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 Post subject: Modulation Hum with Distortion - Stumped
PostPosted: Nov Mon 27, 2017 10:05 pm 
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Working on a Canadian General Electric G-40 (Canadian RCA 94T-schematic in Radiomuseum) and very similar to US RCA model T4-8A. This is a 3-tube (plus rectifier) reflex with the IF/DET tube being a 6B8. Volume control is accomplished by adjusting the gain of the converter. The converter tube in my set is a 6A8. At the low end of the dial, stations come in loud and clear (as clear as could be with this set). Around the middle of the dial, I get modulation hum, and pretty bad distortion. I have tried several 6A8's in case of H-C leakage. Alignment has been done. I had to change out the socket as one of the tags broke off while re-capping. The only differences I see between my set and the T 4-8A in the converter section is that I have no wave trap at the antenna, my oscillator coil has a 600 Kc adjustment. Am I missing something?


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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Nov Tue 28, 2017 12:46 am 
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Location: Mountains of Mourne. Ireland.
Here's an accessible schematic for anyone interested.
http://radio.codegods.ca/node/7704

Greg.


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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Nov Tue 28, 2017 2:30 am 
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Has the original power transformer been replaced? Add a cap from one side of the line to chassis. Try .01 mf.

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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Nov Tue 28, 2017 4:00 am 
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Transformer is original. Awhile back I tried a pair of .001 uF from each side of the line to ground but that didn’t help. I’ll try a higher value per your suggestion and report back. I can’t imagine that helping the distortion problem, though.


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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Nov Tue 28, 2017 4:57 am 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
When it starts humming and distorting, try swamping L8 with a 39 k
resistor in parallel.

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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Nov Tue 28, 2017 5:16 am 
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Ok I'll be able to get back to this project on Wednesday. I'm not too clear on the theory behind your suggestion of adding resistance in parallel to the second IF secondary. I did put a signal tracer on the plate of the 6A8 some time ago and found the modulation hum and distortion present at that point. I'll certainly give yours and Norm's suggestions a try and post the results. Thanks


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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Nov Tue 28, 2017 9:39 am 
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At first glance, it seemed to me that the reflex stage might be oscillating. If that happens,
the audio path grid bias is thrown off. (distortion) Hum because more current , maybe
in the output.

Do the 6A8 and 6B8 tubes have shields ?

Swamp resistor drops the Q.

Scope would show what is happening.

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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Nov Tue 28, 2017 12:52 pm 
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+1 on Oscilloscope; but I have found the area around tubes like 6B8 to be a bundle of trouble. Without going to a lot of trouble to get schematic bigger. It looks like its back biased? If so putting the first filter cap negative to ground & not CT can cause Hum.

I would also check the rotor of the Oscillator part of the Gang, it could be shorting? You do see all tubes carrying RF shielded in some sets. I note that has no Antenna RF ground return unless the chassis is grounded.

Check the gang; If its not that, or wrong voltages, due to failed resistors, eat Chinese, bring home the chopsticks, get it oscillating then use one of them to prod around & see if you can change the oscillation frequency? I am suspecting lead dress exacerbated by modern caps. It is never unusual for me to have to move new parts & add shielded wire in the area around the 2nd detector 1st AF. Keep wires close to the chassis.

Marc


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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Nov Tue 28, 2017 5:57 pm 
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Thanks for all the input. I'll get the scope out when I get back to this project tomorrow night, and try all of the suggestions. There were no tubes existing when I started the project. I put in a bare 6A8G and 6B8G. Awhile ago I tried to put a shield on to the 6B8G and this caused obvious oscillation so I removed it. I changed the 6A8G to a metal type, and this is where I now sit. The sockets for the 6A8 and 6B8 do not have provision for grounding a Goat shield. I will go over the lead dress again, comparing it to my original photos of the underside.


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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Nov Tue 28, 2017 11:00 pm 
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Whilst I have had 6D6"s & "Goat" shields have problems, failure of grounding on a shield can result in instability. I am not yet convinced that you have a single fault.

Marc


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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Nov Wed 29, 2017 2:09 am 
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I find it odd that the schematic is drawn showing the older symbols for tubes, and that the tubes are listed as G types on the schematic, when this is a late thirties-early forties radio, and RCA-GE-Westinghouse sets of that era pushed metal tubes. If pin No. 1 of the tube sockets is grounded, they were probably actually looking for metal tubes in those positions, and just had an old curmudgeon :wink: on the drawing board.

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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Nov Wed 29, 2017 5:20 am 
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Location: Powell River BC Canada
The Devil is in the detail. Here are excerpts from
the parts list from the Canadian RCA Redbook Vol. 1 page 262.
Which I have here.

A nit for picking is the 6B8-G is the 2nd. detector.

( Radio College of Canada was my school) :D

Radio College of Canada schematics from the war years
were in a word, thrifty.

