TV22 metrosil replace with diode?

[Boom]

The TV22's picture is smearing at high brightness and I have found that with a digital meter connected to the CRT A1 that I only have 111V. (The set has been recapped) I had a spare metrosil but that one only brings the volts up to 168V.

My chances of finding a good metrosil must be zero so can anyone recommend a suitable diode that I could fit in this position?

From what I can gather the voltage should be up in the 300+ range but I am not sure how much the loading a digital meter would drag this down by.

[af024]

Hello Dave,

A good question. I must admit that I also suspected my methods of trying to measure the voltage (ie dragging it down).

I did wonder whether there might be a neat direct VDR solution, but there's always this (shown at the bottom of the page) courtesy of Andy Beer. I haven't tried it myself, but it seems perfectly logical. How wlel it needs to regulate though I don't know.

http://freespace.virgin.net/andy.valve/techa.htm

Regards,

Andy

[Boom]

Thanks, I'll try that once I locate the 4007s or a vdr. I found a box of 4001s and put 7 of them in series and switched the set on. I ended up with a whopping 536v A1 and turned it off quick! A metrosil must be an extremely inefficient device.

[Boom]

With immense thanks to Bill AC/HL the VDR (type BRC1500 green paint spot) arrived today and I soldered it into the TV22 in place of the metrosil.

Ever the pessimist I wasn't convinced it would work but lo and behold as the set warmed up a whopping 300V appeared at the tubes A1 as measured on my digital meter. (The duff metrosils output was down to 111V.) The picture was now much brighter and clearer.

The A1 voltage varied slightly depending on picture content and settings but it is clear the VDR does the same job that the metrosil was supposed to be doing.

I couldn't find an answer to what the voltage should be from a good TV22s metrosil into a digital meter load and was put off by the circuit description of the metrosil as a 'high impedance device' which didn't really mean a lot to me but it seems that even with meter loading there should be around 300VDC on the A1 if the metrosil is ok.

I still don't really understand how a VDR works as a rectifier so one other thing I couldn't resist doing was to swap ends over and try the VDR in the circuit the other way around. I was expecting it to give a negative voltage at the A1 but I was totally flummoxed when it still gave a positive voltage from the other end. I.e the VDR gives a positive output no matter which way round it is soldered in like a bi polarity diode.

Many thanks once again to Bill.

[line sync]

Very interesting thread dave.
I wonder how many collectors could do with a replacement metrosil for their Bush TV22's and how few scrap Thorn 1500 chassis are about to take a VDR off.

It would be nice to find a replacement from RS or Farnell.

[Ian - G4JQT]

Does anyone know the part number for the BCR1500 TV VDR? Or even the parameters/specification/part number for the one used in the Bush TV22/TV24? Or know where this information may be found?

It's not referenced in the Bush documentation as far as I can see.

Thanks,

Ian

[Mikeymushradio]

Have a look here, it works too! I have done this mod to mine many moons ago HTH.


http://freespace.virgin.net/andy.valve/techa.htm

Mike

[Ian - G4JQT]

Thanks Mike. I've seen that circuit, but it doesn't appear to have any voltage-dependent-resistor characteristics at the high voltages in which it will be used. Unless the diodes working into the 2M ohm are operating on the non-linear portions of their characteristics.

When it does work, does it work as well as a good metrosil? And if if so, I'd like to know how.

Thanks.

Ian

[ENGLISH VICTOR]

Dave,
Metrosil is a proprietry name for a vdr and works like this: A vdr is a device that only conducts above a certain voltage in either direction. In the TV22 the voltage fed into it is a positive pulse along with some d.c. component adding up to about 500Vpositive peak, the VDR does not conduct until the voltage reaches say 200V so only the remaining 300 V actualy gets through to charge the 0.1uF that is connected to the CRT A1 and chassis. As the vdr conducts in either direction you can wire it either way round, remember the voltage being fed into it is positive going only so the 0.1uF is only receiving short bursts of positive pulses plus some positive d.c.
Hope this is of some use to you.It seems the replacement vdr you are using must have about the same characteristics as the original metrosil.
Good luck.
Victor

[dominicbeesley]

This circuit always struck me as "funny". Given if A1 draws any substantial current then there would be a darkness gradient on screen. If it draws little or no current then any leakage in the metrosil would charge up the capacitor to full voltage...

