[Closed] A Marconi 702 Mirror Lid Part 1

Brian,

On closer inspection, I'm missing C6 (1 µF/1500V on the focus voltage), and the accompanying wire to the screw/tag board.
Is that capacitor located now somewhere else in your set?
(I noticed a replacement on the dismantled power pack parts.
The original has been a green rectangular one. Not dissimilar to the 0.1+0.1µF, but a lot smaller.)

Jac

You're absolutely right, Jac! The cap is missing at the moment as I am having to make one from scratch - I have already made a couple for other people! I was disapointed tofind the oval one in the set as it would be very difficult to re-stuff as it seems to be swaged together like a tin of beans as well as soldered so I decided to manufacture a completely new one.
There's no fooling the guys on this forum

The cable lacing of the replacement wiring loom looks very professional and certainly adds the finishing touch to your restoration. Looking forward to the next installment of this comprehensive account.

Mike

Thanks Mike. The lacing was a thing I learned when in the planning dept. of the BBC. I used to watch the wire-men at work and ask lots of questions!

Especially for Jac, I stayed up late this evening to finish the replica Focus supply smoothing cap for the 702. I had two abortive starts and ended up by adjusting the rip fence on my circular saw as I was trying to make a former for the cap but is was not coming out square. The cap inside is one of "Ask Jan"'s 1.0 mfd @ 1.5kV.

This is very impressive Brian!
Virtually indistinguishable from the original! Mij first impressing seeing the photo was - hey - were did you find this NOS original capacitor!
I must say that I believe that this type of TCC capacitor in general is quite good.
In my 702, which was in an appalling state, both the 1µF and the 0.1+0.1µF capacitor were in good condition (after reforming gradually).
So it must have been a quick repair to replace the one in your set.

Jac

The Large cap (0.1 + 0.1) tested fine at 3kV so I decided to leave it be - here's hoping!
I had a rubber stamp made with the correct script for the focus cap (it's a pity that it's hidden) but it is very easy to mess the print up and then it could be a re-paint job so I modified a mini-drill press to do the job but there was so much lateral play that I had to scrap that idea!

Next is to fit the focus cap, try to straighten the EHT enclosure and get on with the TRF. That one will be a problem if I can't get the metalwork clean enough. No way will I dimantle that one - well, maybe just the tagboard

The PSU is now officially finished. I have not put the text on the new cap as I can't find the box with the rubber stamps and inkpads in it so I decided to go ahead and install it anyway. Should I find the box in the near future, It is a very easy job to remove the cap provided that I have not installed the PSU back into the cabinet. I have a doctors appointment now so will stick it on the variac when I get back. Here are are a couple of pictures - one for Jac:

I have not put the text on the new cap as I can't find the box with the rubber stamps and inkpads in it so I decided to go ahead and install it anyway.

Get Sarah to look for it

That's the general plan, Mike!
I have now tested the PSU and there was a wiring error where I applied two neutrals to the EHT mains Xfmr so no EHT. A relatively easy fault to find. However, on rectifying the fault, smoke issued from under the chassis - one of the first two Rs in the EHT bleeder chain! The smoothing cap in the twin can had gone short circuit - the reservoir cap is OK (4.5kV off load with no smoother) but 'tother has gone short circuit so I will have to open the huge green beast and replace the caps (both as a precaution). I haven't repaired these caps before, only made replicas and of course, they're not the same as the factory made specimens! So instead of getting on with the TRF, I will be messing about with the 0.1mfd + 0.1mfd 5KV cap can which will probably end up with a re-spray. Such is life.

If that's the original EHT cap that's gone 'phut', I wouldn't have trusted it anyway if I'm honest.

I stuck it on 3kV for a couple of hours and it was OK but, I suppose, the extra couple were enough to blow it out!

Hi Brian,

I tested the 0.1+0.1 µF at 6kVDC (the maximum my power supply can supply).
After some time (I didn't make a note of how long it took), the leakage current was in the region of 20 µA per side.
This did not increase again after several hours, and a few days later it was the same.
Negligible compared to the current through the bleeder resistor chain.
Perhaps I am just lucky with the caps in my 702 - this was more than offset by the condition of the rest of the power pack.
This was how the power pack was when I bought the set:

To be honest, the good electrical condition of these large TCC capacitors was an unexpected result.

Jac

Good heavens Jac, That PSU looks terrible! I assume that the set had been standing in water for some time! I assume that you rescued it by stripping and re-plating.

Just to round things off today, I put the TRF (Vision RX) chassis on the bench to see what had to be done for the restoration. It looks to be in pretty good condition with no rust and all components at least looking OK. In fact it looked so good that I thought that I would power it up using my bench PSU. 4VAC and 250VDC was duly applied - nothing. HT current about 30mA. I then switched 'scope to correct input and got 17Vp-p bars which is enough to drive the hexode CRT to full modulation! The bandwidth didn't look too good and the sensitivity pot is faulty but it's a very good start.

I'm not sure how to tackle the finishing of the chassis in this case. It's fairly complicated metalwork wise and I will not strip the electronics out in order to spray it as the RF side is much more likely to cause difficulties during disassembly and reassembly. There are two sets of screening cans - short solid copper ones beneath the valves and thin alloy ones for the valves themselves. I will spray the copper ones and polish the alloy ones but that leaves the chassis itself together with the bracketry. I think that I should take it apart as far as possible and then very carefully mask all the non-paint areas and spray it and the brackets.

But first, the EHT cap re-stuffing - but that is another day!

It's another day now so I should start on that wayward cap block - but wait a minute, isn't there an EHT interlock on the EMI MKI TVs? Yes there bl**dy is and who forgot? What a prat. I was lying in bed trying to get to sleep in spite of Steroids when it suddenly dawned on me - short circuit cap! They normally go leaky but not S/C and then I realised what a fool I was. I tested it this morning with the intelock defeated and got 4.8kV so all is well with the PSU chassis.

Hi Brian,

Some of my best diagnostics or moments of inspiration have taken place when dozing off and even in dreams. Circuits faults going round and round in my head, armies of diodes, caps and transistors marching in my head. Then a possible solution presents itself, Its almost like lucid dreaming. Sometimes it drives me so nuts I just have to get up and go into the workshop to see if it works, I've been known to do that in the wee hours.

Glad your noodle doodlings sorted it for you, onwards and upwards

That's tremendous progress again Brian!

I put in a 3mm screw in the shorting bar, on the "backside" (front-side of the cabinet) to keep the shorting bar "loaded".
Not a clear picture, but it should give you an idea:

The chassis was corroded through and through, and had become too weak to support the heavy transformers.

A great technical guy was prepared to make me a replica chassis, and he has done a fantastic job.

The connector board also had become completely corroded and too weak to support the pull of the heavy wires. The connectors, or what was left of them, could not be saved due to the corrosion again. So I had to make a new one.

As you see this is the early version, and differs from yours mainly in printed text/lines.
I would rather have kept the original chassis and connector board, but in this case I has no real choice.

Jac