| Products > Test Equipment |
| Tektronix 2235 repair thread |
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| David Hess:
--- Quote from: bd139 on November 28, 2018, 09:44:19 pm ---Yes that is correct. TP842 glitches but Q840 collector doesn’t. Looks like whatever is happening is only pulling the voltage down there by about 4 volts. It is weird as I’d expect Q840 to drop a bit there too but it doesn’t. I stuck the scope across both sides of it and did a differential measurement which matched. I have my suspicions the resistor isn’t healthy. It will be replaced with a nice fresh Vishay MRS25 when I do the dividers. --- End quote --- R842 might be bad then but there is nothing special about it and any film resistor can be used. I am not sure about the output impedance at the collector of Q840. That is the feedback point so up to a certain load, the impedance is effectively zero but the maximum load is pretty low. The quiescent current through Q845 is about 4 milliamps. 4 milliamps through R842 can only produce 1 volt but if TP842 is pulled low enough, the collector-base junction of Q840 will be forward biased and unlimited current will be provided through it and CR840 so that could explain the voltage drop across R842. |
| bd139:
Ok have replaced the resistors in this and no change. Did some more debugging and removed the anode supply entirely and the display is (as expected) dull and entirely stable. That means it’s either the multiplier which is shot or the tube is. I can’t find any evidence of flash over on or around the anode cap so my current thought is the multiplier. Fuck! :( |
| tautech:
Good PDA multiplier rebuild guidance here: https://www.eevblog.com/forum/repair/tektronix-465/ |
| tautech:
--- Quote from: bd139 on December 05, 2018, 09:34:20 pm --- That means it’s either the multiplier which is shot or the tube is. I can’t find any evidence of flash over on or around the anode cap so my current thought is the multiplier. --- End quote --- You mentioned a crack when discharging the anode connection. Discharge then reconnect and repower. Shut down and check if the anode is recharged. |
| David Hess:
If the multiplier was causing a problem then I would expect more than just an intensity change; beam deflection also depends on the PDA (post deflection acceleration) voltage. The CRT can be operated without the PDA attached. The deflection should be about half and the trace will be dim and fuzzy but it should still be visible. |
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