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38-690 audio problem - Printable Version

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38-690 audio problem - chucka - 08-05-2012

Are there any documents available that describe the theory of operation of the audio section on the 38-690?

I have one on the bench, with one issue remaining, no appreciable voltage level out of the tweeter transformer winding. Tuning a local station and adjusting the volume for annoyingly loud output I still only measure 500mv to 800mv p-p output.

I've injected audio into the 6R7 grid at various frequencies and have good levels on the 6L6 grids, but still only millivolts on the tweeter winding.

I did find that the "whistle" filter now has the notch at 10.8KHz

Any help appreciated.

Chuck

RE: 38-690 audio problem - morzh - 08-05-2012

http://www.philcoradio.com/tech/tweeters.htm

RE: 38-690 audio problem - Ron Ramirez - 08-05-2012

The tweeters are driven by the driver tube in the 38-690, using a special winding in the audio interstage transformer.

Have you checked all windings of the audio interstage transformer (152) for continuity?

Also see the link Morzh provided above, for more information.

RE: 38-690 audio problem - chucka - 08-06-2012

Ron,

Yes I had seen the service note and made those mods.

Resistance readings on the transformer:

Winding_________Measured___________Schematic

Primary__________850 ohms___________475 ohms
6L6 Grid #1______270 ohms____________260 ohms
6L6 Grid #2______287 ohms____________260 ohms
Tweeter___________8 ohms______________???

Primary seems way high unless the schematic is wrong, tweeter winding, don't know, can anyone take reading on a known good set and check against my reading?

I did manage to redraw and simplify the complete audio circuit from 1st AF to speakers and believe I have a good idea on how the system works.
I was curious if there was a theory of operation paper somewhere.

Chuck

RE: 38-690 audio problem - codefox1 - 08-06-2012

Hold on, most of us need a scope for anything over 10 KHZ. Those nastieis still can make an otherwise nice radio or whatever misbehave badly. Small cap 1000+ VDC , say .002 mf across primary windings of output transformer usually works, especially if golden screwdriver go there before you.

RE: 38-690 audio problem - Ron Ramirez - 08-06-2012

I think there is a Theory of Operation of the 37-690 in a 1936-37 issue of the Philco Serviceman, but this would not do you much good since the tweeters are driven by the output tubes in the 37-690.

I'm not aware of any such documentation for the 38-690. In fact, there does not seem to be a Service Bulletin for the 38-690; only the schematic, under-chassis drawings, and a (typewritten) parts list. Quite bare-bones.

RE: 38-690 audio problem - Ron Ramirez - 08-08-2012

Thanks to Chuck Schwark for the scan...now you can download the February 1937 issue of Philco Serviceman that includes the article on bass amplification in the 37-690:

http://www.philcoradio.com/phorum/showthread.php?tid=5056

RE: 38-690 audio problem - chucka - 08-25-2012

After going through this set many times over, I have concluded that the interstage/output transformer is bad - the reason I have no tweeter signal.

I waited until I finished fixing the bad tweeter in the set, a burned up field coil, and was able to connect both speakers to come to that conclusion.

As an experiment I connected a generic output transformer across the primary of the interstage and connected the secondary to the tweeters, and guess what? I have sound coming from them.

So my options are to find another interstage from a donor chassis or add the output transformer under the chassis.

Does anyone have a parts donor?

The rest of the set has come together quite nicely, here is the IF response at minimum bandwidth and maximum:

[Image: http://www.myvintagetv.com/posts/38-690_IF2.jpg]

Chuck

RE: 38-690 audio problem - chucka - 09-08-2012

I have finally confirmed that the inter-stage transformer is bad in this set and my idea about using a separate output transformer to drive the tweeters won't work.

Below are a couple of pictures of the swept response of the 6L6 grid windings and the tweeter output winding:

[Image: http://www.myvintagetv.com/posts/690_trans.jpg]

On the left is the 6L6 grid winding response, the loss through the transformer is only 2dB, but the loss through the tweeter winding is between 14 and 18dB! The left side loss is the crossover loss.

I tried about 10 different output transformers, but every time I loaded the output winding the level dropped to virtually 0.

So unless I can find a replacement transformer, this one has beat me.

Chuck

RE: 38-690 audio problem - jerryhawthorne - 09-08-2012

Chuck, when at a loss, if you have the specifications for the interstage transformer and you can't find a used good one, you can always have one rewound. I had the problem on a Midwest receiver with a bad power xformer and sent it out with specs and a new looking one came back that fit my end bells perfect. The midwest had a nice pattern stamped on the end bell. They had a perfect match for the core and rewound on it a new one. If they don't, they can rewind on your old one. This radio is just too nice to not have everything right. As I recall, for a large power transformer it was around $100. Yours probably has less wire. I went to a company called "Boeur" I know I misspelled it but some one else can possibly give you a link. As I recall they are in the North West. I was very happy with their service and speed. Not attached in any way with them, just a happy customer. Too nice a radio to not have all of it looking and working right.

Jerry

RE: 38-690 audio problem - Ron Ramirez - 09-09-2012

Never heard of a "Boeur"...do you mean "Heyboer"?

http://www.heyboertransformers.com/

They are in Grand Haven, Michigan. They are great! Icon_thumbup Not cheap, but they do great work.

Now, don't expect immediate service because you won't get it for a "one-off" job; they work us hobbyists in between large orders so you may have to wait 2-3 months.

I think someone I know, who had the same issues as you are facing, had a transformer wound for a 38-690? I'll contact him and get back with you when I hear from him. Hopefully he still has the specs.

RE: 38-690 audio problem - jerryhawthorne - 09-09-2012

Thanks Ron for the clarification on spelling and location. I will say I had about a 1 week turnaround on a power transformer there. Must have been lucky and came in during a work drought.
Jerry

RE: 38-690 audio problem - chucka - 09-10-2012

Ron & Jerry

Thanks for the reply's, and ideas, I have had new power transformers made for some pre-war TV's by Harbach Electronics (Peter Dahl), but you really need the correct specifications or you end up with a doorstop. Without the correct parameters for the inter-stage transformer there's no telling what you will end up with.

Chuck

RE: 38-690 audio problem - jerryhawthorne - 09-13-2012

Chuck, hopefully someone on this forum has specs for that transformer. Somewhere in all the Philco tech stuff I would think this was documented. Sorry I can't help with that. Perhaps the re winder of the transformer can determine the wind ratio of each stage. I know on the one I sent out, we had some controversy on the current in output windings, they indicated no problem. They would measure the wire gauge. Also had them adjust the input level to 125V due to the higher line voltage at my friends house. You might give them a call and see if they can reverse engineer your transformer. Of course, I know nothing. Did love your scope picture of IF response in the low and high "fidelity" mode, helped me to understand what it does.
Jerry

RE: 38-690 audio problem - chucka - 09-13-2012

Jerry,

Not sure what I'm going to do with it yet, this set came in as a "cleanup" of a really bad restoration. The owner doesn't want to sink a lot more money into it, ,he's underwater as it is. There is a little sound from the tweeters.

Doing IF alignment with a digital spectrum analyzer and tracking generator really can spoil you.


Chuck