[MrFixr55]

One would think that switchers would put out a lot of noise, but a lot of audio equipment use them. I was a field engineer and technical specialist for a well known manufacturer of Clinical Laboratory chemistry and hematology analyzers, where quiet power was an absolute must for the photometric and Ion Selective Electrode analog circuits and instrumentation amplifiers. we started out with analog supplies, but "switched" to switch mode power suppies in the mid 1080s. It is what happens after the switching circuit, how well they are filtered and how well the supply is shielded. In fact, shielding may be a very big part of it.
Power supplies for analog instrument amplifiers, Hi Fi, Mic amps and audio mixers must be rock solid. Computer power supplies must be free of noise too. Computers themselves generate enough on their own.

I have tried to build switching power supplies for B+ using 2 different e-pay power supply modules meant for "nixie clocks", and yes, mucho noise, but have not had any issue with commercial power supplies. I may just not have the correct caps in the output, or maybe did not shield the supply well enough. Electrolytics, for some strange reason, have a high resistance (impedance?) to high frequency. Electrolytics on the "other side" of the high frequency transformer are usually bridged by other types of caps of much lower capacity in the filter circuits. Shielding may be an issue.

If you have one kicking around, firing up the radio with the 5V section won't blow anything and won't cost you anything.

I have 2 farm sets that I powered from a 3Volt "wall wart" switching power supply with no issue. I have powered a 1920s radio using the 5V section of the computer PS and a bunch of 9V batteries successfully. I have not tried to use the -5 or -12V sections for bias yet, but bias batteries last a real long time.

When one looks at an old school tube auto radio, or farm sets with vibrator power supplies, these are essentially "switchers" also, just usually at a lower frequency.



Granted, I eventually bought an ARBE III, but more out of frustration and the need to bust a move on fixing and demonstrating several 1920s battery sets.

[462ron]

Terry, I do have the center tuning cap out and yes,the bottom is in better shape but, I can’t flip them and remount as the there are 3 mounting screws, 2 on top, 1 in the middle on bottom! I want to clean this radio up but not really into doing a museum type restoration. I’ve resurrected this until just to experiment with changing the grid leak detector for a diode detector and compare the difference in distortion!

MrFixr, later today I will dig out that computer power supply, fire it up and see what voltages I can find plus I will put my transistor radio next to it and sniff for objectionable noise or maybe better yet, substitute the power supply for the radio’s battery! If all else fails, I will look to get an ARBE III.

Ron

[radiorich]

Hello Mr Fixr55
I agree with you "When one looks at an old school tube auto radio, or farm sets with vibrator power supplies, these are essentially "switchers" also, just usually at a lower frequency."
I have yet used a computer powersupply or switching walwort to power a radio !
Ron , It sounds like you are coming right along with this set !

Sincerely Richard

[462ron]

I have a power supply from an old computer, don’t remember the brand or age at this point. A little research and I found that to turn it on, you just need to ground the green wire. So had it turned on and had my transistor radio nearby and depending on where in the AM band I tuned, it picks up some squeals and other noise. We have a 20 year old Dell computer that will be scrapped so I retrieved the power supply from that unit too. The first power supply is an M-Tech and the other is a Dell. They both have the same connectors with the exact color wires in each. Not being computer savvy, I found that interesting. Seems like computer power supplies are kind of universal. Anyway, when trying both, seems like the Dell doesn’t put out as much interference as the M-Tech so I can at least try either one after I get the AK up and running! Hopefully not too much longer.

Ron

[MrFixr55]

Interesting grounding the green wire, The 3.3V Output is also green. Several individual wires ganged together because the power supply delivers 25A for 3.3V and 25A for 5V so they use multiple pins to prevent burning up the connector. There is a standard called "ATX" for Pentium 3 and 4 (and possibly newer) computers. mine had a separate harness for the power switch that I jumpered.

Those are the colors and values I rattled off in a previous post. Make sure that you have the power supply connected to a grounded outlet or you WILL get noise.

[462ron]

MrFixr, I see in an earlier post where you told me that the green wire is 3.3 V. I googled how to start a computer power supply and it said to jumper the green and black terminals together which I did and it comes on, both supplies worked the same. I took voltage readings from all the pins and what I get is…..
Orange..+3.3V, there are a few orange .
Blue..-12V
Red..+5V, a few reds.
Yellow..+12V
Purple..+5V
Grey..+5V
White..-5V

There is only one green wire and when grounded, turns the power supply on!

Ron

[MrFixr55]

Hi Ron,

So much for standards and convention. You should have several black wires also. They are all at the same potential. For the filaments, use the blacks connected together for negative and the reds connected together for +5V.

Could be that my computers came from the same industrial OEM.

Good Luck

[462ron]

Yup, there are several black wires! The Dell PS is 20 years old, the other one, older yet. Maybe that might matter!

Ron

[MrFixr55]

Dunno. I had a Dell at one Customer site that ran for 15 years with no hickup. Had several others of one manufacturer or another that ran that long. Still others died after about 3 years.

