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492 specurum analyzer -50dB loss


vk2bea
 

I'm repairing a 492 spectrum analyzer.
I fixed a faulty -15V supply (shorted tantalum) but now see that the signal is shown -50dB down from what it should be. (the 100MHz cal signal (and external signals)).
I've injected the signal directly into the first mixer to eliminate the attenuator, limiter and front end relays from contention.
Does anyone have a good suggestion as to what I should look at next or what is likely to be at fault?
I'm stymied a bit since I don't have another spectrum analyzer or power meter I can use. (I have a network analyzer but it is only low frequency (<1300MHz))


John Miles
 

Check the mixer DC bias. It arrives via a long path that can be interrupted in several places.

-- john, KE5FX

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of
vk2bea via groups.io
Sent: Saturday, January 09, 2021 9:40 AM
To: TekScopes@groups.io
Subject: [TekScopes] 492 specurum analyzer -50dB loss

I'm repairing a 492 spectrum analyzer.
I fixed a faulty -15V supply (shorted tantalum) but now see that the signal is
shown -50dB down from what it should be. (the 100MHz cal signal (and
external signals)).
I've injected the signal directly into the first mixer to eliminate the
attenuator, limiter and front end relays from contention.
Does anyone have a good suggestion as to what I should look at next or what
is likely to be at fault?
I'm stymied a bit since I don't have another spectrum analyzer or power
meter I can use. (I have a network analyzer but it is only low frequency
(<1300MHz))



vk2bea
 

Sorry, which mixer are you referring to ? AFAICT the first mixer is passive.


vk2bea
 

Never mind, I see it on the driver board. It's 0.11V at the test point (within what the schematic says it should be). I'll check the other connections to it.