Rank a640 - low EHT and hum

[rank-bush-murphy]

Any ideas? No pic and very very low virtually non existent EHT on 625 and low EHT on 405; adjusting the brightness the picture balloons .
DY802 changed - still the same.

[Steve_P]

I know exactly what that is. A faulty EHT winding, it was all too common.

So there are a few options...

1: Wait for a A640 LOPT to come available. There was a modified kind, with a stick rather than a DY802 - A bit more reliable.
2: Take another LOPT apart, get it's EHT winding and carefully transplant it onto yours.
3: Get yours rewound. (Mike Barker or Ed Dinning would help here.)

Cheers,

Steve P

[Tazman1966]

Hi Simon.

You could always do what I did with mine many years ago and chop the overwind out and fit a EHT multiplier. I used one from a scrapped Thorn 1500 that my Dad found in a skip . It should work a treat. Mine did - this was about 25 years ago!

All the best,

Tas

[rank-bush-murphy]

Thanks Steve; managed to find overwind from scrap A774 and fitted that and now have EHT on both standards

[rank-bush-murphy]

Now got this set running with an A774 EHT overwind, but it has another annoying problem when switched on from cold.

The picture is small and has low height and line cramp on left side: it stays like this for about 20 to 30 mins, then gradually fills to full picture.

The brightness is OK; I suspect the smoothing caps.
Have changed line output valve with new one, but still the same.

If anyone has any good s/h or NOS smoothers, I would be interested, or if anyone thinks this fault is caused by something else.

When the pic is full it has a very faint hum bar running through picture and hum on sound.

[Steve_P]

It could be the smoothers, but check the diode as well. Also, you could have a lazy PCL805.

Cheers,

Steve P

[Hunts smoothing bomb]

I agree with Steve,

It sounds like lack of height is the PCL85/805. I once had one in my KB set that for some reason stretched about two scan lines over two inches of the screen at the bottom of the picture, rest of the screen was fine though......Weird!

Good luck
Lee

[rank-bush-murphy]

How does the pcl805 account for lack of width and line foldover?

I must admit the height is also reduced for first 20 mins - this is what makes me think HT supply is low and my suspicions about smoothing caps, as when it has filled the screen there is slight mains hum through speaker and a hum bar that runs down the screen.

If you change channel the picture doesn't appear to ripple, just has a hum bar.

[Hybrid tellies]

There was two main smoothing caps on this chassis. As they face forward any leakage around the tags or any other bulging can be easily seen.
If the HT voltage only builds gradually from cold, then this is another sign of a tired smoothing cap. I have the full service manual for this wonderful set so any problems let me know.

[Welsh Anorak]

Hi Simon
Remind me of what values they are as I have a load of NOS smoothers!
Glyn

[Hybrid tellies]

Have just dug the service manual out. Values as follows;
The large can is 3C43 and 3C42 both 300uf 300vw
The smaller can is 3C44 and 3C45 both 32uf plus 3C46 which is 100uf, again 300vw. Hope this is of help. I will keep the manual to hand if further assistance required.

[rank-bush-murphy]

See attached pics of set working with a774 overwind.

Although it took 15 minutes to fill the screen, I still think I need smoothers.

[Mikey405]

Hi Simon.

I'm glad you got a picture on the Murphy - That is fantastic.

Kind regards.

From Mike.

[Welsh Anorak]

Hi
Sorry - no sign of the correct smoothers, I'm afraid - I'll keep searching!

Glyn