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 Post subject: Multi-Elmac A54-H transmitter
PostPosted: Oct Sun 28, 2018 2:13 am 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1684
Location: Milwaukee,WI
I finished recapping this transmitter and noticed the previous owner wired in negative feedback in the modulator stages. From what little I know of N.F. I was wondering what you guys think about the values he used.

The schematic is how it was originally wired. The differences now are:

Pins 3 and 8 ( cathodes ) of the 12AU7 have been separated and each now has its own 1K resistor to ground and no bypass caps. Then from each cathode there is a 100k resistor that connects to each of the 6L6 plates. The value and ratio of the resistors seems wrong. What do you guys think?

The unit is untested because I need to build a power supply first. So I don't know how well or poorly it worked as is.


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feedback.jpg
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 Post subject: Re: Multi-Elmac A54-H transmitter
PostPosted: Nov Thu 01, 2018 3:21 am 
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Joined: Jul Sat 23, 2011 9:33 pm
Posts: 845
Location: Long Beach Ms. USA 39560
The original circuit as shown in the schematic is a phase inverter with capacitive coupling from the plates of the 12AU7 driver to the 6L6 grids. Apparently what you were referring to as bypass caps were the coupling caps. By adding a voltage divider to feed back from the 6L6s to the cathodes of the 12AU7 and shorting across the coupling caps he was trying to make a DC coupled modulator. Perhaps he did a lot of math, or was just an experimenter with or without luck, it looks like a challenging circuit change. If you have a good scope and good probes, it might be interesting to play with it for a week or so, but if you want to put it on the air like it was designed and not risk crispy-crittering it "put it back the way it was" (apologies to Al Capp & Lil Abner).

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 Post subject: Re: Multi-Elmac A54-H transmitter
PostPosted: Nov Thu 01, 2018 12:05 pm 
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It appears "they" were wanting to fix the gain of the two stages at X100, 40db. The reason for the changes may have been an attempt to reduce distortion somewhat but who knows?

As it is, It may function OK, but you'll have to get it up and running to find out.


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ElMac54HMod8.jpg
ElMac54HMod8.jpg [ 97.19 KiB | Viewed 407 times ]

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 Post subject: Re: Multi-Elmac A54-H transmitter
PostPosted: Nov Thu 01, 2018 12:54 pm 
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The original designer based his design on making the entire transmitter work as it should, the ham modifier usually focuses on one discrete circuit ignoring any impact his mod may have on other operations. Unless you fully understand the modification and the reason for doing so I would put it back to stock. Vintage radios have enough issues to sort out without dealing with the quirks and glitches added by a "shade tree" modifier.

The A54-H is a great little transmitter and because of its phase inverter type of audio driver it doesn't have the often failed Stancor A53C audio driver transformer to worry about replacing (the later AF series models went with a driver transformer instead of a phase inverter). Going back to this phase inverter circuit is a fairly commonly seen modification of the later Multi Elmac rigs when the driver transformer fails due to the acidic paper attacking the very find winding wire used in these tiny audio transformers.

Rodger WQ9E


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 Post subject: Re: Multi-Elmac A54-H transmitter
PostPosted: Nov Thu 01, 2018 2:59 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1684
Location: Milwaukee,WI
Sorry for the confusion Pehamel. Those other circled parts were on the schematic when I downloaded it and not relevent to my question. I was refering only to the bypass cap at the 12AU7 cathode. The other coupling caps are still there. I should have used Mikeinkcmo's schematic to avoid the confusion. I forgot I had that one too.

Risngl: I understood the reason for the mod but questioned the resistor values and wanted opinions. I had seen this mod done in other radios but the ratio of the resistor divider was much smaller. That plus the big change from 10k down to 1k for the cathode resistor. It seemed different enough for me to question it.

I will try it as-is first. If I don't like the results its easy enough to remove the 2 feedback resistors and put the proper cathode resistor back in along with the bypass cap. Maybe 5 minutes of work.


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 Post subject: Re: Multi-Elmac A54-H transmitter
PostPosted: Nov Thu 01, 2018 8:03 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Monterey California USA
Not to be a trouble-maker, but I submit that Gus Undy was aware of this potential modification when he designed the transmitter, and that the circuit as designed was the best choice.

The A54 was one of the first mobile transmitters marketed to hams and my friend George Goldstone, W8AP (SK), who worked with Undy at the time, told me that they took George's car and installed the just produced prototype of the A-54 in it, parked it at the entrance to a hotel lobby where the '54 ARRL national convention was being held and "created a sensation." Free advertising.

I was fotunate to rescue a clean A54 with the rare AC supply for almost nothing at a flea market this summer, and hope to have it on the air soon. I do dislike those tiny miniductor PA tank coils though, which get caked with dust and then do strange things.

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 Post subject: Re: Multi-Elmac A54-H transmitter
PostPosted: Nov Fri 02, 2018 5:49 am 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1684
Location: Milwaukee,WI
I wish I could say I bought a clean A54H too. On the bright side, it has all good tubes and cost only $5.
It has been repaired, cleaned it up a little after the pic was taken and waiting for me to build a power supply to test it. I cut out and removed the hardened plastic covers over the meter and dial. The meter and it's cover are fine. The dial is partially melted where the cover stuck to it. It must have been stored near a heat source.


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A54H.jpg
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 Post subject: Re: Multi-Elmac A54-H transmitter
PostPosted: Nov Fri 02, 2018 7:29 am 
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Location: Seattle, WA
Would you mind posting an updated picture. This one is making me really sad.


