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@fredenstein.com
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.

The 7854 is based on a ti-9900 16 bit minicomputer emulating
microcomputer. Bank in the vernacular of the time, a "real time
clock" was a counter driven off of the power line. I can't think
of a reason for a 7854 to have such a clock, but I haven't studied
it extensively.

-Chuck Harris

David C. Partridge wrote:

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@fredenstein.com
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.

Same chip used in the TI-99/4A?

Thanks,
Barry - N4BUQ

The 7854 is based on a ti-9900 16 bit minicomputer emulating
microcomputer. Bank in the vernacular of the time, a "real time
clock" was a counter driven off of the power line. I can't think
of a reason for a 7854 to have such a clock, but I haven't studied
it extensively.

-Chuck Harris

On Tue, 5 Feb 2019 16:52:07 -0500, you wrote:

Same family, but likely physically bigger.

64 pin DIL package, I think.

Wonder what would happen if you somehow managed to recode everything
for an ARM processor?

Harvey