Re: 7854 - yet another problem

 

The problem was simple once it got bad enough that I could hear the crackle/hiss of EHT leakage - I removed the CRT (easier said than done), thoroughly cleaned the cable and around the seal where it connected to the PDA mesh (even though there was little if any sign of carbon tracking), and put it back together. Since then (so far) it has behaved perfectly .

PS I think the "real time clock" problem may have been related as it hasn't recurred. I think the real time clock refers to a problem with the "50Hz" signal from the Readout Acquire circuit (schematic <31>). Strangely the actual frequency in this scope was close to 70Hz (even though the timing R & C connected to pin 6 of U2510 were pretty much spot on value). Strangely U2510 isn't shown in the parts list for the Display Board (A29).
It's a 155-0021-01 timing generator and swapping it with one from another readout board made no change.

David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of fred@...
Sent: 05 February 2019 15:21
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7854 - yet another problem

The 7854 I'm trying to resurrect has a problem where the readout starts to
flicker a lot after it has been on for a short while.

Hi David,
if you store a waveform, is it flickering as well? I had a similar problem with the -15V supply on my 7854. It showed OK DC values by got ripple after a short time running. It was a 330uF filter cap in the powersupply.

--
Best regards,

Fred S.

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