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]

Well, all of the bathtubs in the power supply were leaky and I'm slowly opening them up with a dremel tool and putting new caps inside.  Was there are reason Scott used this type of capacitor?  Do the metal bathtubs offer some short of shielding?  I tested the 4uf canned cap on my Sprague Tel-ohmike and there is absolutely no leakage at full voltage.  Would it be a bad idea to keep this one?

Thanks,

Keith

[Norman S Braithwaite]

Keith:

I would not trust the original 4-mfd capacitor even if it did test good.

Norman

[Keith Lowe]

Help, My confidence has been shaken.  When I purchased my Scott the owner told me he had powered it up before I had my son pick it up for me.  The previous owner was a very competent radio restorer so I'm sure the radio was in working condition.  He did  say it had some caps that needed to be changed so I decided  to go through it before powering it up.  Last week I checked all the caps and resistors in the power supply and re-stuffed all the bathtubs.  I believe I checked all the tubes but that was 6 months ago and I can't be certain.  Well I decided to check my work so I hooked up the power supply to the tuner and speaker and slowly brought up the voltage on my variac while monitoring the current.  I can get my MP3  player to play through the phono input but  it's not very loud.  At  first I didn't think the AM and FM tuners were working but then I discovered I could get the AM and FM to work but I  only hear audio when the volume was turned up to the maximum.  I then checked all the tubes and discovered both rectifier sections of the 6H6 tube were dead.  So either this tube was dead from the beginning or most  likely I did something that made the tube blow.  While I'm waiting for another tube can anyone tell me if errors in my work could have killed this tube?  I've got the table  with the resistance reading for all the tube pins but can I do this without having the tuner and speaker hooked up?  Given the weight  of this thing it would be a bear to attempt voltage readings.

Thanks,

Keith

[Norman S Braithwaite]

Keith:

It is unusual for a 6H6 tube to be bad and even more unusual for one to be "blown" with a short duration of use.  Furthermore 6H6 tubes will still work decent if testing low (but not so if testing dead).  A common cause of audio problems in the 800B is bad connections in the Jones plugs.  Try cleaning the Jones plugs.

Norman

[Keith Lowe]

Norman, thanks for you prompt reply.  Very strange, this tube is testing dead not weak using my Triplett 3444 and also on my B&K tube tester.  I'll run it over to a friends house to see what he gets on his tester.  Is there anything unique to the Scott that could cause the weak audio besides the Jones plugs?  I had very weak audio on a Magnavox I worked on recently and it turned out it was an antenna connection.  I'll attack this problem again tomorrow night.

Keith

[Norman S Braithwaite]

Better start with the 6H6!

Norman

[Keith Lowe]

Now it’s becoming a challenge.  Well the 6H6 tube was completely dead so I put in a new one.  I then cleaned  all the Jones  plugs and the AM volume greatly increased but I’m only pulling in AM stations above 1000kc.  Still no FM which surprises me since I had very faint FM before I put in the new tube.  What’s interesting is that the radio is silent in FM mode even with the volume turned up to the maximum (no hum).  I’ve got a scope but I’m just learning how to use it.  Shouldn’t I be able to unplug the FM-IM connection, hook up my scope, and look for a signal?  I ran all the resistance checks last night and a few are way off but I haven’t  found time study the schematic to see where my problem is.  I’m thinking it must be something simple.  Any troubling shooting tips?  I won’t be able to get the pin voltages until I purchase some test sockets.  Here are the results from my resistance checks.  Any ideas are welcome.

Thanks,

Keith

[Norman S Braithwaite]

Keith:

There are two 6H6 tubes in the 800B.  The 6H6 tube on the tuner chassis is dedicated for AM reception and the 6H6 tube on the power amplifier chassis is dedicated for FM reception.  Replacement of the dead 6H6 tube will have affected only AM or FM but not both.  If the 6H6 tube that you replaced was the one on the power amplifier chassis, the AM is working better because you cleaned the Jones plugs.  I will need to consult the schematic before commenting on resistance values.  In the mean time, does the tuning indicator tube for FM close as you tune across strong FM stations?

Norman

[Keith Lowe]

Norman, So far I haven’t touched the tuner.  When I first fired up the power supply after recapping it I had weak AM and even weaker FM (I could barely hear it when the volume was at maximum).  I then discovered I had an open filament on the FM 6H6 tube in the power supply.  I cleaned the Jones plugs and put in a new tube and now I’ve got strong AM (upper frequencies only) but no FM.  In fact the FM is completely silent (could this be the muting relay?).  If the AM/FM relay isn’t working would the lights indicator lights still switch from AM to FM?  I would think I should at least get a bit of hum on FM when the volume is at max but I get nothing.  I decided to re-do my resistance readings.  I’m pretty sure the SAM’s resistance table has errors.  For example, the SAMS table says you should get 175K ohms on pin #1 of tube 19 but that’s impossible since there is a 47K resistor going to ground from that pin.  So this time I decided to use the table in Riders.  I have no idea how close these values should be but here they are.  I’ve put the readings that are way out of spec in red type.  I’m hoping I can rule out the power supply before I dive into the tuner.

Thanks,

Keith

[Keith Lowe]

Norman,Give me a week or two to sort things out.  I just went to Kinko's and I've got the schematics enlarged to 24"X36" so at least I'll be able to see them.  Last night  I pulled out the tuner and found four  tubes that fall into the "reject" category using my Triplett 3444 tester.  I'll order some new tubes, check all my connections etc. and get back to you.  I noticed that the relays are intermittent and I'm not sure if it's the relays or the cable/connections.  I also need to swap out the motor, fortunately I've got a replacement.

Thanks,

Keith

[Keith Lowe]

Norman, I finished my resistance tests and found a bunch of problem resistors centered around the FM mixer and  RF amp tubes.  Unfortunately, replacing them will be difficult due to their locations.  I will replace these but I have one question.  Do these resistors tend to go bad on their own or do you think a problem else where caused them to go bad?  Here's what I found so far.

Thanks,

Keith

[Norman S Braithwaite]

Keith:

The resistors used by Scott Radio Laboratories in the 800B are notorious for changing in value.  Most were made by IRC and have been nicknamed "crustohms" referring to the non-smooth surface of the resistor.  The lower value resistors used in non-B+ circuits are usually OK but I frequently check all of the IRC resistors.  Sometimes all of one value will be uniformly bad.

Norman