... and then I tried one more thing. I swapped U731 and putting the "bad" transistor in the plugin where it wouldn't work before, it now works. U731 is a 7400N and one of the inverters is the drive signal for Q781. I had checked the signal out of that inverter (TP799) and it was almost identical between the two units but I wonder if something is not letting that output to have sufficient current to drive Q781. I may need to check the power supply for that chip as well as what drives the input to that inverter.
Thanks,
Barry - N4BUQ
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----- Original Message -----
From: "Barry" <n4buq@...>
To: "tekscopes" <[email protected]>
Sent: Friday, January 14, 2022 5:10:40 PM
Subject: Re: [TekScopes] A Tale of Two 7D14s
I replaced all three resistors in question and the stubborn transistor still
will not work in that 7D14s. I also checked the coupling cap and it was
extremely close to the correct value using my inexpensive component checker
(which may not be giving me a very reliable reading for a cap that small but
it's about all I have for that).
The same transistor in the other 7D14 will work in either one so I've resigned
myself to swap the transistors and let it go. Both 7D14s work that way.
Thanks,
Barry - N4BUQ
----- Original Message -----
From: "Barry" <n4buq@...>
To: "tekscopes" <[email protected]>
Sent: Wednesday, January 12, 2022 6:57:00 PM
Subject: Re: [TekScopes] A Tale of Two 7D14s
Mark,
I'll try that. It could just be a "perfect storm" of several parts contributing
to the failure.
Regarding one of these plugins, one of the custom ICs has one bad output (the
2^0 bit) which causes odd frequency values but most everything else works on
it. I keep thinking I can retrofit something to take care of either the entire
IC or, perhaps, just handle that one bit's output. I swapped that IC between
the two and the problem follows the IC so I know that's the problem. I wish I
could source one of those ICs but I think that's just about impossible now.
Thanks,
Barry - N4BUQ
----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@...>
To: "tekscopes" <[email protected]>
Sent: Wednesday, January 12, 2022 5:46:13 PM
Subject: Re: [TekScopes] A Tale of Two 7D14s
Barry,
Change the resistors that are out of tolerance to see if that makes a difference
since you say they are almost double the rated tolerance. A "bad" transistor
can work in one area and not another. I have seen this with new in box active
devices before. The specs of the one transistor that will not work could be out
tolerance, e.g. voltage drop at saturation. This number plugin I would like to
get one day.
Mark
