Hello! Here goes with my first post to this great group.
I recently picked up a rather tasty Bush TV165 (A640 chassis), and have been looking forward to getting the old girl going.
After initial cold checks, it was found that On/off switch was open (surprise surprise!) and sections of the dropper had gone O/C. I replaced the open sections with suitable RS “polo mint” types I had in the spares bin, and also replaced a few sad looking Plessey Caps around the Lib-timebases. The PY88 has gone to air so this was replaced also. Now the fun bit...
Upon power up there was heaters, a hum from the speaker and a faint line whistle was heard. No spark or drive could be had from the TC’s of the PL504 or PY88. This being an A640 I swapped the LOPT out for a known good one, I have in a Murphy set just for testing purposes. Again, no EHT. It appears that the DY802 isn’t lighting either, but cold checks prove the Heater winding is okay, as of course the donor LOPT is perfect. Also R22 is smoking and getting all hot and bothered.
Any input as to what could be up would be greatly appreciated,
Regards
Welcome aboard 🙂
So, the screen feed resistor for the PL504 is getting distressed. We can rule out the LOPT, but I would be inclined to first cast an eye over some of those high-value resistors around the line output stage and indeed the cap decoupling the screen grid of the PL504 - C15. Also, have you checked the boost C, C18 for shorts?
The other thing of course is a lack of line drive to the output valve.
Check line drive, screen decoupling cap on the PL504 and boost capacitor.
Crossed post with similar thoughts. ?
For further information, there is a free-to-view fault finding guide on these sets here:
https://www.radios-tv.co.uk/download/1151/servicing-the-bw-sets/41757/bush-tv161-chassis.pdf
You won't be disappointed with the performance of this set - probably the best of their kind.
i agree with Nuvistor and Cathovistor, check the booster capacitor,,
and welcome inhere 😉
Hello! Thanks for your reply, I changed C18 as matter of course as I’ve had trouble with Boost filters leading me a merry dance before. C15 has also been replaced. I’m going to do some resistor checks and see where that leads. Nothing like a bit of Late night repairing on a Sunday!
Best Wishes
Hi,
That's a good start. The reason I mentioned the high-value resistors around the line output stage is because I've been faced with a set that gave the impression of a dead transformer - but it turned out that a high-value resistor of some 8.2Meg had decided to go o/c: the resistor went from the grid of the PL504 to HT as it was part of a so-called 'stabilised' output stage.
After some resistor checks regarding the PL504 and PY88, some are a little High, like R20 for example, but nothing O/C or vastly out of tolerance. This ones turning into a real head scratcher!
I shall return to it tomorrow after having a think overnight.
Best wishes
Have you a scope to check the line frequency? I would also check for -35 to -40V in g1 of the PL504.
Check the condition of the pcb around the base of the PL504 for charring and leaks and have you tried a replacement PL504, it could have a g2 short.
Hello! Yes I shall fire up the scope and do some tests, although I usually like to do things old school before resorting to new fangled scopes! Although, they do make life a lot easier. I’ve inspected the board and it looks okay, not charred or evidence of a burnup. Will get back into the workshop later and take some voltage measurements and see. I did try a new PL504, but may be worth trying another one, as Sod’s law comes to mind! Will keep everyone updated as to how I get on.
Best Wishes
Will get back into the workshop later and take some voltage measurements
Hi and welcome aboard, that would be very helpful and is likely to shed much more light on the problem. ?
Me being a boring old methodical 'Hector', means I'm curious about the power-supply/ H.T. rails as nothing as far as I can see, has been reported on them. All four of them (HT1, 2, 3 & 4) are they operating correctly? Also what are the readings on the PL504, PY88 and CRT base with regards to voltages. Certainly until they are all known I'm with you, old school methods before scope time. Armed with this additional info I'm sure all the chaps who've offered assistance will gain a clearer picture of what's potentially going on.
Field work I rarely had access to a scope but on the bench it quite often the first piece of equipment to use, depending of course on the fault symptoms. Voltage measurements, waveforms etc all done with the scope.
^^ wot Crusty said 🙂
Hello All, some progress today!
We now have EHT, and a raster. The Lack of EHT turned out to be a faulty base on the PL504. With this replaced EHT rustled up and a raster (of sorts albeit very dim) appeared. The raster seems to be more dim than I would expect as the CRT tested great. We now have another issue regarding video. It appears that no video information is getting through to the CRT. Here are some voltage measurements from the base of the CRT-
Pin 2 58v
Pin3-200v
Pin 514v
Pin 6 580v
So an unusual one here!
Many thanks
Welcome to the forum.
Just a tip to help casual readers understand what is going on - and possibly be able to help as a result - instead of giving pin numbers, electrode designations such as k, g, g2 ,a, etc would make much more sense of those voltages.
It appears that no video information is getting through to the CRT.
Well done for aquiring the set, and welcome to the most friendly forum in the world !
PFL200? Could be dry joints too. Otherwise, a good old fashioned fault-finding drill as outlined above (this time in the video stages) will likely show the culprit.
Some excellent tips in the LLJ article mentioned previously.
Let's see some photo's..... my appetite has been truly whetted. ?
What was wrong with the valve holder, bad connection on g1?
Pin 6 of the CRT 580v? Sure you are on the correct pin?
Pins 2 and 6 should be the same voltage on the circuit I have connected to the brightness control.
Yes, it seems the pin in the socket had just carbonised to nothing! Silly me for not using proper g1, K etc... suffixes. Will re-do the voltages later and re post. Bit different when I’m used to doing things on my own, and not explaining things to others!
