Atwater Kent Radio Servicing Question, WAY O.T.

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Model T Ford Forum: Forum 2011: Atwater Kent Radio Servicing Question, WAY O.T.
Top of pagePrevious messageNext messageBottom of page Link to this message  By Jerry VanOoteghem on Thursday, December 15, 2011 - 12:36 pm:

I know there's a lot of old radio guys here so maybe one of you can help me.

I recently bought a Model 10C breadboard. It had an open AF transformer which I replaced. That got the radio working. Everything is fine except the radio is very "microphonic". The slightest bump or touch will cause a ringing in the speaker. It's so sensitive that, if the speaker is pointed at the AF amplifier tubes, it will set up an oscillation just like pointing a microphone at a speaker. I know this problem begins in the AF amp tubes and can be caused by a loose tube element. So, I swapped out several tubes with no change. Any ideas?

Just in case a Model T Forum policeman checks this out: "Model T Ford" There, now I'm back on-topic. ;>)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Christopher Lang - Brentwood Bay, BC on Thursday, December 15, 2011 - 12:37 pm:

I can't help, but I sure would like to see a picture of it.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ricks_-_Surf_City on Thursday, December 15, 2011 - 01:36 pm:

Are the RF sections Superhet, or TRF, or Regenerative? Could it be running too close to regen?

It's been a long time... I have several old radios that I put off restoring, "until I retired." Been retar'd 12 years now, and haven't touched a one of them. Must be that I didn't work on them back then, because I really didn't want to. I still don't. I'll be putting a 1935 McMurdo Silver Masterpiece IV on tbay soon. I've owned it since 1970, and haven't used it very much.


Top of pagePrevious messageNext messageBottom of page Link to this message  By steamboat on Thursday, December 15, 2011 - 01:58 pm:

We used to find microphonics by putting our hands on things. For tubes, especially metal tubes, wear gloves. Are any shields missing? They get tossed at times. Tapping with a pencil my help find the problem. Sometimes capacitors (condensors) and resistors get microphonic also. If a tube is microphonic tape or a rubber band cut from an old inner tube may quiet it.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jerry VanOoteghem on Thursday, December 15, 2011 - 02:51 pm:

Ralph,

It's a TRF. Over-Regeneration would be oscillation. This is not oscillation. This is a ringing sound, in the speaker, in response to even slightly jarring the radio, or even bumping the table it's on.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Gary H. White, Sheridan, MI on Thursday, December 15, 2011 - 03:18 pm:

Sounds like a loose element in one of the tubes. Try tapping each one lightly with a pencil or orange stick.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Gary H. White, Sheridan, MI on Thursday, December 15, 2011 - 03:24 pm:

Never mind. Reread you orig post. Still sounds like some component is loose. Might check for a cold solder joint.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jerry VanOoteghem on Thursday, December 15, 2011 - 03:34 pm:

Gary,

Interesting that you mention a cold joint. In going through this radio I found 2 joints that had NEVER been soldered. They were absolutely clean of any trace of solder. One was for one of the detector filament contacts. It's hard to believe the radio ever worked at all. The wire must have been touching and worked for some time that way.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Howard near Pittsburgh PA on Thursday, December 15, 2011 - 04:40 pm:

201 A tubes tend to be microphonic. In some TRF sets the detector tube is on a cushioned base. Try putting the speaker on a piece of carpet. I have one of these AK breadboards.
Howard


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jerry VanOoteghem on Thursday, December 15, 2011 - 05:00 pm:

Howard,

Good idea about the carpet.

Thanks!


Top of pagePrevious messageNext messageBottom of page Link to this message  By John H on Thursday, December 15, 2011 - 05:23 pm:

Being a breadboard set don't overlook loose terminals. Should one of the terminals be slightly loose, it can create an effect similar to a carbon microphone. And given all the terminals are attached to the breadboard, the "microphone" effect can be difficult to trace to one particular connection.
Another way to narrow down the fault is to simply use a pair of headphones as a signal tracer; start at the detector - if the microphonic characteristics are evident, then it's the detector or preceeding RF stages. The RF stages can be eliminated as being the problem by removing their vacuum tubes and connecting the aerial straight to the detector.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Don Keil on Thursday, December 15, 2011 - 06:35 pm:

There was a lead cap that was used on O1a's to reduce microphonics. good luck finding any.


