I can’t comment on the chip substitution, unfortunately. But I have successfully used my Hakko 808 desoldering gun to unsolder working 40 pin chips for the sole purpose of substituting into a board that had stopped working to see if the chip was the cause. I then desoldered it from the “bad board” and re-soldered it into the old, “good board” and everything went very smoothly without any issue. I can’t imagine unsoldering without it. I remember the hell of solder wick and quick release pumps. When I first got the hakko I thought I’d died and gone to heaven!

73 Eugene W2HX
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From: [email protected] <[email protected]> On Behalf Of Ray Fantini via groups.io
Sent: Wednesday, December 21, 2022 11:31 AM
To: [email protected]; [email protected]; [email protected]
Subject: [HarrisRadio] UGC-74 and the SM-A-9157

Been wasting way too much time on a UGC-74 power supply, the first one had an issue with the microprocessor power supply that provides the +5, +12 and -5 volt source for the controller cards. There is a separate power supply for the communications interface and a third for the drum motor drive. All of the power supplies are pulse with modulators that are feed from the unregulated +24 volt bus.

U1 has apparently failed and is no longer attempting to feed wide enough pulses to the driver device U5 that provides the high current switching. Also, in the process of working on this discovered that there is a capacitor C3 a 100 uf filter just after L2 on the unregulated 24 volt bus went open and replaced that.

Once I obtained another power supply card plugged that in and noticed it had only around three or four volts on the five volt bus and then U1 failed in the same way as the other card. Looking at it found C3 was also open and am starting to think that when C3 goes open the excessive switching noise on the unregulated 24 volt bus causes the PWM chip to die! Sounds like a reach but before U1 bit it had a scope on the unregulated bus and it had around twenty four volts of switching noise on it.

So now I am looking for replacement PWM chips, the chip that they use for both power supplies is a SM-A-915731 that cross references to nothing. Looking at it it’s a sixteen pin dip and it appears to me to be the same thing as if not a primitive version of the SG2524 PWM chip. All the pins are the same and the 2524 is a fairly common device unlike the 9157

Anyone have any thoughts about this? if the unobtainable 9157 can be replaced with a SG2524? I have a spare card now that has at least one functional 9157 on it but cannot imagine how I can remove the chip without destroying it in the process. To remove the one bad chip from the power supply card I want to use did what I usually do when removing chips and clipped the legs off at the device and desolder them one by one but that’s not going to work when trying to remove a device and wanting to save it.

Will be interested in hearing any comments.

Ray F/KA3EKH

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