fisher MPX decoder troubleshoot

Hi Dave: Picking your brains again: Working on a MPX 65 with no separation ( again)
All new tubes , good pin contacts, recapped audio path and resonance caps and electrolityc, good 19k pilot stable lissajous locking @ 38 Kc ( no free wheeling) equal signal both sides , checked diodes OK . At mpx output I can not obtain a regular even sine, nor separation at all. This happens with both 300"s. Any hints?? Thanks Martin
 
If everything you say is true, then either the composite signal is compromised before reaching the detector mixing network, or the matrix stage is not combining the L+R and L-R signals properly. Is the separation control working properly? Are you driving the MPX-65 directly, or from the receiver?

Dave
 
Here goes: #1 is mpx output of channel A. #2 is grid matrix channel A. #3 is grid matrix channel B.
 

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The first pic is basically one of cancellation -- not addition. If you pull the 38 kHz oscillator tube, what happens to the signals at the matrix tube grids?

Dave
 
# 1 signal at grids ( both the same) pulling osc. tube.
# 2 signal with suppressed pilot.
#3 adjusted scope for #2
thanks. Martin
 

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Uh, where's the L+R signal??? How are you modulating the composite signal? What does the signal look like at the plate of the 19 kHx separator tube? And have you check for proper operation of the 67 kHz trap?

Dave
 
I am modulating 1KC left only
Pic #1 is signal on 1rst plate of 19 kc osc., pic #2 is 2nd plate.
The trap was adjusted to minimum @ 80 Kc , early on.
 

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I'm not sure I understand your terminology: The oscillator operates at 38 kHz. The pilot is 19 kHz. There is not a 19 kHz oscillator. The point is, I'm trying to understand if the first pic is at the plate of the 19 kHz separator tube, or the first plate of the 38 khz oscillator. If those are the signals at the two plates of the 38 kHz oscillator, then something is very wrong with the 38 kHz transformer, or the load presented to it. The signal at both plates should be equal in frequency and amplitude, but 180 degrees out of phase.

Dave

Dave
 
OK": The previous pics were obtained from the 19 K pilot tube V 100. The following were obtained from the 38 kc osc plates: #1 from 1 st plate, #2 from 2nd plate.
 

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OK. Going back to your post# 288. The left pic is a shot of the grids at the matrix tubes with the 38 kHz oscillator removed. Please expound further on where the center and right pics originate from.

Dave
 
The ctr pic is from same place( matrix grid) , osc tube pulled AND applying pilot suppression.
Right pic (#3) is same signal as ctr pic, with scope adjusted.(increased sweep freq and sensitivity)


Martin
 
So if the only difference between the left and center pic of that post (288) is that you've pushed the 19 kHz Cal Phase button to suppress the pilot, and haven't changed anything else, then what happened to the L+R audio signal in the center pic, that is apparent in the left pic? What am I missing here?

Dave
 
What does the center pic with L+R signal should look like? By the way the post 228 you are referring to shows as post 221 on me ( but I am sure we are talking of the same post)
 
MY MISTAKE!! I was pushing amplitude cal and not phase. Sorry. I repeated the step
and I get the same as pic #1 but more stable .
 
OK. So post #286 shows that the AM sub-carrier is making it to each matrix grid, and post #288 shows that the main carrier is present there as well. Therefore, the issue must be matrix related -- as in, it's not happening. Just a thought: How is the B+ decoupling cap at the distribution point that services the matrix stage? If it has gone wonky, so will the separation....

Dave
 
I'm not talking about the composit coupling cap on-board the multiplex chassis. I'm talking about the power supply decoupling cap on the main chassis, that supplies the matrix stage on the multiplex chassis.

Dave
 
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