APC Back-UPS XS 1500VA (BX1500), Need part number for Q7

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joro

Active member
Joined
Feb 8, 2008
Messages
42
Location
California
Hello,

My APC 1500 XS battery backup for my studio crapped out yesterday. It's out of warranty, so I'm digging into it myself. I found Q7 cracked in half. There are no markings on it and I haven't been able to find a schematic and I'm not sure what replacement part to order. I have not contacted APC yet to see if they will give me the info or will sell me a replacement part. Does anyone know what Q7 is?

Thanks!
 

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probably a mosfet, are there similar packaged transistors near Q7?

if they run 3 or 4 in parallel then you can steal a part number off a good one,

maybe an IRFZ42 or similar?

look for a 47 ohm resistor coming from pin 1, if you see that then it is a mosfet,
 
Thanks for the reply.

Yes, there are similar transistors. There are 4 in parallel. Unfortunately, there are no markings on any of them. And, they won't tell me and they won't sell one to me. They referred me to an authorized repair center.

Almost all parts are surface mount and really small and the markings are really small and difficult to read. Although, there is one 4R7 2W resistor that may be the resistor you are talking about.

I'll post a couple more photos in case that helps. (sorry for poor picture quality, but it gives you the idea.)
This photo shows the 4 transistors in parallel with large heat syncs, all on the left connected by plastic spacer. (The others are diodes)
 

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This photo shows the missing Q7 and the one 4R7 resistor.
 

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  • APC 1500 Q7 missing and 4R7 resistor.JPG
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Well, I tried. I picked up an NTE2395 (equivalent to IRFZ42). I soldered it in and turned it on. At least it powered up, but as soon as it went on line, it went into "overload". So, now the question is, do I need a different mosfet or is there another problem with the APC that I'm unaware of. Do you think a different value mosfet might keep it from going into "overload"?
 
I'm wondering if the reason I got the overload indicator is because it wants to see a P-channel instead of an N-channel. How likely do you think this is?
 
Joro,

Seems this is a common problem with this UPS.

From here:
I own two XS-1000 and one XS-1500 Back-UPS units (also use BX1000 designation). Each is in service, with the XS-1000 units loaded to about 250 watts and less than 300 VA and the XS-1500 loaded to 500W/600VA. Each of these units has failed with symptoms like those described by other writers (overload light suddenly comes on and cannot be reset normally). Unlike the other writers, I repaired my units myself. So far, none have failed again.

It is significant that I live in a region experiencing frequent power fluctuations and outages, hence need for a backup UPS. It is also noteworthy that circuit boards and circuit designs are essentially identical for the 1000 and 1500 models, with extra battery connections and a fan being prominent additions for the 1500 model.

In all instances, one or more of the six simulated sine wave switching FET transistors (34N20 or similar 20NQ20) failed shorted. This, incidentally, correctly causes the overload indicator to come on, since the on-battery circuit is then definitely overloaded! This protectective action in fact saves major parts of the unit from further damage, including battery overheating and possible explosion.

Qualitatively, the APC units are very well constructed and seemingly well designed. Additionally, the 34N20 FET transistors are a good choice for this application, and thus the failures are surprising. Several possibilities are suggested: (1) There is a latent quality defect in the transistors. Once one of the six fails, others will fail quickly until the DC/DC inverter stored energy is dissipated; (2) A momentary under or over voltage condition causes the AVR autotransformer circuit to switch in at about the same time the unit attempts to go on battery, possibly resulting in energy stored in the transformer being dumped into the 34N20 switching bank and exceeding a transistor's surge capacity; (3) One of the relays sticks or AVR/backup relay combinations "make before break" during a momentary utility outage or glitch.

Whatever the cause, it seems evident based on my three unit failures and those similar ones reported by other writers, that this defect, though perhaps infrequent for most customers, is nevertheless an issue for potential buyers who most need backup protection.

APC telephone representatives, though friendly, are not very helpful; the company is unwilling to provide a circuit diagram or even a block diagram of their products. Shipping costs are high even if the unit is still in warranty. Until this defect is acknowledged and corrected, these otherwise attractive units may not be suitable for use under the very conditions for which they were designed.

So I would say try this other MOSFET (34N20), and also change all 4, not just the one that blew open. You can also meter the MOSFETs with a diode check setting, and see if the others are in fact blown. Usually there will be a short between two of the pins if they are bad.

.99
 
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