[Central-Electronics] Update: MM-2 advice

[K6JEK]

Bob Sullivan gave me some good advice:  check the CRT.   Lucky for me  
I had a replacement.   The trace can now be adjusted from about a  
quarter of an inch below center to the top.   Annoyed that this still  
wasn't centered I checked the pots.   Whooee, were they out of spec.   
The 1 meg Up/Down pot measured 600K and the Left/Right pot measured  
525K.   That's right.   They drifted down.  I replaced these with 1M  
pots.   This did not change the adjustment.   The voltages are now  
closer to spec but the U/D adjustment stayed the same.

Since it is now quite useable, crisp and clean, I'm declaring victory  
and moving on.

Jon, K6JEK


Begin forwarded message:

>
> I brought a dead MM-2 almost back to life but now I'm a stumped.    
> The high voltages are low and I can't position the trace down to  
> center or below.   Anyone have this experience?   Any advice?
>
> I followed Nick Tusa's advice  ( http://www.tusaconsulting.com/ 
> mm2_service_notes.html ) except I haven't replaced any of the  
> pots.   I did replace all the electroytics, the infamous C17, the  
> HV voltage divider resistors, R73, R4, R5, the sweep and audio  
> oscillator micas, all of the paper caps while I was at it and the  
> line cord and AC bypass caps.   I was pleased that it came to life  
> since it had been DOA.   But as I mentioned the trace is high, I  
> have to adjust the pot all the way to D (own) to get it close to  
> the center and high voltages read low.
>
> Out of the power supply, the voltage is fine, 1080 V at pin 2 of  
> the 5Y3, just as the manual says.  But  at A it's 927, not 1000V   
> and  at B 824V not 960, suggesting something is drawing some  
> unexpected current.   Gosh.  I checked the oil filled electrolytics  
> by subbing (then un-subbing) modern electrolytics.   I even checked  
> the ceramic caps C13 and C13A by disconnecting.   I know I'm  
> running out of ideas when I'm checking ceramic caps for leakage.    
> I also checked the 2.2M resistors feeding the top and left  
> deflection plates.
>
> Any suggestions would be most appreciated.
>
> Jon, K6JEK