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.

Dim display is likely caused by aging CRT. The only solution is to replace the CRT but is it worth it?

 

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

Yes, he very likely killed the tunnel diode if it wasn't dead already.

 

Dim display is likely caused by aging CRT. The only solution is to replace the CRT but is it worth it?

I thought so too, but I get brighter flashes with dirty switch contacts, and adjusting the unblanking center adjustment affects the EXT HORIZ time/div setting. I checked both the INTENSITY and FOCUS pots and they check out, another suggestion I received is a possible drift in one or more of the resistors in the intensity and focus chain. So before I consider a new CRT I will check those values.

 

Dim display is likely caused by aging CRT. The only solution is to replace the CRT but is it worth it?

Also, thank you (and everyone who contributed) very much for helping me get this far. I'm always learning something new. Hopefully I can get her running again, and if not, I still gained valuable experience from this!

 

The tunnel diodes arrived and I will be replacing the suspected diode soon, hopefully this solves the triggering issue. Also, I replaced the transistors in the blanking circuit and managed to get a bit ore brightness on the display. One thing I have left to check once I find some time is the other half of the blanking circuit found on the Vertical Output Board. Upon further reading in the manual I found that the blanking circuit is two plates within the crt, one at a fixed voltage and the other variable (driven by two transistors w/adjustment trimmer). When the two plates are at, or close to, the same voltage the crt is unblanked. When the plates have a difference the display is blanked. Before I deem the crt officially tired (although still usable), I will check the output voltage to the second plate of the blanking circuit.

 

I can show you a way of testing the tunnel diode (and other diodes) using a simple curve tracer.

What you will need:

10Ω ¼W resistor
Function Generator to produce sine or triangular wave ±5V max.
Oscilloscope with X-Y function.

Set up the function generator to output a sine or triangular wave ±1V for starters, about 100Hz.
Set up the oscilloscope for X-Y display.
Connect to the circuit as shown.




Start off with a test diode such as 1N4001.
Increase the amplitude of the function generator output gradually until you see the curves shown. When testing the tunnel diode, reduce the function generator output to zero and increase gradually to no more than 1V.

Here are some sample I-V curves for different devices under test (DUT):

10Ω resistor



1N34A Ge diode



1N4001 Si diode



1N5817 Schottky diode



1N3720 Tunnel Diode



1N3720 Tunnel Diode close up

 

I can show you a way of testing the tunnel diode (and other diodes) using a simple curve tracer.

What you will need:

10Ω ¼W resistor
Function Generator to produce sine or triangular wave ±5V max.
Oscilloscope with X-Y function.

Set up the function generator to output a sine or triangular wave ±1V for starters, about 100Hz.
Set up the oscilloscope for X-Y display.
Connect to the circuit as shown.

View attachment 113593


Start off with a test diode such as 1N4001.
Increase the amplitude of the function generator output gradually until you see the curves shown. When testing the tunnel diode, reduce the function generator output to zero and increase gradually to no more than 1V.

Here are some sample I-V curves for different devices under test (DUT):

10Ω resistor
View attachment 113594


1N34A Ge diode
View attachment 113595


1N4001 Si diode
View attachment 113596


1N5817 Schottky diode
View attachment 113597


1N3720 Tunnel Diode
View attachment 113598


1N3720 Tunnel Diode close up
View attachment 113599

Ok, the new diodes check out. I replaced the old one and I'm still not getting it to display a stable trace (whether from the CAL out or feeding it from a signal gen). I get the trigger pulse on the primary of the transformer(T401) that couples the trigger stage and the pulse generator stage but not on the output. I checked for continuity of the windings and found that they are intact and the input and output are not shorted.

Attached is a video the trigger pulse of the inputs of T401, the beginning few pulses shown are me adjusting the trig knob. The end where you here a *click* is the TRIG knob being put into AUTO TRIG.

Here's the video: https://www.dropbox.com/s/60citlm3hj7i6ii/Trigger.m4v?dl=0

 

I can show you a way of testing the tunnel diode (and other diodes) using a simple curve tracer.

What you will need:

10Ω ¼W resistor
Function Generator to produce sine or triangular wave ±5V max.
Oscilloscope with X-Y function.

Set up the function generator to output a sine or triangular wave ±1V for starters, about 100Hz.
Set up the oscilloscope for X-Y display.
Connect to the circuit as shown.

View attachment 113593


Start off with a test diode such as 1N4001.
Increase the amplitude of the function generator output gradually until you see the curves shown. When testing the tunnel diode, reduce the function generator output to zero and increase gradually to no more than 1V.

