Finally starting 800B restoration

[Keith Lowe]

All, I'm finally ready to start on my 800B and I've got the power supply on my bench.  My plan  is to replace all the electrolytics and re-stuff the bathtub caps.  The power  supply has been worked on before and the bakelite canned caps have been relocated to a terminal strip below the chassis.  I'm thinking it would be nice to relocate these back to the cans provided they can be re-stuffed.  Can these be opened without having them shatter?  I'm hoping I don't need to replace the molded mica caps.  I'd love some advice on what to replace or modify in this power supply.  If I need to solder any connections to the chassis I'll need higher wattage soldering iron for sure.

Thanks in advance,

Keith

[Keith Lowe]

All, I've got one more question.  I noticed that my power supply has a different configuration than the 800B posted on the radiomuseum site.  On mine the three capacitor cans are next to each other in a row.  The chassis on the radiomuseum site has the 4uf cap can  located on the other side of the chassis.  How many versions did they make?  Is mine an early or late version?

Thanks in advance,

Keith

[Norman S Braithwaite]

Keith:

You have the early version of the power supply.  The bathtub capacitors from American Condenser Co are notoriously bad.  There is another brand of bathtub capacitors used in the 800B (mostly in the tuner chassis) the name of which I cannot recall at the moment that is unusually reliable and not necessary to replace unless causing voltages to be incorrect or testing bad.  It is unlikely that you will have to replace the silver mica domino capacitors but I do know at least one collector who does not trust the Noma brand silver mica capacitors.  It will be necessary to check the values of all resistors.  Scott used IRC resistors in the late 1940s and these have been found to be unreliable, especially in higher power ratings and B+ circuits.  At some point in time it will likely be necessary to open the shroud over the split commutator and clean the commutator.  You may also want to consider incorporating the tone control modification posted on this site.

Norman

[Keith Lowe]

Thanks Norman, Somewhere I read that the black  capacitor cans were made from Bakelite.  Do you recommend re-stuffing or should I mount the replacements under the chassis?  If I have to unsolder the Noma caps I'm going to need a much bigger soldering iron.  What type of iron is needed to solder onto a heavy Scott Chassis?

Keith

[Norman S Braithwaite]

Keith:

The black 4-mfd filter "Amsco" capacitor above the chassis is Bakelite.  I have never attempted to gut one of them for internal replacement but they are breakable.  Some break from swelling of original internals.  If you are going to try, go from the top (often popped off anyway).  I use a 260 watt Weller soldering gun to melt solder on the chassis, sometimes complemented with the 140-watt gun as well!  I do not use the large gun for solder lugs.

Norman

[Keith Lowe]

Norman, I thought I'd try re-stuffing one of the bathtubs that contained a 25uf electrolytic.  Unfortunately, one of the screws holding the bathtub is attached under filter reactor L-15.  It looks like the filter reactor case cover isn't designed to be removed.  Is that correct?  It appears to be filled with wax.  I believe I can remove all the other bathtubs  easily.  How about the FM-IF transformer covers, are they designed to be removed?  This is the first Scott I've worked on and I don't want create any additional problems.

Thanks,

Keith

[Norman S Braithwaite]

Keith:

The 25 mfd bathtubs definitely need to be replaced.  You should be able to remove the bathtub capacitor without removing the filter reactor.  The choke is potted in the cover using tar with a piece of fish paper located between the tar and the chassis.  The FM IF shields may be removed.

Norman

[Keith Lowe]

Norman, you are correct, there was a screw directly below the filter reactor holding the bathtub in place.  Al the other screws are secured with nuts but this one is threaded into the chassis.  I've got a couple of other questions regarding the filter caps.  A member on the antique radio forum suggested using a film cap in place of the 4uf 600V canned cap since these tend to overheat.  Is this a common problem?  The second question regards a discrepancy between the Scott and Sams manuals.  In Sams there are 8uf caps in series with two of the 30uf electrolytics.  My power supply has been altered so I'm not sure what was there originally.  I've got a pair of 40uf caps under the chassis and was wondering if the previous repairman mistakenly thought that caps were additive in series (30 + 8 is  close to 40).

Thanks,

Keith

[Norman S Braithwaite]

Keith:  I have never had a problem replacing the 4-mfd, 600-volt filter capacitor with older NOS Sprague capacitors of the same value.  The schematic you are attributing to Sams is not from Sams.  It has been modified.  The published Sams diagrams for both the early and late versions show only the 2 x 30-mfd capacitors.  When placing capacitors in series, the equivalent capacitance is less than the lowest capacitor value.  Two 8-mfd, 250-volt capacitors in series are equivalent to a single 4-mfd, 500-volt capacitor.  Therefore the diagram showing the 30-mfd capacitors in series with 8-mfd capacitors is grossly insufficient for the required application not to mention the fact that the 8-mfd capacitors are shown in the circuit backwards (a sure formula for explosion of the 8-mfd capacitors).  Whoever modified the schematic should not be trusted whatsoever.  The use of 40-mfd capacitors as replacement for the original 30-mfd capacitors in this application is fine.

Norman

[Keith Lowe]

Interesting, I've got an original third version  photofact.  It shows the two section 8uf cap in the schematic, the parts listing, and in a photo.  I'd be glad to send anyone a  pdf after I get it scanned.  Attached is the Sams photo.  I thought it was weird and that's why I brought it up. The cap in question is #30. Thanks,

Keith

[Norman S Braithwaite]

Keith: On closer examination of the schematic I see that they show the 8-mfd capacitors electrically in parallel with the 30-mfd capacitors even though shown in a linear fashion on the diagram. I have never seen this version of the Sams Photofact.

Norman

[Keith Lowe]

I guess that could explain why one of the 30/30 caps was replaced with two 40/40.  Do you think that they decided to increase the capacitance later on?  Just wondering what I should use.

Keith

[Norman S Braithwaite]

Either.  It was probably a late measure by Scott Radio Laboratories to reduce hum related to the poor tone control design.

Norman