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453 EHT and CRT: trouble-shooting advice

 

I'm attempting to bring a 453 (FET's) back to life. There is no trace, nor sign of an "off-screen" glow. Both sets of deflection plates are at the correct potentials, and the heater is not O/c.

Initially, the EHT section did give the correct regulated outputs (-1.96/+8kV), but this must have been a transient condition, as it refuses (for the most part) to produce any EHT at all. After a prod around and the odd component substitution, I can see the start of an oscillation in the driver Q930, but this gently subsides over a second or two as the scope warms up. This has happened several times.

This suggests a possibility there is one secondary which is being loaded to the extent that the oscillator formed around the transformer does not have a loop gain of >1. I'd like to disconnect the EHT secondaries one-by-one, but I'm nervous at leaving the CRT connected in case the grid end up +ve.....

1. is it OK to remove the CRT base connector with the anode still hooked up?
2. Is it OK to remove both the CRT base and the final anode connection whilst powering the EHT section?

Cheers

John
 

Well, as there has been no response, I've bitten the bullet and found that it's fine to power the EHT section with the anode and tube base disconnected.

I think the problem lay in the EHT transformer: circumstantial evidence suggests an intermittent insulation break-down between the CRT heater winding and the "cold" end of another winding.

John

--- In TekScopes@..., "John S" <John@...> wrote:


I'm attempting to bring a 453 (FET's) back to life. There is no trace, nor sign of an "off-screen" glow. Both sets of deflection plates are at the correct potentials, and the heater is not O/c.

Initially, the EHT section did give the correct regulated outputs (-1.96/+8kV), but this must have been a transient condition, as it refuses (for the most part) to produce any EHT at all. After a prod around and the odd component substitution, I can see the start of an oscillation in the driver Q930, but this gently subsides over a second or two as the scope warms up. This has happened several times.

This suggests a possibility there is one secondary which is being loaded to the extent that the oscillator formed around the transformer does not have a loop gain of >1. I'd like to disconnect the EHT secondaries one-by-one, but I'm nervous at leaving the CRT connected in case the grid end up +ve.....

1. is it OK to remove the CRT base connector with the anode still hooked up?
2. Is it OK to remove both the CRT base and the final anode connection whilst powering the EHT section?

Cheers

John
 

John,
? Have you considered letting the filament winding float and supplying the CRT filament with a separate transformer winding biased to the minus 2KV supply.? That way?any intermittent short would not impact the EHT supply.? Not having played EHT supplies I don't know if it would take kindly to the change in loading (removal of the filament load) and you might replace it with a load resistor (not connected to the -2KV rail).
?
? I have seen toroid transformers with 3500V insulation ratings for about $10 + Shipping.
?
? The 453 and 454 to me are very fine instruments and worth the effort to maintain?(even to the point of altering the CRT filament supply.)? I especially like to sharp trace that I see on them and the 'A' versions.
Denis

From: John S
To: TekScopes@...
Sent: Monday, July 9, 2012 2:45 PM
Subject: [TekScopes] Re: 453 EHT and CRT: trouble-shooting advice

?

Well, as there has been no response, I've bitten the bullet and found that it's fine to power the EHT section with the anode and tube base disconnected.

I think the problem lay in the EHT transformer: circumstantial evidence suggests an intermittent insulation break-down between the CRT heater winding and the "cold" end of another winding.

John

--- In mailto:TekScopes%40yahoogroups.com, "John S" wrote:
>
>
> I'm attempting to bring a 453 (FET's) back to life. There is no trace, nor sign of an "off-screen" glow. Both sets of deflection plates are at the correct potentials, and the heater is not O/c.
>
> Initially, the EHT section did give the correct regulated outputs (-1.96/+8kV), but this must have been a transient condition, as it refuses (for the most part) to produce any EHT at all. After a prod around and the odd component substitution, I can see the start of an oscillation in the driver Q930, but this gently subsides over a second or two as the scope warms up. This has happened several times.
>
> This suggests a possibility there is one secondary which is being loaded to the extent that the oscillator formed around the transformer does not have a loop gain of >1. I'd like to disconnect the EHT secondaries one-by-one, but I'm nervous at leaving the CRT connected in case the grid end up +ve.....
>
> 1. is it OK to remove the CRT base connector with the anode still hooked up?
> 2. Is it OK to remove both the CRT base and the final anode connection whilst powering the EHT section?
>
> Cheers
>
> John
>

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