[DSP-10] No TX: from frying pan to fire

[Rajiv Dewan, N2RD]

Previously:
This is a follow up on the "No TX out" story.  Essentially, receiver
works great but no there is TX power out that I can measure using the
RF probe built as described.  The DC voltages in the TX section WERE
OK.

Now:
I do not have a means of generating small signals at 144Mhz so I
hooked up my 2m HT to a 2GHz Sampling directional coupler.  This
seemed to generate a very tiny output that was not measurable using
the RF probe and my voltmeter.

I fed the weak singal into input of U4 to no effect. When I fed this
into U5 input, the S meter on the frequency counter hooked to output
jumped full scale reading.  This seemed to indicate that U4 was bad.

I removed the small signal input and re-checked the DC voltages around
U4.  To my surprise I found that now the DC voltages, that had
previously been fine,  were now incorrect.  Voltage at pin3, which I
had previously measured to be 3.6V, was now 7.55V.  Pin 1 was at 0V -
also incorrect.  So essentially the device DC current had fallen from
a proper value of 31ma to 10ma.  I guess, somehow, in all my fiddling
I have destroyed U4  MSA-0686.  Ouch!

So I now have to acquire a MAR-6 mmic and go from there.

Any thoughts?

Regards,


- -
Rajiv Dewan, N2RD
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