Viewmaster 12" Console set

[Sean Williams]

Thanks both.

I will try shuntng the focus magnet as John suggests.

I have also found that despite my previous statement, the CRT voltages are not as per design - the cathode and G1 volts are bots around the 230-250 mark - the brightness control has been modified in order to match the much higher DC voltage level that is coming from the Video Amp.

Boost voltage is however correct at 330 volts.

Cheers
Sean

[FERNSEH]

The cathode volts on the CRT is a bit high. In the ViewMaster circuit the anode of the video amplifier is connected direct to the cathode of the CRT.
Bias for the video amplifier valve was developed across a 15ohm resistor in the negative HT return but things could be different in the set under discussion.
The video amplifier valve might have a conventional cathode bias resistor.

DFWB.

[Sean Williams]

Hi David,

Yes, the arrangement in this set is very different.

The video stage is biased with a cathode resistor.

I have tried adding shunts around the focus magnet as per John's suggestion, this in fact makes matters worse, so I am inclined to think that the focus ring is working as it should.

I may have got the wrong end of the stick here, but on reading Spreadbury, there is specific mention of keeping the cathode voltage low to reduce spot size? in my mind, if the spot size is too large, then there will be no chance of getting a decent focus?

In this particular chassis life is somewhat more confusing due to everything being built on top of each other - almost looks like the vision interference diode was an afterthought.

I think I am going to have to draw some of this circuit out - I cannot help but think the cathode and grid voltages being high are not good for the tube.

[FERNSEH]

Hi Sean,
In essence, the CRT anode volts are low because the cathode volts are too high. Mullard recommend that the first anode volts should be >150V relative to the cathode volts. A good idea, at least for the time being will be to supply the CRT cathode from a resistor potential divider, say two 220Kohm resistors. A 0.1mfd capacitor should be connected across the upper resistor to pass the video signal. Not a nice arrangement for sure because the video DC component will be lost. The CRT grid voltage will have to be reduced.
Perhaps later on the video amplifier could be rewired to the original ViewMaster specification.

DFWB.

[Sean Williams]

Ah, yes, this makes sense.

I can easily reduce the grid voltage, by rewiring the brightness control as per the original design.

I might just take a look at modifying the video amp circuitry to match the Viewmaster design - I think for simplicity I can do away with the vision interference limiter that has been lashed into the current RF chassis - this will allow room for Ye Olde Sausage fingers to get around these horrible B7G bases....

Thanks for the clarification - time to go have a play

[1100 man]

This is turning into an interesting project! I was very glad it had gone to a Forum member so that we could all monitor its progress.

I was surprised they seemed to have gone to some effort to maintain the DC level of the video signal to the CRT. There didn't seem to be many sets that did this at the time.

Good progress so far though- hope you can sort the focus issue!

Cheers
Nick

[FERNSEH]

Circuit diagram of the ViewMaster video circuits.

DFWB.

[FERNSEH]

Circuit diagram of the video circuits of the HMV 2851 with suggested modifications to the video amplifier so that the circuit conforms to the design of the ViewMaster.

DFWB.

[Sean Williams]

Thanks David,

Im gonna leave this for during the week now - Having spent the past three days playing, domestic resistance is becoming apparent

[beery]

Hi Sean,
I'm glad you got this set. I saw it for sale and loved the fact that it has quite a good cabinet (ready made?). It is good that it went to a good home.
I'm currently bogged down in an unexpected house move, so no vintage tellies for me right now.

If you need a replacement permanent magnet focus unit, I have a few knocking around.
Regarding A1 voltage, well it is all relevant, so if the grid and cathode volts are high, then the A1 voltage needs to be higher by the same amount for the gun to work properly.
I look forward to hearing more when you get the chance to get back to the set.

Cheers
Andy

[FERNSEH]

Found it! Details of the ViewMaster Band 3 tuner.

January 1956 Practical Television, pages 359 thru to 363.

From the American Radio History website:
https://www.americanradiohistory.com...on-1956-01.pdf

DFWB.

[Sean Williams]

Thanks Both - No progress to report at present - real life is keeping me busy at present - maybe get some time on this over the weekend