Featured
Latest
Share:
Notifications
Clear all

Forum 135

B&W TV Bush TV125CU

22 Posts
10 Users
10 Reactions
1,022 Views
hitachitv
(@hitachitv)
Posts: 59
Trusted Member Registered
Topic starter
 

Hi All,

I have just acquired a Bush TV125CU off of a colleague at work. It was their parents. 

IMG 20230117 221155678

I've used a lamp limiter to give it a little bit of a electricity but also have been checking for a short circuits.

I have noted one so far - TH32 which did not have continuity but also had a low AC reading. I know that there were issues with this configuration alongside R358. 

orca image  366886623

I'll follow up the heater chain to see if there is are any bad capacitors also.

Warm regards,

Nathaniel 

 

 
Posted : 19/01/2023 12:56 pm
PYE625 reacted
crustytv
(@crustytv)
Posts: 11995
Vrat Founder Admin
 

I'm not sure whether to congratulate you or commiserate you.

Without a shadow of a doubt, you will either already have LOPT failure or will encounter it not long after running the TV for a short while. This range of Bush/Murphy TVs are notorious for it.

Fear not, there is a fix, Andrew (PYE625) covered this in an article for the website. It's a mucky job, but it is a 100% cure, and others have also followed his method with equal success.

https://www.radios-tv.co.uk/vintage-electronics-blog-forum/bush-murphy-lopt/

As for the TH32/R358, I covered that modification in my own TV125CU restoration. The only B&W dual standard I have remaining in my collection.

https://www.radios-tv.co.uk/1963-bush-tv125cu/

Oh, and there will be plenty of bad caps

CrustyTV Television Shop: Take a virtual tour
Crusty's TV/VCR Collection: View my collection
Crustys Youtube Channel: My stuff
Crusty's 70s Lounge: Take a peek

 
Posted : 19/01/2023 1:17 pm
WayneD and hitachitv reacted
hitachitv
(@hitachitv)
Posts: 59
Trusted Member Registered
Topic starter
 

Yes I've certainly heard about the LOPTs on this set failing. I will certainly have a look at Andrew's technique. 

And yes I did have a look at that when researching my set! You certainly did a good job! Has the modification been done on my set? Or is that something I need to cover? I couldn't work out whether the photo of the positioning was before or after swapping!

I'll have a explore for the multitude of bad caps and get back to you! 

Warm regards,

Nathaniel 

 
Posted : 19/01/2023 1:39 pm
WayneD reacted
WayneD
(@wayned)
Posts: 861
Honorable Member Registered
 

@hitachitv crikey! Looks like you've got a lot of work ahead of you there. At least with the TV125CU the LOPT you have Andrew's way of repairing it. Sadly it didn't work on my Bush TV161-chassis Rediffusion so I ended up going down the tripler route.

 
Posted : 19/01/2023 3:52 pm
PYE625
(@pye625)
Posts: 5118
Famed Member Registered
 

I must stress that the method I described will only be of any use if the LOPT overwind is free of shorted turns and has leakage via the fabric tape if it is used. Later ones, such as fitted to the A640 chassis that are of a plastic moulded overwind, will not have fabric tape.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 19/01/2023 6:19 pm
WayneD reacted
slidertogrid
(@slidertogrid)
Posts: 1120
Prominent Member Registered
 

LOPT failure was often the reason these sets were written off when they were 10 years or so old. Fast forward a few decades and add to that damp storage and as Chris says failure is almost guaranteed. I have one set with a so far good LOPT and two others that balloon after a while with the overwind getting warm. I do have a spare that looks terrible the pitch on the overwind is shrunk and cracked. It has been nesting on the workshop radiator for about a year. I am going to run some current through it for a while and then try it in a set. 

Good luck with your set! Are the capacitors wax/paper or the later yellow plastic ones?  

 
Posted : 21/01/2023 12:56 pm
hitachitv
(@hitachitv)
Posts: 59
Trusted Member Registered
Topic starter
 

Posted by: @slidertogrid

Fast forward a few decades and add to that damp storage

As far as I'm aware, I believe that the set has simply been stored in a room in the house by the previous owner's late parents. I'm not sure whether that would also count as 'damp storage'

Posted by: @slidertogrid

Good luck with your set! Are the capacitors wax/paper or the later yellow plastic ones?  

As far as I can see the later plastic ones! I can only spot one wax capacitor.

Warm regards,

Nathaniel 

 

 
Posted : 21/01/2023 2:11 pm
Lloyd
(@lloyd)
Posts: 1958
Prominent Member Registered
 

My Bush TV125 must be a rarity, the lopty still seems to work ok! I haven’t fired it up for a while though..

Regards,

Lloyd

 
Posted : 21/01/2023 5:36 pm
slidertogrid reacted
slidertogrid
(@slidertogrid)
Posts: 1120
Prominent Member Registered
 

You can probably leave the plastic ones until after the set's first power up and change them as required. Some circuits are more tolerant of leaky caps than others and you may get away with leaving some... Hopefully if the room was heated the LOPT won't have absorbed any moisture. What does the overwind look like ? Any signs of pitch-droop? I would still advise warming up the overwind before powering it up. 

 
Posted : 21/01/2023 5:54 pm
hitachitv
(@hitachitv)
Posts: 59
Trusted Member Registered
Topic starter
 

@crustytv 

I'm just wondering what voltage you used to warm up the LOPT overwind on your Bush? 

