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Noise in 680X and 37-690 - Printable Version

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Noise in 680X and 37-690 - Fred R - 12-28-2005

I have restored a 680X, 37-690 and 38-690 a number of years ago. The 38-690 works great especially after reading Ron's notes on the tweeters. Both the 680X and 37-690 have had an annoying static in the audio section, still there with the volume down. It seems to be the driver transformer's primary. The 680X developed a problem and I have to pull the chassis out anyway. This is the version that has the dial under the lid and is the worst design for pulling the chassis as the cabinet has to be laid down to pull it. I am hoping it's something other than the driver transformers. Both sets (the 680X when it was working) seem to quiet down when on for a while. Has anyone who has worked on these sets ever had this problem?

Fred R

RE: Noise in 680X and 37-690 - Ron Ramirez - 12-30-2005

A December 15, 1936 "Changes In Models" sheet issued by Philco gives the following info for Model 37-690:

ELIMINATING NOISY RESISTORS IN POWER UNIT

To eliminate the noise caused by the 80 and 325 ohm sections of resistor (177), (referring to Fig. 6 and the schematic diagram of Bulletin No. 267), two flexible resistors, Part No. 33-3027, 75 ohms and Part No. 33-3121, 300 ohms, are now used in place of these sections. To replace these resistors proceed as follows:

First install a wiring panel (Part No. 03103) under condenser (175) mounting screw. (Be sure condenser lug is grounded after panel is in place.) Then disconnect the green and white wire, from terminal 4 of resistor (177) and connect it to the wiring panel lug. Remove wire between terminal 5 of resistor (177) and terminal 6 of power cable socket. Now connect resistor 33-3121, 300 ohms between the terminal panel lug and terminal 6 of power cable socket. Then add the 75 ohm resistor, Part No. 33-3027 between the terminal panel and terminal 3 of resistor (177). After doing this disconnect the brown wire from terminal 5 of (177) and connect it to any ground on the chassis.

*****

Part (177) is one of those infamous Candohm resistors. In addition to the above changes, Ghirardi (in his "Radio Troubleshooter's Handbook" recommends replacing the 3000 and 2,240 ohm sections of this Candohm with a new unit. Of course you can no longer buy Candohms, so replace with new individual resistors.

Other possibilities of noisy operation in the 37-690: For the Model 680, Ghirardi mentions these possibilities. Hope this helps.

- Fred R - 12-30-2005

Thanks Ron. That is the kind of info I was looking for. I have all the schematics but only a few service bulletins. I do know the crackling noise is not in the power stage but somewhere in the driver stage on both sets. Pulling the 6F6G (or the 42 in the 680X) the power stages are absolutely quiet.
By the way, here's a story on my 38-690. It belonged to a friend of mine who paid someone to restore it. I knew the fellow who did the rebuild and it was a fine job. My friend introduced me to his fiance and the first thing she said to me was "Hi I'm Nancy and I hate radios". Needless to say, within a month he called me to see if I wanted to buy it. I think it cost me about $400 and that was over ten years ago. The only thing I did with it was to remove paint off both chassis. Someone who owned it before my friend had painted both chassis with silver paint. They were not rusted. I can only guess they wanted it to look like a Scott. After about three evenings and paint remover and some q tips, it looks like it should now.

Fred

- Ron Ramirez - 12-30-2005

"Hi I'm Nancy and I hate radios."

That poor schmuck...

- Chuck Schwark - 12-30-2005

Icon_eek Ouch!

"Here is my list of the approved hobbies you can enjoy..."

(Do I detect a royal "We" in there????)

- Frank - 04-13-2006

What wattage resistors should be used for these candohmn replacements?

Noise in output - AI2V - 04-15-2006

Don't go completely nuts in hunting down bad resistors until you have swapped out your output amplifier tubes with known good ones. 42's are notorious for intermittent frying bacon sounds- I've got a box of noisy ones. Noisy 42's can sometimes be used in the audio driver stage, but never in the final stage.

- Ron Ramirez - 04-16-2006

FWIW, the 680X uses 6A3 output tubes; the 37-690 uses the octal equivalents, 6B4G.

- Frank - 04-19-2006

My 37-690 is making the same kind of noise as described in the first post. There must be a common problem with these sets. My set already had the Philco modification done to it. They also replaced the candohm with a new Philco unit that contains only two sections. I went ahead and re-replaced the 300 and 75 ohm resistors that were Micamold resistors with new 5 watt resistors. Still makes that cracking sound.

There is a voltage fluctuation that occurs when this noise happens. I put a lug from my meter on to one of the primary leads of the driver transformer and saw this happen. More tracing needed, I guess....

- Frank - 04-24-2006

Ok, I did some more tracing this weekend, and I think the Philco replacement candohm is causing the noise. When the noise occurs, the voltage at the top of the resistor rises, and at the other lug, the voltage decreses in sync with the noise. This is for the 3000 ohm section. So, I'm going to order some 25 watt resistors and install them in place of the 3000 ohm and 2240 ohm sections. This probably will fix the problem, but at the very least, remove one potential source of this noise.

Will post a follow up when done.

