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. ;>)
I can't help, but I sure would like to see a picture of it.
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.
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.
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.
Sounds like a loose element in one of the tubes. Try tapping each one lightly with a pencil or orange stick.
Never mind. Reread you orig post. Still sounds like some component is loose. Might check for a cold solder joint.
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.
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.
Good idea about the carpet.
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.
There was a lead cap that was used on O1a's to reduce microphonics. good luck finding any.
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.
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.
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.
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
Did you replace the AF transformer or just the AF transformer windings?
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.
Good to see you back here, Art.
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.
Posted this photo in another post.
RCA Radiola III, Balanced amp and UZ1325 horn.
Running 4 WD11s its microphonic-o-rama.
To the person who kindly offered me the tube isolation cap. I lost your email. Please don't think I ignored your offer.
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
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.