Fixing an Aeroflex IFR A7550 Spectrum Analyser – that I found in the bin

I always wanted a spectrum analyser. And then I find one in the skip. It’s an Aeroflex IFR A-7550 with tracking generator.

The unit didn’t power up. I took it apart and got it to work again. And that’s what I’ll go through in this post…

The problem

It powers up for a split second (a pulse of the power LED) then makes a nasty squealing sound until the power is removed. 

I suspected a problem with power supply circuitry. That’s the most common culprit in old equipment since the operation relies on large electrolytic capacitors which are known to dry out and fail over time.

I managed to find the IFR A7550 Operation Manual. I’ve uploaded it here: IFR-A7550-operation-manual.pdf 

Update (13/12/12): Ok, it seems the power supply cannot deal with the initial inrush of current to power the display. I can now use this fine by connecting my bench supply when I turn it on. The bench supply can then be removed and the internal PSU continues to power the whole thing no problem. 

Update (17/11/12): Just got hold of a few original diagrams for the A7550 PSU circuit and assembly. Enjoy it here: A7550 PSU circuit diagram

Update (4/08/12): The squealing fault progressively got worse and now will not power up. This makes me think it could be a leaky capacitor in the power supply (hopefully). If anyone has experience with servicing these or can help me fault-find the SMPSU then gimmie a shout!

Remove case

It’s really rightly packed in here. I wanted to get to the power supply.

Remove power supply

Check power supply

I had a look inside here and poked around with the multimeter. I couldn’t see anything obviously wrong – like bulging capacitors, melted components or failed solder joints.

Side note: it has the 10.7MHz receiver module inside

Some of these units were build with a radio receiver module inside. You can actually tune in and listen to FM broadcast – this analyser even has a built in speaker!

I went a little deeper

I took the CRT and RF modules out. No reason apart from curiosity. And it was Sunday.

 

It powers up when the CRT is not plugged into the power supply

This tells me that either the CRT is drawing too much power (possibly because of a fault), or that the PSU cannot cope with the initial inrush of current that CRTs require. The latter could be explained with faulty capacitors in the power circuit, either at the power supply end or the CRT end – note the big electrolytic capacitors in the CRT module below

Or is there a problem with the CRT controller board?

I noticed what looks like a half melted component with 6 legs (see below). What is this? It looked like some kind of transistor with the multimeter.

This is on the CRT board which is a Computron 115DMX

I was convinced for a while that this odd looking component on the CRT controller board may be causing a short but after testing it, it seemed fine… by fine I mean no shorts. I carried on exploring.

 

I supplemented the power to the CRT with an external power supply (13.8VDC)

Now the analyser powers up every time with no problem. The whole unit and all of its functions work as expected!

It’s working!

After a few hours of desoldering/soldering, checking suspicious looking diodes and caps I get it to turn on for the first time

Conclusion

I left it there. My conclusion was that some of the capacitors in the power supply circuitry were not working at full capacity, and therefore not being able to provide current for starting up the CRT. I replaced them all and everything was good.

Everything seems to work on this unit, including the FM receiver and internal speaker. Nothing looks massively ‘off’ but I’m sure it could do with being properly calibrated.

Here’s the manual: IFR A7550 spectrum analyser manual

Here’s some of the power supply circuit: A7550 spectrum analyser power circuit

Note: I’ve now sold the analyser on Ebay. I no longer play with RF stuff. The guy was happy with it and it worked fine.