- Frank - 04-25-2006

Long story short, the 3000 ohm section was causing the noise. Replaced both candohm sections and the problem is gone. Recommend changing this out to every 37-690 owner with a noise issue.

RE: Noise in 680X and 37-690 - Sam samuelian - 05-22-2012

I just received my newly restored Philco 38-690 and was disappointed that my three other consoles all sound better and have no hum. They are a Zenith 12A58, an RCA 811, and a rare 1932 Story and Clark.

The Philco was completely re-capped, has all new old stock tubes, and bad resistors were replaced. I could NOT get my guy to replace the components in the two tubs. One tweeter was bad and he hooked up a modern tweeter in it's place. He also put in an FM tuner in the rear that can be switched on and off. That tuner is the ONLY sound producer as all other bands are silent. Worst is the heavy hum that lessens as you turn down the bass and also lessens when you turn up the volume. Does anyone have any idea what needs to be done? I now have over $3,400 into it and am far from happy. I bought it since everyone said it was Philco's best and the best sounding radio they ever owned. HELP!

RE: Noise in 680X and 37-690 - Arran - 05-22-2012

Quote:I just received my newly restored Philco 38-690 and was disappointed that my three other consoles all sound better and have no hum. They are a Zenith 12A58, an RCA 811, and a rare 1932 Story and Clark.

The Philco was completely re-capped, has all new old stock tubes, and bad resistors were replaced. I could NOT get my guy to replace the components in the two tubs. One tweeter was bad and he hooked up a modern tweeter in it's place. He also put in an FM tuner in the rear that can be switched on and off. That tuner is the ONLY sound producer as all other bands are silent. Worst is the heavy hum that lessens as you turn down the bass and also lessens when you turn up the volume. Does anyone have any idea what needs to be done? I now have over $3,400 into it and am far from happy. I bought it since everyone said it was Philco's best and the best sounding radio they ever owned. HELP!

Well for starters I would find another radio guy, this guy sounds like too much of a butcher to be working on such a complex set, why it does not play could be caused by a component failure or simply bad workmanship. I don't know what he charged you for this "service" but I would very much contemplate taking him to small claims court after installing an FM tuner in the back of such a radio, if it was something like a Philco 40-180 that would be one thing but a Philco 38-690 is quite another matter. I don't know what tubs you are refering to but if there are capacitors inside those are likely about to fail if they haven't done so already. With regard to the bad tweeter I really hope that you still have it and that it wasn't tossed out, those are not exactly easy to come by if you need one and they can be repaired or rebuilt.
Regards
Arran

RE: Noise in 680X and 37-690 - Ron Ramirez - 05-22-2012

Ditto what Arran said. I hope you still have the "bad" tweeter, original Philco tweeters are pretty much unobtainium.

Now, if by "tubs" you mean "bakelite block capacitors," there is one under the main (upper) chassis that is not a capacitor - it contains the coils for the set's 10 kc audio filter! Never, ever open this bakelite block up!

The 37-690 sets have some residual hum thanks to the directly heated filaments used in the 6B4G output tubes, but the 38-690 should not have much hum at all, if any, since the 1938 version used 6L6G outputs with indirectly heated cathodes.

RE: Noise in 680X and 37-690 - Sam samuelian - 05-22-2012

Hi Ron and Arran,

Thanks so much for your quick replies. You obviously sensed how disappointed I was. There is some good news--I now am getting radio signals (AM, etc). I think the problem was the tiny switch that he mounted on the back of the set to go between AM and the FM tuner he installed (it doesn't effect originality at all--it can be disconnected). I actually asked for that since we have so few good AM stations here anymore and I wanted to enjoy better music. My restorer charged 200.00 per chassis, 300.00 for refinishing and 100.00 for wiring in the FM tuner and installing a new replacement tweeter. I DO have the old one and am considering having the coil rewound--but not until I correct this bad hum. I talked to Alex (he sells on ebay under "Anna17608" I believe) and he suggested I try running a wire between the two chassis to ground them. I haven't tried it yet. Alex's specialty is Zenith radios, something I didn't know when I selected him for the job. I should have looked around more and I definitely made a mistake having Alex go get the set for me (six hour drive) from the ebay dealer because he charged me $350.00 whereas all three consoles that were delivered to me last night from far apart states only cost $400.00. By the way, this small family operated shipping service is great. Very careful with handling, they come when they say they will, and are very cordial. Name is AMTTRANS out of Alabama. If anyone needs great service with very good prices, call Marie (she has a strong British accent!) at 1-256-275-4341. Incidentally, Alex refused to re-do the "tubs" or bakelite blocks despite all my coaxing and offering more money. I sent him the excellent info about doing it on the Phorum. I was careful to tell him to avoid the third one that is a coil. I understand there is also a capacitor that is very hard to see that many restorers miss. I guess all I can do at this point is hire someone locally (possibly my friend who posts here--Fred Rice--to help me. Oh yes--one other interesting fact. Alex told me that when he disconnected the woofer the hum goes away. Does that tell you anything? Also, any recommendations as to who can restore the bad original tweeter? Ever onward.....