Help me fix my latest IC7000 failure

Dan Smith - KK7DS <dsmith@...>

Hi all,

I'm looking for some help with my IC7000. I apologize for being wordy, it's in my nature and I can't help it. I'll provide a little background, in case it's relevant. If not, just skip to the problem description.

=== Background ===

I've used it quite a bit off an on over the years, but we have a bit of a dysfunctional relationship. I used to use it for a lot of portable (early days of SOTA) work in all kinds of conditions. It never got really wet or anything, and doesn't show any corrosion anywhere, but it has been outside quite a bit. Many years ago, it started doing this thing where it would just go deaf (all HF bands at least) on and off. Like, it would be sitting there in RX, showing S5 background noise, then ... silent S0 for ten minutes or so, then back to S5 like nothing happened. No touching, no external stimulus. I made a private youtube video showing it happen, wrote up a bunch of diagnosis and details, and sent it to Icom. They called me back, saying that they couldn't reproduce the problem, refused to look at the video, and thus were just sending it back to me with the ribbon cables reseated. I was super frustrated with this response (it was actually worse than I describe), so when I got it back, I literally put it on a shelf. A couple years later, I pulled it out to use it and got just the click-click-of-death thing. Back on the shelf, but later I diagnosed the shorted tantalum cap in the head, replaced, and it would power up. Back on the shelf without much use. Recently got it back out, and spent a day working it pretty hard on several bands and was thrilled to have my old friend back. Until it let me down again. On to the problem description:

=== Problem Description ===

Yesterday while working it, it started to power off when I would transmit. At one point it got into a really weird reboot loop where the relay that clicks on when you power it up would click on and off, on and off whenever it had external power. Click, click, click .. until I pull the power cable. No head input would stop it. Left it unplugged for a while and then it powered back on normally. Worked it for another hour and then it shut off again during transmit -- for would be the last time. After this, I get no click-click when I hit the power button, no activity at all when it is given power. Just totally dead like a brick. No burning smell either, for what it's worth.

=== Diagnosis ===

After much pouring over the service manual and tracing things out inside, I arrived at the IC1251 3.3v regulator on the logic unit. This (AFAICT) takes the 14v HV line and regulates it down to 3.3v to power the CPU and is required for the PWRK circuit to initiate the startup. This regulator was showing about 2.7v on the input and 2.4v on the output leg. Confirmed on the head connector pin 2 (with the head detached) that it's sitting at 2.4v instead of what I would expect to be 3.3v. I can probe the HV line on the main board right by the connector that feeds the logic board, and see that it sits at 14v when the logic board is out, and drops to 3.7v or so with the logic board in place. This indicates to me that something on the logic board is pulling that down real far.

I did what I could to check the bypass caps along the input and output lines to that regulator, but they're, uh, real small and stuff. So, I decided that a sane course of action would be to pop the regulator out so I could isolate before and after it. I got it out clean (although I pulled a leg off it so I'll have to replace that at least). As soon as I did, the HV line went back up to 14v, so I was hopeful that the regulator was bad. However, I wedged a TO-220 3.3v regulator from my parts box into the spot just to test, and it does the same thing. Without the output leg of the regulator connected to the board, it sits at 3.3v and the input sits at 14v. When I connect it to the output pad, it drops down and pulls the input with it, just like the original one.

As I mentioned, I tried to check what I could that is downstream of that regulator, specifically bypass caps C1252, C1317, C1317 although the latter two are so dang small I have no idea if I'm really on them. But, they don't seem to be shorted, best I can tell. The battery diode D1301 seems fine, and the battery voltage is 3.1v. Other than that, I'm at a bit of a dead end.

I assume I'm correct that ~2.4v at the head connector and output of the regulator is too low, and that the input shouldn't be pulled down that low, if not for something drawing a lot of current. Is that correct?

What else could I reasonably do to poke? Wild guesses, as well as informed ones, are welcomed.



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