Type CA module in a 545


David Kuhn
 

Hello all,

Type CA. All tube sockets cleaned, switches cleaned and the two pots that
can be clean are cleaned.

10volts will only read as 8 volts on either channel.

I swapped the output tube (common to both channels) V383A, 12AT7, with the
multivibrator output tube V3393. Same issue, not enough overall gain, in
both channels.

I haven't quite figured out how that module works. Apparently the
multibrator supplies power to the plates of both channel amplifier tubes
via V3393A. That was one I swapped. I am thinking that V3393A may not be
turning on hard enough, but I am really clueless with this module.

Is there a common issue with the Type CA that causes overall low gain on
both channels? When I install my Type D or Type 53/54K, both are bang-on,
but there doesn't appear to be any head-room to spare. So maybe the scope
needs a gain adjustment?

Thanks for the thoughts and idears....


 

Pardon me if this is a stupid question, but have you performed the calibration process on the plug-in?

There are steps in the cal process that specifically address the gain (step #8) and there are front panel controls (recessed pots adjustable with a screw driver) to trim the gain on both channels.

-- Jeff Dutky


David Kuhn
 

Not fully. With the exception of some trim caps that can not be reached
without and extender (I'm not sure how they adjusted those), the only pot
inside in the Vertical Position/Range pot. The front panel, recessed gain
pots, are at MAX.

Swapping 12AUT's around, and 10volt square wave can almost see 9volts.

As far as the scope chassis, I was thinking may have been marginal, I have
another CA module. With it's gain pot, I make a 10Volt PP SW look about
13volts. So the range is there in the scope.

I have tested some resistors to see if they are dropping too much, but
haven't found any too far out.

I guess I need to insert the working CA module and make voltage measurement
readings (although on the weak one they line up with what is shown on the
schematic), and then record scoped PP values at each stage, then do the
same in the weak module. I am guessing I have some weak tubes in there,
but not the 12AUT (finger burners - lol). It's just that both channels are
identically weak. So I was thinking towards that wonky multiplexer circuit.

I will review the cal procedure again, but I don't know what it can tell me
to adjust to get the gain back.

Dave

On Sat, Jun 19, 2021 at 8:32 PM Jeff Dutky <jeff.dutky@gmail.com> wrote:

Pardon me if this is a stupid question, but have you performed the
calibration process on the plug-in?

There are steps in the cal process that specifically address the gain
(step #8) and there are front panel controls (recessed pots adjustable with
a screw driver) to trim the gain on both channels.

-- Jeff Dutky






David Kuhn
 

Hello again Jeff,

" calibration process on the plug-in?"

I just did the resistance test on both the high gain modules and the weak
one. Everything looked the same except the filament resistance. Pin 15 to
ground is supposed to be ~65 ohms. On the good module is ~74 ohms. On
the weak one, it is 100 ohms.

So I guess I am looking at some old/bad tubes. I have to pull out the tube
tester and get it working.

Dave

On Sat, Jun 19, 2021 at 8:32 PM Jeff Dutky <jeff.dutky@gmail.com> wrote:

Pardon me if this is a stupid question, but have you performed the
calibration process on the plug-in?

There are steps in the cal process that specifically address the gain
(step #8) and there are front panel controls (recessed pots adjustable with
a screw driver) to trim the gain on both channels.

-- Jeff Dutky






Jim Adney
 

On Sun, Jun 20, 2021 at 02:32 PM, David Kuhn wrote:

Everything looked the same except the filament resistance. Pin 15 to
ground is supposed to be ~65 ohms. On the good module is ~74 ohms. On
the weak one, it is 100 ohms.

So I guess I am looking at some old/bad tubes.
I'd be surprised if the actual filaments changed resistance that much. Check the pin to pin resistances right on the tubes and then see if there's a high resistance connection somewhere else. Frankly, 74 Ohms sounds much too high for a cold filament. There must be something else in that path.


Tom Lee
 

I agree with Jim. It's not the filaments per se. They don't age that way. The resistance is coming from someplace else.

