Re: Tektronix 465 No Trace, No Dot


Elliott Li
 

Hi Folks,

There definitely is something going on with the inverter on the low-side. I took a short trip down the troubleshooting tree in the service manual and this is what I found:


I can adjust the "HV ADJ" pot, R1400, and indeed the wiper voltage is adjustable from -8 to +5. This has no effect on the -2450 voltage though, it remains at -490.


The transistor connected to this pot (Q1404) seems suspect. At the base I see 3.61 volts, collector is 4.4, and emitter is 3.0. These are all far off from the voltages in the schematic -- although I did not try adjusting the R1400 pot to attain similar voltages. Unfortunately this is one of the few transistors that is soldered in, so I did not yank it out and test it immediately (will do that tonight probably). I also noticed that the +120 volt unregulated DC rail is running pretty rich at +146 volts. I first noticed this at R1402, the resistor that forms part of the adjustable network at the base of Q1404. The voltage at CR1404 and R1402 is still in-spec at 55.6 volts.


Checking other transistors in the low-side of the inverter showed similar discrepancies to what their expected bias voltages are. Keep in mind I don't have a second scope (yet) to view the waveforms. Using my DMM, I checked Q1418 and found the bias levels near (not exact though) expected values -- emitter was ground, collector was at 21.0 volts (should be 24 I believe but this is close), and the base was at 0.22 volts (seems low though). Checking with the AC setting on my meter showed a ripple at 38 kHz on the collector and base, with voltages of 285mv and 71mv respectively.


Without a scope I can't really place much confidence in these AC readings, but given the low output voltage on the high side of the inverter, I think it's safe to assume something is far off. Lower than expected amplitude and frequency on the low side could cause this.


My plan is to pull Q1404 and test it, and to check the other transistors in that circuit as well. I also may have a second scope tomorrow to test with. Do you guys think seeing +146 volts on the 120volt unregulated DC rail is any issue? Could there be some strange "additive" short from another rail causing the increase? In case you're wondering, the AC line selector is correctly set and the line voltage was 116 volts AC RMS last night.


Your suggestions and comments have been very helpful, thanks again and they are always appreciated!


--Elliott

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