Top of pagePrevious messageNext messageBottom of page Link to this message  By John F. Regan on Thursday, December 15, 2011 - 07:11 pm:

Hi Jerry:

I have an AK 20C (compact model) and having open audio transformers is pretty common problem. If you have a scope you could probably trace to see if any one stage is more sensitive than another to be sourcing the microphonics. In later radios sometimes a capacitor that has been subbed in with too large of a value makes a stage amplify AF when it really is supposed to be RF or IF only. Metal parts that can move next to an inductance such as a rewound AF transformer can induce a signal just like a microphone does.


Top of pagePrevious messageNext messageBottom of page Link to this message  By John F. Regan on Thursday, December 15, 2011 - 07:19 pm:

Jerry I had a thought. Since the volume on these sets is accomplished by controlling the actual emission of the tubes via the filament voltage, I wonder if you might have the volume up way high to get the set to play. That would make the set more microphonic. If possible, try to put a better antenna on the thing so that you get more signal and can then turn the volume down. Likewise if your detector stage is weak you would likely have the volume up to compensate. Not saying this is it for sure but these things are not at all like modern radios and you need a good sky hook to get them to work. Mine plays pretty good.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Garnet on Friday, December 16, 2011 - 03:50 pm:

Here's a few pics of microphonic sheilds for those not familiar with them. They are just a heavy weigh meant to damper motion in the tube. If you have other 201's in the set, swap them. I know it's anoying, but to me its' just another pleasurable part of a great hobby I've enjoyed for just over 30 years now.

Regards all,
Garnet










Top of pagePrevious messageNext messageBottom of page Link to this message  By Michael K Mullis on Friday, December 16, 2011 - 05:14 pm:

I've found that the later "ST" shaped 01As to be far less microphonic.

WD-11s in my experience are the worst, but thats not a problem for an AK BB

Jerry,
Did you replace the AF transformer or just the AF transformer windings?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Arthur J. Solie on Saturday, December 17, 2011 - 12:17 am:

Most of the 201-A tubes were microphonic as were the 200-A and 240 tubes. The grid leak detector and first audio were the most likely to cause trouble. If separate B+ inputs are available, setttine the detector B+ to 22.5 volts may help. A wrap of soft foam or cap as described above helps too. Grandma used to wrap the detector with a soft handkerchif on the AK model 20. Art Solie in Pahrump.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ricks_-_Surf_City on Saturday, December 17, 2011 - 12:36 am:

Good to see you back here, Art.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jerry VanOoteghem on Monday, December 19, 2011 - 12:45 pm:

Thanks guys. John may have the problem nailed. I am playing this set with a modest loop antenna and I do have the rheostats turned way up. John, your 20C is just about electrically identical to the 10C.

Also, Mike's pointing out that ST shaped tubes not being as bad is interesting as well. The AF tubes I'm using are the tipped brass base style. Mike, I replaced the transformer with a modern one made for use as replacements in battery sets.

Also learned a lesson from Garnet. I thought those caps were to shield the tubes from stray electrical fields. (I know that there are styles that really do shield in that way.) I now know the true purpose of that heavy style cap.

Thanks to all. I've got a few things to try now.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Michael K Mullis on Tuesday, December 20, 2011 - 12:08 am:

Posted this photo in another post.
RCA Radiola III, Balanced amp and UZ1325 horn.
Running 4 WD11s its microphonic-o-rama.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jerry VanOoteghem on Thursday, December 22, 2011 - 03:13 pm:

To the person who kindly offered me the tube isolation cap. I lost your email. Please don't think I ignored your offer.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Arthur J. Solie on Thursday, December 22, 2011 - 11:08 pm:

I have a Radiola III with 2 WD-11's and a III-A that has 4 WX 12's as shown in Millis' post above. The III-A has been modified to allow the use of an external mic trans, carbon mic and battery to allow use as a two-way radio by turning up the regeneration a bit. Art Solie in Pahrump


Top of pagePrevious messageNext messageBottom of page Link to this message  By Garnet on Friday, December 23, 2011 - 07:58 am:

Canadian General Electric's versions of the III and III-A had gold trim - very smart looking. A few companies made adapter wafers to use common 01 tubes.

Garnet


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