32V-3 Audio Issue

[NQ5V]

My 32v-3 had been working fine for 35 years or so until recently.  Prior problems were easily identified and (not-so easily) resolved when they arose.  This transmitter is completely stock and except for having Dahl rewind the LV transformer a decade or so ago, and Solid state rectifier replacements, is mostly untouched.  I have replaced one multiplier tube in that time.

The current problem started out as a very intermittent report of "RF on the Audio"  - an occasional fuzz on voice peaks.  I rebuilt the Mic cable and tightened the chassis connector, each time thinking I had fixed it.  It was not, however, fixed, and has been getting worse.  This weekend on 7160, it started doing it more or less constantly.

Here's the symptoms:  at lower modulation levels everything appears normal - voice sounds good in the monitor, modulation current kicks normally.  When it gets up to voice peaks, though, it fuzzes and now on the biggest (but still normal) peaks the modulator current will go up to about half to 2/3 scale and stay there until there is complete quiet or I release the PTT.  Almost like its breaking into oscillation, except its a buzz or fuzz not a tone.  If I keep the mic gain back to a lower level it doesn't  do this, but then everyone says I have puny audio (about 25%, by one report).

There aren't a lot of components in there and I would suspect first the tubes.  I don't think a tube checker will identify this particular issue, but will try to do some swaps when I get ahold of some 6sl7 and 6sn7 replacements.

I occurs to me that this could be a capacitor breaking down under voice peaks.  There are a number of paper capacitors in the preamp stages.  Right now my working assumption is that the problem is in the preamp or driver, not in the 807 modulator finals.

This has me thinking about the speech amp modifications I have read about in other sections of this forum.  Can anyone give me a little further information on these suggested mods?  It looks like they are replacing nonpolarized bathtub capacitors with electrolytics.  Finding nonpolarized capacitors in the sizes and voltage ratings required is pricey. 


I appreciate any advice.  I'd like to get the A Line back on the air.

Steve

NQ5V

[w1vtp]

Has the modulator resting current changed from before?  Sometimes when one tube of a push pull goes soft or has major issues, the result will be a fuzzy sound. The resting current will change too with this problem.  

Can you still hit the same level of modulation as before except that the sound is fuzzy?  Another possibility is that a interstage coupling cap is leaking thus putting a positive bias on the grid of the next tube thus distorting the AF.  Depending on the amount of + bias, it might sound OK until you start to try normal levels and then the positive of the following stage will clip.  The modulator resting current will not change with this problem..

Al

[AB2EZ]

Steve

Adding to what Al suggests:

Try running the 32V-3 into a good dummy load, with a short length of coax between the RF output connector and the dummy load. See if the problem goes away.

It may be that RF is getting into the microphone / microphone cable. If so, using a dummy load may have an effect.

What type of microphone are you using?

Is there any audio processing equipment between the microphone and the transmitter's audio input?

Stu

[w1vtp]

Stu

Absolutely agree.  Probably a good first step whenever one runs into a distortion problem

Al

[NQ5V]

Thanks for the input.  I haven't seen any change in the resting current being drawn by the modulators.  

My suspicions are the interstage coupling cap as you suggested, but I will put the thing on a dummy load to isolate RF in the audio as a possibility.  The current mic is a D104, crystal, no preamp.  I had not had this problem with prior mics I had used in the past, but this one worked for a year before the problem cropped up, plus I have rewired the connector, mic PTT switch, and mic head connector a couple of times on the assumption that RF was the problem.  

The modulator current holding like it is on voice peaks leads me to suspect that RF in the mic is not the issue.  I could be wrong.  

I'll check the operation with the dummy load.  If it is RF in the Mic, It will be interesting tracking it down.  

NQ5V

[W3RSW]

Run a short wire from spring in base directly over to mike input on 32v w/ alligator clips to see if that helps grounding, hum and RF problems.  Does on my coiled wire d104.
No difference on the straight cabled one which is ok anyway.

Also try from direct connections of mike element to 32v. Will have to lash up PTT - short the PTT pin on xmit., etc.  --there's   a lot of wiring and switching in that base for potential trouble. Solder joints 'n stuff.

[NQ5V]

Thanks everyone for the good suggestions. 

Ultimately it was an RF in the Mic problem.  Tests on dummy load showed that it must be.  Audio sounded weak on the dummy load though.

It was very intermittent and I noticed that one particular small movement of the mic cable seemed to cause it.  (this is not the coiled type cable)

I had enough cable to cut the Mic end of it back about 8" and redress the whole thing to the terminal strip in the base.  I think this might have also been the cause of the weak audio - its much more full now, and I will have to recheck levels.

I also went through the modulator bias setup adjustment again, because on checking the manual I realized the modulator resting current was too low.  This may also have a beneficial effect on the audio - haven't had a chance to do a full test or to get on air reports yet.  The mod bias setup procedure on the V3 is a bit of a pain and potentially dangerous.  I doubt if I will have to do it again soon, though.

Steve
NQ5V   

[flintstone mop]

What do you see when you look at your modulated RF using a 'scope? Everything clean even when you are pushing maximum? Oscillations would show up as very fine traces within the display when modulating. There are some very good youtube vids showing how everything should look on a 'scope, and how to setup for the proper display.  A 'scope is almost mandatory for the AM mode.
Fred

[KL7OF]

You asked about audio mods....I did the Tron mods to my V-1...uses electrolytics to replace the bathtub caps in the original circuit and additional electrolytics  added to filter here and there...Also a negative feedback loop coupled in with a cap and resistor...These mods worked well on my radio after I adjusted the values on the cap and resistor in the feedback circuit to get the correct amount of feedback...  The values that Tim specified gave too much feedback for my radio so some adjustment was necessary..I did this a while ago and don't remember what values I wound up using...Good Luck....Steve

[AB2EZ]

Steve

You may also wish to check if the level of RF in your shack has increased since the time before this RF-into-the-microphone problem emerged. I.e., it could be that increased RF in the shack is the culprit, rather than just a problem with the microphone or the microphone cable.

There are lot's of reasons why RF in the shack can increase.

For example, if you have coaxial jumper cables connecting things together (like a jumper cable between the output of your transmitter and a power/SWR meter or a T/R relay)... and if the shield of the coaxial cable in the jumper cable is loose at one end (a high resistance contact between the shield and the PL259 plug)... then the current that should be passing through the shield of that jumper will find another path, if it can. If you have a ground bus connected to various pieces of equipment, then that ground bus can serve as the other path. This will lead to an RF "ground loop" that radiates a lot of power into your shack.

Example: there is a path from the grounded chassis of the 32V-3 through the wire that leads to the shack ground bus... and, from there, through the wire that leads from the shack ground bus to your receiver... and, from there, through the shield of the coaxial jumper cable that runs from the receiver to the TR switch. This path would bypass a defective shield connection at one end of the coaxial jumper cable that goes directly from the 32V-3 to the TR switch.

If you don't have a shack ground bus, then 3-wire power cords can also produce a path similar to the one described above.

You might want to disconnect at least one end of each coaxial jumper cable (if there are any)... and check the resistance from the connector shell at one end of the jumper cable to the connector shell at the other end of the jumper cable.

Stu