Author Topic: Does anyone have a TTi EX752M bench power supply?  (Read 4355 times)

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Offline FF_CCSa1FTopic starter

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Does anyone have a TTi EX752M bench power supply?
« on: March 24, 2014, 10:45:33 am »
Hello, EEVblog forums. I'm sorry to try and leech off of this place with a first post like this.

I got my hands on one of these units for free, broken, but it has sadly been tampered with by a rather inexperienced tech, who did a lot of work on it, but didn't save the components he replaced. I can't trust him to have installed the correct replacements, making troubleshooting it very tedious. If anyone has one of these units, or even one of its smaller siblings, I'd be eternally grateful to find out what IC1 and IC7 on the power board are supposed to be. I'm attaching two pictures of the unit, including one of the two possibly incorrectly replaced chips.

If anybody has any general tips for repairing a unit like this, I'd love to hear them. The only thing that's working in it is the active PFC controller (an ST L4981A), but there seems to be a small switchmode converter built around the two logic chips (a 4013 and a 4069 - but they could be wrong), which is not starting. The entire primary side of this power supply is a bit of a convoluted mess of bits and bobs and thin traces going all over, making it a nightmare to work on. I've checked all the usual suspects (capacitors (including ESR), power devices, low value resistors, diodes, etc.) a hundred times over, to no avail, and I'm starting to grow desperate. It would be such a shame to trash a nice unit like this.  |O
 

Offline seronday

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Re: Does anyone have a TTi EX752M bench power supply?
« Reply #1 on: March 26, 2014, 01:13:41 am »
Have you tried Emailing TTi to obtain a copy of the Service Manual and Circuit.
 

Offline FF_CCSa1FTopic starter

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Re: Does anyone have a TTi EX752M bench power supply?
« Reply #2 on: April 07, 2014, 06:12:20 am »
Yes, and they supplied me with one - free of charge! The sad part is that the ICs are (off-brand) variants of the correct circuits, but I'm gonna try to replace them with proper ST components, once they arrive.

EDIT: After a lot of troubleshooting, I found out that the issue was

A) A broken trace to IC1 causing it to output <5 V gate voltage to the primary side switching transistors
B) A shorted output transistor on channel A, causing the voltage across the 470 µF/100 V cap on channel B to spike high enough for the OVP to kick in
C) An open resistor (R38) on channel A causing the voltage across Q12 to be very high, and essentially cause the regulator to never turn on, and hit the OVP due to underload

I hope this can help anyone else with one of these units. TTi were very helpful in supplying the schematic and service manual. Sadly, they're plastered with "DO NOT REDISTRIBUTE", so I wouldn't feel good posting them here.
« Last Edit: April 17, 2014, 07:09:33 pm by FF_CCSa1F »
 


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