Scott 16 Capacitor Question

[Dave McClellan]

I have a model 16 on the bench for checkout. The current owner has never powered it up and asked me to check it out. It has been electrically restored in the 1985 time frame. The replacement capacitors are a mixture of orange drops, yellow axial film caps, and other film capacitors. So far, all original capacitors have been replaced. But I have a question about one inside the tuner cover, in parallel with choke 4647. The schematic says 0.15 mfd. What is there  is a 0.1 film capacitor in parallel with a Sprague 0.05 5% Black Beauty(?). I was planning on simply replacing the pair with a 0.15mfd axial film capacitor, thinking that the Sprague may be original and thus should be replaced. Advice?

[Norman S Braithwaite]

The Sprague black beauty is not original but is a reliable difilm type.  I would replace the pair and stash the black beauty with my collection of cult audio capacitors!

Norman

[Alex R. Whitaker]

Yes, if the writing on the black beauty is red, it is a mylar film type, and could be reused.  If the writing is yellow, then the black beauty is a paper cap, and is junk.  All original Scott pre-war caps are paper (outside of the mica caps used).  

[Dave McClellan]

Thanks guys. I replaced the pair with a new film capacitor. So now all the paper caps have been replaced, including under the tuner shield. I do NOT want to have to remove and replace that again! What a pain. The filter capacitors are branded Rubycon and are OK after reforming, but they have been there since 1985. I will replace those also. All of the tubes tested good. I next need to check for out of tolerance resistors, adjust the dial drive anti-backlash gears, install a  replacement flat dial drive belt,  and do a badly needed alignment (some broadcast band stations come in at two places on the dial). I have never worked on a Scott before, so please advise any gotchas! 

[David C. Poland]

Make sure you replace the 4 caps in the soldered metal box, mounted on the inside middle partition.

My notes they are .5 mfd.

[Dave McClellan]

I read about that box in the forums. But I do not see it. However, there are several 0.47mfd caps UNDER the chassis. Is (or WAS) this box mounted on TOP of the chassis, UNDER the chassis, or on the center under-chassis partition? 

[Norman S Braithwaite]

You may want to check inside the IF AGC diode shield under the chassis immediately behind front panel.  It may have a couple 0.005-mfd capacitors (two sections in one component) that was overlooked.  And you thought the tuner shield was a pain to deal with!  It looks like the 0.05-mfd capacitors to the RF AGC diode have been replaced with 0.1-mfd capacitors (an acceptable value).

Norman 

[Dave McClellan]

The connections and insulated sleving to the diode shield do not look original to me. So it appears someone has been there before. 

[David C. Poland]

Dave. Here is a photo of my Sixteen - bottom view:

DSCF3495.JPG

Shows the metal box with 4 caps inside. 

Appears your chassis had extensive prior work including replacement of the metal box with discrete caps, which you have now replaced with yellow film caps. I opened up my metal box to replace the caps, and so labeled it

Note my original Candohm B+ divider riveted to the side apron - has 5 lugs - lower left. (I subsequently replaced mine later during restoration with a wire wound Ohmite with adjustable tabs.) The originals like that in the Sixteen, Phantom, Masterpiece and Philharmonic become unreliable - I replace them regardless even if still ok.

A few of the caps are buried and stinkers to access and replace. !

FYI - My Sixteen is in a Accousticraft and sounds really good. My cabinet was really rough, so I rebuilt it as a lift top cabinet.   DSCF3804.JPG

[Dave McClellan]

Thanks. I was curious what the extra holes in the partition were! Yes, the radio I am working on (not mine) has had the caps in the metal box replaced with discrete film caps. The film caps are 1985-1990 vintage based on records left by the previous tech. The Candohm has also been replaced by a large tapped power resistor. I still must replace the filter capacitors, also vintage 1985 (Rubycon brand). I was not planning on replacing any film capacitors. There are several out of tolerance resistors that also need to be replaced. The previous owner or tech removed the phone jacks. He also removed the original filter caps and replaced them with hole plugs and new caps on terminal strips (Rubycon).

I have not found an accurate schematic for this radio. For example there is a wire wound pot on the end of the power supply/output chassis with a 10K fixed resistor on each end. I assumed that this was some sort of balancing adjustment, but it does not change the bias of the output tubes.

Right now I am trying to find the source of a 60Hz hum. It is still there with the 6K7 1st audio and phase inverter tubes pulled. It remains if either 6J5 2nd audio tube is present, but varies in intensity depending on which one is installed. Really WEIRD!

[Dave McClellan]

Well, first of all ignore anything I have written so far about the hum problem. I was using the Instruction Manual, which shows that the back row of tubes is 6B8, 6J5 2AF, 6J5 2AF, and 6J5 Inverter. WRONG!! Riders says 6B8, 6J5 Inverter, then 2x 6J5 2nd AF. Well that cost me a day's work. But now I have isolated the hum problem. The 60Hz hum originates from the PLATE of the inverter which drives one of the 2nd AF tubes. The CATHODE of the inverter which drives the other 6J5 2nd does NOT have the hum. If I pull the 6J5 2nd AF with the hum, or short its grid to ground, the hum goes away (but some of the signal from the cathode of the inverter goes through). I suspect a BAD TUBE SOCKET (inverter). The plate circuit is a much higher impedance than the cathode and thus more susceptible to leakage from the filament or wherever. To confirm, I shunted the plate of the inverter to ground through a resistance box. If I shunt the plate to ground through about 22K the hum goes away, but the radio signal is relatively unaffected.

So it looks like I need to replace the Inverter socket. That will NOT be easy! The sockets look to be the highest quality, and the dimensions are critical. This thing makes my Zenith 1000Z restoration look like a walk in the park.

[David C. Poland]

Dave -  My set disagrees with the Scott manual too.

On mine - the 6J5 inverter is next to the 6B8 AGC tube, and the pair of drivers is to the right by the RF 6U7.

I had an early Sixteen amp with no hum adjustment pot.

Later version has the hum pot located on the end apron of the amp,  opposite the power transformer.

Before you replace that socket see below.

After restoration, I had only fair sound - lacking depth. After listening to Kent's Sixteen with superior sound having good bass, and gave mine a 2nd effort. 

Found one of the driver 6j5's was not functioning. I found a broken lead on the 1 meg resistor if I recall correctly - was about 3 - 4 years ago.  I replaced both the 1 meg resistors with new. That is when I discovered the tube line up error in the manual. Then I put the two closely matched 6J5-G drivers in correct sockets.

If you do proceed to replace the socket, check with Kent as he may have a junker  parts chassis for that embossed 6J5 socket.socket.

[Dave McClellan]

I disconnected everything from the 6J5 inverter tube plate lead and then connected an empty octal socket using clip leads. I assumed this would confirm if the inverter tube socket was somehow "leaky". NO CHANGE. I then did the same for the 6J5 driver tube which is driven from the inverter plate. NO CHANGE. So now I am really stumped. When the 6J5 driver tube which gets its signal from the inverter tube plate is REMOVED, or its grid grounded, there is NO hum. A reduced volume radio signal is still heard with NO hum. This seems impossible to me. The signal from the inverter plate has a significant 60Hz hum, while the signal from the inverter cathode does not. I checked all the resistors in the inverter and driver tube circuits, and they are ok (but not in tolerance). I did change the inverter plate resistor to a value that matched the cathode resistor. 

Does anyone have suggestions? Thanks!