Replace multiple capacitor

[gtr]

Hello everybody,

I have an Eico Model 460 oscilloscope so I almost replaced all the capacitors (paper, molded and electrolytic).

I now want to replace the multiple capacitor C27 (3x20uf-450V) identified C27-A, C27-B, C27-C and C27-D with 3x22uf-450V electrolytic.

On the electronic diagram here https://www.nostalgickitscentral.com/eico/schematics/eico_schematic_460.pdf C27-D does not appear.

The manual here http://pacifictv.ca/schematics/eico460manual.pdf

In the construction manual here http://www.vanade.com/~blc/html/hardware/eico460/Eico460-Construction-Pages.pdf page 5 you can see C27-D.

The positive terminals of the 3 capacitors C27-A, C27-B and C27-C are identified under the housing by a semicircle, a square and a triangle and nothing for C27-D. In the diagram, the 3 negative terminals are not directly connected to ground, but following the path to the left, we arrive at resistor R79 which is connected to ground.

So I weld my 3 positive terminals at the location identified on the diagram, I connect the 3 negative terminals together and I weld on the terminal identified C27-D as seen in the construction manual since C27-D is connected to ground in going through R79 (which is welded to a twist pin of the multiple capacitors case welded to the chassis), is that correct?

Why C27-D (the 3 negative terminals of the capacitors) is not directly connected to ground but goes through R79 so also the midpoint of the transformer?

Please help me because I want to make sure I don't make a mistake.

Thank you! 

[duak]

I think R79 & C28 are there to develop a negative voltage of maybe -20 to -40 V relative to chassis ground.  The allows the cathode resistors of V1 and V7 to have larger values that improve their balance and linearity.  These circuits are called "long-tailed" or differential amplifiers that generate outputs that are 180 degrees out of phase.  Just what is needed for the deflection plates of a CRT.

Those parts may also serve another function.  If you look at V1, V2, V3 & V4 you can see that they are DC-coupled.  That is, there are no coupling capacitors between the stages.  It may be that when the unit is first powered on, one of the tube's ratings was exceeded.  Since the voltage across R79 is a function of the current drawn by most of the circuitry, it holds the voltage down on the plates of V1A & V2A that may protect the following tubes.

[Zero999]

Why do you want to replace all of the capacitors? Do you know which are faulty and which work? Blindly recapping is a waste of time and can often cause more problems, than it solves. Just replace the capacitors which you know are bad.

[floobydust]

It's an interesting puzzle because the schematics at first glance aren't matching as-builts or the construction manual:
"Connect a 500R 5W resistor R79 from TB9-3(C) to the capacitor twist prong which lies between C27-A and C-27D. Solder the resistor to the twist prong, and solder the prong directly to the chassis to ensure a good ground connection"

I see C27 can housing is connected to chassis ground in build pics. C27 may have been a special part with terminal C27-D being the actual common (-) terminal for the internal three capacitors, so the can housing would be isolated. Mainstream can electrolytics all had their can as the common (-). An extra phenolic insulator would be used if the can needed to be isolated from chassis ground, which is what the Eico scope schematics show. Only way I can make sense of it is C27 is an oddball part- or people put in a vanilla can electrolytic(with grounded can) which effectively shorted out R79, C28 and the scope ran without bias voltage which would upset DC performance.

So I would connect all three 22uF capacitors (-) together to some common tie point which then goes to R79/C28/transformer node.
pic from https://www.oscilloscopemuseum.com/oscilloscope-eico-460-s65991.html

[gtr]

Thank you very much duak and floobydust for these excellent explanations, I know what to do now.

For Zero999, see this web page https://antiqueradio.org/recap.htm to understand why you need to replace the old capacitors.

Thanks again to everyone!

[Zero999]

Thank you very much duak and floobydust for these excellent explanations, I know what to do now.

For Zero999, see this web page https://antiqueradio.org/recap.htm to understand why you need to replace the old capacitors.

Thanks again to everyone!

Yes, I am familiar with the reasons why it's often recommended to replace capacitors. I just disagree with it and prefer to keep the original parts, unless it's obvious they're going to fail soon. I've had really old capacitors which are perfectly fine and see no point in replacing them.

If you really feel the need to replace the capacitors then please get decent branded ones, from Digi Key, RS Components, Farnell, Mouser etc. otherwise there's a risk it will go pop.

[gtr]

It's an interesting puzzle because the schematics at first glance aren't matching as-builts or the construction manual:

I forgot to say that there is also this pdf:

http://pacifictv.ca/schematics/eico460addendum.pdf

I have another question. On the Eico there is a sawtooth outlet (J7). Can I with a probe for example connect a second oscilloscope to the J7 output and the mass of the Eico to visualize this sawtooth wave on the second oscilloscope knowing that these two oscilloscops are connected to the mains outlet and which have the same grounding?

Thank you! :-)

[floobydust]

Yes, no problem to look at the sweep output with a second scope. The Eico 460 is not earth-grounded (two-prong power cord) so it is floating. So the second scope if more modern would be earth-grounded and end up grounding the Eico under test.

I would go with the Eico 460 schematic and it using Back-biasing to make a little negative voltage. Although no sign of -ve voltages on the voltage chart because Eico left out R79 node so no idea what it should be.

[gtr]

No me my 3 pin Eico on the power cord, so it has a ground. So in my case I can not see the output of the sawtooth wave on a second oscilloscope?

Thank you!