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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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15th May 2017, 9:50 am | #1 |
Pentode
Join Date: Apr 2014
Location: Kirkwall, Orkney, UK.
Posts: 165
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Skanti MF 24V AC DC Power Supply Unit.
Please could some wiser brains than mine cast their eyes over the schematic of this Skanti MF PSU that I am trying to get some sense out of.
When I switched it on, I found 45V DC at the terminals, with the output light lit. I connected up a 240V 10W lightbulb as ballast, and this glowed gently, with the PSU maintaining its 45ish Volts. even with a heavy load (48V battery) the volts remained at this level. Something wrong with the regulator. I dismantled it and had a very brief check for loose wires etc. Now, when it comes on, there is 12ish volts only on the output, the output light is unlit, and the overload light is lit dimly. I am trying to fix it myself, but am a bit stuck on working out how the foldback circuit works. what is the function of the diodes D1 (which I have marked in red boxes) ? Am I right in deducing that Q1 and D2 (on module 481) form the main voltage regulation reference? (which is 6.2V for zener convenience, sensed from the output by resistive divider R1-R17-R3) Am I right in saying that in overload condition, the voltage at PL8 would rise (with respect to the V at PL5) and this effect is used for the shutdown sensing? thanks |
15th May 2017, 10:57 am | #2 |
Dekatron
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
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Re: 24v ac dc psu
Interesting design.
D1 on all three both boards is a protection diode to stop the load damaging the power supply. D2/Q1 looks like the voltage comparator. It's a bit of a naff circuit though. It looks like D2 is acting as a reference voltage on the right to compare the divider on the left against. When the voltage between PL4/PL5 rises to a certain amount it will make Q1 conduct which will short out the error amp in the bottom of the diagram via Q2. Without further analysis it looks like it compares the ratio of voltages between the divider and the zener reference. The zener ref is quite soggy. PL8 looks like it's "circuit ground" based on the connection to the filter capacitors so we'll assume that to be the zero volts reference. The pass transistors are at ground by the looks so it looks like when there is 15 or more dropped across the pass transistors, the light comes on due to the zener action on D4 which is the overload indicator. As for shutdown, I can't see anything that would indicate that it would. The large number of pass transistors suggests it'll just eat the short until it's fixed, or explode. Whole thing is a bit of an iffy circuit but it probably works. Not sure why people don't just build an LM723 into their designs for these sorts of things. You get all the features for significantly less cost. |
15th May 2017, 10:32 pm | #3 |
Nonode
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 2,169
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Re: 24v ac dc psu
Have you checked the 'output' bulb to see if it has blown?
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15th May 2017, 11:10 pm | #4 |
Pentode
Join Date: Apr 2014
Location: Kirkwall, Orkney, UK.
Posts: 165
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Re: 24v ac dc psu
There is a circuit description in the manual at
http://www.peel.dk/Skanti/pdf/P6001.pdf which I should have included really. |
16th May 2017, 11:58 am | #5 |
Hexode
Join Date: Nov 2003
Location: Ossett, West Yorkshire, UK.
Posts: 483
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Re: 24v ac dc psu
It's unusual to see a power supply that uses silicon trnasistors that is regulated in the negative side.
The most likely cause is a failed BC547 or BC557, possibly one of the zener diodes or the preset resistor. Connect a load and unplug the control card. If there is no output then the fault is on the control card or if there is still a voltage present at the output connect a 100 ohm resistor across Con1 and PL1 on each bank of transistors, disconnect PL2 on both banks and check for voltage from the positive output to each PL2 in turn to determine which bank is at fault. Last edited by See_Mos; 16th May 2017 at 12:05 pm. |
16th May 2017, 11:02 pm | #6 | |
Pentode
Join Date: Apr 2014
Location: Kirkwall, Orkney, UK.
