You Saw where this was headed ~ A 1930's Style Station Part 2

My PMR6A Power Supply Build Progress!

1-15-2020 ~ Final Wiring.

Power OFF -- Neon Pilot Light OFF

Power ON -- Neon Pilot Light ON

The DC Filament Power Supply is Installed.

Plan View of the Power Supply Modules.

The Filament Supply is "Hot".

Work to be done includes final wiring to the terminal strip plus wiring to the Meter and the load tests. The No Load Voltage is + 350 VDC. Another point here is that essentially there is no bleeder resistors other than the radio itself (poor 1950's practice) so there was +350 VDC on one of the pads --for a long time. I will add some protective covering over the areas where there is high voltage.

With the final completion of the power supply I will then turn my attention to the PMR6A. If that is a bust, then I still have a pretty nifty DC power supply for other projects.

Pete N6QW

1-14-2020 ~ More progress on the wiring!

The 12 VDC supply has been shipped and it may be here today. I accomplished more wiring last night but will await the arrival of the DC supply before I install the terminal strips and cable assembly to power the PMR6A. (A board space issue.)

A couple of notes here. I am using a 1.5 Amp fast blow fuse in the cartridge and once I get things working probably will shift that to a one amp. I intend to form the capacitors by using a variac on the primary side at the first "power on" cycle. Once at full power I will let it burn in for maybe an hour. 

We will not connect the PMR6A initially and to substitute a load will install a power resistor on the main HV lead where the VDC = 260 and the Amps = 0.1. So applying E=IR, R value is 2600 Ohms  and the power = I^2*R or 26 Watts. So If I take four 10K , 10 watt resistors in parallel that will give me 2.5 K at 40 watts. You got to love the math!

BTW the Panel Meter will assist in discovering the No Load and Full Load Voltage of the HV supply. It will also provide a bench mark of future power cycles as it should not vary significantly from the Full Load Condition once in operation. An important piece of troubleshooting info downstream.

Now I do not know the actual condition of the PMR6A and so that is where dusting off my Sencore Might Mite Tube Tester will come into play. That also means powering up that old laptop as that is where the tube data for the tester is now hiding. 

Now an old TKT (tribal knowledge tip). Do not pull all of the tubes out at once and throw them in a pile -- you absolutely will not remember what tube goes where. Do it one tube at a time and perform another important step. 

This additional step is where I have a small plastic tub and I spray a bit of De-Oxit into the tub and each of the tube pins is dipped in the tub and are cleaned off before being put in the tube tester. So when each tube is plugged back in -- they are clean pins -- I also give them a quick spray with De-Oxit after testing so now the tube sockets in the PRM6A are cleaned up. RESIST spraying De-Oxit into the tube sockets!

Also I plan that at power on after the tube testing will be the filaments only. If all the tubes light in the tester and the tubes test good then if there are some that do not light once back into the radio -- either I have a wiring issue in the PMR6A or managed to mess up the tube when being put back in the radio.

BTW I also verify the tube number and match that to the tube location in the manual. Some prior owner may have put the right tube in the wrong socket. This is another reason -- DO NOT power ON the RADIO until you test the tubes and assure the right tube is in the right socket! Also note on the schematic that one of two different tubes were used at V2. It is all a process guys! 

Ditto for the wiring -- ugly construction is not a good idea when using HV!! My wiring while not Mil-Spec looks good and also facilitates trouble shooting as you can trace the wires and it is not a jumbled mess! When you know stuff you can do stuff!

1-13-2020 ~ A few changes and a DC Supply

A bit more power supply wiring and a few changes. Initially I had a very small DPDT mini-toggle switch for the main power switch -- it had been liberated from another project. As I was removing the wiring I noted that one of the terminals "wiggled" in the housing. No need to add problems. That is when I switched to a full size toggle switch.

When I bought parts for the supply several years ago, that buy included a 600 Ohm 10 watt resistor. Somehow that resistor got put in a bag with other components and seemed forever lost in my huge junkbox. I substituted a 600 Ohm at 5 watts thinking that would need replacement. While looking for some quick connect terminals for the neon pilot light -- boom there it was and so the 5 watt was removed and the 10 watt installed.

Clever blog viewers will spot a dangling black wire. That will be properly terminated (shrink wrap) and tucked under the transformer housing. Hammond transformers come with a tapped primary for either 117 or 125 VAC. I used the 117VAC and so the black wire which is used for the 125 VAC tap is redundant. There are a couple of more wires to be added today including the power switch, a jumper on the PC Board and adding two additional wires to the transformer primary circuit where 117 VAC is supplied to another module. 

Yet to be done is the wiring for the two terminal strips that will house all of the wiring that will be fed to the PMR6A via a three foot long cable and an 8 pin Socket with shell. That cable plugs into the rear apron of the PMR6A. The other open item is installing on the board and wiring of the cool (and small) panel meter.

