I've already had a duff Plessey cap
When I first started this hobby I was told that Plessey caps were pretty good but I've always found them to be a complete pain, If I've come across more than a handful that were any good it'd be a miracle.
Marc
BVWS member
RSGB call sign 2E0VTN
I never found those Red/Black caps with the yellow label very reliable 40 years ago.
The sets I fixed didn't use a lot of them but I had replacements on the van for the sets we did fix.
Frank
A big thank-you to Marc for sending the replacement EHT valve base cover, it fits exactly. 🙂
At last the LOPT can be shut up in it's little box and forgotten. I added the yellow PVC sleeving and the cable clip to keep it all snug and nicely fitting.
To understand the black art of electronics is to understand witchcraft. Andrew.
...and keep it away from the dog!!!
...and keep it away from the dog!!!
I hurried it into the workshop and fitted it before she even had her tea ! 🙂
To understand the black art of electronics is to understand witchcraft. Andrew.
Looking for the line sync problem, I started measuring voltages around V11 (ECL80 line pulse discriminator) as lightly tapping this valve seemed to cause the line hold to twitch and jump around a fair bit. Replacing the valve removed the microphonic action although after a while, the hold was unstable again. The waveform on pin 2 and voltage was unstable coincident with the on screen instability. The cathode voltage was varying too, and a check on C41 (2uf Plessey) proved it was completely useless (Frank was right to suspect it). I could get no uf reading, but the resistance of the thing was everywhere. Replacing it completely solved the instability.
With the set working ok for half an hour or so now on 405, I thought I would have a try on 625. I was rather surprised to find it working pretty well indeed after tuning and adjusting the 625 line hold. It has to be said that most of the Hunt's capacitors with any appreciable voltage across them have now been replaced, but there are many still in service not to mention a couple more Plessey electrolytics.
To understand the black art of electronics is to understand witchcraft. Andrew.
Earlier this evening I discovered almost by accident that a Hunt's capacitor is in fact effectively across the mains (it is well hidden, tucked away in a corner of the chassis). It is C75, a 0.02 uf 1000v between the input to the rectifier (D2) and chassis. I have replaced this capacitor even though it seemed perfectly happy, as I know it may not remain so and could protest violently at an unsuspecting moment. Not only that, if it was to go short, it could take out a dropper section or two (R64-8) in spite of the fact there is a fuse. I certainly don't want that to happen.
To understand the black art of electronics is to understand witchcraft. Andrew.
Well on the way now, is it the photo or is there a bit of noise on the 625 picture?
Frank
When I was a kid, we used to put those Plessey caps on the fire - Plessey bangers, we used to call them. I'm not sure why because I don't remember hearing them go off with a bang.
The 625 width is too great and I have noticed that C57 is 140 pf and not 100 pf as in the manual. I believe this is the LOPT tuning capacitor?
The picture is noisy, more so on 625, and I think the CRT emission is not that great either. It looks ok in the photo, but turn up the contrast and the focus is poor with washed out whites. A cross-hatch looks terrible.
To understand the black art of electronics is to understand witchcraft. Andrew.
If the EHT is a bit low it won’t help the CRT. Try a 120pf If you have one, if it brings the width in but not enough, go try the 100pf.
Noise on the picture is probably the valves in the tuners, but very rarely I have seen IF valves cause it. Presume you have enough signal going into the tuners.
To say it’s 50 years old it’s doing very well.
edit. Did you change the 625 S correction capacitor?
Frank
Hi Frank, replacing a PCC189 in the VHF tuner and both valves (PC86/88) in the UHF tuner has made an enormous improvement. Perhaps not so visible in the photo's, but boy, there is bag's of gain now.
The width problem is still there regardless of 100pf or the 140pf fitted. I did replace the 625 s-correction cap. I have no idea how to correct this problem as of yet. (Test card F is the 625 picture, naturally).
As for the slightly soft CRT, I've been spoilt by fantastic tubes in other set's I have lol !!
If my photo's look a bit odd, it's because the set is upside down, not a gravitational shift 🙂
To understand the black art of electronics is to understand witchcraft. Andrew.
Another problem I had but now is fixed, was varying height. The triode section of the PCL85 frame output valve takes it's HT via a 490k , the height control, and then from a tapping between a 1.5 meg (via a 1 meg from the boost HT) and a 2.7 meg resistor to ground. At the tapping there is a 2uf 500v electrolytic, C107. I thought I had re-formed this capacitor, but it was still leaky enough to drag down the high impedance HT line for the triode. Replacing this solved the varying voltage, which is even sensitive enough to be dragged down slightly by the multimeter internal resistance.
Here I am re-forming the capacitor, but it had to be replaced anyway. Immediately below the cap, you can see the 1.5 and 2.7 meg resistors that had been replaced long before I got the set. (They are fine).
To understand the black art of electronics is to understand witchcraft. Andrew.
I will have a another look at the circuit if I get time tomorrow, well today really, about the width.
The picture looks better tonight, perhaps the CRT will improve with use.
Frank
The circuit is odd....why not provide two width controls instead of a variable boost control which has no clear indication in the data as to what to set it to. The first anode of the CRT on pin 3 is 560v, so I guess you adjust it for that, but of course it alters if the width is then adjusted anyway.
To understand the black art of electronics is to understand witchcraft. Andrew.
The do provide some width/boost switching, R57 2.2M is only in circuit on 625, I would also replace C51 line output screen decoupler if not already changed it, hopefully the screen feed resistor is W/W.
Presume the Line output and Boost diode replacement valves have been tried.
Frank
Hi Frank, yes all have been tried except replacing the screen decoupling capacitor as this re-formed and is fine. The feed resistor is w/w and in good order along with all the other localised components and R57.
Now, the only oddity here is the VDR. To me, it looks to have been replaced, probably robbed from a scrap PCB judging by the look of it. Here, it can be seen just above the 140pf red capacitor. This picture was taken before I replaced the broken LOPT tag strip and the soldering is much tidier now.
The VDR is meant to be type E298ZZ/01 (no data in 1965 book) but the one fitted looks to be type E298ZZ/06 (950v 2mA), according to the Mullard 1965 data book.
To understand the black art of electronics is to understand witchcraft. Andrew.
Sourcing a replacement VDR will not be easy, if you didn’t know it’s used for EHT regulation. This idea was put into circuits around 1964 or thereabouts.
You probably know that so I will shut up now. 🙂
Frank
May be worth bridging that screen decoupler as a test, it may have reformed but behave differently on the two frequencies, yes perhaps clutching at straws but I never did trust old electrolytics.
Frank
