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  2. 7704A (et. al.) Inverter Regulator Question

Re: 7704A (et. al.) Inverter Regulator Question

n4buq
 

In regards to C3150, Tek used a 1000pF, 1%, dipped silver mica. I don't know if they did that because of wanting to hold a tighter tolerance or better temperature compensation (or both). Since R3150 is a 49.9k 1% so there must be something rather critical going on with that circuit and I presume I should replace it with a dipped SM as well(?).

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "n4buq" <n4buq@...>
To: TekScopes@groups.io
Sent: Monday, September 13, 2021 1:29:59 PM
Subject: Re: [TekScopes] 7704A (et. al.) Inverter Regulator Question

I just checked those with my component checker (without U3105 plugged in as I
wasn't sure what effect that might have) and, yes, I have several bad
tantalums so those will be replaced. Oddly, C3150 is bad (measures about
32pF). I really didn't expect that one and, to be honest, thought the
smaller tantalums would have been okay. I was wrong.

I wish I'd realized all this when I had the board out. Some of those can be
replaced without having to remove the board but some, particularly C3150,
may not be possible.

This poor power supply has had a rough time of it...

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@...>
To: TekScopes@groups.io
Sent: Monday, September 13, 2021 12:55:11 PM
Subject: Re: [TekScopes] 7704A (et. al.) Inverter Regulator Question

Barry,

Goran is right about that area. VR3147 being open will throw the circuit
off.
C3127 and C3129 are marked ,1mfd. Some models use 1mfd in this place. I
have
replaced mine with the 1mfd 63V film types because the ESR of the originals
was too high causing too much ripple and low B+. After replacing the parts,
the B+ voltages at the pins on the 0067 IC were correct. These parts can be
checked while the parts you have on order are being shipped. I also
replaced
the 1 and 2mfd electrolytics on the inverter board with film types of a
higher voltage. The originals were bad in mine.

Mark
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