Here are some sample I-V curves for different devices under test (DUT):

10Ω resistor
View attachment 113594


1N34A Ge diode
View attachment 113595


1N4001 Si diode
View attachment 113596


1N5817 Schottky diode
View attachment 113597


1N3720 Tunnel Diode
View attachment 113598


1N3720 Tunnel Diode close up
View attachment 113599

Would you be able to take a picture of what the output of T401 looks like on your scope?

Thanks,
M.

 

I will do that if you don't mind waiting for the weekend when I might have some spare time.

 

I will do that if you don't mind waiting for the weekend when I might have some spare time.

Yup, no rush!

Thanks,
M.

 

It is very unlikely that the pulse transformer T401 is bad.

Let us recap to recall where we're at.
You have a visible trace in FREE RUN.
You cannot obtain proper SYNC.
You have observed 500mV square wave + 6V DC at the anode of D375 (IN3719) at the junction of D375 and R372.

On my TEK 422 there is no measurable signal on the primary winding of T401 (same as you have observed).
On the secondary winding I observe the sweep generator in sync.

CH1 - Signal at D375
CH2 - Signal at D401

 

It is very unlikely that the pulse transformer T401 is bad.

Let us recap to recall where we're at.
You have a visible trace in FREE RUN.
You cannot obtain proper SYNC.
You have observed 500mV square wave + 6V DC at the anode of D375 (IN3719) at the junction of D375 and R372.

On my TEK 422 there is no measurable signal on the primary winding of T401 (same as you have observed).
On the secondary winding I observe the sweep generator in sync.

CH1 - Signal at D375
CH2 - Signal at D401
View attachment 114411

Hey, sorry for the dark period, things got crazy and I just got some time to look at the scope again.

The image attached shows what I'm getting at the cathode of D401, I should be getting ~.6V signal there but I'm not. I am getting that squarewave at D375, so I presume the new tunnel diode is working correctly as the square wave stabilizes at D375 indicating a trigger on the signal.

It's just not getting through to D401?

 

It is very unlikely that the pulse transformer T401 is bad.

Let us recap to recall where we're at.
You have a visible trace in FREE RUN.
You cannot obtain proper SYNC.
You have observed 500mV square wave + 6V DC at the anode of D375 (IN3719) at the junction of D375 and R372.

On my TEK 422 there is no measurable signal on the primary winding of T401 (same as you have observed).
On the secondary winding I observe the sweep generator in sync.

CH1 - Signal at D375
CH2 - Signal at D401
View attachment 114411

Also I am in Auto Trig, not free run, I get no signal on Free Run

 

Just so you know, you are not being ignored.
I need to spend some time to think this through carefully, perhaps sometime this weekend.

 

I am going to focus on the junction at D401, R403 and D405 on the SWEEP GENERATOR <6>
while on AUTO TRIG.

CH1 INPUT connected to CALIBRATOR
CH1 VOLTS/DIV = 1V
TIME/DIV = .2ms

The signal on the anode of D401 should be 0 to -10V
The signal on the cathode of D401 should be 0 to +500mV

What are you getting?

 

I am going to focus on the junction at D401, R403 and D405 on the SWEEP GENERATOR <6>
while on AUTO TRIG.

CH1 INPUT connected to CALIBRATOR
CH1 VOLTS/DIV = 1V
TIME/DIV = .2ms

The signal on the anode of D401 should be 0 to -10V
The signal on the cathode of D401 should be 0 to +500mV

What are you getting?

First photo is cathode of D401, the second is the anode of D401.

 

With a DMM measure the voltage at the junction of D403, D404 and R404.

Labeled "S" - blk-blu on wht

Also measure the voltages at AK and AJ as shown in Fig 4-9.

 

With a DMM measure the voltage at the junction of D403, D404 and R404.

Labeled "S" - blk-blu on wht

Also measure the voltages at AK and AJ as shown in Fig 4-9.

Here are the measurements I got:

S = 11.80V
AJ = -11.73V
AK = 0V

 

Here are the measurements I got:

S = 11.80V
AJ = -11.73V
AK = 0V

Those appear to be normal.

Are you able to unsolder and lift one leg of D401 and scope the output at the secondary of T401?

 

Those appear to be normal.

Are you able to unsolder and lift one leg of D401 and scope the output at the secondary of T401?

Yup, what am I looking for?

Sorry for the wait, I'm not ditching the project, things just got busy.