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Signal occasionally disappears on 492, needs to be power-cycled by unplugging


dododude700
 

Hi there! New to the group, I posted this on Reddit at /r/rfelectronics first and was referred to here.

I recently got a Tektronix 492 spectrum analyzer on eBay. I paid around $320 for it in a Best Offer, and apart from the issue in the title and the trace rotation needing some adjustment, everything I've been able to test on it does work, so I'm still pretty happy with the purchase.

That said, it does have one quirk I haven't been able to diagnose. It can operate as normal, but if left plugged in, eventually (after anywhere between ten minutes to a day or two), everything but the noise floor will disappear, as if the DUT was simply unplugged. This happens even with e.g. an antenna looking at broadcast signals, so the signal is certainly still there, it just stops seeing it. This doesn't require any kind of physical jolt (I don't think it's a connector being bumped out, for example), can happen when the instrument is on or off (you'll find out when you go to use it again), and can happen as the user is interacting with it or when it's just sitting there. When this happens, the device still "works" - you can look around the noise floor, change settings, all as expected, but no amount of disconnecting and reconnecting the DUT, making sure everything's plugged in right, or fiddling with the knobs will bring the signal back. The only fix is to either power cycle it with the front switch, off for 30 seconds, after which it will work for a few seconds, or to unplug the power cable for about 30 seconds and then plug it back in, after which it's back to normal for minutes to hours until it happens again and necessitates another brief unplug. This seems especially weird given that, as I understand it anyway, the power switch on the 492 is not a soft on/off - I'm surprised there's a difference between power cycling with the switch and power cycling at the plug. The failure is sudden - I've seen it happen, the input doesn't slowly fade into the noise floor, it disappears abruptly and sometimes in the middle of a trace. I haven't discounted that I may be doing something wrong - this is my first "real" RF instrument and I may be missing something obvious - but I'm not doing anything when it fails that doesn't work as normal any other time. I also don't think it's some out-of-spec input signal (too much power or DC on it or something) messing up the frontend, as the failure is temporary and I've seen it happen with just a normal, passive antenna connected. I don't hear relays click when it stops working, so I don't think it's accidentally enabling the attenuator or something.

Given the relatively easy workaround, this is more of an annoyance than a dealbreaker issue, but I'd still like to fix it. What could cause what's seemingly an input problem (given that everything but the actual measurement keeps going) but need a full power cycle and seemingly, time to discharge power supply caps, to resolve? Is there some always-on power rail powering whatever's failing that power cycling from the switch wouldn't affect? Is this a known problem?

Thanks!


Chuck Harris
 

The 492, 494, and 496 SA's are compact devices simply stuffed to
capacity with good RF stuff. To make sure they had absolutely the
optimal space for that good RF stuff, they developed a state of the art
switching power supply with a rather high watts/cubic inch rating.

That high watts/cubic inch power supply gets hot, very hot... especially
if the air filter is clogged, or blocked by walls or junque on your
bench.

Every time I have taken one of these SA's apart, I have found many
of the electrolytic capacitors in that supply that have gotten so hot
that they have shrunk the plastic heat shrink sleeves to a point where
they look like kids that have out grown their clothes.

Start there.

Second point worth noting. These are long instruments, and there are
only a couple of places where it is safe to let them overhang the
edge of your bench with the covers off. If you set them in one of
the places where the bench edge puts the weight of the SA on internal
modules, you will create needless repairs, as you break solder joints
and connector pins in blind spots in the modules.

Third point worth noting, many of the SMA connectors are not well
mounted. Tek did things like bolt the connector to the thin aluminum
can of a module, and solder the center pin to a circuit board that is
inside of the thin aluminum can of the module. If you over torque the
SMA, and make it turn in the can, you will break the center pin off of
the SMA making an intermittent. Use a torque wrench. An intermittent
SMA connector can never be fixed by simply tightening it above the spec.
You can sometimes fix it by loosen it and then re-torque it, but never
above the spec.

-Chuck Harris

dododude700 wrote:

Hi there! New to the group, I posted this on Reddit at /r/rfelectronics first and was referred to here.

I recently got a Tektronix 492 spectrum analyzer on eBay. I paid around $320 for it in a Best Offer, and apart from the issue in the title and the trace rotation needing some adjustment, everything I've been able to test on it does work, so I'm still pretty happy with the purchase.

That said, it does have one quirk I haven't been able to diagnose. It can operate as normal, but if left plugged in, eventually (after anywhere between ten minutes to a day or two), everything but the noise floor will disappear, as if the DUT was simply unplugged. This happens even with e.g. an antenna looking at broadcast signals, so the signal is certainly still there, it just stops seeing it. This doesn't require any kind of physical jolt (I don't think it's a connector being bumped out, for example), can happen when the instrument is on or off (you'll find out when you go to use it again), and can happen as the user is interacting with it or when it's just sitting there. When this happens, the device still "works" - you can look around the noise floor, change settings, all as expected, but no amount of disconnecting and reconnecting the DUT, making sure everything's plugged in right, or fiddling with the knobs will bring the signal back. The only fix is to either power cycle it with the front switch, off for 30 seconds, after which it will work for a few seconds, or to unplug the power cable for about 30 seconds and then plug it back in, after which it's back to normal for minutes to hours until it happens again and necessitates another brief unplug. This seems especially weird given that, as I understand it anyway, the power switch on the 492 is not a soft on/off - I'm surprised there's a difference between power cycling with the switch and power cycling at the plug. The failure is sudden - I've seen it happen, the input doesn't slowly fade into the noise floor, it disappears abruptly and sometimes in the middle of a trace. I haven't discounted that I may be doing something wrong - this is my first "real" RF instrument and I may be missing something obvious - but I'm not doing anything when it fails that doesn't work as normal any other time. I also don't think it's some out-of-spec input signal (too much power or DC on it or something) messing up the frontend, as the failure is temporary and I've seen it happen with just a normal, passive antenna connected. I don't hear relays click when it stops working, so I don't think it's accidentally enabling the attenuator or something.

Given the relatively easy workaround, this is more of an annoyance than a dealbreaker issue, but I'd still like to fix it. What could cause what's seemingly an input problem (given that everything but the actual measurement keeps going) but need a full power cycle and seemingly, time to discharge power supply caps, to resolve? Is there some always-on power rail powering whatever's failing that power cycling from the switch wouldn't affect? Is this a known problem?

Thanks!






ulf_r_k
 

John Miles KE5FX has an excellent "Service Notes" on his web-site that detailsa lot of information about what needs to be done with an old 492/494 Tek Analyzer.
Apart from this information, I also had to replace U1040B (74LS74) in the Phase lock synthesizerhaving encountered the same problem described by KO4BB and the transistor in the CRT high voltagesupply module.

Cheers

Ulf KylenfallSM6GXV