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Elmac PMR-7- Mixer Screen Voltage




 
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Author Topic: Elmac PMR-7- Mixer Screen Voltage  (Read 442 times)
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W3GMS
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« on: November 26, 2017, 11:15:25 AM »

I am trying to help a friend get his PMR-7 working.    

If anyone knows what the screen voltage on pin 6 of V2 is on yours, I would appreciate knowing the value.  This is the 6BE6 which is the 1st mixer stage.  

Currently he is measuring only 14V on the screen.  Here is how the math works out:  

Screen voltage bulk source is 190V DC
Screen dropping resistor is 220K
Screen voltage measured is 14VDC

Voltage across 220K = 190V - 14V = 176 VDC

Screen current becomes:  

I = 176/220K
I = 0.8 MA

Verified correct value screen resistor
Verified screen bypass capacitor is good
Substituted another known good 6BE6 and got the same results.

If I had the RX hear, I am sure I could zero in on the problem.  Trying to help from afar!  

Symptoms are the IF stages are fine and when injecting a single into the plate circuit of the mixer, all is good.  I have been told, injecting a single at the grid of the mixer and nothing is heard.

So just curious what the screen voltage is intended to be on a working PMR-7.  Nothing in the manual showing tube voltage on individual pins.          

Thanks,
Joe-W3GMS  
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KA2DZT
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« Reply #1 on: November 26, 2017, 11:53:33 AM »

Sure there is plate voltage on the mixer??  No plate voltage and you would still hear a signal being injected at the plate.  What frequencies are being injected at the plate and grid??

Fred
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« Reply #2 on: November 26, 2017, 12:59:48 PM »

Hi Fred,

Yes, there is plenty of plate voltage on the plate of the mixer. 

Bill has this wide band square wave signal injector that he uses.  It has frequencies from DC to light.  He hears that signal when placed at the plate of the mixer tube, but not the grid.  I believe he is using 80M as his band to test it on. 

If I was there, I would use my signal generator and insert the proper frequencies.  He seems comfortable in using a BC-221 and I asked him to try that as a signal source. 

I am not convinced without more testing that the mixer is the problem.  Saying that, his 14V on the SG caught my attention since its very low.  The screen max current rating on the screen of the tube according to the tube manual is just under 7MA and as the math shows, he is only pulling 0.8MA.  I know they use a lot of 6BE6's as mixers on the HQ-170's and I know the voltage on the SG in that receiver is much higher, maybe around 100V or so. 

I am also trying to get him to test the RX out on 160M.  I asked him for now, if he can hear his TS-570 when he is on 160M but have not heard the results of that test.

73,
Joe-GMS       
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« Reply #3 on: November 26, 2017, 01:58:09 PM »

Joe

Screen voltage certainly doesn't look right.  Must be the screen resistor like you think.  I'm going to look at the schematics for a HQ-170 and a NC-183D.  Both receivers use 6BE6s.

Fred

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« Reply #4 on: November 26, 2017, 02:42:43 PM »

Since this receiver worked as designed and since the 220K resistor was the original value when shipped, it should be OK. Its measured value is still 220K. 

That is why I am hoping to find someone to give me the voltage they measure on their PMR-6 on pin 6 of V2. 

I first thought that the plate voltage may be missing or very low and the screen was pulling very high levels of screen current, but that is not the case.   

Joe-GMS   
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« Reply #5 on: November 26, 2017, 03:59:10 PM »

Looked at the 170 and 110, both have the same first three stages.  Pin 6 (G2-G4) show they're connected directly to the HV line (about 100 volts) or through a small screen resistor (about 5K).  Looked at the 183D and pretty much the same thing.  RCA manual shows 100 volts and about 7 ma.

220K screen resistor seems not to be the norm.  Could be his measurements are in error.  Another thing that comes to mind, could the receiver be in mute mode, maybe a missing jumper.

Fred
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« Reply #6 on: November 26, 2017, 07:51:29 PM »

Does the socket pin have a carbon track to ground?
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DMOD
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« Reply #7 on: November 26, 2017, 09:51:40 PM »

Unsolder C26 and see if the voltage go up.

