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 Post subject: Silvertone Model 6421
PostPosted: Nov Mon 13, 2017 10:48 pm 
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Joined: Dec Sun 29, 2013 10:11 pm
Posts: 112
Location: Richmond, Va.
Silvertone Model 6421; Chassis no. 101.571 (no -1, A. B. etc.)

Hi all - I have been working (fighting?) with this radio for some days now, and it still has me baffled.

Attachment:
Silvertone Model 6421, cabinet, as found 11-5-17 sm   .jpg
Silvertone Model 6421, cabinet, as found 11-5-17 sm .jpg [ 69.46 KiB | Viewed 563 times ]


Schematic link - http://www.nostalgiaair.org/Resources/download.asp?FN=\M0017543.pdf

It has the following problem(s?). The output from the speaker is barely audible, and very distorted. There is a good signal from 2 or 3 received local stations, (reasonable to me at least, and undistorted) across the volume control, and at the wiper, when fed to my signal tracer. I have checked, as well as replaced, the 6F5 and the 25L6. Both the as-found and the replacement tubes test good on my NRI emission tester and show no shorts, as do all the other tubes. I have replaced the 6F5 with another from a working radio, with the same result. I do not have another radio with a 25L6. I have put a 1N4007 diode across the rectifier tube to reduce any voltage drop in the tube.

I have recapped it (all except the micas which test OK on capacitance, and show no shorts on my Heathkit capacitor checker). I have replaced all out-of-tolerance resistors. I have tweaked the IFs at 455 mhz. I have it powered by my isolation transformer. The field coil resistance is 499 ohms warm; normal is 450 ohms. I did not try it before starting my rebuild. The B- bus and the chassis are separate, connected by a 0.1 uf capacitor. The wiring is according to the schematic. I have carefully checked for poor soldered joints, and shorting solder blobs and wire pieces. I have substituted another output transformer and speaker, with the same result. All 4 IF coils test good (about 12 ohms DC resistance). I have replaced R6 (47K) in the 2nd IF transformer. C13, 0.00015 uf, is a metal leaf with a piece of mica to insulate it from the trimmer capacitor.

Significant voltage readings (taken with respect to B-; “normal” is as shown on the schematic) -
Rectifier cathode = 130 v, normal is 130 v.
Plate of 25L6 = 15 v, normal is 95 v. VERY LOW.
Screen of 25L6 = 87 v, normal is 100 v.
Plate of 6F5 = 16 v, normal is 40 v, VERY LOW. (Here, I have replaced, and rechecked, the R8 resistor, shown as 220M, or K. Could the schematic be wrong? The schematic pasted inside the cabinet agrees with the schematic from Nostalgia Air, above. I have tried several different resistor values for R8, and 220K seems to give the greatest volume. C20, 0.01 uf, has been replaced, and rechecked for capacitance and leakage. C15, 0.00025uf - a square, brown mica(?), and C12, 0.00005uf, a rectangular brown mica(?) - have been checked for capacitance, and leakage at 250 v, and seem OK, I have replaced the 6F5 tube with another from a working radio, with the same result. I do not have another radio that uses a 25L6.)

My questions -
(1) How does the detector circuit in this radio work? (Do the plate and the grid of the 6J5 rectify the audio signal with respect to B-, and capacitors C12 and C13 filter out any remaining RF?)

(2) Can someone, more experienced than I, guide my thinking, and my analysis, of this schematic, and help me solve this problem?

Many thanks in advance for any help from the experts.

CharlesD


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 Post subject: Re: Silvertone Model 6421
PostPosted: Nov Tue 14, 2017 12:47 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1807
Location: Lafayette, CO
Charles, l have the same radio and came to the same point. It is set aside for now as I'm focused on a console radio. Speaker here is not original, l have re-capped it. Craig


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 Post subject: Re: Silvertone Model 6421
PostPosted: Nov Tue 14, 2017 2:24 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 13478
CharlesD wrote:
Plate of 25L6 = 15 v, normal is 95 v. VERY LOW.
Screen of 25L6 = 87 v, normal is 100 v.
Plate of 6F5 = 16 v, normal is 40 v, VERY LOW. (Here, I have replaced, and rechecked, the R8 resistor, shown as 220M, or K. Could the schematic be wrong? The schematic pasted inside the cabinet agrees with the schematic from Nostalgia Air, above. I have tried several different resistor values for R8, and 220K seems to give the
CharlesD

This is the horse dropping amongst a bunch of fly specks...

Check R12, apparently supposed to be a 22 ohm... Either it's up in value or maybe the output transformer has a issue...

The plate resistor of 6F5 should be 220K...


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 Post subject: Re: Silvertone Model 6421
PostPosted: Feb Sat 10, 2018 2:18 am 
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Joined: Dec Sun 29, 2013 10:11 pm
Posts: 112
Location: Richmond, Va.
Greetings all. I am finishing restoration of my Silvertone Model 6421, Chassis 101.571, which has an electrodynamic speaker which looks to be original. (I found the problem I posted about on Nov. 13 - it was a bad capacitor I had unknowingly brushed against with my soldering gun, and caused it to short.) The set is working well, except that the speaker output is very much lacking in base response. If I connect the output transformer secondary to a different speaker, even the 4” speaker in my Heathkit signal tracer, the response is much better. There is nothing visibly wrong with the present speaker other than several tears that have been sealed with glue. The voice coil appears not to rub. I hope someone familiar with this radio can tell me if this is normal for this set, or if there is some hidden fault with the speaker. Thanks for any help. CharlesD (See photo with my Nov. 13 post).


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 Post subject: Re: Silvertone Model 6421
PostPosted: Feb Sat 10, 2018 3:30 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18211
Location: Albuquerque, NM 87123
You should re-check all the capacitors around the audio tubes for correct values.


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 Post subject: Re: Silvertone Model 6421
PostPosted: Feb Sat 10, 2018 3:38 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 37187
Location: Livermore, CA
Hi Charles

What was used to seal cracks in the speaker? If the cone is stiff it will be lacking in low frequency response.

_________________
Norm


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 Post subject: Re: Silvertone Model 6421
PostPosted: Feb Sun 11, 2018 2:12 am 
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Joined: Dec Sun 29, 2013 10:11 pm
Posts: 112
Location: Richmond, Va.
Thanks for the replies to my post. The tears in the speaker have been repaired with radio service cement. The cone is stiff near the repairs, but seems normally flexible otherwise. CharlesD


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