[Test-Equipment] Repair of Avcom PSA-65A

[fwbray at mminternet.com]

Gary Schafer <garyschafer at comcast.net> said:

> Have you ever seen this unit work properly?

Not this one.  I have seen others of this model, working, however.
> 
> Does the trace move up and down as you tune from one station to another?

No, that is the problem.  The trace is completely flat, except for the
downward deflection when the pulse is on.  I have a HP 141 with plugins, so I
have an idea of what to expect with various settings of a working spectrum
analyzer.  The Avcom gives no visible sign of any received signal(s)regardless
of the attenuator setting -- although this does impact the audio level of the
demodulated signal.  
> 
> The 2 db trace being off the scale may be normal. Some analyzers the 2 
> db and 10 db are at the same position only at full scale. The 2 db will 
> drop rapidly compared to the 10 db if the signal is not full scale.
> 
> Are you sure you have the controls set right? It sounds like you may be 
> in the monitor position where the trace may stop. Some analyzers operate 
> differently than others when monitoring audio.

I don't have a manual for the unit.  However, the trace is the same in the
monitor and normal positions, including the downward deflection that coincides
with the pulse.
> 
> The pulse that you hear could be where they momentarily stop the sweep 
> in order to hear audio. Some analyzers do it that way. Not familiar with 
> the avcom though.

The pulse seems to start when the CRT warms up to the point that the trace is
visible.  Sometimes it takes longer, but it always eventually does start. 
Until the pulse starts, the audio is normal and the receiver seems fairly
sensitive.

Thanks for responding and for any further ideas you might have.


> 
> 73
> Gary  K4FMX
> 
> Frederick Bray wrote:
> > Does anyone out there have any experience with repair facilities for the 
> > Avcom PSA-65A specturm analyzer?  I understand that Avcom now provides 
> > only limited support for these units.
> > 
> > The unit I have has the following symptoms/problems: Although the 
> > receiver section does work to the extent of receiving and demodulating 
> > audio, the spectrum display is not working properly.  Since it can 
> > receive WWV at 10 MHz, stations on the FM broadcast band and the 
> > national weather service broadcasts at 162 MHz, the receiver section 
> > appears to be basically functional.  The problem is with the display. 
> > While there is a trace on the scope, it will not display received 
> > signals.  I can center the sharp, but flat, trace on the scope in the 
> > 10dB sensitivity position, but at the 2dB position the trace appears to 
> > be off the bottom of the CRT and cannot be adjusted.  There is also some 
> > type of internally generated  pulse (2-3 times a second) that causes the 
> > trace to deflect markedly downward and is also audible in the speaker.  
> > When placed in the audio monitor position, the pulse causes the last 
> > digit of the center frequency display to jump by one or two increments 
> > and to interrupt the reception of the signal.  This generally starts 
> > within a minute of the time the unit is turned on.  My guess is that the 
> > problem lies in the electronics that drive the scope.
> > 
> > Does anyone have experience with this unit and/or any idea of whether it 
> > is worthy of repair?  I would like to get a few more years of service 
> > out of it if possible.
> > 
> > Many thanks.
> > 
> > 
> > 
> > 
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> > 
> > 
> 
> 
> 
> 



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