### Author Topic: Linear Power Supply Problem - HELP!!!!  (Read 3685 times)

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#### Fryguy

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##### Re: Linear Power Supply Problem - HELP!!!!
« Reply #25 on: September 27, 2018, 11:50:19 am »
This smells like a dead primary - check if there is any current on the outputs . At these low voltages try to connect a small load to the secondarys - e.g. a flashlight bulb or something similar and look if the secondary outputs drop dead or the lamp lights up . Your multimeter alone has a too high input impedance to show that .
May the forces of evil get confused on their way to your home !

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#### ArthurDent

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##### Re: Linear Power Supply Problem - HELP!!!!
« Reply #26 on: September 27, 2018, 01:49:16 pm »
I had previously posted: “The A.C. voltage from BE to BE should be the same as from YE to YE and perhaps about 10 to 12 VAC. The D.C. voltage from D5 to common should be the same as from D7 to common and perhaps 12 to 15 VDC.” (actually YE-YE might be a little lower)

So let’s analyze one regulator and see if the voltages you get make sense. Looking at the +/-15VDC supplies, the A.C. voltage from RE to RE should be the same as from OR to OR and perhaps about 17 to 20 VAC. The D.C. voltage from D1 to common should be the same as from D3 to common and perhaps 20 to 25 VDC.

If you make a block diagram of what just the +15VDC supply would have to have for voltages to work, what you’re getting is not right. To get +15VDC out the A.C. secondary voltage would typically be a few volts higher and 17-20VAC is a possibility. When this is rectified you will have the diode forward voltage drops for 2 diodes but the capacitor will try to charge to the peak value of the rectified A.C. so you might have 20-25VDC, depending on load. The regulator works like a variable resistor so the input has to be a few volts higher than the output to compensate for drops across the pass transistor, the current sense resistor, and for low input line voltage. To get a constant +15VDC out of the regulator you would need a few volts higher on the input for the regulator to work properly.

When you say: “new results: YELLOW: 2.2 BLUE : 2.9 RED: 4.5 ORANGE: 2.2 (all readings VAC) -that's across the secondary windings - relative to ground each is 27-30 VAC” the secondary voltages are way low as I explained, above. The 27-30VAC you measured to ground is just capacitance coupling and not really there. Put a 1K resistor between any wire to ground and that measured voltage will drop to near zero.

If there are shorted turns in the transformer it would be really overheating and/or blow a fuse. If you are now powering the transformer through the other primary (RE-BE), measure the A.C. voltage across the original primary wires (GY-BR) and you should measure about 120VAC as well.

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##### Re: Linear Power Supply Problem - HELP!!!!
« Reply #27 on: September 27, 2018, 04:29:40 pm »
Refer to your description of symptom 1 and now you said you have the primary resolved? Can you measure the ohms from the hot and neutral to confirm. Should be around 6.1 ohms, otherwise clean all contacts, fuse contacts, fuse etc...

symptom 1: there is an open load across the hot and neutral of my power cable.

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#### jaunty

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##### Re: Linear Power Supply Problem - HELP!!!!
« Reply #28 on: September 28, 2018, 09:14:27 am »
HEY thanks for the help guys - i wasn't online yesterday so didn't check ... but i made HUGE headway tonight ... i was chasing a red herring - i thought there was maybe an intermittent short in the primary too - but part of that was the result of not understanding that the primary circuit routed OUT of the power supply and i had to jumper something -but then the transformer was very much (through mental inertia) still under suspicion - so i ripped the whole thing apart again (part of the inherent evil of this design due to the 'wrapped' wire and soldered terminals which are a BITCH to undo .... measured all the secondaries LIVE with dim bulb (no bulb lighting) and THEN i decided to pull ALL the diodes just because ... 'what else could it be???'  for the second time - and I actually found ONE SHORTED DIODE (GAAAAHHHH) - i guess i never found it before somehow because of it's awkward location ... difficult to probe even ... so yeah .. OF COURSE it HAD TO BE that one ... (grrr) - anyway - I'm happy at least i found something now - after checking the transformer - all secondary voltages looked great - very much within spec ... so i feel confident the transformer will live on for some time now ... THANKS for the help and i'll update if i can get this fixed up and running ...

now i just need to figure out how to build a replacement IR 10DC-4 diode pair ... seems impossible to find online NOS ...

#### jaunty

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##### Re: Linear Power Supply Problem - HELP!!!!
« Reply #29 on: September 28, 2018, 09:19:22 am »
I had previously posted: “The A.C. voltage from BE to BE should be the same as from YE to YE and perhaps about 10 to 12 VAC. The D.C. voltage from D5 to common should be the same as from D7 to common and perhaps 12 to 15 VDC.” (actually YE-YE might be a little lower)

So let’s analyze one regulator and see if the voltages you get make sense. Looking at the +/-15VDC supplies, the A.C. voltage from RE to RE should be the same as from OR to OR and perhaps about 17 to 20 VAC. The D.C. voltage from D1 to common should be the same as from D3 to common and perhaps 20 to 25 VDC.

If you make a block diagram of what just the +15VDC supply would have to have for voltages to work, what you’re getting is not right. To get +15VDC out the A.C. secondary voltage would typically be a few volts higher and 17-20VAC is a possibility. When this is rectified you will have the diode forward voltage drops for 2 diodes but the capacitor will try to charge to the peak value of the rectified A.C. so you might have 20-25VDC, depending on load. The regulator works like a variable resistor so the input has to be a few volts higher than the output to compensate for drops across the pass transistor, the current sense resistor, and for low input line voltage. To get a constant +15VDC out of the regulator you would need a few volts higher on the input for the regulator to work properly.

