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 Post subject: Is this Silver Mica Disease?
PostPosted: Nov Thu 16, 2017 5:02 pm 
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Location: Potomac, MD 20854
I have a lovely HQ-180AX and I have read so much about the dreaded disease. What I hear are some static crashing type noises WHEN THE RECEIVER IS IN THE SEND MODE, WHEN I AM TRANSMITTING. Not too bad, but... the crashes are pretty constant for the first 20 seconds or so of transmission and then settle down to almost nothing but the "typical receiver background hiss" over time.

Is this SMD, or is SMD only apparent when RECEIVING?

paul


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 Post subject: Re: Is this Silver Mica Disease?
PostPosted: Nov Thu 16, 2017 7:25 pm 
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Hi Paul,

Static from SMD would always be present in RCV mode.
Whether or not it would be heard in XMIT depends on how the receive audio is muted.

SMD is only present in permeability-tuned transformers.
If your IFTs have trimmer capacitors, you don't have SMD.

- Leigh

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 Post subject: Re: Is this Silver Mica Disease?
PostPosted: Nov Thu 16, 2017 9:03 pm 
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Location: Potomac, MD 20854
Leigh,

According to the manual "In the send position, the B+ to the RF amplifier, the 2nd mixer and the 2nd IF amplifier is removed, muting the receiver." I thought he Hammarlund HQ-170/180 series all used permeability-tuned transformers? Anyway, I don't see trimmers under the cans.

In normal receive operation, all is fine. I was just wondering about the noise when "muted".

paul


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 Post subject: Re: Is this Silver Mica Disease?
PostPosted: Nov Thu 16, 2017 9:04 pm 
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I doubt if you have SMD in your HQ-180AX. The early production HQ-170/HQ-180 have it because of the open mica leaf capacitor construction used in the IF transformers but that was done away with long before your AX model was produced. Your receiver has encapsulated silver mica caps which RARELY fail.

And although SMD does exist it is far from the number one reason for unexpected noise and static although lately it seems to be the "go to" reason people jump on. You may find the problem is a tube, capacitor other than silver mica type, poor connection, etc. Under the wrong conditions, typically high DC differential across the capacitor, silver will begin to migrate and gradually reach the point where it intermittently flashes over (the crackle) before curing itself temporarily and starting all over again.

You will find the same basic mechanism at work in the tube type lab scopes from Tektronix when they are exposed to atmospheric pollution and the silver begins to migrate across the barriers between the many terminal strips used in the scope.

I expect the problem you are experiencing lies in the audio section or possibly in the relay used for your standby/receive switching.

Rodger WQ9E


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 Post subject: Re: Is this Silver Mica Disease?
PostPosted: Nov Thu 16, 2017 9:33 pm 
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Joined: May Tue 16, 2017 3:03 pm
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Location: Potomac, MD 20854
Thanks for the explanation!

p


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 Post subject: Re: Is this Silver Mica Disease?
PostPosted: Nov Thu 16, 2017 10:33 pm 
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K3STX wrote:
Thanks for the explanation!

p


You are welcome.

The first time I ran into this problem was with the phasing network in a CE sideband exciter and it could be a big problem there because it uses precision silver mica caps. However the capacitance value remained the same and the only required fix was to place a large value DC blocking cap in series with the troublesome mica cap to prevent the DC differential induced problem.

Any component can fail and several years ago a few months after restoring a Pierson KP-81 receiver it failed to operate. Fortunately the problem was a shorted silver mica cap in the IF section which was easy to replace. However that failure concerned me because it was a brand new production dipped mica rated at 500 volts that never saw more than 10 volts across it AND I had replaced all of the old mica caps in the very difficult to access front end module with caps from the same order.

My first HQ-170, purchased at a hamfest in Baton Rouge, had the original open leaf mica construction and a few months after I got it they begin to develop carbon tracks to ground. I removed all of them and replaced them with modern dipped mica types and never had another problem with the IF section.

Rodger WQ9E


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 Post subject: Re: Is this Silver Mica Disease?
PostPosted: Nov Thu 16, 2017 11:25 pm 
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This is from the HQ 110 and it was not the problem, but you can see what the transformer looks like and what the SMD looks like as it starts to migrate.

Norm


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 Post subject: Re: Is this Silver Mica Disease?
PostPosted: Nov Thu 16, 2017 11:38 pm 
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Norm,

Nice photo of the same nasty issue with my HQ-170! I suspect units that spent their life in a low humidity clean area worked a long time with that open construction but it is still difficult to believe an engineer felt that was a good idea.

If you find this issue with one IF transformer in a Hammarlund receiver then it is best to address all of them because it is highly likely others are on the verge of failure.

You will find the same issue in "postage stamp" mica caps, but hidden from view, when they are exposed to the migration causing DC differential.

The reason the Hammarlund IF capacitor problem is so great is they are also exposed to all sorts of atmospheric contamination and debris so even without silver migration they are subject to failure.

Rodger WQ9E


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 Post subject: Re: Is this Silver Mica Disease?
PostPosted: Nov Fri 17, 2017 12:43 am 
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Joined: May Tue 16, 2017 3:03 pm
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Location: Potomac, MD 20854
Norm, what am I looking at here? Is the little plastic sheet wit the "single" white thing normal and the double "abnormal"? Or is ALL the black abnormal?

Paul


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 Post subject: Re: Is this Silver Mica Disease?
PostPosted: Nov Fri 17, 2017 1:37 am 
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The black is tarnished silver--not a problem-- but the black fingers creeping toward the edge of the clear mica sheet--that is where the problem is. At least that is my understanding. This one is getting to the point it could soon cause the crashing noises as small current leaks jump the decreasing gap.

Norm

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 Post subject: Re: Is this Silver Mica Disease?
PostPosted: Nov Fri 17, 2017 2:03 am 
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Norm Johnson wrote:
The black is tarnished silver--not a problem-- but the black fingers creeping toward the edge of the clear mica sheet--that is where the problem is.

Correct.

Silver oxide is just as good a conductor as metallic silver, so it doesn't matter.

When the electroplating extends to the point where it shorts between two metal areas,
it creates a very small conductor that immediately blows open like a fuse element.

It's that continual forming and blowing open that sounds like "static".

- Leigh

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Click "Grebe Stuff" for Synchrophase info


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