Did that pot have a short trace of silver between the end of the resistor to the terminals? It can oxidize, and open. I've seen a lot of that type that failed, but they were mostly other brands. I don't like open frame pots, but they are cheap so a lot of companies used them in non-critical applications.
Michael A. Terrell
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-----Original Message-----
From: Adrian <Adrian@...>
Sent: Jul 28, 2018 4:43 PM
To: [email protected]
Subject: [TekScopes] Slightly OT - Bourns trimmer pot failure
I finally succeeded in breathing life back into my Tek 634 display today but, as it turned out, only briefly.
Thanks to google, an expired ebay link and a really helpful chap in the US of A I got my hands on a spare A4 HV Multiplier board and also tracked down a failed transistor and diode on the A6 interface board and all came back to life. I ran it for about an hour keeping an eye on the current drawn by the HV board and everything was fine.
The contrast was a bit horrid and the display seemed slightly dimmer than I remembered but as the HV board was a NOS replacement I figured it would need setting up.
I ran through the procedure which worked fine up to the point of setting the cut-off level to make the background raster just invisible. This is a bias applied to Grid (2?) of the CRT and is derived from a separate winding on the HV transformer with a 5M Bourns 3355 series trim pot as part of a divider network. These pots are a PCB vertical 'skeletal' type with a blue molded plastic thumbwheel type adjuster.
At first it responded as I expected but then the screen suddenly started getting very bright and the current to the HV board started climbing so I switched of sharpish! Investigation revealed the trimmer was open end-to-end and neither end was connected to the wiper. I opened it up - with extreme prejudice - and cannot see any evidence of over-heating or flash-over and the resistive track seems intact and continuous but is just no longer connected to the solder tag stake at either end.
On that basis I will find something to replace it and try again, but for reassurance I'm just wondering if these trimmers were used widely by Tek and if so if anyone has seen this sort of (mechanical?) failure mode before?
Thanks,
Adrian
Michael A. Terrell