开云体育

-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Tom Lee
Sent: Saturday, March 26, 2022 6:04 PM
To: [email protected]
Subject: Re: [TekScopes] Some weirdness about a 485

Hi Zen,

TDs are quite thermally sensitive, so try cooling CR751 alone and see what happens. If that proves to be the sensitive element, first take a screenshot of initial conditions, then give R755 (the trig sensitivity
trimmer) a little tweak. It may very well be that CR751 has drifted in characteristics. If it hasn't drifted too far, adjustment of R755 will restore functionality. I generally don't advise random tweaks, but here it might be warranted.

Just a thought.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070


On 3/26/2022 14:03, Zentronics42@... wrote:

Some new observations of the fault. The fault is thermally dependent.
Cold scop triggers just fine warmed up scope triggers miss behave.
Fired up the air compressor and cooling down the sweep board Left side
of scope pops the trigger back temporally. I have also reseated all
the combs on both boards

I should also note that this a now a trigger fault. 50 ohms 1meg and external ALL behave the same way.

Unfortunately a thermal camera would help troubleshooting on this one but I don’t have one of those. Letting the scope cool off to reseat some transistors this most DEFFANATLY behaves like a capastitvely coupling issue. Higher frequances making it through to the trigger circuits better.

At the moment I think skipping ahead in the calibration document will not help due to the scope not triggering on signals of 8 division of display. And I can watch it fail. As the scope warms up it gets flickery then trigger falls off this is with not changing any controls on the front panel. Signal source is known good and checked so it is not flaking out.

Zen

-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of David
Templeton
Sent: Saturday, March 26, 2022 4:29 PM
To: [email protected]
Subject: Re: [TekScopes] Some weirdness about a 485

I’ve not seen that scope, does it have an LF or HF rejection on the trigger coupling options?

David

On 26 Mar 2022, at 19:24, Zentronics42@... wrote:

?I am working on doing a restoration on a Tektronix 485 for the lab and I have been arguing with it for a little while. I am curious if anyone has run in to a scope that does not like to trigger at low frequencies? I am currently working on the 50 Ohm calibration and it seems that the scope in 50 Ohm mode REALLY does not like to trigger under 1.2Mhz. So it is struggling with 50 Khz cal signals. Vertical amplitude is good trigger is flakey. I have heard that some of these the TD’s go kind of deaf at the high speed side of things but not the low side. Any ideas what might be causing this. I am continuing to investigate further.
Zen