Re: Type CA module in a 545


David Kuhn
 

Well, it's down to the 6AU6 tubes. I got them confused now between the
good module and the bad module. I can make the bad module the same as the
good module by swapping the 6AU6 tubes. I now have the eight 6AU6 tubes
all confused and intermixed - lol.

I have an eico-625 tube tester that I just fixed. It really can not see
the subtle differences between all eight tubes. Some read "630", some read
"640". Not much difference; all read good.

On Thu, Jun 24, 2021 at 12:04 PM David Kuhn via groups.io <Daveyk021=
gmail.com@groups.io> wrote:

"You should check the voltage divider from pin 15 of the plugin connector.
That pin needs to sink 150 mA at 75 volts in all plugins. The 75 volts is
derived from the +100 in the mainframe dropped through a couple of 12.6
volt tube heaters. In many plugins including the CA it is connected to a
combination of tube heaters and divider resistors with taps to provide bias
for various parts of the circuit. In the CA there are also interconnection
networks with the +100 and +225 pins. One of those voltages from the
divider network is +37.5 that supplies the suppressors of the output
amplifier tubes. If that voltage is not right it could affect the gain of
those stages. There are also +69 and +12.5 volt taps that are used for DC
balance adjustment and to set operating conditions for other parts of the
circuit."

Hello Morris,

I should be checking those voltages and dividers this afternoon. The
filament resistance did go back to about 70 ohms once I wiggled most of the
tubes and pulled them and re-seated them. The sockets do not look
corroded, but the tube legs themselves are. Getting the
filament resistance down, did nothing for loss of gain in the CA module.

Dave



On Tue, Jun 22, 2021 at 12:42 AM Morris Odell <vilgotch1@gmail.com> wrote:

You should check the voltage divider from pin 15 of the plugin connector.
That pin needs to sink 150 mA at 75 volts in all plugins. The 75 volts is
derived from the +100 in the mainframe dropped through a couple of 12.6
volt tube heaters. In many plugins including the CA it is connected to a
combination of tube heaters and divider resistors with taps to provide
bias
for various parts of the circuit. In the CA there are also
interconnection
networks with the +100 and +225 pins. One of those voltages from the
divider network is +37.5 that supplies the suppressors of the output
amplifier tubes. If that voltage is not right it could affect the gain of
those stages. There are also +69 and +12.5 volt taps that are used for
DC
balance adjustment and to set operating conditions for other parts of the
circuit.

Let us know what you find.

Cheers,

Morris









Join TekScopes@groups.io to automatically receive all group messages.