Re: 475 Triggering Issue -- Will recent 468 Triggering Issue Thread Help Me?


I've uncovered what I think is a significant symptom. If I inject a 1 volt square wave, at 1khz or above, set v/div to 20mv/div, and play with the position control, I can get the trigger source lamp to light. If I try the same stunt with a sine wave, I need at least 500khz to get the scope to trigger. This symptom seems to play into what Max was suggesting by lifting one lead of a tunnel diode, and then determining if I can get trigger with high slew rate sources. Of course, with the scope setup to the above test conditions, it's useless.


--- In TekScopes@..., "stan_katz" wrote:

Hi David,

I sampled rear panel ch2 vertical signal per your suggestion. It's coming out at only 1/20th the signal true amplitude. So maybe R403 should be examined? However, a confounding symptom, occurs when I try 1v to 20v sine waves at 60hz with line triggering. I couldn't get a lock. no lock at any other freq. for line source. Another bad sign, which has presented itself since I started exercising all controls through full range...neither of the v/div controls will advance past 5v/ 10/20/50v positions possible on either channel. Is this op. error, or what? Ch 2 v/div control skirt illuminator is not lit. Just burned out, or is illuminator driven by other circuitry?


--- In TekScopes@..., David wrote:

I used the same military manual from Bama.

I had two reasons not to suggest that the tunnel diodes are the

1. The common failure point for the symptoms does not implicate the
tunnel diode part of the trigger circuit. External triggering and I
would assume line triggering both work and neither of them go through
U370 which could cause all of the problems.

2. The tunnel diodes are fragile and difficult to replace so fiddling
with them if they are not definitely the problem is asking for
trouble. :)

On Sat, 26 Jan 2013 23:56:53 -0000, "stan_katz"

Hello Tom,

I will gladly avoid touching the tunnel diodes, and will start with David's suggestion. The only service manuals I have are from BAMA, which are the Army TM--- model manuals. Will these do, or can you point me in a better direction? BTW, Bert's post is a warning to me that the 475 has potentially unobtanium parts. Right now, Tucker has Tek tunnel diode kits for sale. Can you, or Bert, post the proper kit part number for my scope? I should buy a kit now if it's one of those listed at Tucker. Tom, I noticed my thanks for your input on my 3325 repair effort got cut. Let me thank you here.


--- In TekScopes@..., "Tom Miller" wrote:

If you do work on the tunnel diodes, be sure to use a heat sink right up against the diode body. Ge tunnel diodes are easily ruined with heat.

However, I think David's comments are more likely the issue. That is because of the external trigger working.

Do you have the correct service manual for your 475?


----- Original Message -----
From: Bert Haskins
To: TekScopes@...
Sent: Friday, January 25, 2013 5:09 PM
Subject: Re: [TekScopes] 475 Triggering Issue -- Will recent 468 Triggering Issue Thread Help Me?

On 01/25/2013 03:46 PM, stan_katz wrote:

I have an ancient 475 scope which works fine with an external trigger, but will not trigger on any of the internal sources. Should I start by reviewing the recent 468 triggering issue thread to get me going? If not, can anyone recommend a thread I should start with? Of course, I'd be delighted to receive help on this thread.


Probably not.
Going from what I had on 465Bs the triggering problem is often that
the tunnel diodes have changed in value.
There are many! threads about this in the archives.
It may be worth your time to try swapping the TDs from the B sweep section
with the A section to see if things change.
I had this problem come up when I was hunting for a process control network
problem while working in factory in a different country.
I swapped the TDs and the scope has been near perfect ever since.
Go figure.

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