Topics

High-Risk Ceramic Strip Locations in Types 535/545


Dave Wise
 

PSA: CERAMIC STRIP LOCATIONS WITH RISK OF ARCING IN TYPES 535/545

David Wise
October 2020

1. Delaying Sweep chassis, lower left corner. The two rightmost terminals on this strip are +500V and ground respectively.
Symptom is a snap and a blown V794 (12B4).

2. Secondary High-Voltage compartment, rear. The two center terminals on this strip are -1350V and -400V respectively.
Types 531/541 also have this problem.
Symptom is dim or absent trace.
Tek could have ameliorated it by simply kinking the resistor lead and using another terminal.

I cleaned them, grinding off ceramic to break tracks already built.
Then I coated both terminals and the space between with Corona Dope.
This should block silver migration in the future.

There's a third spot, in the power supply, but it blew before my time; the tech removed the wires from the ruined terminal and wire-nutted them in the air.

There are a number of other high-potential spots (keep an eye on the Horizontal Amp) but these are the worst.


Tom Lee
 

This is very helpful information, David. If you haven't already done so, perhaps sending it on to tekwiki would be worthwhile.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 10/20/2020 10:51, Dave Wise wrote:
PSA: CERAMIC STRIP LOCATIONS WITH RISK OF ARCING IN TYPES 535/545

David Wise
October 2020

1. Delaying Sweep chassis, lower left corner. The two rightmost terminals on this strip are +500V and ground respectively.
Symptom is a snap and a blown V794 (12B4).

2. Secondary High-Voltage compartment, rear. The two center terminals on this strip are -1350V and -400V respectively.
Types 531/541 also have this problem.
Symptom is dim or absent trace.
Tek could have ameliorated it by simply kinking the resistor lead and using another terminal.

I cleaned them, grinding off ceramic to break tracks already built.
Then I coated both terminals and the space between with Corona Dope.
This should block silver migration in the future.

There's a third spot, in the power supply, but it blew before my time; the tech removed the wires from the ruined terminal and wire-nutted them in the air.

There are a number of other high-potential spots (keep an eye on the Horizontal Amp) but these are the worst.







tek_547
 

Thanx for the info David. I had the same experience with trace intensity variations on a 547.
After several hours searching and measuring it was a voltage variation on the grid of V800, and so increasing the HT. Inspecting of the ceramic strips around I saw a kind of black sooting at the solder joints so time to clean them all with IPA.
After that the voltage around V800 was stable and also the intensity, so happy at all that it was not the HV transformer.
I think it must be that after warming up the scope the sooting gets conductive wich give voltage variations at the grid of V800.
grtz and stay safe, René


Dave Wise
 

535-545-Ceramic-Strip-Flashover.zip uploaded to tekwiki at w140.com . I figure Kurt will put it in the 500-series Scopes section but I don't want to second-guess our esteemed curator.

Revised, expanded, with photos.

Regards,
Dave Wise
________________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Tom Lee via groups.io <tomlee=ee.stanford.edu@groups.io>
Sent: Tuesday, October 20, 2020 10:54 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] High-Risk Ceramic Strip Locations in Types 535/545

This is very helpful information, David. If you haven't already done so,
perhaps sending it on to tekwiki would be worthwhile.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 10/20/2020 10:51, Dave Wise wrote:
PSA: CERAMIC STRIP LOCATIONS WITH RISK OF ARCING IN TYPES 535/545

David Wise
October 2020

1. Delaying Sweep chassis, lower left corner. The two rightmost terminals on this strip are +500V and ground respectively.
Symptom is a snap and a blown V794 (12B4).

2. Secondary High-Voltage compartment, rear. The two center terminals on this strip are -1350V and -400V respectively.
Types 531/541 also have this problem.
Symptom is dim or absent trace.
Tek could have ameliorated it by simply kinking the resistor lead and using another terminal.

I cleaned them, grinding off ceramic to break tracks already built.
Then I coated both terminals and the space between with Corona Dope.
This should block silver migration in the future.

There's a third spot, in the power supply, but it blew before my time; the tech removed the wires from the ruined terminal and wire-nutted them in the air.

There are a number of other high-potential spots (keep an eye on the Horizontal Amp) but these are the worst.