Help! MPI-4 cathode voltage is -28V

[jdsalinger]

The CRT on my MPI-4 does not display any trace or dot. I have checked all voltages going to the CRT and all are OK except that I am not getting -1000V after R317 (measured 229K). I only get -28V.

I measure at least -1000V ahead of R317 (my multimeter only goes to 1000V). So, the transformer is OK.

I tested the zener diodes D304-D308 (all good) and they have continuity between each other and from D308 to ground.

I have replaced C301, C302, C303 and verified continuity between all and between C302 & C303 and ground.

What could cause there to be almost no voltage on the down-stream side of R317? Short somewhere?
Am I looking in the wrong place? Any advice appreciated!

Circuit is on page 8 of the service manual. http://akdatabase.org/AKview/albums/userpics/10004/MPI4 Service.pdf

 

[jdsalinger]

Made some progress. I pulled the 1000V side of R317 and measured over -1000V.

Then, I removed the 2.2 Meg resistor R316 and got less than -12V, so the problem must be upstream from there.

Then removed R319 and measured around -600V on the -1000V side of R312.

So, the problem is somewhere between the LDR network and the Focus and Intensity pots.

 

[petehall347]

instead of pulling resistors if you measured the voltage drops across them and used ohms law it may save you some work .
might also reveal a flaky resistor .

 

[jdsalinger]

Ok, I want to try replacing the power supply zener diodes. It is PN 070-060 200V zener diode.
It is D304, D305, D306, D307, D308 on the MPI-4 power supply section schematic 154-600.

They see -1000V on one side and should drop 200V each. I don't know what the current is. I tried searching Mouser but I'm not sure how to find a zener that is rated for at least 1000VDC AND drops 200V and since I don't know what the current is....I have no idea whether to select one with a high current or if it needs to be sized for the current that needs to be flowing in the diodes.

Does anyone have a recommendation on a new production zener diode I can use, or know if I should just call McIntosh to ask. ?

Thanks

 

[dcgillespie]

I would try disconnecting the lead from the -965 volt tap at in the Intensity/Focus voltage divider, that goes over to the CRT cathode terminal. This terminal is tied to one of the heater terminals, meaning that if the CRT heater winding in the power transformer developed a short, it would put a serious hurt on your negative CRT supply voltage. Even if the winding is (hopefully!) good, disconnecting that lead will still allow you to eliminate all the components attached to that tap lead.

Dave