Those multi section caps have been known to go bye-bye.


Phil
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« Reply #8 on: November 27, 2017, 09:42:47 AM »

Looked at the 170 and 110, both have the same first three stages.  Pin 6 (G2-G4) show they're connected directly to the HV line (about 100 volts) or through a small screen resistor (about 5K).  Looked at the 183D and pretty much the same thing.  RCA manual shows 100 volts and about 7 ma.

220K screen resistor seems not to be the norm.  Could be his measurements are in error.  Another thing that comes to mind, could the receiver be in mute mode, maybe a missing jumper.

Fred

Hi Fred,

I agree about the screen resistor seeming way to high of a value.  The color bands do indicate that its 220K which is the value noted on the schematic.  Plus, Bill measured it and indeed its 220K. 

Joe
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« Reply #9 on: November 27, 2017, 09:46:55 AM »

Unsolder C26 and see if the voltage go up.

Those multi section caps have been known to go bye-bye.


Phil

Hi Phil,

I had him unplug the tube and when he does that, he gets the same voltage on each side of the screen resistor.  If the cap was pulling the voltage down, he would see that when the tube was unplugged.   So with the screen current being 0.8MA, that current through the 220K is what is bringing the voltage down so low. 

Someone must have a PMR-7 out there that can measure the screen voltage for me  Smiley.   

Joe-GMS

 
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« Reply #10 on: November 27, 2017, 11:36:33 PM »



Hi Phil,

I had him unplug the tube and when he does that, he gets the same voltage on each side of the screen resistor.  If the cap was pulling the voltage down, he would see that when the tube was unplugged.   So with the screen current being 0.8MA, that current through the 220K is what is bringing the voltage down so low.  

Someone must have a PMR-7 out there that can measure the screen voltage for me  Smiley.  

Joe-GMS

Bummer.

My calcs show that for an average Pin 4 current of 6.5 mA and 100V (from tube sheet data using external excitation) and using 190V-100V/6.5 mA = about 13k so I would expect a resistor value of from 12k to 15k to be in there, unless they are doing something funcky like using a starved screen voltage system to get a higher stage gain.

Is it possible he could measure the voltage with a VOM, say a 20k or 10k/V VOM?


Phil
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« Reply #11 on: November 29, 2017, 09:06:58 AM »



Hi Phil,

I had him unplug the tube and when he does that, he gets the same voltage on each side of the screen resistor.  If the cap was pulling the voltage down, he would see that when the tube was unplugged.   So with the screen current being 0.8MA, that current through the 220K is what is bringing the voltage down so low.  

Someone must have a PMR-7 out there that can measure the screen voltage for me  Smiley.  

Joe-GMS

Bummer.

My calcs show that for an average Pin 4 current of 6.5 mA and 100V (from tube sheet data using external excitation) and using 190V-100V/6.5 mA = about 13k so I would expect a resistor value of from 12k to 15k to be in there, unless they are doing something funcky like using a starved screen voltage system to get a higher stage gain.

Is it possible he could measure the voltage with a VOM, say a 20k or 10k/V VOM?


Phil

Hi Phil & all,
 
Well Bill has put this receiver aside out of frustration at this point.  I have never heard of a mixer running this low of screen voltage, but at this point, based on all the measurements, I am beginning to think that is what it was designed to be that way.  I just don't have anymore suggestions for him concerning the mixer. 

If I had the receiver here, I am sure I could find the problem and it may indeed not be the mixer! 

Thanks to all that tried to help! 

Joe   
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« Reply #12 on: November 29, 2017, 11:34:54 AM »

Joe.

I looked at the schematic and the the mixer runs off a 250 volt line.  The screen resistor (R-26) is after the plate resistor (R-27).  I could not find online any parts list or any other info except for the schematic.  The oscillator is a separate triode and signal from it is applied to the grid between G-2 and G-4.  The incoming signal is applied to the main grid.  Only other thing that looked strange is there is a shielded line that runs from the mixer plate to the first IF xfmr.

The very low screen voltage remains a mystery.

Fred
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