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

[Matthew J. Wolf]

Yes it's a hobby.

Your assumption is correct, I have not done that much troubleshooting in 
the past.

Well W7 is good I see the same voltage on both sides of the wire. I see 
the same voltage on ether side F2 and at L5 and at pin A1 of D1. Pin K 
of  D1 is not zero! It is 4.5 volts. Also when I measure the resistance 
between D109-D110 (pin K) to the A1 pin of D1 I get 0.5 ohms. Should I 
replace L7?

I can receive the local repeater if I connect a random length wire to 
ether side of C23. Also when I transmitted some CW it seemed to work. It 
was a quick test.

-Thank You
   Matthew Wolf KC9DQP


 >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