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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Mon 28, 2016 2:37 pm 
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The 3 cords from the power supply are connected at the box and at the radio so the unit is removable. I am using some miniature connectors but I think due to the low current of this radio they should be ok. The box is too thick for the connectors, and switches etc. so I will attach a metal plate after drilling large holes in the box. Here they are, note the colors match the radios original wiring color (red, black, green, yellow) Blue is the LED cord.

http://www.ebay.com/itm/182286565254?_t ... EBIDX%3AIT

http://www.ebay.com/itm/161479119462?_t ... EBIDX%3AIT

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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Mon 28, 2016 4:01 pm 
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Location: Westland, MI, USA
If you want to simplify the design further you can skip the "A" supply regulator. Use 2 silicon diodes connected in series with a limiting resistor to the transformer secondary. The diodes will maintain approx. 1.4VDC (the junction voltage across them in series) needed for the tube filament supply. Partition the current around 20% through diodes, 80% filaments to keep regulation. It will also allow you to use the supply in a different set. Not as efficient as a regulated supply, and you will need a larger filter cap (5000-10000mfd) but its sufficient to get the job done.


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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Mon 28, 2016 6:20 pm 
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That is simple but the regulator is a tiny module with input and output connectors all on a tiny pc board. Bolt it on (the board) plug in the wires tighten the set screws, done. :)


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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Mon 28, 2016 6:30 pm 
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Location: Santa Rosa, CA
I would stay with the regulator for the filament supply. It's cheap (especially because you can buy board already assembled on Ebay) and you can set it to the exact voltage.

A diode has a 0.7V drop at very low currents. The drop can be over 1 Volt at higher currents, so you must "choose wisely" when selecting the series resistor in the 2-diode scenario.

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Rich


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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Tue 29, 2016 2:30 am 
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Location: Westland, MI, USA
That chart is a bit different than what I'm used to seeing for diode characteristics. Perhaps due to the log plot?

As to the selection of the resistor maybe I'm either lucky or a wise chooser. For the power supply performance I get a Voc =1.6V and V@400ma =1.55V measured. My supply uses 4001 diodes. I appear to be achieving a lower voltage drop than the characteristics would indicate.


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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Tue 29, 2016 4:05 am 
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That chart is the way power diode datasheets show Vf curves.

The chart you have shown is a generic one and shows the reverse characteristics. The forward characteristic is compressed to the point where one would think that the Vf was nearly constant. We don't care about reverse when we are using diodes in the forward mode. The diode, in this case, is just a voltage dropper.

Rich


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 Post subject: Re: Power supply for farm radio
PostPosted: Nov Tue 29, 2016 4:41 am 
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I appreciate its a generic chart that shows all the characteristics, forward and reverse, and the 1N400X data shown are forward characteristics only. However the former are not "compressed", but rather the 1N400X datasheet is a log plot which obscures the high non-linearity observed in the generic chart forward characteristics region.

The approach of using series diodes takes advantage of that property. Notwithstanding its not a regulator in the sense of a 317 it provides a robust enough result sufficient for the purpose at a fraction of the cost.


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 Post subject: Re: Power supply for farm radio
PostPosted: Dec Sat 10, 2016 5:56 am 
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Location: Missoula mt
I dont like the 90 volt not being isolated from ground, on most sets the b- is grounded to the case of the receiver, good way to get a shock


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 Post subject: Re: Power supply for farm radio
PostPosted: Dec Sat 10, 2016 2:43 pm 
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This project done yet?

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 Post subject: Re: Power supply for farm radio
PostPosted: Dec Sat 10, 2016 2:58 pm 
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Chas wrote:
This project done yet?
No. I have assembled the parts but have not done any soldering yet. I have been waiting on a soldering iron which I just received. The radio itself is not refurbished (capacitors) but I have those now as well. I am in no rush.


Last edited by Ozo on Dec Sat 10, 2016 3:13 pm, edited 1 time in total.

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 Post subject: Re: Power supply for farm radio
PostPosted: Dec Sat 10, 2016 3:08 pm 
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wa7opy wrote:
I dont like the 90 volt not being isolated from ground, on most sets the b- is grounded to the case of the receiver, good way to get a shock


I believe B- is grounded to the case. Here is the radio schematic. I did not alter the radios original engineering.
http://www.nostalgiaair.org/pagesbymode ... 013306.pdf


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 Post subject: Re: Power supply for farm radio
PostPosted: Dec Sat 10, 2016 3:30 pm 
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Thanks for update...

Practice soldering before showtime. Its a fun skill, I have the scars on my fingers t prove it :)

Get some solder wick and some form of a heavy duty solder sucker too. Good tools will always pay dividends and you can pass them on to your kids :wink: On a guess, some 80% of my tools are hand-me-downs. All famous U.S. names, many no longer made.

I have one regret about soldering tools. When I was working I ordered for the shop a PACE triple re-work station. I can remember exactly the model number but it was in an attractive sculpted cabinet. That one bit of equipment literally made a technician hero out me. One of the best re-work soldering systems I have ever used. That and a China charcoal barbecue were my key equipment for repairing SM circuit boards. Barbecue? I'll tell what it did later, If you really want to know... Oh, the regret? Apparently the model was discontinued by PACE, I cannot find any data to even locate one of these machines used...:(

BTW, there is a manual, very hard to find that shows what the dozens of properly soldered circuit joints look like and grades the appearance of the joint. I do not have a copy,, I did peruse a copy when I worked at Vectrix electric motorcycle. FWIR it was an ASTM handbook.

I have taken so long to post this reply that I see an interesting topic has just appeared...

