Tek. 454 no triggering on A or B

#179621

I have a 454 that will not trigger on A or B timebase. I bought knowing of
the triggering problem. I have scoped the signal and it will not go through
T670. Overhauling the unit did not help. The power supply voltages are
right on and the small 1mfd decoupling condensers are now film types. The
voltages at points BE, TP646 and CB are what the manual says by the
adjustments. The 8393s tested very good to new. I pulled out known working
tunnel diodes from another unit that works. That did not work. Swapping the
transistors from the known working one to this one did not work. Slight
adjustments of the pots to the three above points did not work. I tested
the diodes for leakage. They are fine. This looks like something common to
both like the power supply or trigger preamp. Those are working as they
should. The power supply outputs are within mV of the rated voltages. Using
the EXT. trigger input did not work. I checked the connectors on the boards
for corrosion and found none. I did tighten the female ends before
reseating them back on the pins. The unit is clean so dirt should not be a
problem. I know the A SWEEP TRIG'D light is good. I ohmed it out. I cleaned
the transistor sockets and moved the transistors up and down a few times to
make sure they were making contact. I resoldered connections in case one or
more were bad. Nothing I have done will make it trigger. I have worked on
this for a few days and cannot find the problem(s). Does anyone here know
of something I missed or what? I waited to ask here until I did what I
could. At this point, I need help. Thanks.

Mark

#179648

Hello Mark,

Maybe I can help you a little bit.

I have a 454 oscilloscope.

You can use my oscilloscope as a working reference unit.

Set up your 454 displaying the 1 V PP calibration signal
into channel 1 via a coax cable and I will do the same.

Tell me which test points that you would like me to check
on my oscilloscope. I will report back to you what I see.

Regards,
John Crighton
Sydney

Show quoted text

----- Original Message ----- 
From: "Mark Vincent" <orangeglowaudio@...>
To: <[email protected]>
Sent: Monday, March 01, 2021 12:09 PM
Subject: [TekScopes] Tek. 454 no triggering on A or B

I have a 454 that will not trigger on A or B timebase. I bought knowing of
the triggering problem. I have scoped the signal and it will not go through
T670. Overhauling the unit did not help. The power supply voltages are
right on and the small 1mfd decoupling condensers are now film types. The
voltages at points BE, TP646 and CB are what the manual says by the
adjustments. The 8393s tested very good to new. I pulled out known working
tunnel diodes from another unit that works. That did not work. Swapping the
transistors from the known working one to this one did not work. Slight
adjustments of the pots to the three above points did not work. I tested
the diodes for leakage. They are fine. This looks like something common to
both like the power supply or trigger preamp. Those are working as they
should. The power supply outputs are within mV of the rated voltages. Using
the EXT. trigger input did not work. I checked the connectors on the boards
for corrosion and found none. I did tighten the female ends before
reseating them back on the pins. The unit is clean so dirt should not be a
problem. I know the A SWEEP TRIG'D light is good. I ohmed it out. I cleaned
the transistor sockets and moved the transistors up and down a few times to
make sure they were making contact. I resoldered connections in case one or
more were bad. Nothing I have done will make it trigger. I have worked on
this for a few days and cannot find the problem(s). Does anyone here know
of something I missed or what? I waited to ask here until I did what I
could. At this point, I need help. Thanks.

Mark

---
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#179659

John,

Thank you for your offer to help. I appreciate it. I used the 1V cal. output for testing the triggering and the signal went to TP656. That is where the signal ended. Looking at the cathode end of the tunnel diode showed some signal on the working and nonworking units. C689 is 15V that should be replaced with a higher voltage type. I did that as mine was leaky. The voltages at the transistors are right. I know the resistors are accurate, They are now new ones at 1% types. The originals were out of tolerance. The resistor replacement did not help with triggering, only getting voltages at the E, B and C right. The Delay-Time Multiplier does work.

Mark

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Tek. 454 no triggering on A or B