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Old 15th Apr 2018, 6:35 pm   #1
George G4EUF
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Default Iso-Tech IPS1603D power supply

Hello All,
Does anyone know where I can get hold of the circuit diagram for the above power supply.
I have been given one that is faulty.
Regards.
George.
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Old 15th Apr 2018, 8:03 pm   #2
MrBungle
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Default Re: Iso-tech ips1603d power supply

Looks like a Mastech OEM unit although I can't find which one. Same as a Leader 760-3D though.

What's up with it? These tend to have a couple of regular failure modes:

1. Tap switching relay gets stuck, which causes pass transistors to blow up and go short and suddenly you get 75-80v across the output.
2. Ceramic resistors on the control board burn out.
3. Cheap capacitors go short.
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Old 16th Apr 2018, 7:49 pm   #3
George G4EUF
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Question Re: Iso-Tech IPS1603D power supply

Hello Mr Bungle,
Thank you for your reply, the fault is as you say, some 88 volts on the o/p, one of the 741s completed cooked (U202) that has now been replaced. I should mention that this PSU was subjected to a very strong electrical discharge in an adjacent room, so its any bodies guess!!
Have taken a look on the net to see if I can find any circuit info on the Leader 760-3d which as you rightly say is a very near the same machine.
Again my thanks.
George.
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Old 16th Apr 2018, 8:09 pm   #4
MrBungle
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Default Re: Iso-Tech IPS1603D power supply

Ok sounds like the usual suspect failure mode. Circuit is pretty straight forward for these usually. They're all derived from a single design with slight variations but that's usually enough to debug them. I've attached a schematic of the "generic" HY3005 power supply which is the parent of all variants I've seen. It may be different but the usual case is they aren't that different. Usually variation is in part costs, pass transistor counts and voltage references.

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There's a CC/CV regulator which consists of two 741 opamps OR'ed together via the front panel LEDs. There is a voltage and current sampling network connected to each of these. These opamps drive a couple of transistors in a darlington arrangement which drive the array of 2n3055's.

There are usually two or more comparators connected to the output using an LM324 which switch transformer taps in and out depending on the output voltage. This keeps the power dissipation of the 2n3055 pass transistors down.

There is a separate voltage reference circuit connected to a transformer tap which uses a 7812 and a TL431 as a reference.

Metering is passive and they are Chinese clone ICL7017 "blobs" or a real DIP IC if they pushed the boat out on costing.

The failure mode here is probably one of the relays failed. This causes excessive dissipation and a pass transistor went short. This caused a cascade failure which blew up the opamps and then drove the whole thing hard into the ground. Seen this a couple of times!

Repair steps are usually:

1. Check that the pass transistors (2n3055's) are not short from collector to emitter.
2. Check that the driver transistors, usually on the control board are not short from collector to emitter.
3. Check for open resistors (usually the ceramic ones if any go open).
4. Desolder and check the relays out of circuit or it'll blow up again.
5. Replace both the LM741's.
6. Replace the LM324 (this usually blows up first)
7. Visual inspection for any other damage. Not seen anything else blow up (yet).

Edit: forgot schematic!

Edit 2: I actually rather like these supplies because you can repair them. Some of the newer ones, particularly £1000+ Agilent/Keysight ones aren't any better and aren't even remotely repairable. Also as a fine example, they put all the metering trim pots right near live mains terminals in the Agilent ones!
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Old 16th Apr 2018, 8:39 pm   #5
George G4EUF
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Default Re: Iso-Tech IPS1603D power supply

Hello again,
Thank you so very much for the info together with the schematic I shall now plod on, again my thanks.
George.
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