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Old 6th Oct 2017, 9:01 pm   #1
Niechcial,Steve
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Default KB VC1 width problem

I picked this set up on Ebay and it looks like it has been dry stored and is generally in excellent condition. The problem that is driving me mad is the width slowly creeping in on both sides. After an hour of operation it's in about an inch on both sides on both standards.
I've checked by substitution every component in the output stage except the LOPT and the scan coils (tried several new valves). Scoping the drive to the PL36 shows it remains stable throughout. No drop on the HT line
I'm beginning to think it must be the LOPT but I've never known one do this. The EHT is good with no blooming.
Anybody got any ideas?
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Old 6th Oct 2017, 9:09 pm   #2
Phil G4SPZ
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Default Re: KB VC1 width problem

Is the EHT stable and not rising?
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Old 6th Oct 2017, 9:55 pm   #3
Niechcial,Steve
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Default Re: KB VC1 width problem

I haven't put a meter on the EHT , but it seems to be stable. Boost voltage drops.
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Old 6th Oct 2017, 10:35 pm   #4
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Default Re: KB VC1 width problem

Quote:
Originally Posted by Niechcial,Steve View Post
I haven't put a meter on the EHT , but it seems to be stable. Boost voltage drops.
Are you saying that the boost voltage drops over the 1 hr time frame ?

Since the set would cool down over a shorter time frame than 1hr, turn the set off say for 10 minutes, let it cool, turn it back on, is the raster still 1 inch in on either side ? This might help determine if it is a temperature related issue or some other issue that is time dependent.

Can you post the circuit of the line output stage & yoke ?

Obviously one of the circuit constants is changing, and you have ruled out the grid drive voltage, HT and most of the components.

If there was moisture absorption in the EHT overwind in the LOPT, and the EHT was increasing with time over 1hr, then in theory at least, not only would the width decrease with time, so would the height (the whole raster scan)...so is the height stable or changing over the 1hr, or not ?
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Old 7th Oct 2017, 5:41 am   #5
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Default Re: KB VC1 width problem

Almost certainly the LOPT. Very common with these old receivers that the damp has not been driven out by regular use.

You could try passing a small current via a variable power supply through the overwind for a couple of days. Adjust the voltage so that the overwind is just warm after 30 mins. John.
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Old 7th Oct 2017, 11:29 am   #6
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Default Re: KB VC1 width problem

HKS wrote:
"You could try passing a small current via a variable power supply through the overwind for a couple of days. Adjust the voltage so that the overwind is just warm after 30 mins." John.

Do as John suggests and I reckon you could save the line output transformer.
Start of with about 20 volts, measure the current through the overwind, 20mA should be sufficient to warm up the transformer windings.

DFWB.
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Old 8th Oct 2017, 1:45 pm   #7
Niechcial,Steve
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Default Re: KB VC1 width problem

Thanks guys. I'll try the transformer warming and let you know how it goes.

Steve
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Old 8th Oct 2017, 3:37 pm   #8
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Default Re: KB VC1 width problem

I heard somewhere that the VC1 lopt is different to the later Vc4-53 series chassis.
I would expect the overwind would be the same though.
If warming it up doesn't cure it you could try a replacement from a similar donor transformer. I have had some success with replacing the overwind on a duff transformer. I replaced the overwind on a dual standard GEC transformer with one from a later single standard transformer. The same with a Bush A640 transformer the overwind from a A774 fitted and worked perfectly.

Rich.
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Old 9th Oct 2017, 8:11 am   #9
Niechcial,Steve
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Default Re: KB VC1 width problem

Thanks- that's a good idea, but unfortunately I don't have any suitable LOPTs in stock.
I will slow cook the overwind for a week. If that makes no difference I will do some checks to verify it is actually the overwind causing the problem. I have a stand alone variable EHT generator so I can hook that up with the set's overwind disconnected and see what happens.
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Old 9th Oct 2017, 4:09 pm   #10
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Default Re: KB VC1 width problem

Quote:
Originally Posted by Niechcial,Steve View Post
I have a stand alone variable EHT generator so I can hook that up with the set's overwind disconnected and see what happens.
Just disconnecting it won't do I'm afraid. It needs to be completely removed from the transformer as a sure test. I can't remember if it is a separate winding or wound on top of the underwind. I think the former making it possible to remove without damaging the rest of the LOPT. John.
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Old 9th Oct 2017, 9:29 pm   #11
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Default Re: KB VC1 width problem

Width control in the VC1 chassis was achieved by adjusting the value of the resistors in the anode supply to the boost diode, a common practise in a lot of dual standard and convertible sets made around 1961 to early 1963. It was same in early Pye model 3 and 11 models. All STC VC2 and subsequent dual standard sets employed stabilised line output stages.
As a result to the difference in design to the line output stage the VC1 chassis employs a different line output transformer. The VC2/3/4/51 series chassis all use the same type of LOPTx.

