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Grandstand "Astro Wars" / Epoch "Galaxy II" Repair.

Thread starter #21
Thanks for your input, I have to say that result surprises me. It doesn't match what I see on my scope but it's clear your equipment is far better than mine. Mine is just a basic self-built scope and the large input voltage measured into the diode was over its intended range(!).

HOWEVER, when I used my scope the output voltage from the disconnected anode (of S06) was -39v as I expected based on the measurements in my circuit (taken when S06 was connected in circuit).

I have seen the V3 board in the past and yes now you mention it I do remember two zeners in series. From memory, although I never identified them, I think S06 and S011 are identical and just one of those zeners could replace S06 in the V2 board. That is to say, I think S06 in the V2 board is the same as the S06 in the V3 board and S011 is also identical to those.

As we know we're talking about different board revisions, I suppose it's possible there are differences, and I know the VFD can be run from a much lower voltage than -39v from the testing I have done previously.

Sorry to ask, but are you able to confirm the voltage measured at:

R01, pin 1
R02, pin 1
VFD Pin1 and Pin2?

No problem if not.

I like your idea of combining batteries to create a large enough test voltage!
 
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#22
Yes it surprised me too. Has a sense of "its not right" doesn't it ? It was in circuit, 'scope is new and I'm new to this too so I could easily have missed something. I will get you the voltage measurements no problem, but It will likely be Thursday as I am away just now. I'll also try that battery idea see if I can test the Zeners. Your theory that the two in series are the same could be the case as if one goes short the second will still clamp the voltage down, could just be a move by the designers to halve the chance of the failure we are seeing.
 
#23
2 zeners can dissipate twice as much power as a single one of the same size. Mind you, one can get a higher wattage zener to replace smaller ones.
Their design may have tried to handle the heat dissipation of the zeners, or they used two common values to get to an uncommon value, who knows...
 
#24
Here is what I measured on my working unit

-39V R01, pin 1
-39V R02, pin 1
-31.8V VFD Pin1
-32.8V VFD Pin2

I also tried to measure the Zener voltage of SO6 and SO11, but I got to 89V before I ran out of PP3's and still didnt manage to get either of them to conduct in reverse bias.
 
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Thread starter #25
Thanks fella. The voltages on the pins are what I expected.

But wow at the S06 and S011 test. I didn't expect that at all! So it does appear that those components are not 39v zener diodes after all.

What else could S06 be?

Anyone?

(BTW I know one leg is lifted in this photo but it's the only closeup I have to hand.)

 
#26
Well I bought some more PP3's... connected 17 in series to give and actually measured the "pack" at around 160v DC. In reverse bias SO6 drops around 140 to 144 volts across it... SO11 did the same. It does seem high and I didnt see a 140v zener in a glass package for sale anywhere. However if the 39V zener you tried gave you 15v then perhaps a higher value zener would get you closer to the expected voltage.... For me unfortunately its time to solder it all back together and put it back onto the shelf... good luck and let me know if you know anyone who needs a lot of PP3s
 

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