The filament problem is resolved. All Electronics (here in Southern California) sells a 12 VDC supply at 5.6 Amps for under $10. That supply will be mounted vertically (smaller footprint) on the open space to the right of the PC Board. BTW All Electronics has flat rate shipping --so I bought some other needed supplies. They have a website!

The delivery of the DC supply will not be until Tuesday so we are a couple of days away from having a fully operational PMR6A  power supply. I also suspect that having a DC filament supply will help a bit with AC hum so often heard a low volume levels on "Toob" radios.

1-12-2020 ~ Wiring the Power Supply!

Dabbling with our hobby would be no fun unless we had a few problems that actually cause us to grow and learn. In the quest to build a 1930's station I started by actually trying to finish something initiated several years ago and that is to get on the air a Multi ELMAC PMR6A Receiver. That in turn would cause me to build a suitable power supply first, as I had most of the components (bought at the time I picked up the receiver) but these were never assembled. 

So I have "noodled" several approaches and settled on building a master circuit board which would be mounted on a wooden base. Initially I have specified one size of base and then in a stroke of luck picked up a piece of "cut-off" lumber at Home Depot which I cut in two. The other half --yes will be for the transmitter! The size now for both boards is now 8.75 X 9.25 inches by 7/8 inch deep. This is bigger than my initial approach; but has proven to be a good thing because of an issue I encountered. 

But first some progress photos.

The above photos show how the PC Board was cut on my CNC and then mounted on the wooden board. To the right of the board is open space that will be used to solve one of the problems you will read about further on in this post.

The PMR6A has the ability to turn on the power supply but I will not use that feature; but instead use a local mini-toggle switch. Next to that switch (rectangle with a red inside rectangle) is a 115 VAC neon pilot light. There will be no mistaking that the supply is hot! 

On the back side is the IERC three pin power jack and a fuse so that we have the PMR6A meet the current electrical standards. Something to be added on the front panel at the middle  is a cool (and small) 0 to 500 VDC panel meter. That along with the neon pilot light will certainly add to the 1930's look.

A construction note here. While at Home Depot I picked up some 3/8 inch nylon spacers and 1 inch long wood screws. The power transformer has been elevated above the wooden base by 3/8 inch. Why? Wire management. I will have to run wires to the power jack/fuse and to the 115 VAC neon pilot light and power switch.  I also do not intend to cut the power transformer wires. Thus this space under the transformer promotes wire routing and management as well as providing a space to bundle up excess wires. When you know stuff, you can do stuff! 

BTW having both a CNC and manual milling machine sure makes cutting rectangular and circular holes in various materials a lot easier than the old drill holes and use the file method.

The power supply schematic should have been a clue that there was a problem brewing! But I missed it.

Many of the PMR6A's are marked on the back cover 6V or 12 V and this bypassed me. My PMR6A has no marking (as I discovered). The clue in the power supply diagram is at the very top of the power transformer that shows a switch for 6V or 12V. The actual Schematic in the manual I downloaded shows two wiring configurations for either the 6V or 12V. 

The first problem is that my power supply transformer only has a 6.3 VAC winding at 3.5 amps so while that would be OK for the 6V version --a bit short for the 12V version.

I popped the hood (actually it was the bottom cover) and after a bit of tracing in the rats nest of wires -- mine is the 12V version.

So we are left with a few alternatives for the filment supply:

  1. Rewire the filaments so that all filaments are in parallel and thus would operate from the 6.3 VAC winding. Easier said than done because of the tight space. Doable but not without a lot of hair pulling and swearing. Not totally discounted at this point and involves no additional cost.
  2. So OK take the 6.3 VAC and build a voltage doubler supply and then run it through a voltage regulator so that you get 12 VDC. (The PMR6A filaments can run on either AC or DC). A possibility … but more hardware to build and we are talking amps so big power pass transistors.
  3. Just bite the bullet and purchase a 12 VAC filament transformer say at about 3 amps and install it on the board. Certainly a pretty simple hookup but does require some added board space. Hmmm TRGHS in that now the wooden base has a larger foot print.
  4. Purchase a 12 VDC 3.5 amp power supply and install it on the board. Such supplies tend to be pretty small and should fit on the board. Now we have to evaluate the cost of the supply versus the cost of a transformer.
  5. In looking at items 3 and 4 -- a 12 VAC transformer at 3 or 4 amps is roughly in the $13 to $15 range. I did find a 12 VDC 3.5 amp DC supply for about $8. Still noodling this one but will make the decision today. I can finish the build of the HV part of the supply but can not power on my PMR6A until I get the filament supply whacked!

Today was the first day I actually looked inside  my PMR6A and it is not unlike the photos that I used in the earlier post. A bit dirty -- and it contains many "dog bone" style caps -- no brown colored resistors ( which may be evidence of overheating). Keep in mind this is a journey that we started by building the power supply. A smoke test is in the very near future! 

The garage right now is about 39F -- "a bit nippy" said the topless stripper as she rode the metal pole!


Pete N6QW

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