Marconi TF1331 Oscilloscope

[Skywave]

Hi Lawrence,
Yes, yes, by all means fit dummy loads to simulate the full-load conditions as a final test 'soak', but prior to that, for fault-finding conditions, we only want enough current flowing so that the cct. has something to 'bite on'. If we were to go straight for a full-load test, the consequent currents might just be enough to burn out a resistor, say, that was on the edge of going O/C, or a valve that has a weak gm and won't work properly at its normal, designed Ia, thus adding to the faults we are trying to locate.

I use that approach when I'm testing / repairing switch-mode PSUs, be they for a computer or in a T.V., e.g. the line O/P stage.

As for my health, etc., things are much the same - just trying to cope with the slowly-worsening arthritis; thanks for the enquiry.

Al.

[pwdrive]

Hi Al, yes written in haste! I have been banging nails in for the last few days, maybe low current dummies to start off with, just to prove that the PSU is all workie workie etc or as you say do a blanket job. Those 6080's will take quite a bit of temp. stress, mine did!

Lawrence

[AlanBeckett]

Good evening Gentlemen.

Having tidied the Greenhouse, started up the Propagator, chopped down the Raspberry canes, applied BFB, disposed of all debris, mowed (yes it's been that warm and dry), ferried people about and eaten a light but nourishing lunch I decided I could have a go at my stuff.

So .....

Fear not young Joe, I still have the old Electrolytics so I could re-stuff them. However, I quite like the collars, and at least I didn't hang the new ones in the wiring.

Unfortunately Al your missive arrived while I was on the job, as the Bishop said to the Actress. However, I did broadly follow your approach - see below.

Yes Lawrence, it is a bit 'upside down', in fact fairly typical in my experience of the (non-electrical) DO of that era. Everything is actually joined to the right places, but not necessarily logically.

Step one was to measure the voltages properly and apply some Switch Cleaner to the pots - see attached.
After that the -150 set up OK and the -85 settled down to the correct value.
Similarly the +300 set up OK.
The +400 was a bit high but then it us 'Unregulated'.
The +365 was OK.
The +150 didn't respond to RV601 at all although the voltage on the Slider and the Grid of V607 (Pin 7) did vary. After much head-scratching and measuring I gave V607 a strong wiggle in the base and a blast of Switch Cleaner, and it all came to life and set up as it should.
I was standing there congratulating myself on a job well done when R634 started to singe. I'm not quite sure why at this stage. It measures OK and the voltage across it is about right. Possibly I'd pushed it against an adjacent Resistor as it hasn't happened again.

There are indeed a lot of high value Resistors in there which could do with checking and possible changing, along with a few Hunts and Plessey Caps. RV601 is near the end of its travel so that area in particular will warrant some investigation.

So what next? It's looking promising so I suspect some more cleaning and measuring as Al suggests is probably a good idea, followed by the EHT Generator. I can't actually measure the 8.5kV at the moment, apart from dabbing the AVO on.

Just in passing I'm working on it on the Bench upside down. As one of the characters in 'The Dambusters' said 'This is bloody dangerous'. I kept thinking 'I'll just lean my hand on the Chassis rail while I look at the meter'. That's a bad idea 'cause there's a long Tag Strip with all the HT Voltages on it above the Electrolytics and near to the edge. I've avoided it so far, but I fear it's just a matter of time



           

[Skywave]

R634 getting too hot: you may be right, of course, that you inadvertently nudged it against some other electrically-active point (including chassis), but I would check C610 for leakage. C610 may even have re-formed itself, hence the heat from R634.
As for RV601 at end of its travel, if the resistors in series with it check out O.K., I'd suspect C605 (0.25 µF) as leaky. There is always the possibility of an internal short in V607, of course, but then a check at the anode (pin 6) should reveal all: drwg. indicates + 50v., which sounds about right. If much less than that, the valve is passing too much current, possibly due to a leaky C605. Of course, if you fancy an arithmetical exercise, you can always calculate the voltages at each end of RV601, assuming that the grid current of V607 is negligible in comparison to the resistor drain current.

I've just had a look at the CRT drwg.: hmm. If you're poking around in the area of the mains transformer, keep well away from the LT4 winding, since one side of it is connected to -1.25 kV at pin 4 of the C.R.T.  Yo!   Now you and I know why they've done that (to minimise the h-k voltage stress), but even apart from safety reasons, I still don't like it since it places one helluva strain on the insulation-to-core of the LT4 winding. It's obvious that the mains xfmr. for this item is a 'special'. Should it 'go down', I reckon that you'd be well and truly up the Swanee!

Al.

[AlanBeckett]

Al,

That sounds eminently logical, although C610 is one of my new ones. I'll run it for an extended period as soon as other activities permit.
The Drawing are loading OK for me so I suspect your ISP.

Alan

[AlanBeckett]

Onward and upward .....

R634 didn't show any signs of distress on an extended run, and hasn't done since, so I'm ignoring it.

There was no sign of high voltages round the Tube so off with the EHT Unit cover.
Shock horror, it's different, see attached.
This version, which I can only think is older than the Diagram has different valves, 5 Thermionic Diodes and a Pot.
The Oscillator isn't running. However, there's a cracked Hunts Capacitor - C502 - near the Oscillator valve so I think changing that will be the next step. I'm not sure whether there's enough lead to 'snip and tack' so I'll have to do a bit of dismantling.

Alan

       

[Skywave]

The QV03/12 is another name for our old friend the 5763. In Class-C, good for about 5 watts up to about 70 MHz; a pretty rugged valve. (Was one of my favourites in the ol' days of designing VHF transmitters). The EL822 is a bit of a 'special': no known equivalent, apart from a CV2382, although an EL84 comes close on spec. The 12AU7 = ECC82. I suspect that an ECC82 or an ECC88 would work equally well in that cct.

If C502 is a bit 'iffy', then the osc. ain't going to osc. very well, since it tunes the primary winding. Of course, if it goes S/C, the QV03/12 is in for a hard time!

Just a few thoughts,
Al.

[AlanBeckett]

Al,

That was pretty well what I had in mind. If C502 is S/C then the current will possibly only be limited by R500. However, it looks as though the grid is generally a long way Negative so probably that's what limits the current.
What to you make of C503 at the top? Of course this circuit may be entirely different.
It's warming up a bit now so I'll go and play.
Keep thinking.

Alan

[Skywave]

C503? Boost HT capacitor, a-la valve T.V. practice of that period.

Al.

[Skywave]

C503? Boost HT capacitor, a-la valve T.V. line output stage of that period.

Al.