Posts: 165
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Re: 24v ac dc psu
Quote:
"OK Jenkins, and where are we at with the P6001 regulator?" "well we tried this and we tried that but we think the most cost effective solution will be to use loads of pass transistors. That way, it'll either cope with a short circuit for ages or ... um... explode!" "Hm. Well, it'll keep the customers on their toes I suppose. Right, what about the SWR protection? or shall we just fit a flameout device onto the antenna lead-in....?" |
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17th May 2017, 7:11 am | #7 |
Dekatron
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
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Re: 24v ac dc psu
Had a look at the manual and it makes more sense now. It should indeed shut down. I say should because the method used is somewhat iffy which is sampling the pass transistor dropout voltage. The regulator input DC at high loads is going to have a ton of ripple so the thing is only periodically going to be seen as regulating 43A so your mileage may vary. Usually you sample after the pass transistors and then sample the voltage to compare with the reference after that using a divider.
I've seen an article that used 100x 2N3055's in a 1Kv DC supply. This turned out to be a joke no one got Last edited by AC/HL; 17th May 2017 at 11:20 am. Reason: OT aside deleted |
18th May 2017, 7:56 pm | #8 | |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,536
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Re: 24v ac dc psu
Quote:
Reality in power electronics is often stranger than (science) fiction!
__________________
....__________ ....|____||__|__\_____ .=.| _---\__|__|_---_|. .........O..Chris....O |
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18th May 2017, 11:37 pm | #9 |
Dekatron
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
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Re: 24v ac dc psu
Impressive. I'd love to see something like that. It's entirely the opposite end to RF where like to play. I get worried if I have to deal with anything more than a couple of amps!
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25th May 2017, 1:44 pm | #10 | |
Pentode
Join Date: Apr 2014
Location: Kirkwall, Orkney, UK.
Posts: 165
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Re: 24v ac dc psu
Quote:
I did discover that Q3 was faulty, and on replacing that with a NPN, I can get 42V at the output again, but only if I disable the rest of that area (ie, by lifting Q2, Q6 & Q5) What is the purpose of Q2, Q6 & Q5? I have a gut feeling that Q5/6 form the "latch" in case of overload, which must be reset by powercycling... is that right? if so, then what is the purpose of Q2? if Q2 saturates, then this would appear to just short-circuit the rest of the Q5-Q6-Q3 area. it seems more like sense that Q2 is controlling the voltage on the positive rail of the Q5-Q6-Q3 circuit, in response to the Q1 comparator...? I am starting to wonder if there is a fault further back in the rectifier, 42V DC across the filter seems like too much to start with? |
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25th May 2017, 5:01 pm | #11 |
Nonode
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 2,169
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Re: 24v ac dc psu
Do you know where this power supply came from?
The Primary of the mains transformer may have been wired for 110/120V. |
25th May 2017, 6:35 pm | #12 |
Pentode
Join Date: Apr 2014
Location: Kirkwall, Orkney, UK.
Posts: 165
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Re: 24V AC DC Power Supply Unit.
Yeah, that was my first thought too. it came from a UK building, and the jumpers were set for 230. I set them for the full monty ie 240V before starting work.
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26th May 2017, 10:53 am | #13 |
Hexode
Join Date: Nov 2003
Location: Ossett, West Yorkshire, UK.
Posts: 483
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Re: Skanti MF 24V AC DC Power Supply Unit.
I think this is how it works:-
Q5 and Q6 are a current limit latch. If too much current is drawn the supply tries to maintain the output voltage by increasing the drive to the transistor arrays. This causes an increase of voltage across R9, Q1 B-E and R1 ( for example). The increase in voltage will eventually turn on Q6 which in turn turns on Q5 which turns Q6 on even harder causing an SCR effect which pulls down Q2 collector removing the voltage from the drivers Q3 and Q4. Capacitor C3 prevents the pair from latching up when power is first applied as well as preventing momentary surges from causing the trip. Q2 amplifies the voltage from Q1 which is the voltage comparator. Referenced to PL8, if the output voltage is high then Q1 collector should go higher turning on Q2, the collector of Q2 goes down reducing the drive to Q3 and Q4 and then the transistor arrays. The collectors of the arrays go higher and the voltage difference, the output, between the storage capacitors C1 to C4 = positive output and the negative output terminal reduces. |
26th May 2017, 11:29 am | #14 |
Pentode
Join Date: Apr 2011
Location: Swansea, Wales, UK.
