HP 8566B display problem

[WA4AOS]

I have an HP 8566B Spectrum Analyzer which has worked great for years. Recently, I turned it on and the display had a blurry dot, about a half inch in diameter. The brightness not focus has any affect. My initial thoughts are power supply, probably tantalum caps,  for the display unit. I have not pulled it and have not done any troubleshooting this go around...

I was wondering if someone else has seen this problem and any words of wisdom. I have changed out some of the tantrum caps before but I do believe there are many in this unit.

Best regards aka 73,

Glenn WA4AOS
DSM Labs

[james_s]

Does the display work other than the dot, or is the dot all you get?

[WA4AOS]

No, the display ONLY has the bright blurry spot/dot, no indication of anything looking normal....Pretty sure it's in the power supply but curious if anyone has seen this b4

Thanks
Glenn

[james_s]

The power supply is certainly a good start. Checking the voltages should be fairly quick and easy and will either point you in the right direction or eliminate a likely cause.

[tkamiya]

I have an entire display section with a bad CRT.  If you need any parts, I'll be glad to pull it out of that and send it to you.  If you are local to Orlando Florida, and you can pick it up, you can have the whole thing.

I'm sorry, but I cannot send it due to my physical ability.

[jcline01]

Hi Glen

Have you found the solution to your issue?

I just acquired a 8566B / 85662A display with the same issue you described. On my unit, the IF section -15V supply is apparently being loaded (down to -2 V, and the LED light is very dim. I substituted in a good A1A6 board (has the -15V supply), and it made no difference. So, I assume that the A1A6 supply board is okay, and the load must be coming from another board, perhaps from a shorted tantalum capacitor or zener diode.

I will be substituting other known good boards to try to isolate the load, but would appreciate helpful guidance from anyone who knows these display units.

Joe, KN5U

[jcline01]

Problem Solved!

Turns out that a $2.00 tantalum capacitor brought down the whole IF display unit. It was located on the -15V supply line on the A4A7 board. Fortunately the HP engineers had included an over-current shutdown circuit in the PS.

Tantalum capacitors were used extensively in this vintage HP equipment and are notorious for shorting out over time.

In any event, here is the method I used to isolate the problem:

On a good display unit, I measured across the -15V supply and found that the resistance was about 143 ohms. On the bad unit the load was 2.7 ohms.
I left the ohmmeter connected across the -15V supply, then sequentially lifted each board out of its socket until the ohmmeter indicated that the short was gone.
With the bad board removed, I measured across each tantalum cap. The one with the lowest resistance was lifted on one end to confirm that it was indeed shorted.
I also measured across the PCB holes from the vacated capacitor to ensure that no other parts were shorted on this board. Eventually, I replaced all of the tantalum caps on this board for good maintenance purposes.
After reinstalling the board, the 8566B booted properly and had a proper display.

I hope that helps someone who may be monitoring this thread!

Best regards,

Joe, KN5U

[WA4AOS]

Hi Joe,

Excellent info and your approach helped me find 2 very leaky Tantalum caps. Thanks for sharing.
I know for $1500 I can buy a newer MUCH smaller SA and may at some point. However, the 8566B has been such an outstanding workhorse that upgrading to a smaller box is LOW on my priority list.
I did purchase the DS Instruments TG 6000 tracking gen and that has added new utility for filter test. Of course, the Max Hold feature built into the 8566B does almost as good.

73
Glenn WA4AOS

[james_s]

Always nice to hear a followup on threads like this, glad you were able to get it going. As long as you've got space there's nothing wrong with these old boat anchors, I'd have a collection of them myself if space was plentiful.