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-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Roger Evans via Groups.Io
Sent: 09 December 2018 08:27
To: [email protected]
Subject: Re: [TekScopes] 466 Channel 2 problem

Colin,

If you press the beam finder and then switch Ch 2 to 'invert' does the position of the channel 2 trace move? This is a quick check on whether the channel 2 imbalance is before or after the >invert circuitry (Q220 - Q226).

Yes, it does. It starts off at about three main divisions above the graticule centre-line and then drops to about three below the centre-line when "invert"-ed.

I am puzzled when you say that you 'mostly cannot see anything untoward' on channel 2 voltages and then say that the voltages on TP11 and TP14 are very different. Are they the only >places where the DC voltages on the two halves of the paraphase amplifier are noticeably out of balance? Since the amplifier is DC coupled throughout a failed transistor or diode tends to >upset the DC levels through other parts of the chain and finding the failed component can take patience. If the transistors are socketed then just pulling them out for testing, or swapping >with the corresponding channel 1 parts if you don't have a transistor tester is an easy route. If they are soldered in then you need to measure Vbe and Vce to check that they are consistent >with a transistor operating in its linear regime. Don't forget that the switching diodes CR314 - CR318 are part of the DC circuitry so a failed diode will also upset the amplifier balance.

I understand your puzzlement. I had actually only checked a few voltages, but I had probed with another scope the signals at the various points indicated in the manual and seen the signal going through, though there was a voltage offset. I had only recently decided to check the voltages. My hope was that there was a common failure that someone with more experience would immediately point out to me. Mea culpa. I will check out the other voltages more rigorously.

The probablility is that you have a failed semiconductor but it is possible that a resistor has drifted badly over time.

Yes, I am inclined to agree that there is a failed semiconductor. Now to find which one...

Roger

Colin.