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Old 26th Jan 2021, 4:13 am   #1
nemo1956
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Default Icom IC 251

I have recently picked a IC 251. Its not a A or E version.
The problem is the power supply. It has been set to 110 volts AC input, But I have changed this to 220 volts AC input.
Now there is a short link from the two 220Uf caps at the on the input PCB does this have to be moved as well as the link on the transformer.
There is no mention of this in the manual or about the voltage change link.
Does this other link need to left where is is now or do I have to remove it?

Br Paul
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Old 26th Jan 2021, 2:30 pm   #2
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Default Re: Icom IC 251

Hi Paul,

I only found documentation on the A/E version, but I assume that what you have has the same PS.

For 220VAC operation the strap or link needs to be removed from between the rectifier "~" input and the mid-point of the two electrolytic filter capacitors.
This is just like the mains voltage switchover circuit in the AT-type SMPS power supplies of the PCs.

The link at the L2 transformer also needs to be moved over as you mentioned.

Regards, Peter
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Old 28th Jan 2021, 10:18 am   #3
nemo1956
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Default Re: Icom IC 251

Hi there orbanp1.
Thank you for that but I'm not to sure if I'm getting this correct. The link on the L2 has been moved to the 220 volt position. But at the moment the other link is connected to the two electrolytic caps.
Should I move this link to the " Ac side of the rectifier or lever it connected to the two caps?
Can you please confirm this.
Many thanks.
Paul.
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Old 28th Jan 2021, 10:30 am   #4
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Default Re: Icom IC 251

Hi again I forgot to ask about the shorting plug. I don't have one of these so which pins do I need to short out to make it run from the internal power supply.
Thanks again
Paul.
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Old 28th Jan 2021, 5:03 pm   #5
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Default Re: Icom IC 251

Hi Paul,

The schematics of your radio is available at the mods.dk web-site. That should help in verifying the details.

Enclosing two annotated sections of the schematic, the power supply and the connection of it to the radio.

The power supply schematic shows the links and connections.
For 220V operation the link at the rectifier (at the top in the drawing) needs to be removed, shown in red, and the transformer wiring needs to be set to the 220V point.
The "+STDBY" supply keeps the memory powered up in the radio.
The "P1" connector of the power supply goes to the "connector section" of the schematic, it mates to "P12".

For the +13.8V to reach the radio on the "DC connector" the top two points need to be connected as shown with the "red wiring" there.

Hope this helps.

Regards, Peter
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Old 29th Jan 2021, 3:57 am   #6
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Default Re: Icom IC 251

Hello to you again orbanp1.
Thank you for this info very useful. It looks like it had been half changed as the link between the caps and the rectifier has been removed but the transformer had not so it was set to 110 v AC input.
Ok I will replace the caps on the filter board and in the switch mod PSU at the same time and go from there.
The radio is working but only has about 5 watts of RF output.
So expect more problems with it and may need your help again soon.
Many thanks.
Paul.
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Old 29th Jan 2021, 1:15 pm   #7
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Default Re: Icom IC 251

Hi Paul,

It is possible that the radio was still operational with the strap missing at the rectifier (set for 220VAC), the transformer set 110V, and the radio run from 110VAC.
The 110VAC rectified is about 155VDC, 220VAC rectified is about 310VDC. The switching regulator turns that into +13.8VDC.
With the feedback the regulator circuit does pulse width modulation of the switching transistors to produce the 13.8V.
It is possible that running the switching circuit from 150VDC the regulation is still within its range for producing the +13.8V at light loads.
It is also possible that the regulation is not within range at 150VDC with high loads, the +13.8V voltage sags at high loads, and the radio does not reach full power.

Regards, Peter

Last edited by orbanp1; 29th Jan 2021 at 1:21 pm.
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Old 2nd Feb 2021, 1:17 pm   #8
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Default Re: Icom IC 251

Hi. Many thanks for that very interesting information.
Thanks to all.
I will let you all know how I get on when the psu is running.
De HS0ZLQ /G0MIH.
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Old 9th Feb 2021, 10:21 pm   #9
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Default Re: Icom IC 251

Hi all. Well updat on the 251 PSU. Sadly after doing the mode it blow the fuse big time so I'm thinking it's not very happy and someone has done it in before.
So I'm going to look at the two SG switching transistors to see if they have been distroyed.
I hope the regular IC is intact.
Any other thoughts would be appreciated.
Thanks.
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Old 10th Feb 2021, 3:03 pm   #10
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Default Re: Icom IC 251

Hi Paul,

Do check all the components, including the rectifying diode.
Also do check all the electrolytic capacitors, preferably with an ESR-meter.
Both the transistors and the IC are available, and at a very reasonable cost, e.g. at Digikey.

Regards, Peter
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Old 10th Feb 2021, 11:45 pm   #11
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Default Re: Icom IC 251

Hi this is a block diag of the ps also description
On the remains of a pu i have here the red link above is in place only change is the tx is linked for 240v hope this helps Mick
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Old 11th Feb 2021, 7:34 am   #12
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Default Re: Icom IC 251

I've come across this before. It's a common scheme in Icom radios of that period. The set I was asked to look at was an IC761. It had been bought by an American amateur in the US and then shipped when he moved to Australia. He had someone change it to 220V. They found the tap change for the iron transformer and moved it correctly. They failed to notice the link at the rectifier... on that set it was located on the board, with the bridge rectifier mounted over the top of it.

