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Author Topic: IC-251 (pre-A) -- Dark Operation - FUSES ???  (Read 215 times)

KB6LWN

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IC-251 (pre-A) -- Dark Operation - FUSES ???
« on: March 02, 2022, 09:05:28 AM »

Normally I prefer the 'Dark Mode' for my operations, not in this instance...

Stepping into the back room the other evening after an all-day online session, I found my Icom IC-251 (pre-A) was dark...   Cycled the power switch, checked the AC cable connection, checked the rear apron fuse holder...  Fuse tested good, still no joy...

Checked the eCopy of the manual I have for the A/E version, read that there are TWO values of fuses; 2A for DC ops, 5A for AC ops.
Only see one fuse holder listed in the docs, is there an additional board or inline mounted one internally ?

Removed the top and bottom covers, pulled the PS chassis, no more fuses evident.   Consulted the Schematic, see no other fuse symbols other than the one located on the rear panel.
I have not pulled the main chassis or any of the sub-boards, and hope I won't have to.

There is no physical evidence of damaged components, no burn marks, exploded parts, acrid odor, etc...   As far as I can tell, no components have been 'leaking smoke'.

The rig has been operating on receive acceptably for the last two weeks on a tabletop using AC power, and no power has been applied to the TX circuits.   Been waiting for a mic and resonant antenna...

I have not done any voltage measurements yet.
That will be my next task, to re-install the internal PS, get ye ol'DVM out,  decipher some pinouts - and take some readings.

Any constructive/positive input welcomed...
It will be about a month before I can get my hands on a 12vDC PS and I can supply power to the 12v rear panel connector then, and see if it comes back to life - pointing then to a failure of the internal AC PS.
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W3PX

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Re: IC-251 (pre-A) -- Dark Operation - FUSES ???
« Reply #1 on: March 02, 2022, 10:15:57 AM »

Fuse values shown in your post are actually reversed, the DC fuse is 5A and the AC fuse is 2A.

The fuse holder on back of the rig is the AC input fuse for the rig's internal 12V power supply. This is shown in the user manual on page 13.

The DC fuse that comes with the rig's parts kit is intended to be used in the ICOM supplied DC power cable.

Given your symptoms, I suspect the internal power supply has failed or the main power on-off switch is bad, providing the external AC fuse checks OK. Powering the radio via an external DC supply will easily verify.

You should be able to at least perform some AC and DC measurements on the internal power supply's input and output, and check the rig's main power switch, before your other 12V power supply arrives.

Good luck and 73,

Frank W3PX




« Last Edit: March 02, 2022, 10:22:34 AM by W3PX »
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"So, we just went ahead and fixed the glitch" - Bob Slydell

"I’d say in any given week I probably only do about fifteen minutes of real, actual, work" - Peter Gibbons

W3PX

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Re: IC-251 (pre-A) -- Dark Operation - FUSES ???
« Reply #2 on: March 02, 2022, 11:13:21 AM »

Also, check to make sure the 4-pin Molex DC jumper plug is properly connected at the rear of the rig, that it hasn't worked loose, and that the red jumper wire hasn't become broken or loose. This jumper plug is needed when using the internal AC supply.

73 Frank W3PX


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"So, we just went ahead and fixed the glitch" - Bob Slydell

"I’d say in any given week I probably only do about fifteen minutes of real, actual, work" - Peter Gibbons

WA3SKN

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Re: IC-251 (pre-A) -- Dark Operation - FUSES ???
« Reply #3 on: March 02, 2022, 11:51:15 AM »

It sure sounds like "power" to me, luckily a simple voltmeter can be used.
Start at the wall socket and work into the radio because you should be getting some lights somewhere.
Receive does not use much power, but lights should light!
73s.

-Mike.
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KB6LWN

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Re: IC-251 (pre-A) -- Dark Operation - FUSES ???
« Reply #4 on: March 03, 2022, 08:59:07 PM »

Thank you Gentlemen...

I'll begin the voltage checks in the morning.
Doubt it is the panel power sw as it was energized when it failed.
Checked the molex conn for DC on rear in the initial checks.
My bad on reversing the fuse values.
Since there are no other fuse blocks internal on the boards, it would seem to point to an internal PS fail.   As AC cords seldom fail.

But will start with metering the IEC end of the AC Cable, re-connect the internal PS and move inward from there.

If I test with the PS chassis connected, but outside the case, do I need to strap the chassis to the radio case ?   Or is that connection taken care of by a line through the interconnecting loom(s) ?
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