[R-390] R-390A meters

[David Wise]

Pete, I have a request.  I am very curious as to how this will
affect the AGC action and the meter reading, and I'd like it
to be as scientific as possible.  If it's convenient and
interesting, please do the following:

A. Before the mod

1. Set the IF gain to the manual's standardized value.

2. Set up an emulated carrier meter out of an 18-ohm
resistor with a sensitive voltmeter across it.
Record the exact resistance.  Zero the "meter".

3. For 10dB signal increments from (undetectable) to
90dB above that, record (a) the AGC voltage, and (b) the
meter reading.  Also record one of the signal levels
(from the generator's point of view) so you can
reproduce it later.

4. Scope the AGC line.  For one of the above 10dB
transitions, measure the AGC time constant in the
SLOW position.

B. The mod

1. Record the coil resistance of your substitute meter.*
(We've been saying 100 ohms, but a quick look around the
net shows that this is arbitrary.  I saw everything from
18 ohms to 1K.)  That way, those who find something
different can (provided it's lower) simply add series
resistance and use your findings without repeating the
experiment.  With that in mind, it might be a good idea
to pad your own meter to some reasonable value (150 ohms?)
if it's one of the low ones.

* Don't use an ohmmeter!  Unless you know for a fact
that it puts out <= 1mA.

C. After the mod

1. Set the IF gain to the manual's standardized value.

2. Using the same signals as step A3, record (a) the
AGC voltage, and (b) the meter reading.

3. Scope the AGC line and measure the time constant.

Actually, this setup will also help you establish the
resistor values for best accuracy and linearity.

Thanks,
Dave Wise

> -----Original Message-----
> From: r-390-bounces at mailman.qth.net
> [mailto:r-390-bounces at mailman.qth.net]On Behalf Of pete williams
> 
>  Thanks guys  for the  lucid  explanations. ---i've  got 3 rx to
> adapt/ modify as they lack both originals.
>  I'll attack one IF in next week and  let assembled  company know
> results
>   Pete
> Pete D . Williams
> METUNG 3904
> Australia
> jupete at bigpond.net.au