Warm regards,

Nathaniel 

 
Posted : 05/02/2023 8:05 am
slidertogrid
(@slidertogrid)
Posts: 1120
Prominent Member Registered
 

There are a couple of handy threads . Drive Out Moisture from LOPT Overwind - Radios-TV  and Drying out an RBM Pitch Overwind. – Black & White Television – VRAT Forum (radios-tv.co.uk)  

I started with 30V and then upped it to 40V. another transformer from a later Bush A640 but still pitch I didn't run any power through it I just left it on the central heating radiator in my workshop for three months last winter. That transformer so far has been OK. I didn't run the transformer before drying it out though so I can't be sure it would have failed but the set had spent some time outside in damp conditions.

Coaxing a Rank A640 back to life. - UK Vintage Radio Repair and Restoration Discussion Forum (vintage-radio.net)

Good luck with it. 

Rich

 
Posted : 05/02/2023 8:30 am
hitachitv reacted
irob2345
(@irob2345)
Posts: 584
Honorable Member Registered
 

I believe that "pitch" is called "Berry-Wiggins Compound".

At least, that was what was used on LOPTs by Pye in Marrickville, Australia.

The LOPTs were impregnated and coated with this stuff in a WW2 vintage facility that used to make ignition coils for planes (that had to survive tropical conditions).

It was one of the most dangerous looking facilities I've ever seen. There were fire extinguishers placed all around it and it was in a WW2 era timber building. But there was never a fire! It would have been spectacular if there had been, like the Thermite plant at Brookvale.

LOPTs made in this facility from the mid 1960s were quite reliable, after some problems in the early '60s. It was used for the colour TV LOPTxs too, up until the Philips chassis changeover in 1980, and I never encountered a bad one. I think a lot of the reason for this was they had some very good engineers, notably Russ Hauser, who went on to work for Panasonic.

 
Posted : 05/02/2023 10:23 am
hitachitv
(@hitachitv)
Posts: 59
Trusted Member Registered
Topic starter
 

Hi All,

An update on the Bush! I've been heating the LOPT for several days now at 30V, I heated up for a few days a few months ago but decided to give another bit of heat just to be sure!

I've noticed that C3/46 (Mains Filter) has no capacitance and will need to be replaced. 

That capacitor is a TCC D741/13 0.1uF 1000V. I've tried to find a direct replacement but to no avail.

Warm regards,

Nathaniel

 

 
Posted : 23/04/2023 6:09 pm
sideband
(@sideband)
Posts: 4228
Famed Member Moderator
 

You need to use an 'X' class capacitor rated for mains operation. Typically they will be rated at 275Vac. Available from the usual suppliers or even eBay. So a 0.1uF capacitor 'X' class will be perfect for the job. The set will work equally well without it. 

In the days when the TV was made, 'X' class capacitors had not been developed so typically a 1000V DC cap was used instead.

 
Posted : 23/04/2023 6:43 pm
hitachitv reacted
hitachitv
(@hitachitv)
Posts: 59
Trusted Member Registered
Topic starter
 

A replacement capacitor is on order!

I have however, also discovered that the HT Line Smoothing Resistor a 2k ohm 3w wirewound resistor (3R64 in the workshop manual) has cracked. It disintegrated as I was attempting to inspect it to get a reading!

I'm trying to find a replacement however, I'm only getting results for 2.2k ohm 3w ceramic wirewound resistors. The tolerance of the original is 5% +/- so 2.2 would just be over that. 

Any advice is appreciated.

Warm regards,

Nathaniel 

 

 
Posted : 25/04/2023 8:02 pm
slidertogrid
(@slidertogrid)
Posts: 1120
Prominent Member Registered
 

I would expect a 2.2 would be fine. You could experiment with another resistor in parallel to get it to 2k but that would not be as tidy and probably wouldn't matter anyway. 

 
Posted : 25/04/2023 8:25 pm
hitachitv reacted
Nuvistor
(@nuvistor)
Posts: 4623
Famed Member Registered
 

Agree 2.2k will be fine, most of the WW were 5%, I think Bush would have used a 10% if it was cheaper and available.

Has slidertogrid states you could always put one in parallel if there is a problem.

Frank

 
Posted : 25/04/2023 8:58 pm
Till Eulenspiegel
(@till)
Posts: 4947
Famed Member Registered
 

My calcs work out a parallel 22K resistor should give approximately 2000ohms.

Till Eulenspiegel.

 
Posted : 25/04/2023 8:59 pm
Till Eulenspiegel
(@till)
Posts: 4947
Famed Member Registered
 

Before fitting a replacement resistor check for short circuits across the HT line. It's a long time since I worked on these sets, getting on for fifty years. Do remember the performance was very good on 625 lines, thanks to the three stage vision IF amplifier.

Till Eulenspiegel.

 
Posted : 26/04/2023 10:11 am
hitachitv
(@hitachitv)
Posts: 59
Trusted Member Registered
Topic starter
 

Thank you all!

I have checked the HT line and so far no sign of any short circuits. I'm planning on going through it again today however.

The replacement resistor has been added.

I'm going to replace R358 with a 10w 10R resistor, as I believe this has not been done. R358 and TH32 however, appear to have been modified previously, possibly if the set was repaired or before being sold? 

The CRT is lighting up, however I'm only getting a singular vertical line:

20230428 124515

Warm regards,

Nathaniel 

 
Posted : 28/04/2023 1:12 pm
Page 1 / 2
Share: