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Old 3rd Mar 2013, 5:27 pm   #1
Heatercathodeshort
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Default Vidor 9" table model model CN369A. 1948.

It's time to restore a proper television receiver. A real 'truckers' telly. A 9" table model from early 1948, Vidor's first post war model. Nice lethal mains EHT..Gas relays for timebase generators.. [Thyratrons] and enough different valve bases to confuse the man from Mazda.
The build quality is incredible with the I.F. [yes it's a superhet] sound amplifier/output and video stages mounted on a raised chassis supported by thick metal stilts above the main chassis. The EHT rectifier sits on a dias above the substantial mains transformer and it's uninsulated top cap could be a severe and lethal hazard to the unwary.

At this point I must say that if you are not familiar with the dangers of television receivers with mains derived EHT systems, you should stay well clear of them as they present a lethal shock hazard. Great care must be taken. They are a silent killer.

This delightful little receiver was kindly passed to me recently by another vintage television enthusiast with the warning that the wiring was in a bad way and the tube was U/S.
First examination proved this to be so with wires that were once protected by India rubber, now fully bare and exposed. To be honest it did not look too bad and other Forum members, myself included have tackled receivers in far worse condition than this one.

I like to get first light as quickly as possible so the chassis was removed for a very quick check to make sure there were no obvious short circuits. The chassis top was covered in what looked like an exploded rabbit, matted with dust, oil, grease and dead insects. This is a great sight and proves the chassis has not been attacked by the metal buffers and wrong connection society. It also coats the metal work with a protective layer that once cleaned off presents a nicely aged chassis that can be worked on with pleasure.

I pulled the H.T. rectifier valve and connected my croc clips across the mains input. The original mains lead resembled a dried up snake and even with my bravado decided it was definitely only fit for the bin, shame.
I connected my EHT meter across the EHT supply and with just a slight hesitation hit the power switch. With the workshop lights extinguished I could see the valves begin to glow but my eyes were more interested in any purple antics that may be about to start inside the anode assembly of the Mullard HVR2 B4 based EHT rectifier. This sits as mentioned earlier in a lonely spot on top of the massive mains transformer. It's heater appeared very dim but low and behold the meter began to rise to record 6kv steady. Rather disconcertingly the 6kv remained when the power was switched off revealing that the chain of resistors known as the 'bleeder network' across the EHT supply had gone O/C as they always do, leaving the .1uf capacitor fully charged to 6kv, just waiting for you to grab hold of it..
The EHT/mains transformer was manufactured by PARMEKO and is of military quality. I have never seen a transformer in a domestic receiver of such quality. The smoothing capacitor is also an excellent DUBILIAR .1uf 7kv and I have never known one of these to fail.

Now at this point with no H.T. available a defocused spot about three inches in diameter should appear on the tube face that will be unaffected by any of the controls.
Nothing, and a visual check of the heater confirmed it was not lit. The AV0 8 proved the Mullard MW22-14C's heater to be completely O/C so it was decided to remove it and tidy up some of the thick goo on the top of the chassis.
I used commercial truck engine cleaner applied with a soft toothbrush, mopping up the liquor with paper. The workbench looked more like the aftermath of an autopsy with slop everywhere. [Must be mad?]

The next job was to replace the five 4.7M and one 270K across the EHT supply. They were all O/C and probably had been for most of the receivers life.
With this completed and the mains applied, 6kv appeared and died away within five seconds of switch off, a lot safer.

The next job will be entering my tube loft and snatching a 9" Mullard tube. It's quite a scary experience as it's gloomy up there and they all tend to gang up on me..
Hopefully this will produce 'First Light' and with the timebases sorted it will be time to replace some of that rotten wiring.

This is not going to be a quick job like David's Masteradio and it might drag out a bit so you will have to be very patient.

Regards, John.
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Old 3rd Mar 2013, 5:41 pm   #2
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Default Re: Vidor 9" table model model CN369A. 1948.

Good work so far John, looking forward to the next instalment!
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Old 3rd Mar 2013, 6:03 pm   #3
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Default Re: Vidor 9" table model model CN369A. 1948.

Could the charge alone from the EHT smoothing cap on a mains EHT set still kill, once mains was off if discharge path was O/C?

I'm a sparky by trade and respect electricity but the thought of (n)ever working on a mains EHT set frightens the *@%> out of me!

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Old 3rd Mar 2013, 6:46 pm   #4
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Default Re: Vidor 9" table model model CN369A. 1948.

What a great set and previously untouched! Were Vidor behind the times producing a mains EHT set in 1948 or was it still common? I once owned a Philips 663A from 1948 and got excited when I saw the large mains transformer and double HVR2s, then discovered it was an oscillator circuit.

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Old 3rd Mar 2013, 7:55 pm   #5
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Default Re: Vidor 9" table model model CN369A. 1948.

You would be very unlucky if the charge from eht smoother would kill you.Its not relevant what has charged it,mains or flyback generation.
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Old 3rd Mar 2013, 8:40 pm   #6
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Default Re: Vidor 9" table model model CN369A. 1948.

Flyback at 10kc/s only requires a capacitor in the region of .001uf. Mains 50c/s supply requires at least .1uf at 6kv and that can deliver a hell of a kick across the body. It's a real firey crack when shorted out.
Not to be experimented or messed with. John.

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Old 3rd Mar 2013, 8:49 pm   #7
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Default Re: Vidor 9" table model model CN369A. 1948.

Quote:
Originally Posted by Magic Eye View Post
Were Vidor behind the times producing a mains EHT set in 1948 or was it still common? Jon
Not really Jon.
Pye released their AC/DC set, the B18T with flyback in September 1948. This was followed the following year by many companies but HMV continued with one model until 1954! Ekco used a fearsome voltage doubler supply from a 6kv mains transformer again late in 1950/52/53. [15" models]Very odd because they had excellent success with early flyback receivers.
1949 was the year when it started to fade away quickly and by 1950 most had gone over to flyback or for just a year or less the unpredictable separate R.F. oscillator. Regards, John.
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Old 3rd Mar 2013, 10:32 pm   #8
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Default Re: Vidor 9" table model model CN369A. 1948.

There is a lot of people out there who tend to dismiss Vidor radio and TV products as of low quality and of little interest. The truth is Vidor has real heritage because these sets are really a legacy from from Burndept whch was a respected make before WW2. Vidor pulled out of TV manufacturing in 1955, the company made some 13 channel sets ready for the start of ITV.
You can read about the Burndept-Vidor story in the Setmakers book, pages 199 to 203.
Always discharge the EHT capacitors before working on any TV receiver which has mains derived EHT. There is a lot of power in a 0.1microfarad capacitor with a charge of 5000 volts, and many sets have two EHT capacitors.

DFWB.
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Old 4th Mar 2013, 8:29 am   #9
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Default Re: Vidor 9" table model model CN369A. 1948.

I cannot stress this point more strongly David. Mains derived EHT is lethal but perfectly safe in the right hands with knowledge of the dangers.
Flyback EHT has very poor regulation and tends to collapse when a heavy load such as the engineers hand shorts it out! A small burn usually results and in fact the same process is used in hospitals to seal blood vessels. [I expect they have something more manageable than a Pye V4]
With a 50c/s low impedance supply it does not collapse producing a dangerous shock situation and a very nasty deep burn.
An old service guy friend of mine, now sadly no longer with us used to show me the badly distorted end of his thumb that he had managed to get caught on the top cap of a HVR2 rectifier. I was a young lad at the time but had been playing around with PYE D16T's for quite a while. I was aware of the dangers but that thumb certainly made me even more cautious. I don't want to put guys off but this is not TV22 stuff. If your not aware or are in any doubt, leave these early receivers alone for a while. We don't want the guys in yellow coats to divert all our old junk to the crusher do we?
Regards, John.
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Old 4th Mar 2013, 10:59 am   #10
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Default Re: Vidor 9" table model model CN369A. 1948.

