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 Post subject: Help needed on ARBE circuits
PostPosted: Nov Thu 29, 2018 10:37 pm 
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Joined: Nov Thu 29, 2018 9:09 pm
Posts: 3
Hello all;

I am new to this forum and I am fairly new to collecting antique radios.
Primarily I am more mechanically inclined and a tinkerer, but I am determined to learn about old radios.

A little while ago a friend promised to give an Eisemann Freed battery op radio. In preparation for this I had an ARBE made per a circuit diagram I found on line.
Since then I picked up a Philco Model 623 battery op radio for restoration.
I've already read that I need to likely replace all the caps on this before powering it up.....

But the ARBE I now have does not have the 135V output required for the Philco 623.

So I have modified the circuit diagram on the ARBE and I am looking for some input from someone who knows more than me on the subject.

The First image below is the schematic I had the ARBE made to.
The Second Schematic is one I revised to provide the needed 135V output.

Here are my questions:

1. Does the revised schematic look correct for adding the 132V tap by inserting 2 additional Zener diodes?
2. If #1 is correct above; is the 2K /2W resistor in that portion of the circuit still sufficient?
3. Also, if #1 is correct; is the 100 micro farad capacitor still sufficient?
4. I've recently read other comments regarding these home brewed ARBE's and how some do not filter the AC well enough resulting in the radio humming. Does this circuit provide adequate filtering to prevent this?

Thanks a whole bunch for any help on this, it is appreciated!!!!

Paul H


Attachments:
PowerSupplySchematic.jpg
PowerSupplySchematic.jpg [ 40.67 KiB | Viewed 3483 times ]
ARBE Power Supply Schematic revised for 132V.jpg
ARBE Power Supply Schematic revised for 132V.jpg [ 67.99 KiB | Viewed 3483 times ]
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 Post subject: Re: Help needed on ARBE circuits
PostPosted: Nov Fri 30, 2018 3:56 pm 
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Joined: Jan Fri 02, 2009 11:32 pm
Posts: 618
Location: Equinunk PA 18417
Hello and welcome to ARF!

You can add a couple of zeners to the +B divider chain, within limits. By adding zeners you are gaining a 132 volt tap and simultaneously trading away useable +B current. You'll get approx 132 volts as long as the total current drawn from the +B supply remains quite low, I guess around 7ma or less. With more +B current drawn by your radio, there will be higher voltage drop across the 2k resistor. If that drop becomes too much, the +132 volt tap will no longer have 132 volts on it and once that happens, there will be higher AC ripple on the +B voltage.

You could add a second 100uf cap, on the other side of the 2k resistor. That would reduce ripple somewhat especially when the +B current draw increases. I don't see any reason NOT to add a second filter cap even if you don't use additional zeners.

The 2k resistor's value and overall success of this supply is dependent on the power transformer. The spec'd Hammond transformer is small-ish with a 6VA max rating. VA stands for volt-ampere and is used to describe the max load the transformer was designed to handle. When overloaded, it may over-heat and no longer deliver the voltages/currents it is spec'd for. The resistor value you asked about, 2k, was chosen in part to limit the total current drawn through the +B transformer winding. The more current used by that winding, the less current is available for use by the filament output (without exceeding the the transformer's VA rating). The zeners draw a significant portion of the total current available at all times, through the 2k resistor. Resistor choice is therefore a compromise and the 2k value chosen seems to be okay. I have issue with the 2 watt resistor rating, because I calculate it may need to dissipate as much as 2.5 watts. A 5 watt resistor* would have been better. That is no longer an issue if you add zeners for a 132 volt tap.

* edit: I originally wrote 10 watt, but 5 watt is probably okay unless trapped in an air-less space

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-Richard


Last edited by richfair on Nov Fri 30, 2018 4:45 pm, edited 1 time in total.

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 Post subject: Re: Help needed on ARBE circuits
PostPosted: Nov Fri 30, 2018 4:43 pm 
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Joined: Jan Fri 02, 2009 11:32 pm
Posts: 618
Location: Equinunk PA 18417
I just looked up Philco 623. Your supply is not suitable. For one thing, you need two different bias voltages, -3 and -7.5. Your supply's optional bias section should be able to do that with minor modification, by adjusting the regulator's "select" resistor to produce 7.5 volts (1200 ohms should do it), then adding a 3 volt tap across the 7.5 volts output (try a 2k resistor in series with zener). But, the filament output of your supply is insufficient. Philco 623 requires 2 volts at 630ma, if I added up the tube filament currents correctly, which is more current than your supply can produce. You're out of luck there.

