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Old 21st Sep 2020, 6:27 am   #1
Clay1905
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Default Power Supply for AC/DC portable

HI folks,
Presently I have a Philips "Town & Country" model 168 on the bench. The mains supply transformers' H.T. secondary has gone open. After some searching, I came across the power supply on the projects pages at this site.

https://www.vintage-radio.com/projec...y-set-psu.html

I'll be darned if I can't get the HT side of the supply circuit to work as per the description. The best I can get is about 45VDC at the output. This isn't quite enough to get the valves functioning.
I've modeled the circuit in LTSpice, and voltages I'm measuring are what the simulation suggests I should find.
Now, my theory isn't very strong, but I would have thought that as the multiplier circuit does full wave rectification, the input, 15VAC, should be multiplied by 1.41, then doubled twice, as that's what the circuit should do. Accordingly, I should expect an ideal 84.6 volts at output, or realistically, about 80 VDC allowing for diode drops etc.
However, the same calculation, omitting the 1.41 bit (i.e. doubling the voltage twice), gets me very close to the actual measured voltages. For why?

There's a very good chance I'm overthinking things, and/or I have messed up the spice simulation too.
Any help or advice would be very welcome, I can tell you.

Files for the spice modeling are here:
https://drive.google.com/drive/folde...Z8?usp=sharing

Clay.
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Old 21st Sep 2020, 2:27 pm   #2
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Default Re: Power Supply for AC/DC portable

Hi Clay,
I can definitely say that the circuit works.....I built one! It gives me just short of 85V on load. The pictures on the website that Paul used are of the one I built. If you are only getting 45V then half of your quadrupler is not working. Make sure it's not something silly like a diode or capacitor connected incorrectly. I built mine straight from the circuit onto Veroboard over 10 years ago and it worked first time. I think I had to adjust one resistor value slightly but that was because I mounted the LM317 on a small heatsink and needed to reduce the dissipation......nothing whatsoever to do with the quadrupler.

Unless you have a dud diode or capacitor in the mix (which can happen even with new ones....and on that subject make sure the diodes are marked correctly with the band as cathode), I don't see how it can't work.

I don't have Spice so can't help there I'm afraid.
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Old 21st Sep 2020, 3:11 pm   #3
Clay1905
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Default Re: Power Supply for AC/DC portable

Thanks, Sideband,
I've been more or less coming to the same thought, that I need to lift some components and see just how well or other wise they are performing (and aligned). The output being as close as it is to half expected makes me suspicious. No such thing as coincidence, and all that.
I was fully expecting the supply to work, the web page has a ring of genuineness about it. And it comes in the right mains voltage for local conditions.
I also needed to add a bit more dissipation capacity to the L.V. circuit. Mine got a 47 Ohm 10W resistor and a big heatsink out of an old printer. The radio pulls 300mA for the heaters, so I've also added a bit of extra capacity in the transformer.
I built mine on veroboard too, only with a more long and thin aspect ratio. It fits easily into the battery compartment, and the transformer drops straight into the hole where the original went, which is handy.
Come the morrow I'll investigate further.
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Old 21st Sep 2020, 6:17 pm   #4
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Default Re: Power Supply for AC/DC portable

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Originally Posted by Clay1905 View Post
I was fully expecting the supply to work, the web page has a ring of genuineness about it.
Thank you! It is my website although it has been severely neglected for years. A freshen up is on the long to-do list though.
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Old 21st Sep 2020, 7:52 pm   #5
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Default Re: Power Supply for AC/DC portable

I am a raw novice using spice but according to this video (simple ac voltage source only) the specified amplitude is the peak value not RMS.
https://www.youtube.com/watch?v=VLOV...hannel=TimDean
Changing your sine amplitude to 21 volts gives just under 80 volts before the 1k resistor.
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Old 21st Sep 2020, 7:57 pm   #6
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Default Re: Power Supply for AC/DC portable

I have to admit that I've robbed the LM317 out of mine for the moment so it isn't fully operational but if I'm in the workshop later I can at least prove that the quadrupler works.....
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Old 21st Sep 2020, 8:35 pm   #7
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Default Re: Power Supply for AC/DC portable

It's quite possible you have your output points mixed up. For instance if you take the output from across C4, you'll have about 45V. Make sure that the electrolytics are the right way round....when you are dealing with a doubler or a quadrupler, it's easy to get confused as to what way diodes and capacitors go especially when translating the circuit diagram straight on to veroboard....and of course you have to cut the tracks in the right place as well.