Hi John,
Has your vidor TV got the channel switch for London and Birmingham?
I Know the CN377 has.
Returning to H and S matters. Remember a 0.1microfarad capacitor charged up to 5000 volts will initially release 10mA if a resistance of 500Kohms is connected across it. 500Kohms is the typical skin resistance of humans so with that kind of current flowing though someone it would be enough to send you off to Valhalla.

DFWB.

Last edited by FERNSEH; 4th Mar 2013 at 11:05 am.
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Old 4th Mar 2013, 4:51 pm   #11
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Default Re: Vidor 9" table model model CN369A. 1948.

Mr Blooby has arrived!
As you can see by the pictures I have attempted to get 'first light' but it was not so simple as I thought. I must say I think it is the most uneventful first light I have ever been presented with but it is a start.

I unearthed an unknown Mullard MW22-14C and tackled some of the rotten wiring. This was not so bad but the workbench surface looked as if I had crunched a whole tube of Smarties over it.

I used stripped out rubber flex and the recovered innards was just right. I did the minimum required. I removed the timebase valves and very roughly fitted the tube into place. With the H.T. rectifier removed and the mains applied, nothing! Then it all fell into place. The Mullard tube is a tetrode requiring a bit of H.T. on it's first anode to produce anything at the flat end...
With great caution I fitted the 5Z4G rectifier [It should be the much beefier GZ32] and switched on while metering the H.T. line. It slowly crept up to around 100v and what do you know, the expected blob appeared proving the tube has probably got a bit of emission left.

The four huge H.T. electrolytics are all completely U/S and will require restuffing. That will be the next job when time permits.
Regards, John.
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Old 6th Mar 2013, 1:20 am   #12
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Default Re: Vidor 9" table model model CN369A. 1948.

hi John,
I reckon that you will have your Vidor set up and running long before my Masteradio T851. I'm having problems with the incremental switch tuner.

DFWB.
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Old 6th Mar 2013, 5:29 pm   #13
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Default Re: Vidor 9" table model model CN369A. 1948.

No excuses David, I reckon yer chickened out. Now you know why we kept Masteradio and McCarthy out of the south.. John.
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Old 6th Mar 2013, 6:13 pm   #14
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Default Re: Vidor 9" table model model CN369A. 1948.

Why are small things such a pain? Short guys, Small dogs and 9" television receivers.

Well I just 'ad to 'ave a go at it this afternoon. I removed the four large smoothing electrolytics from the bottom right corner of the chassis and decapitated them. I dug out the 'contents' which resembled the curry you consumed at 11pm that returned to haunt you at 11.45.
I have fitted modern replacements inside the old cans and they don't look too bad.

With the mains reconnected the H.T. was now around 200v and a much better 'blob' appeared on the CRT face. It was decided to replace the very tired incorrect 5Z4G with a more suitable U52. This brought the H.T. up to around 280V with no funny smells or pops. It was time to insert the EL38 and it's gas relay T41 generator. The scope was connected to the anode of the T41 and the receiver powered up. No trace and Mr Blobby refused to move.

A few voltage checks were made around the Thyratron and it gave some very odd readings. The cathode voltage was very high, about the same as the anode. The reason for this was an O/C line hold slider in the cathode circuit being O/C. With a temporary pot clipped into circuit a rough saw tooth waveform appeared on the scope and a poor line scan appeared on the tube by way of a thick horizontal line. It was very ragged and replacing the timing capacitor 1uf made a big improvement.
The brightness control refused to dim the raster due to being O/C. It is a Colvern 30K with a long spindle. Again a temporary pot was wired into circuit and this produced a very smooth brightness control action.

