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Author Topic: ic-28a  (Read 3900 times)

Posts: 3

« on: February 02, 2005, 06:01:19 PM »

if the internal battery is dead how will it affect the radio, i can't get the unit to power on

Posts: 69


« Reply #1 on: February 02, 2005, 11:35:38 PM »

That shouldn't stop the radio from powering on (he said before looking at the schematic).  What you should see is all the channels at "144.000" or something like that, with other settings "forgotten".

I don't remember how easy it is to swap that battery, but it wouldn't hurt to see if RS has one, and try that first.

(I have an IC-28H here.)


Posts: 1045


« Reply #2 on: February 03, 2005, 05:55:55 AM »

If the rig won't power on, I would expect oxidation or dirt in the switch contacts from non-use.  

Posts: 10248


« Reply #3 on: February 03, 2005, 06:39:29 AM »

I guess I'm in for a rude awakening one of these days. My 28A is 29 years old and it has never had the battery replaced. Ummmm.

Alan, KØBG


Posts: 17411

« Reply #4 on: February 03, 2005, 12:03:52 PM »

29 years old?  No, that might be the IC-22A, which was
diode programmable.  I bought my IC-28A new in 1986 or so,
so 19 years old is more like it.

Oh, and I've never had problems with the internal battery
either.  However, if the backlighting for the display
doesn't come on, the problem is in the main power lead
rather than the memory backup battery (if it has one.)
I don't even recall seeing an internal fuse, but there
could be one.  (I'll have to look at the schematic.)  It
is possible that the power switch is broken, or that the
input voltage was reversed and blew something inside.

Posts: 3

« Reply #5 on: February 03, 2005, 12:16:50 PM »

well i have checked the battery and it is good, check power coming into radio and it was correct, haven't found any internal fuses, i must say there might be one and i don't recognize it.  It probably is not worth having someone to repair it, probably cost more then a new radio

Posts: 17411

« Reply #6 on: February 03, 2005, 04:45:29 PM »

I don't remember the 28A as being that difficult to repair,
particularly the type of fault that would cause a loss
of DC power to major portions of the receiver.

What are you using to determine whether there is power
to the radio?  The display backlight? audio output with
the squelch open?  I suppose the backlight could go out
and leave the rest of the radio operating.

Was the radio working properly and just quit one day?

Maybe I'll try to find the schematic this evening and see
if it gives me any ideas...

Posts: 3

« Reply #7 on: February 03, 2005, 06:28:42 PM »

The schamatic i have is not very detailed so any thing you can pass on will be appreciated.  I used to work on these things years ago but my eyesight is not the best in the world



Posts: 69


« Reply #8 on: February 03, 2005, 09:23:34 PM »

MODS.DK has the service manual.

It shows a reverse polarity diode and a small cap across the DC input.  If either is shorted, it's not going to work, but you'd likely pop a fuse in whatever's powering the thing.

There should be a fuse in the power harness supplied by Icom, too.  Not to bring up the obvious, but....

Once the DC source gets past that, it goes over to the power switch on the volume control, and into one of the modules (I can't tell which - too much detail [grin]).  Given the amount of power required when transmitting, it should be the finals, but....

There are a couple of connectors involved internally between the rear panel and the volume control.  I'd look at those first.  Re-seat everything else that looks like a connector too at this point.  The backup battery is visible - I think it's on the underside of the unit but wouldn't swear to it.  I wouldn't spend the money for a new one until you can trace other problems out first.

You should be able to trace the power wiring in, and see what's getting it to the power switch.  The panel lights may have died years ago, but you should be getting something out if the problem is past the switch....

If you don't mind a Yaesu, the FT-1500M may still be around for about $130.  That's what replaced my IC-28H. The FT-1500M loses it's mind once in a while when powered up cold, and comes with about the crappiest mic I've run into in years, but it also comes with built-in PL (no need for an add-on module).  'Bout the same size, but with a mount that defies description.  Kenwood style power connector if that means anything....  The FT-2800M looks to be a better radio and is still available, I think, for a couple bucks more, but I was told that it tended to lose it's mind more readily.  Either one can be programmed via a PC, but the software wants Win9x or better (which blew away thoughts of leaving an old 386-25 notebook in the car for emergencies).  Cable & Software run about $40.

I think I've had the IC-28H for about 18 years.  My daughter was a baby when I bought it.  She's driving in from college this weekend to beg for money or something....


Posts: 17411

« Reply #9 on: February 04, 2005, 12:48:44 PM »

Well, the reverse polarity diode is a relief - makes it
less likely that something is really messed up.  The first
thing I'd do is to check the power lead with a DMM or
VOM and see if you can confirm the forward voltage drop
on the internal diode.  If it is blown due to over current,
it is possible that the rig could be toast.  Next step is
to check continuity from the DC input to the power switch
on the volume control and/or the RF power module (when the
switch is closed - may be easier to check).  Also check
that the negative lead is connected to the chassis (again
an ohmmeter check from wire to chassis and/or antenna
connector ground is a good start.)

The only long-term problem I've encountered with my IC-28A
is that the ground braid on the antenna connector pigtail
is wearing thin where it enters the back of the radio.
I have already repaired the output module a couple of
times and replaced a switching diode in the front end.

Posts: 3161

« Reply #10 on: October 29, 2012, 10:29:29 PM »

Icom IC-28 (A-E-H) Service Manual

This radio has a CPU RESET button, inside radio and
a solder-in lithium coin cell (based on radio age, check for battery leakage).

Looking at the schematic diagram:

Q17 is a 2SA1359 PNP transistor.
The emitter is connected directly to the 13.5 volt power input, right after the reverse-polarity protection diode.
C54 is a 10 uF tantalum capacitor (polarized) between the collector of Q17 and ground.

It is not uncommon for this part, C54, to fail.  If so, replace C54 with an electrolytic version.
Q17 is the APC control tethered directly to the PA IC-6 /SC-1019/(SC-1022 in 45 watt models).
Q-17 is controlled by the APC amp (IC-7 and Q-18).
APC amp circuit gets its drive from the phase detector a pair of 1SS97 diodes (D12,D13).
The other 1SS97 drives the RF-Indicator amp.

The pinout of the SC-1019 PA amp if you look at it from the front with the 4 pins down is, input, V1, V2, output.
The heatsink tabs are ground.
« Last Edit: October 29, 2012, 10:43:17 PM by W9GB » Logged
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