Do we have a current/voltage curve (or current / resistance) for the Metrosil? I'm just wondering as I've got a pile of unknown VDRs in a box somewhere and it would be interesting to see if any would be suitable replacements...

D

[ENGLISH VICTOR]

D
There would be no brightness gradient across the screen because the 0.1 uf would not be able to discharge sufficiently over the line period to modulate the A1 supply. I should mentioned that the start of conduction of the VDR and the D.C. component of the pulsed voltage to the should equal the desired final voltage i.e. 300V. Thus if we need 300v and the D.C. is 250V the conduction of the VDR would need to start at 50V. The figures I gave in my earlier post were in fact poorly chosen and would possibly not work, the pulse voltage in the case of the TV22 is possibly quite low say only a hundred or so volts above the boost voltage. The VDR only boosts the the A1 voltage by a small amount to reach the target figure of 300v. In early days the boost rail rarely ever reached more than about 50 or 60 volts above the HT line due to poor efficency cores in the line output transformers and scanning coils and the circuit configurations with low heater cathode insulation of the booster diodes used. Mullard stipulated an A1 voltage higher than was easily obtained thus the wierd circuits sometimes seen. Take a look at the Bush TV 11A it employs a resistive voltage divider across the EHT to obtain the voltage.

The current the A1 of the crt draws only a few microamperes. with leakage through the VDR the voltage would cause the voltage across the 0.1uF to fall a little not charge up to the full peak voltage. It is the non linear conduction of the VDR that makes the function as it does. Caution is needed when selecting VDRs by trial and error in case the peak pulse voltage causes over heating of the device, in the case of the TV22 the pulse voltage is not very high, even so I would incline towards puting a series limit resistor in circuit of a 1000 Ohms or so in case you try a vdr with a voltage of 15 volts or so as sometimes is the case or the VDR under test is in fact a thermistor.
Sorry my explanation is not is not as clear as it might be.
Enough before I end up in hot water.
Victor.
Victor.

[PJL]

Just summarising what Victor has said. The signal applied to the Metrosil has a DC component of the boost volts (280?) with an AC signal whose main component will comprise a large +v going flyback pulse. Initially the VDR will conduct during the +ve flyback pulse to raise the A1 voltage above the boost volts until the VDR conducts in the opposite direction thus creating an A1 voltage of Boost + VDR conduction voltage.

The VDR should be more stable than the replacement circuit which has quite a large potential divider (1/2.5) so is very dependent on the flyback voltage.

[dominicbeesley]

Thanks, that makes more sense. I wonder what the actual A1 voltage is supposed to be when its not being dragged down by measurement, I'm sure it can't be 300V otherwise why bother with the complication of the Metrosil when the boost voltage is 280V it would be easier to run A1 at that voltage and drop a few volts off the cathode?

[ENGLISH VICTOR]

Same thoughts here, my service manual states 300V and subscript with a note that the voltage must be measured with an electrostatic voltmeter. It seems strange that they have added the extra component just for an extra 20V but I guess they had a good reason even to ensure the CRT was operated exactly as Mullard suggested. In the scheme of things the extra cost would have been minimal just the metrosil and perhaps the 0.1uF cap from A1 to ground.
Victor.

[dominicbeesley]

I've had a read through of the datasheet for the CRT and it looks like it will work with anything from 160 to 300V on A1 (with an adjustment of g/k voltage)....I wonder if it is more to do with somehow adjusting the A1 voltage in sympathy with the EHT on dark/bright scenes to maintain focus and brightness?

[PJL]

I had also wondered about that which would also apply to mains voltage correction. If that's the case it is probably only conducting on the +ve pulses.

[FERNSEH]

Reference to my thread about the restoration of a Bush TUG24.

The A1 volts were low in this set. First suspect was the decoupling capacitor and although it had a >5Mohm leak it was replaced.
The A1 volts rose to 123V. Still too low. It is certain that the Metrosil is faulty. A BYX10 silicon diode was tried and sure enough the A1 rose to 430 volts! Much too much for this set. The correct figure is 300V.
So it follows that a fast silicon diode can be used as a replacement but if one is used a potential divider will have to wired into the circuit in order that correct A1 voltage is realised.
The voltage dependant resistor is the best replacement component.

DFWB.