Here is the SWEEP GENERATOR board, circuits and manual text in FREE RUN mode.

 

Here is the SWEEP GENERATOR board, circuits and manual text in FREE RUN mode.

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Just probed around the points indicated, getting no square wave to start off with at the input of the sweep generator. Perhaps a problem with the Sweep Trigger? On the input of T401 I'm just getting noise, would it be possisble to get a picture of what that point looks like on yours?

 

Hold the phone! Traces! And she sweeps! Was just poking around with my scope probe and started seeing a signal where I wasn't, and bam there is a trace now. But it won't trigger on the call signal, so I think my hunch of a trigger problem may be correct.

 

Ignore the Sweep Trigger for the moment.
Set your 422 TIME/DIV to 0.5ms/DIV
Set LEVEL to FREE RUN.

Set your working scope to input VOLT/DIV to 20V/DIV.
Set TIME/DIV to 5ms/DIV

Probe the lower end of R447. You should see a 30V amplitude saw-tooth wave.

 

Ignore the Sweep Trigger for the moment.
Set your 422 TIME/DIV to 0.5ms/DIV
Set LEVEL to FREE RUN.

Set your working scope to input VOLT/DIV to 20V/DIV.
Set TIME/DIV to 5ms/DIV

Probe the lower end of R447. You should see a 30V amplitude saw-tooth wave.

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Here's what I see, 30v saw-tooth.

(Sorry about the glare)

 

Sorry, my post #44 was made before I saw your post #43.
So you have a trace and confirmed that the SWEEP GENERATOR is working.

Using your TEK 5441 can you confirm that the 422 CALIBRATOR OUTPUT is correct, -2V 1kHz?

 

Sorry, my post #44 was made before I saw your post #43.
So you have a trace and confirmed that the SWEEP GENERATOR is working.

Using your TEK 5441 can you confirm that the 422 CALIBRATOR OUTPUT is correct, -2V 1kHz?

Yup, 2V square wave, I also fed a 1kHz sine wave and it just scrolls by the screen with the LEVEL having no effect on the trigger.

 

Connect a 1x jumper cable from the 422 CALIBRATOR to CH1 input.
Set CH1 input to 2V/DIV.
Set LEVEL to AUTO.

Probe the signal at D401, R403, R405 junction.
You should see a 500mV square wave.

 

Connect a 1x jumper cable from the 422 CALIBRATOR to CH1 input.
Set CH1 input to 2V/DIV.
Set LEVEL to AUTO.

Probe the signal at D401, R403, R405 junction.
You should see a 500mV square wave.

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Not getting anything there, but I do get it at H which is at the collector of Q364(just before the slope switch in the sweep trigger section).

 

Side note: the display is very very dim, you have to try pretty hard to see the non-triggering cal signal.

EDIT: We may have to fix this brightness issue to continue, because at times it's visible and other times so dim I can make out a trace at all.

EDIT 2: I got the trace to display brighter by adjusting the unblanking control, it'll do for now till the trigger problem is solved.

 

Actually upon further probing I am getting a 20mV square wave at D401.

 

We now have to look at the SWEEP TRIGGER circuit <5>.

With the 422 CH1 INPUT on CALIBRATOR,
LEVEL set to AUTO,
probe the signal at the base of Q364. The test point is on R352 in the board picture below. You ought to see a 500mV p-p square wave at 1kHz centered about 0V.

 

We now have to look at the SWEEP TRIGGER circuit <5>.

With the 422 CH1 INPUT on CALIBRATOR,
LEVEL set to AUTO,
probe the signal at the base of Q364. The test point is on R352 in the board picture below. You ought to see a 500mV p-p square wave at 1kHz centered about 0V.


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Here's what I get at that point:

EDIT: I trace that 500mV square wave all the way to C377, once it passes through C377 it drops in amplitude to 20mV, the signal on the output of T401 is down in the noise.

Also, if you probe just before C377 you can adjust the TRIG LEVEL and watch the signal disappear and come back around 12 o'clock where the scope is supposed to trigger. So the trigger section is triggering but not relaying that info to the sweep.

 

I am going to guess that D375, tunnel diode 1N3719, is bad and needs replacing.
This is a very sensitive Ge diode and will be difficult to source at this stage.
Let me know if you find any.

 

I am going to guess that D375, tunnel diode 1N3719, is bad and needs replacing.
This is a very sensitive Ge diode and will be difficult to source at this stage.
Let me know if you find any.

Ok, I will look for one and buy it if I find it. So for the time being, what could be causing the dim display? I got it brighter by adjusting the unblanking center, but it is close to fully clockwise. The manual says it should be around the middle of the pot. I also took some voltage measurements and found the +48V points around Q863 and Q864 are at +20V. Adjusting the unblanking center gets it to +42V, but then you can't see the trace and the pot is fully counter-clockwise.

EDIT: It also looks like someone before me has replaced D405 (1N3714) and D455 (1N3713) which are both Tunnel Diodes also.

 

I tested the Tunnel Diode, and it seems to test fine, low ohms reading, and low voltage with a diode tester. Does that mean it's still good? And perhaps the problem is something else like a drifted resistor?

 

You cannot test a tunnel diode with a DMM diode test. You have to put it on a curve tracer.

 

I seem to remember that testing tunnel diodes with a multimeter is bad for their health - too much current.

 

You cannot test a tunnel diode with a DMM diode test. You have to put it on a curve tracer.

Ok, I'll borrow a friend's tomorrow. In the mean time I check R377 which says 39Ω but actually measures 390Ω on more that one of my meters. I don't have this value, but I'll get one tomorrow.

 

I seem to remember that testing tunnel diodes with a multimeter is bad for their health - too much current.

It's still measuring around 3Ω, and I did some quick research and a bad tunnel diode is said to have a high resistance reading when it fails. But all the same I just found a seller and ordered two of them.