Re: Tektronix 454


erik erickson
 

Thanks for all the help. For some reason channel two is working perfectly right know. Trace shows with invert switch on or not but it moves down from the center, one horizontal line. Also the beam finder does the same. I assume this is normal or maybe I didn’t let it warm up enough. I will check the voltages and probe it later but for now I’ll just see how it works.
Sent from my iPhone

On Jan 27, 2018, at 4:48 AM, Roger Evans via Groups.Io <very_fuzzy_logic=yahoo.com@groups.io> wrote:

I found some more useful information in the previous thread which I assume is still valid.

You said that Ch2 would flash on when you pulled the Ch2 Invert switch and also that with the invert switch half way out the trace would stay on screen but not showing any signal.

With display set to Ch2 and Invert pushed in (normal) press the beam finder switch. Is there a line showing that the trace was originally off screen? Now pull the invert switch and press beam finder. Is the trace off centre in the same direction or the opposite direction? If the direction changes the Ch 2 imbalance is probably before SW181, if not then the fault is probably after SW181 (Q184, Q194 or the switching diodes, note that Q184, Q194 are PNP Germanium).

The manual gives the DC levels to be expected in the Ch2 preamp with no signal applied. Start at the collector of Q124 and then the emiiter of Q134. You should be able to get Q134 emitter close to zero with the Ch2 vertical position control.

Q164 and Q174 should have almost identical voltages on their emitters (-0.78V) and collectors (+2.0V) and their bases should be close to 0V, but since the observation implies that the balance is way off at SW181 you may have found something wrong by now. Since the preamp is DC coupled it may not be obvious whether the fault is where you first observe a 'wrong' voltage or due to the DC operating point being wrong because of a fault later on. The observation using the invert switch should help to narrow things down. Another test is to swap Q164 with Q174 and see if the off-screen direction changes and similarly with Q184 and Q194. You can also use the good Channel 1 on the 454 to probe Channel 2 preamp. Display Ch1, put your probe on Ch1 and connect Ch2 to the calibrator, Ch2 set to 1V/div. Then look for where the signal disappears.

Regards,

Roger


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