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Lafayette Comstat 25B

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prcguy

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Is the receive audio ok? If so most of the circuits for receive audio are the same on transmit except for 1/2 of tube V5 which is a 12AX7 mic amplifier. That would isolate your problem down to a smaller area.

Did you ever run an amplified microphone on the radio? If so you might check or remove C53, a .001uf cap that limits high frequency audio in the transmit chain. Running a lot of transmit audio will sometimes exceed the voltage rating of this part and it will short out.

If you have an oscilloscope available it should be easy to trace the microphone audio from the mic jack through the mic amp then from the 6AQ5 audio amp to the final tube through R55, C37, D12, etc.

All of this is assuming the microphone is ok as mentioned before. Boa sorte!
prcguy
 

principiante

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Lafayette Comstat 25B

All components checked,ok.
Bird 4431 whit sample for osciloscope, dummy load 50.ohms.
Attach.pics.

73"-Jorge.
 

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prcguy

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Good you have an oscilloscope to play with. Can you look at the output of the microphone with the scope to see what the peak to peak level is and if there is any distortion when you make a tone into the mic? Then check again when the microphone is plugged into the radio. The peak to peak level should drop a little with the mic plugged into the radio but the scope display should not change much.

Do you have another similar microphone to compare? What kind of microphone are you using? Your radio needs a high impedance mic like an old crystal or ceramic element or an amplified mic. A modern low impedance dynamic mic will usually not put out enough voltage to fully modulate the radio.

Here is a diagram for the radio if you need: http://www.cbtricks.com/radios/lafayette/comstat_25b/graphics/lafayette_comstat_25b_sch.pdf

With the scope you can check for audio gain from the grid to the plate of V5 A and B. Then put the radio in receive with no squelch and the volume turned up and measure the peak to peak voltage at the plate of V6 with the scope. Then transmit and measure the peak to peak voltage at the same location while speaking into the mic. The levels at the plate of V6 between receive and transmit should be similar and if its very low on transmit then the problem might be in the mic preamp area around V5 A.

If the transmit and receive audio levels are similar at the plate of V6 then unplug V7 (osc tube) and measure the transmit audio at the plate of the final tube V10. It should be similar but a little lower than you measured at the plate of V6. If not it could be the components in the audio path between V6 and V10.
prcguy

All components checked,ok.
Bird 4431 whit sample for osciloscope, dummy load 50.ohms.
Attach.pics.

73"-Jorge.
 

RFI-EMI-GUY

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a fine radio, worthy of repair!

I had one. Only problem was one of the crystals in the "synthesizer" kept failing taking out several channels.

C67 dried out?

The Range Boost circuitry and switch?

Does the Mod Neon blink at all? It could mean nothing as those wear out.
 
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principiante

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Lafayette Comstat 25B

Components associated with v5a and v6 checked.
Waveform on the v5a pin and pins 1-7 on v6 as shown in the photo.

Substituted C-67
All functions are ok.
frequencies ok
range boost ok

Jorge.
 

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RFI-EMI-GUY

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Components associated with v5a and v6 checked.
Waveform on the v5a pin and pins 1-7 on v6 as shown in the photo.

Substituted C-67
All functions are ok.
frequencies ok
range boost ok

Jorge.

Are you saying the modulation envelope is now fixed? Does that flat topping go away if you reduce the audio drive level, and if so what does the envelope look like?
 

RFI-EMI-GUY

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Joined
Dec 22, 2013
Messages
6,859
Is the receive audio ok? If so most of the circuits for receive audio are the same on transmit except for 1/2 of tube V5 which is a 12AX7 mic amplifier. That would isolate your problem down to a smaller area.

Did you ever run an amplified microphone on the radio? If so you might check or remove C53, a .001uf cap that limits high frequency audio in the transmit chain. Running a lot of transmit audio will sometimes exceed the voltage rating of this part and it will short out.

If you have an oscilloscope available it should be easy to trace the microphone audio from the mic jack through the mic amp then from the 6AQ5 audio amp to the final tube through R55, C37, D12, etc.

All of this is assuming the microphone is ok as mentioned before. Boa sorte!
prcguy

Would removal of C53 introduce some unwanted RF rectification on D12?

Also I seem to recall that mistuning the PI network on these radios caused some "negative modulation". This from tinkering some 40+ years ago!
 

principiante

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Dec 31, 2017
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curitiba-brazil
Lafayette Comstat 25B

Thanks a lot for the help.
The result I got was this.pics
Changing the tubes should look better.
btw, where I get the lafayette comstat 9k scheme.
In manualman it is very expensive.

73"-Jorge.
 

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RFI-EMI-GUY

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I had the 25A. The audio was great when I paired it with a Turner +2 mic! ;)

Yup; In looking back I actually had the 25A not the B, and I had a blue Turner M +2 mic. Sold it off in Maryland to someone. Wish I had hung on to it. It was a very good performer, very sensitive.

Had another box, a "Lincoln Linear" that was commercially made, brown wrinkle finish and although it was some sort of linear amplifier, the output tube appeared to be only good for 15 watts or so, maybe 30 ICAS. I could never find any information about this.
 

TheSpaceMann

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The audio was amazing on the 25A. When I turned up the +2 mic, I could hear my voice vibrating in the tubes!! :)
 
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