When you say: “new results: YELLOW: 2.2 BLUE : 2.9 RED: 4.5 ORANGE: 2.2 (all readings VAC) -that's across the secondary windings - relative to ground each is 27-30 VAC” the secondary voltages are way low as I explained, above. The 27-30VAC you measured to ground is just capacitance coupling and not really there. Put a 1K resistor between any wire to ground and that measured voltage will drop to near zero.

If there are shorted turns in the transformer it would be really overheating and/or blow a fuse. If you are now powering the transformer through the other primary (RE-BE), measure the A.C. voltage across the original primary wires (GY-BR) and you should measure about 120VAC as well.
thanks arthur dent for your characteristically thoughtful and kind input! but yes i was getting 117V across the BRN/GY pair as expected ... after disconnecting the secondaries I ended up with about 21.5 for both orange and red windings - then about 14 then 10 for the remaining two ... so it sounds about right to me ...and a big monkey off my back ... thanks so much for that ... sometimes you just have to take everything apart ...

#### ArthurDent

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##### Re: Linear Power Supply Problem - HELP!!!!
« Reply #30 on: September 28, 2018, 01:24:47 pm »
Without a photo it is hard to tell but looking at the parts listed on the schematic it appears the rectifiers are three lead full wave with just 2 diodes.  The 10CR-4 has a + out and the diodes are reversed in the 10CR-4R to give a – out. Using the two types together form a more common four lead full wave bridge and that is what I’d use for replacement. Here is just one listing for hundreds on Ebay that should work.

https://www.ebay.com/itm/5Pc-FULL-BRIDGE-RECTIFIER-2W06-2A-600V-Full-Wave-Rectifier-FREE-SHIPPING-US-SHIP/163263426481

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#### jaunty

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##### Re: Linear Power Supply Problem - HELP!!!!
« Reply #31 on: September 29, 2018, 05:46:40 am »
Without a photo it is hard to tell but looking at the parts listed on the schematic it appears the rectifiers are three lead full wave with just 2 diodes.  The 10CR-4 has a + out and the diodes are reversed in the 10CR-4R to give a – out. Using the two types together form a more common four lead full wave bridge and that is what I’d use for replacement.

yes - they are 'dual diode' packages with three legs as you say ... they look just like this though i can't find the voltage/current capacity ratings for this specific model ... and I DO have 1N4004s on hand ... i fabbed one up and I will try it with a dim bulb i guess and see what happens - though the 'main' fuse is only a 500mA ... so I'm guessing that the 1.0A rating of the 1N4004 might be enough .. (?)

#### ArthurDent

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##### Re: Linear Power Supply Problem - HELP!!!!
« Reply #32 on: September 29, 2018, 01:32:27 pm »
1N4004 diodes would work to make a replacement bridge but I would check any of the 10DC-4 diodes you remove to see if they are the problem. It could be the diodes are o.k. and it is something they are feeding that is the problem.

Old electrolytics are a common failure point in older pieces of equipment and I would suspect them before the diodes.  It is pretty common to just replace all old electrolytics in old equipment before powering it up as a safety measure and I would replace all the 2200, 1000, and 220 uF caps. It still could be some other component but if the caps aren't bad now they could fail at any time plus they are probably dried out so the value is far less than the stated value.

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#### PKTKS

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##### Re: Linear Power Supply Problem - HELP!!!!
« Reply #33 on: September 29, 2018, 07:30:52 pm »

2 cents  of opinion....

- It looks a rather simple PSU.

- By today standards I would ditch that PSU aside...

- and quickly  put together a "3 pin IC" discrete like thing equivalent using 78xx/79xx or clones

Just in case I would stress the transformer or just ditch it altogether with a new one

CHEAP. SIMPLE.  Low and common voltages and simple topology.
May use an island board even single sided with no further problems...

and bye bye old PSU

#### jaunty

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##### Re: Linear Power Supply Problem - HELP!!!!
« Reply #34 on: October 03, 2018, 03:00:07 pm »
YES - i did precisely that Arthur - the 1N4004s are in and have been doing well for the last three days or so ... i'm wondering if maybe i shouldn't replace ALL the old diodes now

#### jaunty

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##### Re: Linear Power Supply Problem - HELP!!!!
« Reply #35 on: October 03, 2018, 03:02:14 pm »
not so easy to do considering the sense lines and all ... it's an old school 'safety feature' i guess ... but i'm not confident enough with my skills yet maybe

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##### Re: Linear Power Supply Problem - HELP!!!!
« Reply #36 on: October 03, 2018, 11:27:23 pm »
First:  you have to verify the primary of the transformer you have to get high ohm reading for primary for the secondary low ohms
a) in this case you have to unsolder the wire of the main transformer just only the first wire for either bridge and with the multimeter in ohms range take the reading.
b) is everything is fine verify the voltage from secondary you have to read around 18vac to 20vac, could be more up to 25vac because the caps on the secondary are all 35vdc, but could be less depending of the transformer wiring.

Second: Verify the bridge rectifier if you don't know how to do it, read this,
1: Put the negative test lead of your multimeter in the positive sign of the bridge rectifier of course you have to isolate both bridge rectifier from the board. with the positive test lead of your multimeter test the pins of the bridge (if the bridge is inline) the second pin is one diode voltage drop so se third and the fourth is two diodes voltage drop if you check the bridge with test leads reverser i should not read any voltage.

Third: You said that you replace the caps but check the zener diodes and transistors in case you solve the rest.

#### james_s

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##### Re: Linear Power Supply Problem - HELP!!!!
« Reply #37 on: October 05, 2018, 12:34:55 am »
No need to redesign the supply, there's nothing wrong with the original design, it just needs to be repaired. You'll get to the bottom of it, just follow along the suggestions in this thread and post your findings. If you get stuck, set it aside for a few days then come back to it.

Smf