I'll see if I can get some typical voltage measurements if that helps, unless of course you find the problem with yours.
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Old 21st Sep 2020, 10:36 pm   #8
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Default Re: Power Supply for AC/DC portable

OK a few quick checks. I'm getting just over 90V off load so the 84V I mentioned earlier was with a typical 10mA load. There is approximately 22.7V across each diode so if you add those together you get 90.8V. There is 22.7V each across C1 and C2 and around 45V each across C3 and C4. The voltages across C3 and C4 effectively add together to give the 90V available across C5/6.

Be careful probing around while it's on....90V bites!

....and to prove it works....

90.6V is off-load HT and 22.7V is the voltage across each diode.
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Old 22nd Sep 2020, 8:35 am   #9
Clay1905
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Default Re: Power Supply for AC/DC portable

Hello again gentlefolk,
Thanks all for your replies.
I went and looked at the circuit again, comparing with the schematic. There was some little niggle, but I couldn't put my finger on it. I made a rough sketch of the vero board with component locations, but much enlarged. Then the mysterious niggle became glaring. I had left the negative end of C3 just going nowhere useful, and the -HT terminal connected directly to one end of the transformer secondary.
A wire link and a copper strip cut later, and the picture says it all.
Thanks again, I'm muchly obliged for the prompting.
Now, to install it in the radio case....
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Old 22nd Sep 2020, 11:19 am   #10
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Default Re: Power Supply for AC/DC portable

Just a quick update.
With the PSU installed, heater voltage is at 1.43VDC and neither the dropping resistor or the LM317 are even warm.
+B is at 107VDC. However, the nominally 15VAC secondary is being generous and supplying 19.5VAC.
All in all, rather pleased. Thanks.
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Old 23rd Sep 2020, 8:09 pm   #11
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Default Re: Power Supply for AC/DC portable

Well you can tweak the final output down to 90V if necessary by adjusting the series resistor. I wouldn't want the LT any higher though. The 1.4V filaments are rather critical.
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Old 24th Sep 2020, 4:05 pm   #12
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Default Re: Power Supply for AC/DC portable

Hi.

I built mine several years ago and it has been very useful. I used tagstrips rather than stripboard. A couple of pics can be seen in post#15 in this thread https://www.vintage-radio.net/forum/...d.php?t=145072

Regards,
Symon.
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Old 25th Sep 2020, 7:14 am   #13
Clay1905
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Default Re: Power Supply for AC/DC portable

Quote:
Originally Posted by Sideband View Post
Well you can tweak the final output down to 90V if necessary by adjusting the series resistor. I wouldn't want the LT any higher though. The 1.4V filaments are rather critical.
Thanks. I will look at getting the anode voltage down, although it really shouldn't matter too much.
Filament voltage yes. I've been reading a bit on the effects of under and over voltage on valve life expectancy. Seems thorium can be upset by anything that isn't in a data sheet.

As an aside, now the radio has power, it's looking like one of the trimming slugs in an I.F transformer has dropped off its screwed rod.

Clay.
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Old 25th Sep 2020, 7:49 am   #14
Clay1905
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Default Re: Power Supply for AC/DC portable

Quote:
Originally Posted by Philips210 View Post
Hi.

I built mine several years ago and it has been very useful. I used tagstrips rather than stripboard. A couple of pics can be seen in post#15 in this thread https://www.vintage-radio.net/forum/...d.php?t=145072

Regards,
Symon.
Very neat it is too. I'm inspired to build two more. One is for a H.M.V./ Marconiphone T24-DAB. This will be a good exercise, as these were "transformerless". The chassis won't be connected to the mains any longer, and that pleases me. Some of the supply dropping resistors have wandered off tolerance, and frankly, right out of the paddock.
Another, something like you've built as a stand alone unit, would be a most useful thing in the future.

Cheers,
Clay.
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