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IFR 1200S Looking for Help with PSU fault

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fredastra
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Posts: 2
Joined: Wed Oct 05, 2011 11:49 am

IFR 1200S Looking for Help with PSU fault

Postby fredastra » Tue Nov 26, 2019 11:53 am

Hi Guys

I have IFR 1200S. I switched it on recently after it had been unused under the bench for about 8 years. It seemed to work OK at first switch on, but after about 1 minute it suddenly went dead as though it had been switched off. I figured that this was probably a mains fuse so I thought a quick way to confirm this was to give it an external 12 volt supply (from a 7ah battery). This worked OK for about 30 seconds but then it went dead again. I have now opened it up and found two fuses blown, F1 (1 amp) on the mains input and F1 (5 amp) on the battery charger PCB. I have the maintenance manual, but I have limited skills in electronics. I checked all the capacitors for visual damage but I can't see any. After opening up the inverter supply I was expecting that C3 & C4 might be ballooned, but they appear OK.

I will be grateful for any guidance as to where the fault might be or how best to find it. The serial number is 6270

The units had an easy life, I bought the unit at an auction in 1989 and used it for amateur radio. About 10 years ago the CRT trace disappeared (intermittent) but as my business ruled my life then I never found the time to look at that. If I do fix the PSU fault I will doubtless be back here looking for further help. Paying for a commercial repair is out of the question but as I retire next month I will have plenty of time to try myself

This is my first post on here so admins please go easy on me if somethings wrong or in the wrong place.

Thanks G4GCO (lapsed)

jry
Posts: 402
Joined: Fri Feb 06, 2004 3:14 pm

Re: IFR 1200S Looking for Help with PSU fault

Postby jry » Wed Nov 27, 2019 9:14 am

so I have repaired more of these than I can count.

You really need a professional de-soldering tool to work on these. Without one chances are you will damage the PCB.

The DC-DC section is likely where your problem is and usually they need to be re-capped regardless of the tops of the capacitors.

Usually the internal DC-DC board is what I re-cap.

The external board may also have one of the tantalum caps shorted.

You should be able to easily see which one of the outputs is shorted ...disconnect the output connector to isolate the short to the power supply or one of the other internal assemblies.

Usually its either the +5 or +12.

The DC-DC converter can be tested externally and separate from the unit.( two allen screws on the back, remove the external reference internal cable ..SMA side of the SMA to BNC, remove the external DC connector, remove the AC power connector and ground screw

The +5 V is separate from the +12V ( has the -12v and +40 volt output as well ) so you can test those separately with a 3 to 6 ohm load.

The SN is mid-range on the 1200S series so it's a decent version.

If you do not have the experience to repair correctly you may wan to sell as is and get something that works.

fredastra
New User
Posts: 2
Joined: Wed Oct 05, 2011 11:49 am

Re: IFR 1200S Looking for Help with PSU fault

Postby fredastra » Wed Nov 27, 2019 12:25 pm

Thanks for that Jry, I have now removed the DC-DC supply (in its enclosure with the battery charger PCB attached) as you suggested, and I have tested the output pins for shorts but nothing obvious there. I have tested the caps in situ all seems ok except for C1 which is on the input to the battery charger regulator, but not sure if bad reading may be caused by the regulator inputs. With regard to testing the DC-DC supply externally, I was thinking of giving a 12 volt supply to the fuse output on the PCB and then checking outputs. Can you give me an idea of what if any load resistors would be needed? Not sure if you meant 3 to 6 ohms for all outputs?

I have been thinking about replacing all the supplies with modern supplies, possibly a miniature PC supply plus 40 volts from somewhere. With the battery removed there will be plenty of space. What are your thoughts on this ? Thanks!

jry
Posts: 402
Joined: Fri Feb 06, 2004 3:14 pm

Re: IFR 1200S Looking for Help with PSU fault

Postby jry » Wed Nov 27, 2019 3:15 pm

12v is the minimum to run the dc-dc converter. you have to ground the on off control as well as providing DC power

usually a 6 ohm load would work on both to start with.

I would just repair what you have unless the PCB is severely damaged.

In that case you be able to substitute a single IC type regulator ....the 5V circuitry can be easily replaced by a single IC and leverage the output filtering .

Like I said I would only do that if the PCB was very damaged and burnt through.


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