Boat Anchors... it's always something!!!

[KX5JT]

So apparently I have a coupling capacitor or "something" breaking down under high voltage in my modulator.

The other day I had headphones on and I was monitoring myself while in a QSO.  I was hearing some strange things with my audio.   A subdued crackling and rushing sound.   I had asked people if they could hear it but on 40 meters on the edge of the noise floor, it was not a reliable way to find out.   They said they couldn't hear it.

This morning Don K4KYV mentioned he heard something on my audio that sounded like a rushing sound and maybe some component is breaking down under high voltage.    I shut all the receivers and listened close.   Just idling, I can hear something that sounds a lot like a hard disk drive being accessed by a computer.   Seriously this is the perfect analogy.  You know when you touch your computer case you can feel the vibration and hear the hard drive "purring" as it's stepper motor moves the heads back and forth?  I hear this.  My audio is way down too... it's subdued compared to normal when these sounds are occuring. 

Here's a BIG clue too.  It comes and goes with the AUDIO drive on the front panel.  The more open and higher the audio gain, the louder "flickering" sound and the more I feel the vibration on the Vikings case.   Could this simply be the audio pot?   Coupling capacitor?  I changed the 807s just because I have two pair (NOS recently purchased) but the problem is still there.

This is now the priority, the noise on the computer and the VFO are going to wait until this is resolved now.  Love this hobby!!  Any advise and direction is appreciated.

How do you go about safely pinpointing the component making the noise?   I am only comfortable sticking my head so close to a live chassis.... which is not too close.  Stethoscope?  But that has metal pickup..... is there something like a stethoscope with a rubber tube?

John

[KA2DZT]

May be the mod xfmr making the noise and/or vibration.

[kb3ouk]

A piece of rubber hose held up to your ear on one end and using the other end to listen at the chassis might work.

[WQ9E]

John,

When you say it increases with audio drive level, does it do this without audio  input (i.e. disconnect the mic, or cover the mic head, if it is PTT only).  It it is related to audio gain but not input level it sounds like you have something going unstable in the audio, perhaps RF into the audio system.

Is the power output steady?  I heard a similar sizzling/light crackling noise in a Hallicrafters HT-31 amp I was restoring for a friend.  In that case it was due to the DC blocking cap in the final output tank leaking.

If it is something arcing it will get worse and to the point of causing damage so proceed cautiously with testing.  Try looking at the rig in a darkened room, pull any unneeded indicator bulbs and block the light from tubes as necessary.  Feed through insulators are often the culprit.  A loose connection can also cause this type of noise.  I had a sizzling noise in my Desk KW that was due to one of the chokes in the 872A anode leads that had a slightly loose connection at the plate transformer.  Since it came with SS replacements I removed those chokes since they serve no purpose without the noise causing MV rectifiers in use.

[WA2SQQ]

If the noise goes away when you disconnect all the audio input sources than it might be something external being fed to the radio. Perhaps RF getting into something externally. Does the noise change when you decrease the RF output?

[W3RSW]

Need more info. What's the circuit?  Is it a tube type modulator? Early audio stages may have a microphonic tube or similar Intermittant tube or other component breakdown. May not be high voltage in the later stages but lower voltages in the early stages.
I trust you've changed out the tubes in experimentation.

[N2DTS]

Could it be feedback?

Nothing is going to break down just because the mic gain is turned up (with no audio).
With modulation, then look at the plate coupling, bypass and fixed loading caps if any.
Without audio its just the 1st stage mic preamp, with audio its anything after the mod transformer or the mod transformer itself.

[flintstone mop]

Do you have a 'Scope or SDR to SEE what your signal looks like in an RF kinda way?? The modulator is in some sort of oscillations?

Fred

[KX5JT]

It happens with the mic disconnected but the audio gain on the front turned up more than 0.  The higher it's turned up the more pronounced and louder things are.   I think it might be coming from the interstage transformer.  

[WQ9E]

If it is happening with the mic disconnected you either have instability in the audio section or RF getting into it.  Otherwise the most that should be seen in the audio channel is a little hum as the gain is run near maximum.  It sounds like something is oscillating in the audio section, probably at supersonic frequency, and you are hearing the audible side effect of this oscillation.


Which Johnson rig are you using and is it stock or modified?

[N2DTS]

Then I do not see how it could be anything after the mic gain control.

It happens with the mic disconnected but the audio gain on the front turned up more than 0.  The higher it's turned up the more pronounced and louder things are.   I think it might be coming from the interstage transformer.  

[KX5JT]

This is a modified Viking II.  It has some work done on it.   Valiant Modulation transformer but....I am fairly certain I found the (or at least a) problem.  

The interstage transformer seemed to be where I was hearing all the activity so I pulled it out and lo' and behold....

How do I fix this?  The lamination is loose.  What originally held this together?  Glue?  That part of the transformer doesn't appear to have outer metal that keeps the entire form together.  It's a gap there and the so the layers are coming apart now.... Certainly that's where I was getting all the fun rushing and talk back and clickety clatter?

[KA2DZT]

Glue it together, use a clamp overnight.

[KX5JT]

Excellent.  Anything special as far as what kind of glue?

[KA2DZT]

Any glue that will hold metal.  Epoxy it you have any.  Glue between the laminations doesn't matter.

[KX5JT]

Okay great, I have epoxy and clamps.. will do it!

[N1BCG]

There certainly could be something wrong with the interstage xformer but if the noise varies in level with the audio gain pot (R6) then it could also be C2, the 0.003uF coupling cap. It keeps the V1 plate voltage off the grid of V2 via R6. It will certainly be heard if it's going bad and developing a resistance.

[N2DTS]

Sorry, but you are not going to hear loose laminations over the air.
Mod transformers can talk back in the shack, and power transformers can buzz, but you are not going to hear that over the air unless it gets in the microphone.
Interstage iron is low power and you would not hear anything from that transformer, on the air or in the shack.
I would also replace C2....

[WQ9E]

Agree with DTS re the lamination.  You aren't going to hear that over the air and turning up the audio with no audio drive should not be doing anything to cause the lamination stack to make noise in a normally operating transmitter.  The direct noise you may be hearing  from the lamination stack is a symptom of the underlying issue in the audio chain.

A bad coupling capacitor or open bypass capacitor in the B+ distribution within the audio chain could be the issue as could RF into the audio.  If the 1 meg input stage grid resistor has gone open this can cause instability also.  At this point looking at the audio section with a scope is needed.  If you could post a schematic of the modified audio section that would also be useful. 

[flintstone mop]

I kinda think that listening for audible sounds in the transmitter case and no mention of using an Oscilloscope to monitor the RF, either from a radio with an I.F. out, or actually picking up the transmitted RF with a probe, will make things harder to diagnose. I'll take a risk and say that the laminations of the interstage transformer have nothing to do with the problem. It may even be defective...??

Fred

[KX5JT]

I have the transformer epoxied and clamped and will return it to it's rightful place probably Monday night when I'm off again.   I also have my scope now on the bench.  So once the transformer is back in place, I'll take it from there.  I suppose the coupling capacitor will get my attention next. 

At least that transformer will look a lot better after it's all set!

[w4bfs]

a good noise source is a small carbon comp resistor carrying current