Les Lawry-Johns wrote an interesting article on this chassis in the Feb and March 1976 editions of Television. I find his articles on anything of extreme interest and this article is no exception for obvious reasons.
Among the various faults that can crop up, the one of poor sync caught my attention. I had this issue during the brief power-up test I conducted previously. I need say no more and allow you to peruse the following.....
To understand the black art of electronics is to understand witchcraft. Andrew.
Yes the feed from the heater supply, G2 supplied from the rectified heater supply, if the diode shorts the sync goes, same problem with a duff cap.
The TV135R had an interesting sync problem, the smoothing cap on the G2 of the video output failed and resulted in poor LF response. The picture wasn’t too bad but there was little or no frame sync. The cap was fitted up against the valve and dried out, cure new cap on the print side, official mod.
Frank
Today the paint for using on the repaired dropper arrived so I set about spraying it. I also replaced the Hunts 8uf capacitor on the signal's panel with another 8uf Hunts. This one is rated for 350v use and is blue. How nice.
But what is really nice is this. I placed a wanted post on UKVRRR and received not only a brand spanking new dropper, but a LOPT from an A774 chassis with a suitable overwind to go on the A640 LOPT. Neil29 on there was the kind gentleman who supplied the items. I needed to extend the wires from the overwind slightly to suit the new location, but the main thing is that it oscillated very nicely with my skantest device. Looking good then !
Here below are the order of events today but in pictures. New cap, painted dropper (when dry installed into set), LOPT with new overwind and finally.... wow. I am over the moon. Thanks Neil. 👍
To understand the black art of electronics is to understand witchcraft. Andrew.
Fantastic! Good to see it's working, how's the repaired dropper? I might have to try doing the same with a few of my sets here that have dead droppers. Do you have to make sure the turns don't touch? I have an ancient Ekco radio dropper here that's wound on a ceramic former about the same size as a bog roll tube, but the turns of resistance wire are very tight up to eachother, which makes me wonder how it doesn't just measure short!
Regards,
Lloyd
Cheers Lloyd 👍
So far the dropper is holding out, but with a smell rather like a BBQ ! I expect this to die down a bit as the paint hardens. I now have a spare dropper in case it does not though. More importantly is my hope that the overwind holds out.
If faced with an open dropper again, I will not hesitate trying a rewind. The turns must not touch and with more care and patience than I have, I am certain most droppers could be rewound. Has to be worth a go.
To understand the black art of electronics is to understand witchcraft. Andrew.
That's an excellent result Andrew. I've visited Neil several times and I know he's a good chap and a big fan of RBM mono sets.
Over on UKVRR, Fernseh (alias our own Till Eulenspiegel) has a dropper with open circuit section on his newly acquired HMV 1920. The dropper is a unique to the set and my first thought was to wonder whether the faulty section could be re-wound. It would seem the answer is 'yes'.
I might just have to point him in the direction of this thread...
Steve
Cheers Steve 👍
So far the dropper and the rest of the set is holding out well. It has been on for some time today, an hour here, an hour there. The smell from the new paint on the dropper has gone away now. The number of mixed dilectric capacitors in the set seem to be in good order with no leakage. I took out the boost HT cap and tested it, half expecting it to need replacing but is fine. I have resoldered the mains RF bypass capacitor and reduced the mains plug fuse from 13A to a 1A fast blow. This capacitor can stay as long as it behaves.
I saw the thread about the HMV1920 and was pleased it went to a good home. Also had seen it on ebay. The open section of the dropper in that set could well be rewound.
If it were not for Neil, then the Murphy would not be operating with an overwind for sure.
To understand the black art of electronics is to understand witchcraft. Andrew.
reduced the mains plug fuse from 13A to a 1A fast blow
Well, it did just that. Blew fast upon switch on from cold. Nothing wrong with the set, so in goes a standard 3A fuse in the plug top. Working fine now.
I guess a 1A fast blow is not up to the slight current surge of charging up the HT caps. Probably explains why they fit a 1.25A anti-surge type in the set in the first place !
To understand the black art of electronics is to understand witchcraft. Andrew.
I would remove or replace the mains filter cap, when go they very often take the on/off switch with them.
Yes when sets started to use silicon rectifiers anti-surge fuses became the norm.
Frank
I will heed your good advice Frank. I cannot run the risk of wrecking the switch.
To understand the black art of electronics is to understand witchcraft. Andrew.
This morning I removed the tuner from the chassis in order to simply clean it. Other than that, there was no real reason to. It is working perfectly. It was a bit of fiddling about getting it re-installed in order for correct alignment of the standards change over mechanism to operate correctly. This meant full movement of the slider switches in either direction. The foam washers on the tuner buttons have long since rotted away, so I cut out suitable sized washers in felt. They provide the same effect and may last longer than the foam ones.
I briefly tried the set on 625 and all seemed well there. Below is the tuner along with the standards switch bracket.
To understand the black art of electronics is to understand witchcraft. Andrew.
Here is a better photo of the set in all it's glory on 405. The top three tuner buttons are set for UHF 625 (BBC2, BBC2, ITV1), the lower also UHF for BBC1, whilst the other two (BBC1 and ITV1) are VHF 405. The positions can of course be altered by turning over a small metal key on the tuner front. But it needs to come out of the set for that. A bit of trivia really, as what's the point of altering them nowaday's with no transmission other than digital rubbish.
To understand the black art of electronics is to understand witchcraft. Andrew.
On 405, there is good achievable height of the raster and it appears quite normal.
On 625 slightly less height, it can be increased, but with top foldover not noticable on 405.
To understand the black art of electronics is to understand witchcraft. Andrew.
What may not be helping matters is that the 360 ohm cathode resistor (3R47) of the PCL805 has risen to just over 500 ohms. I thought the cathode voltage was a bit high, meant to be about 19v, it is about 22ish. Not only that, the height control has to be almost at maximum.
To understand the black art of electronics is to understand witchcraft. Andrew.
No problem now, a Welwyn 3watt 390 ohm is the nearest I had. So complete with a ceramic bead, it has been installed with a big improvement to the height and has cleared the top foldover. The frame linearity presets required adjustment, but now a good linear raster is achievable with plenty of available height.
To understand the black art of electronics is to understand witchcraft. Andrew.
Thus far, excluding the rewound dropper section, the component count to repair the set has been quite small. Not bad for a 53 year old set, 26th Feb 69 was found stamped on the top right corner of the timebase PCB (albeit faded and hard to see).
I won't discard the old overwind, just in case it may be possible to have it rewound at some point.
To understand the black art of electronics is to understand witchcraft. Andrew.
Make that two PY88's....
I had a scare this evening when I switched on, just after the picture appeared there was interference consisting of white flecks on screen and fizzing from the speaker. The effect is similar to when an arc is drawn with an insulated screwdriver on PL/PY valve top caps.
However, the effect cleared after a few seconds. Immediate thoughts went to the overwind breaking down. After a bit of prodding around and checking all line timebase plug connections, I noted that the PY88 produced the same symptoms when lightly tapped. I replaced the used PY88 that I had fitted with a new one. No more problems. Re-fitting the old PY88 again produced the same symptoms, so it was despatched to the bin without hesitation. An intermittent internal cathode connection is my guess.
To understand the black art of electronics is to understand witchcraft. Andrew.
The set has had a few hours use now and so far is behaving very well.
It must be time to return it to Beardy and non-Beardy by now. (Those familiar with LLJ's anecdotes will know what I mean.) 😉
So let's have a pint at the working men's club, blow the fag smoke out of our eye's and settle down for some good old entertainment...
To understand the black art of electronics is to understand witchcraft. Andrew.
