IFR (Marconi/Aeroflex) FM/AM/1500

[TheMG]

Was using it normally today and suddenly it just shut off and unable to turn it back on. Found the DC fuse had blown (was powering it from AC at the time, the AC fuse did not blow, but the DC fuse which is in-line with the built-in power supply is the one that did).

Put another fuse in, the unit powered up for a few seconds (LEDs coming on and relays clicking as it would during its normal power-on process), then the fuse blew again.

So I connected a bench power supply to the DC input jack, put a new fuse in and briefly powered on the unit to measure the current - 12A!!! The fuse (verified it's the correct rating) is a 7.5A fuse so no wonder it was blowing.

Something, somewhere, inside this device is drawing an abnormally high amount of current, or a power supply voltage has risen to higher than normal.

Problem is, I have been unable to find suitable troubleshooting information or schematics. The only copies of the maintenance manual I could find online (that aren't behind a paywall) seem to have the troubleshooting and schematics sections missing.

Pretty hard to figure anything out without at least schematics, I don't even know what all the power supply voltages are supposed to be or anything, and it is not silkscreened onto the PCB anywhere.

Anyone have a copy of the full maintenance manual or a link to where I could get one (no paywalls!)?

[Andy Watson]

Have you tried:
https://elektrotanya.com

Search for "ifr 1500". The version that has 669 pages appears to have schematics in it plus the sections that are missing from the 237 page version.

[TheMG]

Keep forgetting about that site for some reason. Thanks!

Looks like the power supply has gone open-loop.

5V is sitting at 6.7V, 12V is sitting at 16.5V. Unloaded the 5V skyrockets to 14V!

Come to think of it, the fan did seem louder than usual during the few minutes that it was on before the first time the fuse popped. I really hope nothing got fried especially in the 5V digital logic.

Step 1: repair power supply.

[George Edmonds]

Hi

User group at.

https://groups.io/g/IFR-Monitors

This is the place to ask for help.

G Edmonds

[TheMG]

R53, 4.75k resistor in the feedback loop, completely open. No signs of physical damage or overheating (which is virtually impossible as this resistor would never see anywhere near to its 1/4W rating).

Replaced the resistor and now the power supply behaves as it should and outputs proper voltages.

Luckily, so far everything seems to work, so I think I got lucky and nothing got fried by the over-voltage.