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 Post subject: Hoffman A300, undocumented mod.
PostPosted: Feb Mon 05, 2018 9:50 am 
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Joined: Nov Wed 08, 2017 11:52 pm
Posts: 15
Location: Arizona
Does anyone have a copy of Sams Photofact for this Hoffman A300 radio, I have the two page "manual" that is available on line. Except for the line cord and the electrolytic capacitor which looks like were replaced long time ago everything else looks original on this radio. In the process of replacing all capacitors I also checked all resistors. I found something realty unusual which I am hoping the Sams Fotofact may shed some light on it. Across the primary of the 2nd IF transformer (in parallel) there is a 100K resistor which is NOT on the schematic and it looks original from the factory.

The only reason I can see they would shunt the primary of the IF transformer with a 100K resistor is if you would want to lower the Q of primary resonance and in a way to widen the IF bandwidth. I am not even sure that 72 years ago they had the proper test equipment to perform accurately such a test. Has anyone ever seen this done on any radio and without documenting it on the schematic?


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 Post subject: Re: Hoffman A300, undocumented nod.
PostPosted: Feb Mon 05, 2018 2:49 pm 
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Joined: Nov Tue 03, 2015 9:43 pm
Posts: 42
Location: Naperville, Illinois
Greetings Upsss:

Is your two page manual schematic the same as Beitman's Volume 8 (1948) page 70 schematic for the Hoffman A300?

See (> http://www.americanradiohistory.com/Archive-Rider/BEITMAN-SUPREME/Beitman-1948.pdf <)

I'll check the Sams Set 4, Folder 41 for the Hoffman A300 but I recall that the Beitman and Sams schematics are the same.

John


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 Post subject: Re: Hoffman A300, undocumented nod.
PostPosted: Feb Mon 05, 2018 7:21 pm 
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Joined: Nov Wed 08, 2017 11:52 pm
Posts: 15
Location: Arizona
Hi John, thank you very much for the nice Beitman's schematic collection. My Hoffman A300 schematic looks the same as in that collection and it also doesn't show that extra parallel resistor on the primary of the 2nd IF transformer. However I did find two radios in that collection, a Montgomery Ward and a Pilot that had a similar resistors on their schematics so I guess other companies did that too. One thing that struck me in that old collection is the way they drew some of the schematics with the tubes upside down (the cathode at the top and the anode at the button), I never seen a tube drawn that way before.

I am still curious why Hoffman didn't show that parallel resistor on their schematics. If you have the Sams Fotofact for this Hoffman A300 I would appreciated it very much if I can have a copy of it. Most of the Sams Fotofact were very detailed in their manuals so hopefully they mention it if that was something that Hoffman did do at the time.


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 Post subject: Re: Hoffman A300, undocumented mod.
PostPosted: Feb Tue 06, 2018 11:05 pm 
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Joined: Nov Tue 03, 2015 9:43 pm
Posts: 42
Location: Naperville, Illinois
Greetings Upsss:

Hopefully this will make the cut before the web page goes down.

The attached PDF is the schematic that you're looking for. I can't say when or if Sams started to redraw the radio schematics, but I can say that from Sets 1-30 that I have seen many (about 25%) are in lock-step with Beitman's for the same radio model.

Having worked at Motorola some years back, if the customer wasn't paying for a design they did not get updated releases as long as the item still passed the final specifications. I have to guess that was the same for the radio and television groups.

On the fly changes due to a change of suppliers generally resulted in a marked print at the plant. Adjustments for different suppliers was the biggest culprit. All changes were documented in Engineering Change Notices but the 'vaulted' print might never be updated. Same for schematic releases to Sam's, Rider and Beitman. If changes were made and published all three companies would go broke attempting to keep up-to-date.

Unless a radio or television model was planned to be popular, the production life of a model was the tool life of the stamping dies that made the chassis. A four slide die set might mean 1,000 to 2,000 finished metal chassis / production assemblies before the tooling would wear out.

I think that you are right. Hoffman may have run low on IF transformers from supplier 'A' but IF transformers from supplier 'B' would work if they were just tweaked a bit. So there is a resistor that allows the A300 to pass the final audit. Maybe 'A' was on strike in 1948 or there was an earthquake.

I can understand the 256kB max. this gives everyone a chance to show a bit of a taste. For me 256kB is a bit tough as I make scans that I can enlarge enough to see the paper fibers. Not easy to squeeze one down.

John


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James 1999 UP Photos 420.jpg
James 1999 UP Photos 420.jpg [ 255.33 KiB | Viewed 115 times ]
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 Post subject: Re: Hoffman A300, undocumented mod.
PostPosted: Feb Wed 07, 2018 12:32 am 
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Joined: Nov Wed 08, 2017 11:52 pm
Posts: 15
Location: Arizona
Hi John and thanks again for your reply. Your latest schematic is of Chassis 100, mine is 100S which has a PM speaker and it looks like the Tone control circuit is also slightly different. The reason I wanted to see a copy of the Sams Fotofact is for their detail assembly drawings even if the schematic was not updated maybe they have a note about the additional resistor across the primary of the IF transformer. At this point I am convince that the resistor was added at the factory so I will probably leave it alone.

Once I am done with replacing all the capacitors and the speaker which is shot, I will do a complete alignment with a sweep generator and look at the bandwidth with a scope. Just for curiosity, I may check the bandwidth with and without the resistor across the primary of the IF transformer and see what kind of a difference does it make.


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