Due to a perished rubber sealing gasket the Dubilier .1uf 7000v EHT smoothing capacitor had a slight leak from it's base. Stripping it down and fitting a modified tap washer in place of the worn gasket soon cleared this up and the whole lot was reassembled and tested.

With the EL33 frame output valve together with it's T41 inserted I was hoping for at least a 1/4 inch scan but it was not to be.
A check around the EL33 base revealed a complete lack of voltage on the screen grid pin 4. For a moment I thought I was checking pin 3 the anode and had nasty thoughts of O/C frame output transformers but luck was on my side.
The screen is fed via a 3.3k from one of the H.T. lines and decoupled with a 4uf capacitor. This was S/C and a replacement produced a sort of bobbing scan.
At least it was something!
With a signal applied there is almost a test card but the I.F. decoupling must be O/C as it is covered with herringbone patterns.
The rear slider controls are all U/S and will have to be dealt with.
Watch this space.
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Old 6th Mar 2013, 6:16 pm   #15
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Default Re: Vidor 9" table model model CN369A. 1948.

A few more pictures of the little horror! J.
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Old 6th Mar 2013, 10:01 pm   #16
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Default Re: Vidor 9" table model model CN369A. 1948.

Hi john
This vidor is causing you a bit of trouble.
Great work so far as always.
You do a very neat job of rebuilding the electrolytic cans, I also go to this trouble and have various ways how I open up the can.
I do somtimes worry about has the new capacitor I'm fitting inside the can has got a high enough ripple rating.
I know the first electrolytic in the power supply after the rectifier will need a high ripple rating so I would always rebuild using a can type with solder tags.
Keep up the good work, I'm looking forward to seeing a picture on this set.

Robin
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Old 7th Mar 2013, 8:00 am   #17
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Default Re: Vidor 9" table model model CN369A. 1948.

Yes I agree about the ripple current Robin. I used a 16UF 500V 'surge proof' [well that's what it said on the can] for the reservoir. I do stretch a point here but with modern capacitors of good repute, they seem to hold up very well. These 47uf 450V were removed from an almost new switch mode power supply that has yielded eight so far!

These are all normal age related/storage faults and quite straightforward. It's more a case of working out exactly what is going on due to faults in every section of the receiver but I think I may have the better of it now.
The little Vidor certainly appears to have given very good service to it's owners back in 1948. I wonder how many crowded into the living room back then to watch the Coronation on it's tiny screen?
I'm busy over the weekend so it will have to wait for a while. Cheers, John.
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Old 7th Mar 2013, 9:38 am   #18
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Default Re: Vidor 9" table model model CN369A. 1948.

John, did you have to top-up any lost electrolyte in the leaking 7kV cap before resealing or was there sufficient in there?
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Old 7th Mar 2013, 9:52 am   #19
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Default Re: Vidor 9" table model model CN369A. 1948.

Hi John.

Looks like you're working for a living with this little Vidor! A great write up and progress so far.
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Old 7th Mar 2013, 1:24 pm   #20
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Default Re: Vidor 9" table model model CN369A. 1948.

Quote:
Originally Posted by newlite4 View Post
John, did you have to top-up any lost electrolyte in the leaking 7kV cap before resealing or was there sufficient in there?
Neil
Hello Neil,
It was just a bit damp with dust stuck to it. These 'condensers' are oil filled and of very high quality. Dubiliar were top of the list together with B.I.C. [British Insulated Callenders]. No oil loss as such.
These are completely different to the TCC type VISCONOL with the Bakelite case. These go leaky and just damp the EHT with flyback or R.F. supply but get very hot with a high voltage mains transformer supply. If this is not noticed they usually explode with a bang that will frighten you to death sending bits of Bakelite flying everywhere. [Glad you survived Peter from L.] Now tell me mains EHT is harmless..
I will tackle the I.F. deck when time permits. That way I can see what effect a signal has on the timebases. Such fun! John.
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