Model 800 Acquired Yesterday!

[Joseph W Strickland]

Yesterday, following a Vintage Antique Radio & Phonograph Society (VRPS) meeting in the Dallas, TX area, I visited the home of one of the club directors. He gave me two items completely free! I now have a Silvertone AM/FM/SW radio-phono-wire recorder console and an E H Scott model 800. I worked on a model 800 years ago for a good friend and the only thing wrong with it at the time was a shorted bypass capacitor in the FM IF strip. All its tubes were still good.

I am looking forward to restoring this E H Scott model 800. This one may be an early version of the 800. It seems to only have 3 IF stages in the FM IF rather than 4. I need to check closely and take some pictures of the units. I also will look for the serial number and any date codes on components to get an idea of when this unit was made.

Unfortunately there is no electro-dynamic speaker with this one, nor is there a cabinet for it. If anyone knows of a source for the speaker, please let me know. There is also no record changer or turntable with it, so that is another item to search for. I think I still have a Garrard RC-88 changer somewhere in the attic. I need to search to find it or else locate a suitable replacement turntable.

Joe

[Norman S Braithwaite]

Joe:

You are correct regarding the location of the VR tube. I was visualizing the location from the bottom of the chassis while describing the top! The VR tube was moved in an effort to reduce hum, a pernicious problem with the 800B. Yes, your 800B definitely would have had the field coil speaker. The only 800B sets I have encountered with the PM speakers were sets that were sold very late long after production of the chassis ceased in the Summer of 1947.

Norman

[Joseph W Strickland]

Norman;

I appreciate any hints you may have on the hum issue. Hum can be a difficult problem to solve in older chassis. One thing I have seen happen is corroded connections between tube socket saddles and the chassis ground partly due to the rivets being of a different type of metal in some radios. That can set up galvanic action and lead to hidden corrosion. I have a nice Fluke 8600A digital meter and on its lowest resistance scale I will ground one lead to the chassis metal and then touch each rivet head from underneath the chassis and also the metal ground lances of terminal strips and the tube socket saddles themselves. If I see a high resistance at any such location I know I need to either drill out the rivet and install screws, nuts and star-washers or find a nearby good ground I can solder a wire to or use braid from discarded coax cable ground shield and re-ground the connection. Of course extra care has to be exercised in an RF circuit in dealing with longer ground leads and component leads.

I do see that the tallest electrolytic on the power supply is swollen. It is the one with a large nut and star-washer to hold it to the chassis. It looks to be an OEM part with considerable corrosion on its surface. There are two other can electrolytics on the chassis which appear to have been replaced some time in the 1970s, if I am reading the date codes correctly. I see a number of bathtub type capacitors I think E H Scott calls "dry electrolytics" scattered throughout the receiver. These may actually be paper in oil types and perhaps not bad. One of these failed in a Scott 800B I worked on for a friend. It was a bypass in the 1st Limiter stage and I replaced it using 500V disc ceramic capacitors plus replacement of the burned B+ dropping resistor to that stage.

I need to put together a list of parts I need to order. I have a fair collection of polypropylene and polyethelyne film capacitors on hand, but need to check the values this set uses before I order. Antique Electronic Supply is were I get most of my electrolytics from. Mouser is only about a 2 hour drive from here and I often order from them by US Mail parcel lowest rate. The orders from both usually arrive within 3 days.

Joe

[Norman S Braithwaite]

Joe:

Most if not all of the "bathtub" capacitors in the 800B are in fact wax-paper types and most are leaky, especially the 3x.01 types in the tuner.  Scott Radio Laboratories published a number of hum reducing modifications that can be found in the Sams Photofact data but they failed to recognize the high impedance cathode of an audio tube to be extremely sensitive to hum pickup.  The cathode of the audio tube was high impedance to accommodate an unusual tone control circuit allowing tone controls to be out of circuit at mid position.  Not only was the unusual circuit responsible for a stage of amplification being sensitive to hum pickup, the range of tone is limited by the circuit.  I have identified changes to the tone control circuit that greatly improves the tone of the receiver and eliminates the sensitivity of the audio amplification stage to hum pickup on the page where I have the photos of the tuner chassis lever mechanism.  One coupling capacitor in the identified modification is superfluous.

Norman

[Joseph W Strickland]

Norman;

Thanks for the extra comments on the high impedance cathode circuit and its sensitivity to hum pick-up. I have already saved the two pages of information and will give them close attention while restoring the receiver. On the bathtub capacitors - I appreciate the information on the internal contents. I might look at gutting the internal parts and putting new types inside. In an IF strip or front end disc ceramic bypasses are better choices than coiled up paper-foil types. Of course in that time period some of the modern capacitor types did not exist. We are fortunate that there are so many choices today.

I just took a look at the tone control circuit. V10 has a 10K ohm resistor plus a 1.3K resistor in the cathode circuit. That is indeed far higher than modern circuits that perform the same task. It is strange that they used a 25mF bypass on the 1.3K cathode resistor but left the 10K ohm resistor un-bypassed thus depending on the parts in the tone control circuit to provide the effective bypassing of the cathode. I looked at the changes you made and I agree that the re-positioning of the tone controls and tying the V10 cathode to ground makes good sense. The increased value of coupling capacitors there will allow an extended low end.

Did you increase the values of the electrolytics in the power supply? There are two 5Y3 rectifiers, likely to provide more current capability in the power supply and reduce internal resistance. Has anyone tried using 5AR4 rectifiers? They have a lower internal resistance than 5Y3 rectifiers if I remember correctly. I need to check filament current requirements though. And it is important to keep in mind the recommended capacitance for a given rectifier tube. This power supply design uses a small value of capacitance followed by the 5H choke then a larger capacitance which will mediate the application of larger amounts of capacitance at C94. 60mF is a fair amount of capacitance, but higher might be better. I will have to look at that when I get to that part of the restoration.

Joe

[David C. Poland]

good reading.

As for cabinets, the big Chippendale cabinets do turn up empty. At least one Scott collector dismantled one and rebuilt it at about half the original width. Result was a nice console size cabinet with all the Chippendale details right down to the original doors over the original speaker grill. In such a rebuild, one could preserve the drop front roll out receiver platform and also hinge the top for additional viewing. Would also provide additional material to add cabinet height to accommodate an overhead record changer.  

Could be an interesting project, but a lot of work. Or find one of the 800B cabinets (there are 2 additional styles) to use as intended. 

The correct 15 inch Jensen coax speakers turn up on ebay - one quite recently.