[DSP-10] Re: 5V at D1-A1

[kd7ts]

Hi Matt,

I'm going to make an assumption that you haven't done a lot
of troubleshooting in the past. Please correct me if I'm
wrong.

When your meter shows 4.935,4.983,5.007 and 5.017 volts, you
can take that to mean 5 volts. There are no voltages other
than 5 or 0 from the Qa - Qh pins. So when you observe .004
volts increase on Qe, it is not significant. Each pin Qe -Qh
can be either +5 or ZERO according to how they are
programmed from the serial data.

So here goes...

There are two T/R diode // attenuator chains from U107

1> Qa-Qd --> R137-R140 --> D107,D108 --> D2 --> L25

2> Qe-Qh --> R141-R144 --> D109,D110 --> D1 --> L7

Your question concerns the second chain of componenets, and
the voltages on R141 - R144. 

So, what is going on ?? The PIN diode ( D1 ) near the end of
this chain is a current operated device. For the device to
work, there needs to be a current source ( U107 ) and a
current sink ( L7 ). By changing how much resistance is
between the source and the sink, we control how much current
flows in the circuit :

Qe-Qh --> R141-R144 --> D109,D110 -->W7--> D1 --> L7

-----------------variable-source----------PIN-----sink


"Should I being seeing lower voltages at R141 - R144?"

I'd have to YES, at different gain settings these voltages
should be ZERO or 5 volts, and ONLY one at a time will be 5
volts.

So how is it possible to have 4 pins (resitors) showing 5
volts at the same time ?? 

The first thing to check is the loading of the serial data.
I think it is OK, from what you report, but I'm still not
clear on that. It's not required for the following check,
unless there is no voltage at all.

The second thing to look at is the voltage at D109-D110 (pin
K). This is the common point of the two diode pairs. We
really don't care at this point what the voltage is, but
need to know the value for the next measurement. The wire,
W7, takes this voltage to F2 (right next to L8) where we
should measure the SAME voltage on both ends of F2, then L5
(same voltage in and out) to the A1 pin of D1. Pin "K" of D1
should measure ZERO. If it doesn't measure ZERO, then we
need to look at L7. 

Make sure that wire #W7 is connected correctly at both ends. 

D109-D110 (pin K) to the A1 pin of D1 can be checked with an
ohm meter, and should read ZERO resistance.

A little background information might be useful, as in, it
works OK in receive but the gain is low. Transmit is OK. Or,
I just got to the checkout stage and nothing works... This
is the first thing I checked, and I don't know if anything
else is correct. Or, everything seems OK except this
reading....

It's a hobby, right ?? 

Let us know how you do..

73
Mike KD7TS