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]

Thanks Norman,

I had gone over the service data on the antenna's but it still wasn't clear to me how to  hook them up.  The manual says the loop antenna with the 4 prong receptacle is sufficient for receiving strong AM/Shortwave stations so I hooked that one up with a modern FM dipole antenna.  I wasn't certain what the ribbon antenna was for and I'm trying to work on this this outside the cabinet so I didn't have the cabinet antennas hooked up.  Most of the resistors in the tuner are at their maximum tolerances so I'm going to replace all of them.  It might be my imagination but I think some have gone out of spec since I started working on this thing.  So far the only caps in the tuner I have changed are the  electrolytics and the canned "american condenser"  bathtubs.  I haven't tested the "Gudeman" bathtubs yet but I believe you said that they were usually ok.  I will now test every component out of circuit and get back to you when I'm done with my replacements (this might take a week or two or longer if I don't have the parts in stock). I still have a horrible feeling that my problem is in still in the Jones plugs but there's no harm in a complete rebuild.   My FM tuning eye is staying open but the socket is open between pins 2 and 4 so I'll open it up and replace the 1 meg resistor and see if it closes.  Should I be able to get decent FM reception using a simple dipole antenna?

Thanks for all the help.  I've rebuilt a dozen or so radios but none have been as challenging as this one.

Keith

[Tom Jardine]

Tom Jardine said:

Norman and Keith, I just started working on my 800B s/n 3187 which has the later p/s amp. I wish there were updated mechanical drawings and schematics for these. My amp also has wax, dual 8 uf caps but in parallel with the C93 dual 30uf aluminum cap which should increase C93 to 38uf on each side. This set, as is, works but FM is weak and a little distorted. It has the Thorens 78 changer with crystal cartridge, and the H.H. Scott dynamic scratch suppressor. I'd like to add an early GE VR cartridge and preamp but the tone arm has no stylus pressure adjustment leaving it too heavy for the GE, I think.

Tom Jardine

Norman S Braithwaite said:

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]

Tom, I can send you a version of the schematic that has the 8uf caps.  I need to check it to see if it has incorporated other design changes.  It sounds like we're in a similar boat when it comes to the FM.  Let me know what you find and vice versa.

Keith

[Tom Jardine]

Emailing that would be very kind of you. I'm finding the 800B a little overwhelming. It's peculiar in that it's not a complicated circuit, but circuit layout is very cumbersome. The amplifier chassis with the FM IF circuitry is difficult to troubleshoot. I can only work on it an hour at a time before getting frustrated. I retired a couple of years ago and have many radios that need my attention. Last year I restored a Capehart 400M.

[Keith Lowe]

Tom, send me an email.  I sent you a message with my email address.

Keith

[Keith Lowe]

All, I'm at a complete loss as to what's going on.  I've checked and double checked everything and replaced all the out of spec caps and resistors and tried every type of antenna configuration.  I've found and corrected lots of problems including a broken solder connection on a Jones plug.  Currently, AM and shortwave work normally but FM is barely audible with the volume control turned up to max.  It picks up lots of stations on FM but the volume is incredibly low (tuning eye doesn't close).  Well tonight I was playing with it and all of a sudden it came to life working perfectly with the tuning eye closing and volume working perfectly too.  I shut it off and when I turned it back on it was back to the barely audible FM again.  All of the tubes test strong on my Triplett 3444.  So how does one track down an intermittent problem like this one?  I've only restored a few tube radios with FM and nothing as complex as a Scott.

Thanks in advance.

Keith

[Keith Lowe]

I turned it on this morning and FM worked perfectly, but I noticed that when FM is working AM is completely dead.  This sounds like a bad contact in the FM/AM relay correct?

Thanks,

Keith

[Norman S Braithwaite]

That is most likely and a good place to start.  It is possible that the bad connection is elsewhere but is triggered by the AM-FM relay.  At any rate, the problem appears to be with switching and not the circuit or alignment.

Norman

[Keith Lowe]

Wow,  I finally got this thing running and I never realized I could have multiple failures at the same time.   Not counting the leaky caps and open or  out of spec resistors the main problem before I started working on this was a unsoldered wire on the Jones plug going to the AM/FM relay.  One of the first things I did on this project was to test all the tubes using my Triplett mutual conductance tester.  I found an open  6H6 filament on the FM side and numerous tubes that tested weak.  I replaced all the tubes that tested weak with strong new or used tubes.  This is where my inexperience is coming into  play.  The lesson is that I should have tested the tubes again (!)after I started working on this project.  The bad FM was   caused by an intermittent 6SJ7 tube.  That was the first tube I’ve ever encountered that tested like new on my B&K emission tester but failed to amplify on my mutual conductance tester (the needle didn’t even move).  I took this tube over to a friend’s house and he tested it twice on his Hickok mutual conductance tester, the first time it tested completely dead and the second time it was good.  Replacing the 6SJ7 tube completely fixed my weak FM.  Now with very sensitive FM that sounds great I turned my attention to the AM section.  I retested all the tubes and found a grid short on a 6SK7.  This was a NOS tube that I had recently purchased and tested fine before I put it into the radio,  so the short developed after I started using the tube.  As expected ,the now radio sounds very good.  The next step is  to swap out the tuning motor and do the tone modifications.  I spent most of my time thinking that I had made a mistake in reconnecting wires after re-stuffing the  bathtubs and it never occurred to me that new tubes could go bad or that they could fail during my troubleshoot process..

Thanks for everyone putting up with me.   Has anyone written up anything on the  tuning motor?  I suspect I need to find out why the first one burned out before I swap in a replacement.

Thanks again,

Keith

[Y2K Bruce]

Hi Keith-Congratulations. This has been a very informative thread. Got room for a Fisher yet ?

[Norman S Braithwaite]

Keith:

The usual causes of a burned out tuning motor are a stuck motor relay or a dirty split disk commutator (station finder).  The symptoms are hunting at the station or motor runs tuner to end of travel and keeps trying.  Both cause continuous or near continuous motor operation.  The motor was designed for intermittent operation.  A simple way to prevent motor failure is to turn the set off if the motor fails to stop as it should.  Honestly, I can't understand how these motors get burned out because someone has to be present for the motor to start and that person must fail to think that something is wrong when the pointer darts to the end of the scale and keeps trying or it bounces back and forth across the station for several minutes.

One other unlikely possibility is that some thoughtless person has moved the control voltage supply wire to a higher voltage tap on the control transformer.  This will supercharge the relays and motor (make them work in spite of built up dirt and grease) but may toast these components.  Of the many 800B receivers on which I have worked, I have a vague recollection of finding one where someone had moved the control voltage supply wire to a higher voltage tap hence my recollection of a need to check.

By the way, do not dispose of the burned out motor.  You or someone else may need the fiber gear as a replacement for a stripped gear.  The tooth count on the gear differs from that used on the otherwise identical pre-war motors employed by Scott and possibly otherwise identical motors used by other radio manufacturers.  Check the tooth count before trying to install a replacement motor if not known to be from a Scott 800B.

Norman

[Tom Jardine]

Norman, thanks so much for your excellent advice and patience with so many of us!

Tom