Posts: 144
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Re: Skanti MF 24V AC DC Power Supply Unit.
42 volts off load seem reasonable to me Cresta. You need a load of, say, 10 ohms all the time to give the regulators a chance to regulate.
If you removed the regulator card and ran it from a variable psu you could check that it is working as Seemos describes. Best of luck, Andrew, hater of high current psus. |
29th May 2017, 8:39 am | #15 |
Pentode
Join Date: Apr 2014
Location: Kirkwall, Orkney, UK.
Posts: 165
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Re: Skanti MF 24V AC DC Power Supply Unit.
See_Mos & Andrew, thanks very much for the help and in particlar the theory of the Q5/6 latch.
my 7W 240V xmas lightbulb load may not be low z enough to make the pass arrays do their thing, possibly.. will poke around and revert. |
30th May 2017, 12:31 am | #16 |
Pentode
Join Date: Apr 2014
Location: Kirkwall, Orkney, UK.
Posts: 165
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Re: Skanti MF 24V AC DC Power Supply Unit.
Fixed it. Q5 and Q6 both very dicky looking on the meter (junctions dropping 0.2-0.3V both ways on diode check for one, and base shorted to collector on the other)
also Q1 didn't seem to be playing ball in-circuit (though it checked out OK when out of circuit) so I replaced it. strange that so many transistors appeared to have failed. I wonder if it had suffered lightning. it makes a very passable effort at welding with the foldback transistors disabled. Thanks very much for all help. I will post up my notes on voltages and component locations if appropriate on the site? or is it just too obscure for anyone to be interested in? Tris Incidentally, it does not seem to require any load for the voltage regulation to function. |
30th May 2017, 10:58 am | #17 |
Pentode
Join Date: Apr 2011
Location: Swansea, Wales, UK.
Posts: 144
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Re: Skanti MF 24V AC DC Power Supply Unit.
Well done Cresta.
I see the output and the regulator circuit can be switched to heavy duty external batteries. Those batteries would need their own charger. Two things come to mind which could damage the regulator circuit. 1. The batteries could be accidentally connected the wrong way round when being replaced. 2. The battery could be disconnected and the charging voltage rise to a high value. Have you got the radio on air yet? 73, Andrew |
30th May 2017, 11:46 am | #18 |
Hexode
Join Date: Nov 2003
Location: Ossett, West Yorkshire, UK.
Posts: 483
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Re: Skanti MF 24V AC DC Power Supply Unit.
Good to read you fixed it.
From experience I have found that BC547 and BC557 fail quite often and are not as reliable as the BC182 and BC212 which have the same basic specification. If I get a unit with 547 and 557 and any are faulty then I replace them all. |
30th May 2017, 11:49 am | #19 | |
Pentode
Join Date: Apr 2014
Location: Kirkwall, Orkney, UK.
Posts: 165
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Re: Skanti MF 24V AC DC Power Supply Unit.
Quote:
the project radio, a Sailor T128 is not quite on air yet. I wanted a nice PSU to drive it with (rather than 2 tractor batteries) which is why I got this Skanti P6001. now I have that I can press on with the T128. As part of the deal of the P6001, I got its associated TRP6000 MF transceiver, which is quite a nice retro unit itself. PA3FUN has done an EPROM mod on the TRP6k to do free tuning so I may look into that later. Might start a "fishphone through the ages" exhibition.... or perhaps start a MW talk radio station. attached are some notes for anyone that cares. |
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