An ARBE-III type supply could do it. Or, you could select a beefier transformer with higher current capability, and that would allow more +B capability as well. Only thing to consider along with doing that is to make sure there is enough heat sinking on the filament LM317 regulator. It will run hotter.

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 Post subject: Re: Help needed on ARBE circuits
PostPosted: Dec Mon 03, 2018 10:18 pm 
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Joined: Nov Thu 29, 2018 9:09 pm
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Richard;

Thank you very much for the excellent feedback.
I now know what to do in regards to the 132V tap.
In regards to the negative voltages required.... I knew I needed these and my ARBE is not set up for these.
My thought on those was to use a couple independent power supplies. I went thru my dig box of DC adapters and found a 3VDC supply and an 8VDC supply, each rated at I think 300 milli-amps that could be wired in as negative power supplies. Do you see any issues with this?

Also a big thank you for taking the time to determine that the 2 VDC needs 630 ma. I did not know this and thus need to figure out something for this.

I've also learned that my friend who made this ARBE built in some added protection for me. The diagram shows a 200V / 100 micro farad cap on the B+ supply. I checked over the weekend and my unit has a 400V 200 Micro F. Think I should check the transformer as that might be beefier as well.

Thanks again
Paul


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 Post subject: Re: Help needed on ARBE circuits
PostPosted: Dec Tue 04, 2018 5:24 pm 
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Location: Equinunk PA 18417
You can use batteries to provide bias voltages. Button cells or standard AAA batteries in series, chemistry doesn't matter as long as the voltages are near correct. The batteries will last for years because the current drawn from them is vanishingly low. Some early radios actually used battery cells soldered into place and many of those are still good today!

Alternatively, use your 8vdc wall wart to generate both bias voltages. A 3 volt and 4.3 volt zener in series with a 1/4 watt resistor is simple and will work nicely. Most unregulated supplies will put out considerably higher voltages when not fully loaded so some sort of voltage regulation is needed, I am assuming yours will actually put out somewhere between 8 and 12 volts dc. 3.0 and 4.3 add up to 7.3 volts, which is close enough. In actuality, I expect the 7.3 volts to be closer to 7.5 anyway depending on the voltage of your wall wart and part tolerances. Only thing to worry about is whether there is filtering built into the wall supply. If not, you will want to add a 470uf (or more) across the supply's output.

The 0.1uf caps are optional. They are good practice to help ensure no unintended rf leakage through the power supply. The output marked "+C/-A" is the most positive output from the bias supply. It is actually connected to the radio's ground, which is also typically the filament negative "-A" connection.


Attachments:
Biasx3.jpg
Biasx3.jpg [ 9.77 KiB | Viewed 3304 times ]

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 Post subject: Re: Help needed on ARBE circuits
PostPosted: Dec Fri 07, 2018 10:58 pm 
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Joined: Nov Thu 29, 2018 9:09 pm
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Richard;

Thank you very for much this useful info.
I need to mull it over to figure out what works best.
Might just go with the batteries as you say.


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 Post subject: Re: Help needed on ARBE circuits
PostPosted: Dec Sat 08, 2018 2:40 am 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 537
Location: Fayette County, Pa
Attachment:
Variable_B_Supply.gif
Variable_B_Supply.gif [ 8.32 KiB | Viewed 3248 times ]
You don't specify the voltage on the B Secondary of your transformer, but here is what I use on my bench. It is a variable B Supply that provides between 15 and 150 VDC at up to 100 ma. Both transistor are NPN silicon.

Note the output pass transistor MUST be mounted on a heat sink as it will get warm especially in cases where you use 80-100 ma. at low voltages

One thing I caution is to put the control in an area that once set for your particular load it is hard to accidentally change. (It would not be good to accidentally bump the voltage up to 90V on a 22V radio!) You set the output to whatever you want using the meter then connect your radio to it Regulation is fairly good from minimum to max load. (Within 5%) I built mine out of spare parts I had laying around my lab, values are not extremely critical.


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