So the iron transformer which powers the ancillary side of this switch-mode supply was happy, but the rectifier was in its 'voltage doubler mode' and it tried to make about 600v instead of the intended 300v. There was a loud bang when he plugged it in in Australia. He never got it fixed before he was moved again, this time to the UK. The dead radio travelling as part of his household goods.

Once he and his family had settled in the UK, he got round to sending the unit to a UK retailer for repair. He was told that the power supply had overvoltaged all of the rest of the set and that there was so much damage it was beyond economic repair.

The kindest interpretation of what he was told was that they hadn't really looked at the set. The other interpretation is that they wanted to sell a new radio.

After this I was asked for a second opinion. Tracing what had happened, I found the link under the rectifier and understood what had happened. No, the link is not made clear in the manual! I applied 13.8v to the radio downstream of the power supply, and it worked fine. I checked everything worked. Damage was confined to the power supply. It needed new chopper transistors and a new control chip. I replaced the main reservoir electrolytic capacitors as a precaution.

So, do not power up that unit on a >200v supply with that link in place at the rectifier.

If you have already done so, there is probably damage.

Back in 1990, the control chip Icom used across a range of sets was no longer generally available. Icom seemed to have reserve stock for repair and they also used it in later radios (IC765 for example) so I bought one via Icom's service operation.

David
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Old 11th Feb 2021, 12:30 pm   #13
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Default Re: Icom IC 251

Hi all. many thanks for your information.
I have replaced all the caps in this PSU before I started on it as they where in a bit of a mess. Anyway the link from the AV side if the rectifier had been removed but the standby one had not been moved. I think the gut just plugged it in and it didn't work so just left it.
I have changed the input on the standby transformer so that's ok now but it still when bang on me. I will need to test the two switching transistors but I'm guessing both are short and they have damaged the controller chip too.
I will need to get all three parts.
Anyone have an input on where to get them from?

Any input would be very much appreciated.

73's.

Paul.
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Old 11th Feb 2021, 1:48 pm   #14
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Default Re: Icom IC 251

One thing I'm not sure off here is the link on the main PSU board.
For 220 volt input. should the wire link be to the capacitors or going to the AC input of the rectifier.
I'm still unclear on this point.

BR.

Paul.
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Old 11th Feb 2021, 2:01 pm   #15
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Default Re: Icom IC 251

I got a controller chip from Icom UK, but that was around 30 years ago.

David
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Old 25th Feb 2021, 7:28 pm   #16
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Default Re: Icom IC 251

Hi all just an update on the 251 PSU problem.
I've now replaced the bridge rectifier both switching transistors and the control chip.
Nothing. Still not working.
All the caps have been replaced and I havr tested the transistor that does the switching on/off for the back up supply. All good but nothing it's died.
Any suggestions.
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Old 26th Feb 2021, 1:54 pm   #17
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Default Re: Icom IC 251

Hi Paul,

Remove the load and see if the PS is working. It is possible that the radio itself is shorted.
You should also touch up every soldering in the PS, it could have a cold solder joint.
If that does not help do check every component in the PS.
Do check the PCB wiring too, there could be a crack in the traces, do not assume blindly that "wiring conducts electricity".

Regards, Peter
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Old 26th Feb 2021, 2:00 pm   #18
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Default Re: Icom IC 251

If worst comes to the worst then you need to run the power supply up safely on the bench.

Use a lab power supply to do the job of the iron transformer auxiliary supply.

If you have a high voltage lab supply to substitute for the rectified mains, use it. Otherwise I used a variac and an isolating transformer. Together they allowed me to bring things up gently AND to be able to ground the HV DC negative so I could go scoping around.

David
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Old 27th Feb 2021, 8:30 am   #19
nemo1956
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Default Re: Icom IC 251

Hi David.
Well I have been doing some more work on this PSU. I have found that the opto copler is OS on both sides.
So what I did I put a resistor across it to bias the Q1 transistor to it on state.
This got the low voltage side working and with no AV on the main side I can here the chip running.
So i connected up the main input and via a veriac and a dim bulb I ran it up but nothing at all.
Looking at the schematic I just can see how the rectifier gets AC to both sides of it.
It can't work like that.
The other side of the AC input is doing to the center of the two big capacitors and two resistors.
So how does it work?

BR
Paul.
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Old 27th Feb 2021, 10:48 am   #20
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Default Re: Icom IC 251

With one end of AC mains connected to the midpoint of the two big reservoir capacitors, it must be in voltage-doubler mode for 110v AC mains supply.

Operation in this mode uses only two of the four diodes in the bridge rectifier. One half-cycle of mains charges one capacitor, the opposite half cycle of the mains charges the other capacitor. So it's really two half-wave rectifiers stacked on top of each other. It's pretty hard on the capacitors.

David
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