Attachment:
RCA 94T parts   shields 1.jpg
RCA 94T parts shields 1.jpg [ 31.15 KiB | Viewed 424 times ]
Attachment:
RCA 94T parts   shields 2.jpg
RCA 94T parts shields 2.jpg [ 74.36 KiB | Viewed 424 times ]


Attachment:
G40 FROM Canadian  General Electric  1930 to 1942 page 369.jpg
G40 FROM Canadian General Electric 1930 to 1942 page 369.jpg [ 248.17 KiB | Viewed 419 times ]

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Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Nov Wed 29, 2017 6:15 am 
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Shield: When its listed in the parts list that's rather a definitive answer. If its not there it should be. 6D6 is basically electrically the same as 6U7 all that is different is the base. 6U7 without a shield is a virtually uncontrollable nightmare: It looks like sorting that lot out is the first priority.

With that sort of carry on, you may need to re-check the IF alignment, after you get rid of the oscillation.

Marc


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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Nov Wed 29, 2017 6:22 pm 
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I was looking for an equivalent to this set in the US Redbook which is readily available to get more of a clue, but could not find anything. What you just posted confirms what I've been thinking. I checked the 6B8 socket and found that yes pin 1 is grounded, but pin 1 of the 6A8 is not. I had recalled seeing in the past on another radio set, a small clip which wrapped around pin 1 of the tube base and extended up the base so it comes in contact with the Goat shield. This must be the S-2064 clip referred to in the parts list you posted. I was planning on trying this out first before anything else when I get back to this tonight. I also see from the wiring diagram you posted that I may have an issue with parts placement in that area. Thank you everyone for the input. I'll be sure to report back


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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Nov Wed 29, 2017 11:08 pm 
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What is not often appreciated is the amount of RF flying about in the area of 6B8 & family. In many sets some shielding is marginal & in others decoration; Adding more metal to make any form of Faraday cage, adds cost.

The second and most annoying issues can arise from the modern capacitor construction. The "Outside Foil" type were constructed such that the "foil" encircled the body of the cap. This then formed a shield and the foil was always put at the most negative end. This being most effective when the foil was to the metal chassis (Zero volts). You do not have this "Luxury" with the modern cap so they are more prone to being affected by anything that can induct into them.

This is where lead dress comes into play. Wires that are away from the metal chassis can become radiators (base stations) and if that radiation can induce in to any part that will allow it, too use it to destabilise the whole show. A 6U7 with no shield, would be one of the worst tubes for doing this sort of thing. They seem to be able to find everything that can be used to cause oscillation and use it. Perhaps they should of followed Philips who put shielding on the tubes (Metalisation). Be prepared to add shielding.

Marc


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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Nov Thu 30, 2017 5:41 pm 
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I spent some time with this last night working with the suggestions and realized that we got a bit off track. I didn't detect any unwanted oscillation with the scope, and a properly grounded shield around the 6B8 had no effect. I also played around with the lead dress in the area. But, the original problem was modulation hum on all stations, and audio distortion around the middle of the dial. I removed the 6B8 and 6K6, leaving the 6A8 and rectifier. Using my signal tracer with RF probe, I had the same situation occurring at the plate of the 6A8, ahead of the 6B8. I'll concentrate my efforts on the converter when I get back to this on the weekend. Somehow that darned AC is sneaking its way in there. I refuse to give up; I know it's going to end up not seeing the forest for the trees!


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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Nov Thu 30, 2017 10:12 pm 
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Voltage chart below.
Sets with external aerials may have burned out windings,
Volume controls - open , dirty

And, that 5 meg resistor is different from the rest.
Attachment:
G 40 voltage chart.jpg
G 40 voltage chart.jpg [ 109.69 KiB | Viewed 286 times ]

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de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Nov Thu 30, 2017 10:32 pm 
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I see no action on the first IF & that's worth looking at. A clean monotone from the signal generator (keep the strength down) 5-12 uV at antenna. and look at the oscillograph in various spots

If you changed 6A8 and that did nothing, you still could have a bad ground at that end, or a mistake in the filtering. Hum modulation with pentagrids like 6A8 does happen.

I would like to think that the oscilloscope will see hum. The reason I calibrate with one, is so as I can spot distortion. Did you check resistors? The plate resistor of 6B8 and family have an attrition rate, as do grid resistors on OP tube. If the 6B8 plate reads 82V with a DVM and they did not use one, the resistor is likely high. That applies to screen voltages & others where resistors of around 100K+ are used, elsewhere as well.

6A8 does not like low volts. Make sure resistors & caps are the right ones?

Marc


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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Nov Thu 30, 2017 11:17 pm 
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Radiotechnician, thank you; THAT'S what i wanted to find, a voltage chart to help out. As a matter of course, when changing caps, I always check each resistor, (open circuit of course) and like to replace it if out more than 10%. My checklist for the weekend did include checking the filtering. And yes, I will re-check those resistors for correct values. I have checked the antenna coil windings. Definitely continuous anyway. The volume control wasn't original and they put in a 5K instead of a 7.5K, but there was no effect when I temporarily tacked in a fixed 7.5K in series with the 300R, so no problem there. One is never too old to learn - that's the positive part of working on a tough dog.


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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Dec Fri 01, 2017 4:29 am 
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The radio that does not play ball is the one you learn the most from. Invariably you are forced to learn how various bits work & interact, so you can understand why they won't.

Left of field, in a world where enforcement of RFI rules has gone awry. Do short out the antenna with a 0.01mfd cap. That will give an indication as to the hum, actually being hum and the source of the problem being in or external to the set. CFL's & Wall Warts with switch-mode power supplies, are often the enemy.

Marc


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