proven faulty, I would leave alone. It has been replaced on my MkI.
2. Sounds like a replacement. The MkI uses 8+8 mfd, which I have replaced with 32+32.
3. The Y shift on my MkI also seems inverted to the usual convention.
4. No visible variation. Although the eht is not stabilised, the changing sensitivity of the X & Y deflection should not be aparrent when the trace is centred. Check the eht voltages, and the eht smoothing.
5. Take heart. Valve equipment is vastly easier to work on than a transistor board with 100 components to a square inch. I have to use a magnifying glass now.
wme_bill.

[ColinTheAmpMan1]

Hi Bill,
thanks for your information. I first came across one of these 'scopes at Secondary school. I don't recall which Mark it was, but nostalgia got the better of me concerning this one.
I follow your thoughts on mass replacement of capacitors, but for once I thought it was not a bad idea as almost all of the capacitors are Hunts, with just three waxies. I have so far found that C3 (Y amplifier anode to Y2 plate) is leaky and I suspect C11 (timebase anode to X2 plate) is also, since the horizontal trace cannot be centred. Since C1 and C10 are waxies and easy to get at, even though probably not stressed, I am inclined to replace them. Also, as one timebase range seems wrong and four out of five of the capacitors on the timebase switch are Hunts and not too difficult to get at, I just might replace them all. It might also serve to improve the waveform of the timebase generally, as it seems a bit non-linear - though with such a simple device, I am not expecting perfection. C8 is a replacement anyway and not a very nice one, in my opinion.
I think I might take your advice over C22, the voltage doubler cap, since it is Dubilier and probably ok, as the EHT seems fine.
The tasks that I am thinking might be tricky are replacing caps on the component board near the CRT connections. The Hunts 0.05uF (C17, C21) caps might be ok, but there is also a waxy (the aforementioned C11) and two more Hunts buried in there.
I have poked about checking components where I can and have found that the resistors have, in general, drifted a bit high, but since they were 10% or 20% tolerance originally, I am inclined to leave them alone.
Colin.

[ColinTheAmpMan1]

I have received the capacitors now, except that the 1000uF, 6V electrolytic which decouples the cathode of V1 is an axial, where a radial would have fitted better, I think. I might try to source one from elsewhere.
I have now found C11 (0.25uF, 350V waxy - coupling to the X2 plate), C6 (0.01uF, 350V Hunts - anode cap of V2) and C3 (0.05uF, 350V Hunts - anode cap of V1) are definitely leaky. As a consequence, I am reckoning on replacing the three waxies and just about all of the Hunts. The tricky bit is replacing those capacitors that are on the circuit-board by the CRT base. There is a total of four there and it looks a bit of a puzzle as to how to get at them. This being so, I might as well replace them all if I can get at them. One is the leaky C11 waxy, so that has to go, anyway. The others are Hunts.
Has anyone any experience of changing these caps? The circuit-board that they are mounted on is fastened with four screws to a bracket which is supporting the back end of the CRT. While removing these screws loosens the board, it is still connected with a few electrical connections to an earth-point, the doubler diodes, a wire to the Y2 plate and the CRT base. The trouble is, all of this is sandwiched without much room between the mains transformer and the bracket supporting the back end of the CRT. I am thinking that I need to release all of these electrical connections and loosen and remove the CRT base to get at all the caps.
Any advice would be useful.
Colin.