Cheers
Tom

Sent from an iThing; please excuse the terseness and typos

On Jun 21, 2021, at 6:59, "Jim Adney" <jadney@vwtype3.org> wrote:

On Sun, Jun 20, 2021 at 02:32 PM, David Kuhn wrote:

Everything looked the same except the filament resistance. Pin 15 to
ground is supposed to be ~65 ohms. On the good module is ~74 ohms. On
the weak one, it is 100 ohms.

So I guess I am looking at some old/bad tubes.
I'd be surprised if the actual filaments changed resistance that much. Check the pin to pin resistances right on the tubes and then see if there's a high resistance connection somewhere else. Frankly, 74 Ohms sounds much too high for a cold filament. There must be something else in that path.






Morris Odell
 

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


David Kuhn
 

"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






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










David Kuhn
 

Sorry, I accidentally hit enter and the message was sent before I was done.

The funny thing is it seems one tube being weak appears to drag the gain of
both channels down. Both channel one and channel two tubes sets are
basically in parallel except for the input grids. Any measured voltage
differences between a good gain module and lower gain module are very
subtle.

So at this point, I am not sure what to do, if I really want two good CA
modules (which I really do not, I just like the challenge, or it's my OCD
that drives me to have both the same). They are about $10 each on ebay. One
vendor is selling them "tested" for 99cents each. To purchase four "good"
boxed ones for $10 each is $40+shipping. I'm not sure my OCD is worth
that. lol

It's weird that the module could be that sensitive in design to a slightly
weak tube. Maybe all my 6AU6s are weak and I just do not know the
difference? With the recessed Gain Adjustment, how much overhead gain is
typical for a Type CA module? I would think, best case, I would be
adjusting the gain down a bit instead of having it "just" calibrate with
that pot fully CW.

On Sat, Jun 26, 2021 at 2:43 PM Daveyk021 <daveyk021@gmail.com> wrote:

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










 

Where are you seeing CA plug-ins for $10 (much less 99 cents) on eBay? All I see are $39 + $9.99 shipping.

Not that I need another CA, but at those prices I might have a need for a parts mule or three.

-- Jeff Dutky


David Kuhn
 

No, no, no, sorry, the 6AU6 tubes.

I fixed the problem. I cheated. I kept going back and forth between the
"good" and "Weak" module. The differences were so subtle. I do think I
have a few weak 6AU5 tubes, but I can not prove that to myself anymore.
Anyway, I cheated and turned the gain of the 545A chassis up (R1027) up a
bit. Now all modules can be calibrated for proper gain.

On Sat, Jun 26, 2021 at 3:42 PM Jeff Dutky <jeff.dutky@gmail.com> wrote:

Where are you seeing CA plug-ins for $10 (much less 99 cents) on eBay? All
I see are $39 + $9.99 shipping.

Not that I need another CA, but at those prices I might have a need for a
parts mule or three.

-- Jeff Dutky






Tom Lee
 

Sorry to be late to the party, but first, congratulations on getting it to work.

If you ever intend to revisit the repair, I’d suggest measuring the filament voltages, just to be sure that something global wasn’t causing the tubes to run on the cold side, as the marginality you’re seeing is somewhat surprising. It is of course possible that you have a group of tubes that have aged similarly ungracefully, but I’d look at other possibilities too.

— Cheers,
Tom

Sent from my iThing, so please forgive typos and brevity.

On Jun 26, 2021, at 12:53 PM, David Kuhn <Daveyk021@gmail.com> wrote:

No, no, no, sorry, the 6AU6 tubes.

I fixed the problem. I cheated. I kept going back and forth between the
"good" and "Weak" module. The differences were so subtle. I do think I
have a few weak 6AU5 tubes, but I can not prove that to myself anymore.
Anyway, I cheated and turned the gain of the 545A chassis up (R1027) up a
bit. Now all modules can be calibrated for proper gain.

On Sat, Jun 26, 2021 at 3:42 PM Jeff Dutky <jeff.dutky@gmail.com> wrote:

Where are you seeing CA plug-ins for $10 (much less 99 cents) on eBay? All
I see are $39 + $9.99 shipping.

Not that I need another CA, but at those prices I might have a need for a
parts mule or three.

-- Jeff Dutky