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 Post subject: Re: Multi-Elmac A54-H transmitter
PostPosted: Nov Fri 02, 2018 11:44 am 
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My A54H was a gift. I was coming out of a used bookstore and one of the area hams asked me where I was parked and told me to wait at my car for a moment. He then drove up, popped his trunk, and gave me a A54H and PMR-6A receiver that his son found stuck in the closet of a house he had purchased. His father was strictly a modern SSB type so he had no interest in the vintage stuff.

The A54H has utilitarian styling that manages to cross the line into beautiful.

Rodger WQ9E


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 Post subject: Re: Multi-Elmac A54-H transmitter
PostPosted: Nov Fri 02, 2018 3:31 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1684
Location: Milwaukee,WI
I have a PMR-6 and 7 receiver I can pair up the 54H with. They are both in good condition. I'll take a pic of the radio now after it's initial cleanup.

On the subject of the negative feedback mod. I found out that 54H mod makes it the same feedback circuit as my Babcock MT-5B transmitter.


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feedback2.jpg
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 Post subject: Re: Multi-Elmac A54-H transmitter
PostPosted: Nov Fri 02, 2018 4:06 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1684
Location: Milwaukee,WI
This was after a damp rag wipe down. It should clean up better with more effort later on. I'll have to find a couple of small pieces of clear material to make new covers. If I'm lucky I might be able to find a better dial scale.


Attachments:
54a.jpg
54a.jpg [ 88.3 KiB | Viewed 309 times ]
54b.jpg
54b.jpg [ 60.98 KiB | Viewed 309 times ]
54c.jpg
54c.jpg [ 87 KiB | Viewed 309 times ]
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 Post subject: Re: Multi-Elmac A54-H transmitter
PostPosted: Nov Fri 02, 2018 4:14 pm 
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Location: Seattle, WA
Nice! Thank you for posting the new pictures! And hats off to the guy who made sure to find it a good home!


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 Post subject: Re: Multi-Elmac A54-H transmitter
PostPosted: Nov Fri 02, 2018 4:19 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1684
Location: Milwaukee,WI
It was a guy at a swapfest selling off an estate for a friend. I also bought the 4 piece setup in my Babcock transmitter thread from him. That turned out to be a great setup.


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 Post subject: Re: Multi-Elmac A54-H transmitter
PostPosted: Nov Thu 08, 2018 3:59 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1684
Location: Milwaukee,WI
I used a power supply for a different radio to do a quick test on the A54h. Its a bit underpowered for the B+ at only 300 volts but it should be enough for the test. I've got the vfo frequency all the way up to the 6AQ5 driver tube but almost no power coming out from the 807 final. On the 10 watt scale of my meter the needle barely moves off the bottom. I didn't notice any sag in B+ voltage when keyed up so I'm guessing the final tube is not working. If it was and I had a bad connection to the output connector I would still see the voltage sag a bit. I tried an NOS tube next, no difference. So its time to put on the glasses and grab my VOM to figure out whats missing. I didn't test the modulator section so I guess I should find a mic too.


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 Post subject: Re: Multi-Elmac A54-H transmitter
PostPosted: Nov Thu 08, 2018 6:35 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1684
Location: Milwaukee,WI
I did some wire tracing. Always difficult on these old radios with faded color wires bundled together in harnesses around the chassis. The mistake was on my part. I unsoldered some parts and wires while doing the recap and replacing diode and resistor in the modulation transformer secondary which feeds the RF final. I made a mistake when wiring it back together. I got confused because the schematic shows the A54 and A54h versions of that section side by side and I goofed by placing one wire according to the wrong circuit. Lack of color on the wires didn't help there. DUH on me.


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 Post subject: Re: Multi-Elmac A54-H transmitter
PostPosted: Nov Thu 08, 2018 9:59 pm 
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Location: Liberty, Missorui
Things like that happen. If that's your first demerit, not to worry, there'll be more to keep it company before the final curtain call.

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 Post subject: Re: Multi-Elmac A54-H transmitter
PostPosted: Nov Fri 09, 2018 4:20 am 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1684
Location: Milwaukee,WI
Testing showed it works on all bands and with the underpowered power supply it was putting out only 5 watts dead key but with clear modulation. Its time to give it a better cleaning and put it back together.


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 Post subject: Re: Multi-Elmac A54-H transmitter
PostPosted: Nov Fri 09, 2018 7:15 pm 
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Quote:
If that's your first demerit, not to worry, there'll be more to keep it company before the final curtain call.


Great quote.

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 Post subject: Re: Multi-Elmac A54-H transmitter
PostPosted: Nov Sat 10, 2018 4:17 am 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1684
Location: Milwaukee,WI
Does anyone here have a good scan of the tuning dial. I would like to make my own since finding one for sale is probably a futile effort. It doesn't seem to be a popular radio these days.


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 Post subject: Re: Multi-Elmac A54-H transmitter
PostPosted: Nov Fri 16, 2018 2:59 pm 
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Joined: Jul Thu 30, 2009 3:32 pm
Posts: 35
I was born and raised less than 1 mile from the old Elmac shop on 10 Mile Road in neighboring Hazel Park Michigan.
When the A-54 was being built, I was but 0ne year old, however. Very early on in my ham radio career at age 10 I finally first met the little A-54 and fell in love.
Got to know Mr. George Goldstone as well as several other pivotal people from the early days at "Multi Products" (Elmac).
Over the decades I have rescued literally dozens of Elmac and Multi Elmac radios just for the sheer joy of seeing and hearing them back on the air, where they belong.

A few years ago I picked up the earliest A-54H that I have yet seen at, fittingly the Hazel Park Hamfest. It is Serial Number 63. I immediately built a nice little power supply to complement the new treasure.

Worth noting, the first few A-54 came with VFO dials and meter faces that were printed with ink on paper and glued in place.

If I get the chance, I will try to scan a VFO face and post it here.


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