Boonton 8210 modulation meter deaf / IF LO at >= 500MHz in

[philpem]

Once again I'm never sure whether I should be posting these questions in "Testgear" or "Repair"... but seeing as this is very definitely a repair question, I'll drop it in here 

I've got a Boonton 8210-01-S/3 on my bench at the moment. This unit differs from a standard 8210 in that it has a slightly different set of filters (the -01 option) and a the SRAM is battery-backed with two AAAs (the -S/3 option). The point of the latter option is to speed power-on; the calibration factors are saved across power cycles. This saves a minute or so of waiting every time the power is switched on or off.

Problem is, at any input frequency below 500MHz with -30dBm or more of signal, it works fine. As soon as I take it to or above 500MHz, I get an "IF LO" error on the display and it refuses to lock.

The spec is 1.5GHz, though it wants 30mV RMS signal for that. I set my Marconi 2022E to 30mV and still got "IF LO", and even increasing to 60mV (i.e. in case the Marconi was assuming a hi-Z load in voltage-output mode vs. 50-ohm load in dBm) didn't get me anywhere.

I've done a full realignment on it, but it's no better. A3R79 is fully counterclockwise (RF gain all the way down) as a result of that, so it seems the IF stage is working OK...

Does anyone happen to know anything about these beasts, or have any suggestions as to where I should be looking for the fault?

The manual doesn't seem to list "Falls down over/under/at a certain frequency" as a possible failure, unless I've missed it somewhere...

(The manual for the standard 8210, including Theory of Operation and schematics is here: http://www.davmar.org/pdf/Boonton8210.pdf )

Cheers,
Phil.

[philpem]

Ohhhhkayyy.... this is going to be one for the "embarrassment factor nine" heading! 

This evening I powered her up and noticed she was reading flat zero. No oscillation from the RF board VCO. Whoop.
This time a Schottky diode (CR13, 1N6263) had failed shorted. Of course, I didn't figure this out until I'd desoldered and checked half the rest of the VCO circuit (desoldering a minimum number of parts - I think it was six? to check everything in isolation). Winner. A New diode later and she's whistling again. I also swapped out C32 (100uF 25V) with a Panasonic FM because there was no way I was going to solder the original electrolytic back in having desoldered it... may as well put a new one in if I have it in stock. Saves me a potential failure in future.

Next I hooked up the signal generator (Marconi 2022E), set everything up for AM per the manual and hooked an MP3 player up to the ext-mod in on the Marconi (it's more interesting than a 1kHz tone). The Boonton locked in perfectly.

Cranked up the frequency... lost lock. Figured I'd leave the Boonton for a bit, so I hooked the Marconi up to the spectrum analyser for a quick test.

All fine up to 250MHz, but as soon as you go past that... Levels 8dB down (and increasing the higher the frequency went), and the AM vanished...

Needless to say, the signal generator has now been cursed at, sworn at and repaired. It was a bloody OM345...

Re-tested the Boonton last night, realigned it and it's now passing the RF Sensitivity test with the Sensitivity Trim control set fully counterclockwise (minimum). So I'm pretty pleased about that
I've had to swap the JFETs around (they were socketed!), the mixer and oscillator are getting a bit old and cranky (the oscillator especially). Figure I'll get a couple of spares in as they're only a few quid on ebay.

Lesson learned? Sometimes the most reliable tool in your workshop isn't as reliable as it seems! 

Cheers,
Phil.

[poida_pie]

Ahh, but the most versatile tool you have WAS working fine (your brain!)
I would not feel any need for embarrassment at all. Well done.