Leave the "A-" floating in the power supply design. From the "A-" to the AC power input ground connection connect a .0047/630 capacitor. This provides an RF return should the antenna/ground system used to bring the signals to the radio be insufficient. This connection can, however, introduce noises from the household electric system into the radio. The connection to AC power ground via a capacitor can be made optional, with a external wire connection at the supply.

GL

Chas

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 Post subject: Re: Power supply for farm radio
PostPosted: Dec Sat 10, 2016 3:51 pm 
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[quote="Chas"]Thanks for update...]quote

I finally started refurbishing the farm radio. It is my very first attempt and I am making good progress replacing wires and caps. And you are right, it is great fun. :D

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 Post subject: Re: Power supply for farm radio
PostPosted: Jan Sun 15, 2017 4:01 am 
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The banana plug sockets are installed but I am waiting on white plugs so the colors correctly match the wire color on the farm radio. Red, yellow, green, and white. I was going to install a plate for the sockets but then I found some that fit the thick box. The box lid is not hinged but is a nice tight fit. The box is 7" X 5" X 4" (LxWxH).

There are currently no vents but I will drill some holes for ventilation later. I have to enlarge the switch opening a little to straighten that out.

Note the ground (green wire) is attached with a screw to the top of the transformer (far side). No solder there but it is soldered to the input. I will change that and use a crimped ring when I get some.



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Last edited by Ozo on Jan Sun 15, 2017 6:32 am, edited 11 times in total.

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 Post subject: Re: Power supply for farm radio
PostPosted: Jan Sun 15, 2017 4:16 am 
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Chas wrote:
This project done yet?


See photo above.

I have all the components gathered for the power supply and am nearly finished assembly. The 1.5 volts DC is finished. The 90 volt DC is close. I have the DC voltage output from the bridge rectifier and am about to add the resistor or pot to bring that down to 90 volts. The voltage at the rectifier is higher than anticipated but I am confident that I can deal with that. Tomorrow, I will finish it.

The DC banana sockets are installed in the enclosure as is the switch/fuse/switch/pilot-light module. The board holds the transformer, DC regulator assembly, rectifier, three caps, and the potentiometer. I may add a bright LED inside which will display a cool if somewhat annoying light show through the vent holes. I should put a switch on that.

I used a bridge rectifier like in the photo below with wire leads so it would easily mount on the board. It is small but 3 amp 600 volts I think. I had the large type but the terminals were too big.

EDIT: The 90v DC was more work than anticipated. Regulating the rectifier output was accomplished with a VR-90 tube in parallel as well as a 3 watt resistor between the caps. This way the radio is protected from any surge on startup. A Zener diode string could be used instead of the VR-90 as in the schematic below.
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Last edited by Ozo on Jan Fri 27, 2017 6:42 pm, edited 2 times in total.

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 Post subject: Re: Power supply for farm radio
PostPosted: Jan Sun 15, 2017 9:53 pm 
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Location: pensacola fl
Your B battery does not go to the radio ground. Its negative goes through a resistor to develop the bias for the output tube. Your power supply must therefore have the B supply floating.


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 Post subject: Re: Power supply for farm radio
PostPosted: Jan Sun 15, 2017 11:12 pm 
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Audioman wrote:
Your B battery does not go to the radio ground. Its negative goes through a resistor to develop the bias for the output tube. Your power supply must therefore have the B supply floating.


No offence but I think you misunderstand, the green wire in the photo is the power plug earth ground (center pin). The B negative goes to the yellow wire on the radio (90v DC in). See the schematic posted below.

EDIT: The additional modification was added for surge protection/regulation.

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 Post subject: Re: Power supply for farm radio
PostPosted: Jan Sun 29, 2017 9:22 pm 
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It is finished and supplies 90v DC and 1.5v DC. I had to make a change on the 1.5v DC side because the transformer secondary failed. I added a wall wort (6v DC) for that. The transformer still supplies power to the 90v DC side. I used the VR-90 tube for regulation because I do not have any Zener diodes yet. I will experiment with those later.

I hope I did not screw up but I was having trouble getting the 90v DC to drop to exactly 90v by installing resistors in series as in the schematic even with a dummy load so I put a 10k ohm 2 watt variable potentiometer in parrallel to do that. I will keep an eye on the heat from the potentiometer in case 2 watts is too small. I will put the dummy load on it for a few hours to test it as I do not have a real load right now.

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Last edited by Ozo on Feb Sat 18, 2017 10:04 pm, edited 1 time in total.

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 Post subject: Re: Power supply for farm radio
PostPosted: Jan Sun 29, 2017 9:50 pm 
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Ozo wrote:

I hope I did not screw up but I was having trouble getting the 90v DC to drop to exactly 90v by installing resistors in series as in the schematic even with a dummy load so I put a 10k ohm 2 watt variable potentiometer in parrallel to do that. I will keep an eye on the heat from the potentiometer in case 2 watts is too small. I will put the dummy load on it for a few hours to test it as I do not have a real load right now.

I would suggest that you refer to a tube manual, either paperback or on-line, and determine what the maximum plate voltage is for each tube. If all are over 90 volts, then you have nothing to worry about AFA being exactly at that voltage.
Actually since it's a consumer circuit, most likely any voltage within 10-15% is probably safe.

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 Post subject: Re: Power supply for farm radio
PostPosted: Jan Sun 29, 2017 10:02 pm 
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The schematic details of the one non-working farm radio I have indicates that its B supply is 90 volts and the A supply is 1.5 volts. Battery drain is 250 M. A. (A) and 8 1/2 M.A. (B).

The variable resistor with 10k ohm dummy load is pretty hot so if I continue to use a pot in parallel it will need to be higher wattage. Still not sure that is correct , waiting for member response.


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