DFWB.
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Old 23rd Oct 2017, 7:38 am   #12
Niechcial,Steve
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Default Re: KB VC1 width problem

Unfortunately cooking the overwind has not made any difference, so I'm now trying to remove the overwind from the LOPT as suggested. I will then run the set measuring the boost voltage and see whether it still drops, or remains stable.
Unfortunately, having taken the lopt out, I can't see how to split the rectangular core. It doesn't look laminated, neither is it bolted together like modern ferrite cores. Any ideas please?
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Old 24th Oct 2017, 12:54 am   #13
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Default Re: KB VC1 width problem

Hi there,

Could you please forward some photos of the Lopt so we can advise further?
Sorry to hear your efforts didn't work.

Cheers
Brian
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Old 24th Oct 2017, 5:40 am   #14
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Default Re: KB VC1 width problem

It seems that the primary problem is that it has been proposed on this thread that the changes in scan width is due to unstable EHT voltage, and now the EHT overwind is attracting a lot attention. It has already been cooked with no effect, that says something too, likely it is fine.

But the whole idea that changes in the overwind or EHT voltage causing ONLY a change in deflection width doesn't make a lot of sense does it ? I asked the question on post 4, is the Height changing as well as the width?

If it is not then you can assume a lot here; firstly that it is unlikely the EHT voltage has anything to do with the fault, if the EHT was going up and down the whole raster size would be changing geometry, not just the width. Also, if the height is stable this says a lot about the stability of the HT voltage in the set.

You can check the boost voltage stability without having to pull the LOPTY apart which might be an unnecessary move.
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Old 24th Oct 2017, 5:50 am   #15
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Default Re: KB VC1 width problem

From my experience with these chassis, I agree with the conclusion that the LOPT is faulty.

The ferrite material core will separate; gently warming it with a hair drier may well help. The two halves of the core will almost certainly have a very thin gap spacer fitted - be careful not to lose these, as the LOPT will not work correctly without them.
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Old 24th Oct 2017, 7:29 am   #16
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Default Re: KB VC1 width problem

Quote:
Originally Posted by dazzlevision View Post
From my experience with these chassis, I agree with the conclusion that the LOPT is faulty.
Assuming the EHT is stable, based on the fact that no change in the height of the raster has been reported (yet), wouldn't it be more likely that the B+ boost voltage was unstable due to a problem in one of the components associated with that ? On post 3 it was said that "boost voltage drops", but I don't know if that was confirmed.
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Old 24th Oct 2017, 8:36 am   #17
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Default Re: KB VC1 width problem

I still think it is the LOPT overwind that is causing the problem.

The overwind is a pitch insulated assembly that allows moisture to enter and I suspect it also alters it's character. It gave little trouble in it's service life but of course we have moved on 55 years!

Can you measure the actual EHT voltage? I suspect it is dropping with the decrease in width.

The HT supply to the height control is derived from the boost rail stated in the VC1 manual as being 670v. Does this vary?

Reducing the actual H.T. line on my VC1 causes the height and width to reduce due to lack of stabilization of both the boost voltage and the supply to the frame oscillator usually taken care of by a couple of VDRs.

I presume you have used a serviceable capacitor for the boost. C126 .1uf 1000v.

Regards, John.
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Old 24th Oct 2017, 9:58 am   #18
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Default Re: KB VC1 width problem

Quote:
Originally Posted by Heatercathodeshort View Post
I still think it is the LOPT overwind that is causing the problem.

Can you measure the actual EHT voltage? I suspect it is dropping with the decrease in width.

Regards, John.
Its the other way around. (others should chime in on this)

The amount of deflection (in magnetic deflection) is inversely proportional to the square root of the EHT voltage.

This means that if the EHT value goes higher, all other things equal, the deflection (both width & height) will be lower. Or if the EHT drops, the raster will be bigger.

This is why in some sets when the EHT regulation is poor, as you turn up the contrast or brightness and the CRT beam current increases and the EHT voltage drops, the picture blooms to a bigger size.

The problem still could possibly be with the LOPTY overwind though, but some of the data here is not perfectly consistent with that, but it doesn't hurt to rule it out.

Lower width means lower yoke current at the end of scan. The line output stage is powered by a combination of the HT and the B+ boost added to that.

Presumably the HT is stable or checked, that leaves the B+ boost voltage to check. The drive to the output valve has been checked I think.
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Old 24th Oct 2017, 6:57 pm   #19
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Default Re: KB VC1 width problem

We all know that but this lack of width and low EHT fault is due to saturation of the transformer reducing both scan power and EHT at the same time. The transformer will usually be very warm to the feel after 15-30 mins.
This is a very common fault in vintage receivers and has been proved many times by Forum members. I must have experienced it at least 50 times in the last 20 years.J.
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Old 24th Oct 2017, 7:51 pm   #20
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Default Re: KB VC1 width problem

Hi John,
Do you have any theories as to exactly how the transformer is failing? Obviously heat is the key element here: everything is fine when the transformer is cold but what is happening within the LOPT as it gets hot? I guess the resistance of the windings will increase, but I wouldn't think that would affect things much. Presumably shorted turns would have a much more significant effect than a small loss of width? You mention core saturation- does that get worse the hotter it gets?
Some LOPT's do seem to get worryingly hot after 1/2hr or so. Some of the Ekco chassis from the late '50's (of which I have several)have a 2" hole under the LOPT which will take a small cooling fan. This dramatically reduces the temperature of the transformer and hopefully increases their lifespan!!
All the best
Nick
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