IFR A-7550 woes - Page 1
Forum section: electronics / repair

author: bob91343
author_rank: Super Contributor
timestamp: January 05, 2022, 06:41:41 pm
content: I have a problem with this unit, a spectrum analyzer. Something is wrong with the sweep. The display looks good for the graticule, nice rectangles and indicators and menus. But the signal is wildly unstable horizontally, zipping back and forth. The SWEEP output at the rear is also unstable.Where to look? This is a bear to work on, very compact and most stuff inaccessible. I have the manuals.

author: BreakingOhmsLaw
author_rank: Frequent Contributor
timestamp: January 06, 2022, 12:06:58 am
content: Well, the 7550 is a superhet design. I would expect that something is wrong around the VCO for the LO, that would cause such a behaviour.First, check all voltages for voltage and ripple.

author: bob91343
author_rank: Super Contributor
timestamp: January 06, 2022, 12:13:44 am
content: My first suspect are the electrolytic capacitors in the power supply. The unit does pass all self tests. It's the sweep circuit that seems to not synchronize. Now when I turn it on I not only see the problem but the entire screen is pulsing and very bright.Next idea is to find the power supply and do some testing. The manual is not helpful.

author: m k
author_rank: Super Contributor
timestamp: January 08, 2022, 01:19:10 pm
content: The manual I found has a very clean composite video monitor.(IFR_A7550_Spectrum_Analyzer_Service_Manual)So if the whole picture has the problem you should see it in some general voltage level or video input intensity.The device seems to have also a 12V battery power possibility.

author: bob91343
author_rank: Super Contributor
timestamp: January 08, 2022, 06:52:00 pm
content: I have progress, of a sort.The display includes an electronic graticule which works fine. The data is scrambled. Then, without provocation, I got very bright rectangular areas on the screen that mostly obscured the picture. So I tapped around with my screwdriver handle and suddenly things cleared up but there was no data.Then I boosted the gain and saw a nice baseline but there is no sensitivity to signal now. The unit finally seems stable and will not respond to tapping or wiggling. In general it responds to all button presses and rewards me with beeps when I do.At this point I am out of ideas. I have a great working analyzer that cannot see any input signals. Incidentally, the baseline has noise, as I would expect, but no signals. I pumped in a 0 dBm carrier and the analyzer cannot see it at all. It acts as though either a signal channel isn't working or one of the oscillators has died.

author: m k
author_rank: Super Contributor
timestamp: January 08, 2022, 07:34:35 pm
content: My guess is that you lost a voltage.Monitor uses 12V and graticule 5V.Some OpAmps use -12V, -5V and -7V were also balancing something, 40V I couldn't find..02 for -12V.

author: bob91343
author_rank: Super Contributor
timestamp: January 08, 2022, 10:48:04 pm
content: m kk I am having difficulty understanding your message.However it's entiely possible I am missing a voltage but I have to find where to measure them. The monitor and graticule are working correctly.

author: m k
author_rank: Super Contributor
timestamp: January 09, 2022, 11:12:55 am
content: Sorry about the short hand.Pretty compact is this machine.Maybe you should just take it a part and put back first and hope that the problem is a bad contact that dissappeares.Manual page 190/274 has a sweep digitizer Module.Its upper right corner has a sweep out and its final OpAmp has -12V.Upper left is its power input.Page 189/274 has a Log Amp Module.Its lower left corner has many voltages.One can guess that those zeners are actually close to that physical location.

author: bob91343
author_rank: Super Contributor
timestamp: January 09, 2022, 06:55:42 pm
content: Thanks m k. I will get to look at the unit again later today. Will check for as many voltages as I can find.You are probably right about taking it apart and reassembling but I am leery because it's a big project with many potentialities for errors.