[462ron]

Well I received my Atwater Kent parts chassis, model 37. It has the brass pulleys and belts that I needed for my model 35. I would like to transfer the tuning caps from the parts chassis to my set as my originals are rusty and on the parts chassis they are like new. They will fit physically however, my model 35 has 6 fins on the rotors, the parts set has 8. The corresponding RF coils that they resonate with are physically the same length and diameter between both sets. What’s strange is all RF coils on my model 35 are close to each other in uh. The parts set RF coils measure similarly to the model 35 except the #3 coil is a good 20uh more! The tuning capacitors on the parts set are about 100pf more which makes sense as there are 2 more sections of fins. So, my dilemma is should I just swap out the tuning caps and keep the RF coils that are on my model 35. Should I swap both the tuning caps and RF coils that go with them from the parts set? Makes no sense to me that both sets have the same RF coils for the left and center stages while the tuning caps are about 100pf more! The right hand stage has a RF coil about 20uh more on the parts set!

Ron

[MrFixr55]

I wonder if the difference is due to the fact that the detector in a 37 is a 27 which has an indirectly heated cathode and much different internal structure than the 26s used in the 1st and 2nd RF as well as the 1st Audio. a 26 is essentially an 01A with a very thick 1.5V heater. the lower voltage, thoration and heavy current likely helps the filament not induce AC Hum into the set. The 26 and 27 as well as the 71A and 80 were developed for the first radios to be designed to run off AC. (RCA's eqrlier radios were DC sets that ran on AC due to the "Socket Power Unit" in the Model 104 speaker putting out 3.3V DC for the UV199s in the radio.)

I say leave the coils and swap the caps. Doesn't the 37 have "binocular" coils as opposed to the concentric ones on the 35?

BTW, Navel jelly does a good job of cleaning up the meal on the old caps, issue is that it takes off any plating that may still be there. Keep it off the insulators and the aluminum though.

[462ron]

   
 
Coils on both chassis look to be the same and in fact they measure within a few micro Henries of each other except for the 3rd stage which is about 20uh more on the parts set. Attached picture shows my set but the parts chassis looks the same!
MrFixr I believe I will do as suggested, I will change just the variable caps and then the coils if need be!

Ron

[Chas]

Swap the coils and the tuning condensers. No worries. IMHO the range of tuning may be greater in the AC model as opposed to the early DC model. Besides corrosion can be going on in the 35 coils that could manifest itself later.

How are the 35 audio transformers? If replacement is needed try to get OEM or at least try the ones from the 37. Often the first audio is 5:1 but not always.
Do not judge ratio by the DC resistances. Use an audio oscillator at either 400 or 1k with 10k isolation and measure voltage gain on secondary with VTVM or 'scope. Primary DC resistances are important for proper function of driving tube as well as detector, insufficient load = distortion.

I also suggest chasing the threads in the case for the bottom plate. These often freeze the screws resulting in the threaded insert turning in the case. Hold the threaded insert from inside the case when running a tap through.

Note: Setting up tuning bands, a small amount is "slop" in turning the master to each of the outer tuning condensers is by design. Since there is no tracking adjustment, "bumping" the tuning knob pushes the outer tuning condensers back and forth, user listens for best peak. Further: The single bearing in the tuning condensers is a cone, held in place (seated) by a phosphor bronze leaf spring, too tight of bands will force the cone out of place by overpowering the leaf spring shifting the tuning condensers value disproportionately. Some tuning condensers may not have the leaf spring but the "slop" must still be present. The mounts for the tuning condensers should be slotted with a lower slot on the main bracket to insert the blade of a screwdriver to correct the adjustment. The optimal alignment frequency is usually 1mhz, but if better reception on shorter wavelengths then lower, set shaft alignment frequency to 950 to equalize response across the entire BC band. A-K recommended lubricant is Nujol a mineral oil and petroleum jelly for the rheostats...

Another bit, use 6 volts for the "A" that establishes a consistent cycling of thorium in the tubes, then, if overall gain is low with all other bits of the circuits correct, and good tubes try reducing the values of the grid resistors in 100 ohm steps, when the RF stages become unstable, go back to the last resistance value used. DO use a short direct ground to EARTH not the AC line or the bed-springs, 50 to 75 feet of antenna up at least 10' to 35 feet should be fine, there is voltage gain in the tubes and each RF coil so there is ample sensitivity in most suburban locations...

YMMV

Chas

[vinzer]

i would repair the 37 as it is better radio and it is ac powered no power supply needed but rebuilding the power supply is not fun.

[MrFixr55]

That's great info, Chaz!

@Vinzer, that is the big conundrum, that happens to me all the time. I wanna fix both the parts set and the set that I bought the parts set for. That's why I have 3 Radiola 16s and even the parts set works.

True, the 37 is an "AC Set" but between the 71A and the 26s, there is quite a bit of hum if a modern speaker is used.
Amazing what people accepted in terms of fidelity, distortion and hum just to not have to deal with a storage battery and expensive, disposable B battery blocks in the living room.