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Hi,

I recently had a lucky find with a 454. It's working mostly but the delay picoff circuit has a strange issue which prevents the delayed sweep from running.
Up to the waveform at junction Q985/R982/C979 I can measure exactly the outlined waveforms and voltages from the schematics but after C979 it becomes rather incomprehensible.
The waveform at tp979 only has the AC coupled spikes but not the "DC" part which can be seen on the shematics. The bias on the base of Q975 is around 3.1V and the voltages at TP970 and junction R973/Q975/Q965 are drawn down to -8.5v and 2.1v respectively.
Both transistors measure fine in hfe on my 575, so I don't think they are the root cause?
this all doesn't really makes sense to me, so i don't know where i can start to look for a bad component.
I'm curious if any of you might have a clue on what is going on.

Benedikt

Benedikt,

See if the voltages at Q975 are correct. The collector should be -4,15V and the emitter 3,6V. You saying the emitter is off, see if 12V is on one end of R973. If not, trace that back. If so, that resistor is suspect. See if R971 is bad also. There are a number of resistors in this, the A version and the 453/A series that are too small in wattage. The connectors to pins on the boards can be dirty. Clean any that look suspect. See if R1257 is open. That is 12V dec1.

Mark

Hi Mark,

you seem to be right, R1257 was about 10% off.
I'll also check all the other carbon resistors in the decoupling networks and see if things get better after it.

Benedikt


-----Ursprüngliche Nachricht-----
Von: [email protected] <[email protected]> Im Auftrag von Mark Vincent
Gesendet: Freitag, 20. September 2024 23:08
An: [email protected]
Betreff: Re: [TekScopes] 454 delay time pickoff issue

Benedikt,

See if the voltages at Q975 are correct. The collector should be -4,15V and the emitter 3,6V. You saying the emitter is off, see if 12V is on one end of R973. If not, trace that back. If so, that resistor is suspect. See if R971 is bad also. There are a number of resistors in this, the A version and the 453/A series that are too small in wattage. The connectors to pins on the boards can be dirty. Clean any that look suspect. See if R1257 is open. That is 12V dec1.

Mark

Mark,

thanks again for the hint with the underspeced resistors,
this really solved the issues on the board.

what threw me off was maybe that all the static voltages seemed to make no sense and I didn't imagine that the circuit is that sensitive to a wrong supply voltage.

I will add the issue to the TekWiki article.

Benedikt