My ITT CVC7 tale
I got this set from Neil earlier this week.
I left it for 48 hours in my back room with the dehumidifier running to gently warm it up.
Tonight after work I decided to give it some mains and see if it would come to life.
And it did 🙂 .....for about an hour. Then the picture shrank to a rectangular, out of focus green square in the middle of the screen. "Oh dear" I thought, probably a shorted cap, or at worst the Tripler.
I've checked C310 (this looks like it has been replaced), C261 & C308 on the lopty panel. They all check out ok. The voltage on the grid of the PL509 goes down to -15v.
I've measured the current across the 400mA fuse holder and it rose to over 500mA I switched off then!
Disconnecting the tripler makes no difference
I've read about the polystyrene caps in the line osc circuit and mine are the mustard coloured polyester flavour so I don't think it's those.
I fear for the LOPTY. Can anyone give me any clues as to where to look next?
0.47uF 1000V 'boost-cap' will cause your probs, but suspect you've already checked it.
There is another cap--or was it a diode--that will cause the same sort of fault too--Been so long, I cant remember....
Line output Tx are pretty good on these, although I did have occasion to change just the one years ago....
Triplers were the main probs in the line-stage I found, and sometimes a dodgy PY500A--usually with lots of arcing where the cathode sags and shorts the filaments!
I'm going to try new PY500A & change those caps to be on the safe side. There was a faint blue glow in the PL509,but it was only visible in a darkened room.
Having changed the suspect caps (C261, C303, C308 & C310) there is no change.
I can lay my hands on a Decca Bradford lopt, which I've read somewhere, can be used in one of these ITT's.
Has anyone here tried it?
I have fitted Rediffusion MK1 loptx to this type chassis many years ago. Worked fine. The 22/26 inch Bradford and the Rediffusion MK1 loptx are all the same.
Sorry if you've already done this, the components on the PCB panel holding the LOPT should be checked/replaced, (I don't have the circuit on me so not sure component numbers off hand). I remember one capacitor causing lack of width on this board when defective. Also check the solder on the pins to the main PCB as dry joints cause strange symptoms.
Does the transductor look ok?
Thanks for the replies Malc & Brian.
I've replaced the PCF802 and the line drive coupling cap (C301), it was split along both sides. The transducer looks ok and isn't getting warm. The PY500A is getting quite hot, but not having much experience with valve line output stages, i'm not sure how hot is too hot.
Disconect the tripler input. Do you have a scope to check for line drive? Have you replaced the line output valves? The pl/py do run quite hot, the heaters should be glowing red, but nothing else inside the valves should.
You haven't missed a spring-off that's sprung off have you?
Try saying that repeatedly after a few pints ....
Hi Malc, Ive tried disconnecting the tripler. to no avail. I only have a PY500A, PL519s seem to go for silly money these days.
I will have to dig my old scope out, remove it's earth lead (no isolating Tx ) and check the line drive waveform.
Hi Doz, Nowt's sprung open and no fuses blown.
A PL509 is fitted to this chassis as far as i can remember. One of these will be fine. The PL519 is a bit beefier.
I think the PL519 is new, it looks shinier than the others and Neil29 who I got the set from says he renewed it some years ago. I'll try the scope this weekend.
I can recall various line output troubles with this chassis, mostly caused by poor earthing continuity on the line output stage printed circuit board.
The earthing points, where the chassis lugs are soldered to the printed circuit, would go resistive. they were also used as an earth carrier point for different sections of the LOPT stage circuit.
The answer was to resolder the joints with a high wattage soldering iron. I would also solder insulated
wire links between the earthing solder joints, to ensure continuity of the earth line.
I also seem to remember a 10mf electrolytic in the cathode/grid circuit of the line output valve, giving problems.
Don't get too sidetracked by the sound vanishing. It is deliberately muted until the line timebase gets properly going on these series of sets. How are the voltages on the PCF802? In particular check anode volts.
Excellent! Have you tried ditching the colour killer? I must admit on the set I had I ended up snipping the CK diode which permanently ended stubbon intermittent no colour problems! There's arguably no need for it now all transmissions are txd as colour, as they have been since the late 1980s.
Never seen a CVC7 before. Did the original LOPT turn out to be defective?
Yes the original lopt was faulty, I have had it rewound by Mike Barker but a couple of weeks ago a nice chap off UKVRR gave me a lopt from a scrapper.
The set has been on for a few evenings to see if any faults showed up and one did, the picture and sound went off leaving a blank raster. Fortunately I had a copy of Eugene Trundle's excellent servicing article from the service library, which helped me trace the fault to one of the "lockfit" transistors in the IF strip. I replaced all four transistors mainly because it was such a pain removing the IF subpanel.
I've been having a play with it tonight. I disabled the colour killer and was rewarded with unlocked colour, a tweak on R311 and now I have colour