problem with my 4SQRP Ozark Patrol Radio (Fixed!)
Herb Parsons <radioherb@...>
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Thank you for pointing me to the problem. When I got to #4 voltage check on your list, I found 0 volts on R3. I found the connection from R3 to the pot was open! As soon as I connected a wire from R3 to the pot, the set came alive. Thanks so much for your help. It is so appreciated!
From: n5ib_2 <n5ib@...>
To: main <main@4SQRP.groups.io>
Sent: Mon, Jun 1, 2020 12:59 pm
Subject: Re: [4SQRP] problem with my 4SQRP Ozark Patrol Radio
I'm presuming you've fixed the earlier problem of low voltage on the Q3 collector?
Before replacing any more parts, do a little voltage detective work for us. This will give some clues as to whether the problem is Q1 itself, or its biasing circuit. With the regen pot fully counterclockwise (minimum resistance, maximum regeneration) measure the following with respect to ground (record the full resolution of your voltmeter):
1) battery voltage, at the battery +terminal, or at the top of R1
2) voltage at the junction of R1 and R2, or at the + lead of C3
3) voltage at the top of the regen pot, or + terminal of C13
4) voltage at the junction of R3 and the regen pot (other end of the pot)
... also check that this voltage decreases as you swing the regen pot through its range towards full clockwise
... then put the pot back to full counterclockwise for the rest of these checks
5) voltage at the R3 / R10 junction, or the base of Q1
The too high voltage at Q1 collector could be due to a short to ground at the base of Q1, or it couod be due to an open circuit in its base bias circuit. The above measurements ought to narrow it down.
Now, if you have time... temporarily unsolder and lift the base lead of Q1 and repeat the measurements above. For (5) above probe the PCB pad for the base of Q1 , not the base lead itself.
Neither a backwards nor forwards D2 and/or D3 would short Q1-B to ground unless *both* diodes were shorted.