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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Nov Tue 21, 2017 11:45 pm 
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Original schematics for the early version of the AW-23 are rare. I am fortunate to have a copy annotated to show the alternative phonograph hookup (original phono input also shown) although it does not show the 400k resistor. Yes, the phono input attaches to one end of the volume control.

Attachment:
AW-23 schematic_20171121.jpg
AW-23 schematic_20171121.jpg [ 126.22 KiB | Viewed 670 times ]


Norman

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Nov Wed 22, 2017 2:43 am 
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Thanks Norman. So some of the early AW 23 sets did have a pre-volume control phono input. That will be a nice feature.

I wonder why the 400K resistor, maybe to protect whatever was attached to the phono?

The bad news is that whoever worked on this before has this attached to the ground terminal and nothing is attached to the phono terminal. This makes me mighty suspicious and now will need to verify every connection. Not that big of a deal as I was going to be verifying all of it anyway.

There are two 600 Ohm resistors that are variable.

One is attached to the tuning meter and the other is attached to the noise suppressor circuit. Are these variable so they can be adjusted to allow the appropriate amount of deflection in the tuning indicator and the appropriate suppression level?

Here is a photo of the variable 600W resistor.
Attachment:
variable resistor.jpg
variable resistor.jpg [ 99.23 KiB | Viewed 652 times ]


I have measured almost all of the coils and one of the two beat oscillator coils might be open. I am reading something right around 8 ohms on one set of contacts but unable to get a reading on the third contact with either of the other two on the coil form.

Does anyone know what wire gauge was used for the beat oscillator coil?

Cheers

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Nov Wed 22, 2017 3:33 am 
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The 400k resistor prevents the magnetic cartridge from loading down the signal from the detector when listening to the radio. The 600 ohm resistor for the noise suppressor is adjusted to 9-volts as part of the IF alignment procedure. The other 600 ohm resistor is adjusted for best eye tube performance.

Norman

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Nov Wed 22, 2017 3:53 pm 
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So that appears to be part of the phono input, but there is no resistor in that circuit. In the early AW23, it connects directly into a 6C6 grid, in the 7-knob set that came later, Scott added a radio/phono switch and the input is passed through a 0.15 mfd cap. I'm not sure what the resistor is doing, but it certainly appears to be original. I'd carefully identify the two connections and see where they are on the schematic. It might become clearer then.

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Nov Wed 22, 2017 4:04 pm 
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No resistor would be required if the phonograph were equipped with a crystal cartridge.

Norman

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Nov Wed 22, 2017 4:20 pm 
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kent3256 wrote:
So that appears to be part of the phono input, but there is no resistor in that circuit. In the early AW23, it connects directly into a 6C6 grid, in the 7-knob set that came later, Scott added a radio/phono switch and the input is passed through a 0.15 mfd cap. I'm not sure what the resistor is doing, but it certainly appears to be original. I'd carefully identify the two connections and see where they are on the schematic. It might become clearer then.


Hi Kent. Well that is sort of the quandary here. Here are the connections:

One end of the 400K resistor is connected to the GROUND terminal of the phono input terminal set. The other end of the resistor is attached to low loss rubber wire like everything that is used through the RF / IF grid and plate sections. The other end of the wire is just hanging out and not attached to anything currently. It looks like the other end of the wire was at one time attached to the volume pot. The PHONO terminal of the phono input terminal set has nothing attached to it at all (there is a witness mark where something was attached at one point).

Norman - thank you for all of the clarifications you have been providing. I am taking stock of exactly which capacitor sleeves I have and will post on this in the next day or so. The holes through the chassis are 1/2 inch. Also it looks a lot like the schematic snippet you provided showing the alternate phono connection point is what was originally going on here.

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Nov Wed 22, 2017 4:53 pm 
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Which of the phonograph binding posts measures 0 ohms to the chassis, the one marked "Phono" or the one marked "GND"?
If the wire is reconnected to the volume control, the set will work as intended if the "Phono" post is connected directly to the chassis and the "GND" post is insulated from the chassis.

Norman

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Nov Wed 22, 2017 5:31 pm 
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Braithwaite wrote:
Which of the phonograph binding posts measures 0 ohms to the chassis, the one marked "Phono" or the one marked "GND"?
If the wire is reconnected to the volume control, the set will work as intended if the "Phono" post is connected directly to the chassis and the "GND" post is insulated from the chassis.

Norman


The binding post marked "GND" measures 0 ohms to chassis. This is where the resistor is hooked up currently.

The other binding post (phono but not marked) measure no connection to chassis as there is nothing connected to it.

My theory is that this resistor was removed from the phono post at some point in the past.

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Nov Wed 22, 2017 5:43 pm 
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I agree with your theory wholeheartedly!

Norman

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Dec Tue 05, 2017 2:05 am 
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Hi All,

Have been busy working on the survey and BOM for this receiver.

So all of the coils metered out just fine. I was originally worried about the beat oscillator coil.
From the beat oscillator switch to the plate of the 76 measures 8.3 ohm.
From the coil side of the 25 mmfd capacitor to ground through the coil measures 143 ohm.

The big ceramic voltage divider measures 4.4k / 400 ohm / 4.4k so that is great news.

The bad news is that almost every other dog bone type resistor in the receiver has nearly doubled in value. All of them are well out of tolerance. Anyone have a theory on why all of these would be so far out of whack?

I have taken a bit of time to work on molding resistors and can now present molded resistor attempt #3 (it is the middle in this photo).
Attachment:
MR try 3.jpg
MR try 3.jpg [ 80.83 KiB | Viewed 505 times ]

Attempt #1 was the continental that is in the power supply (going to re-do that one also). Attempt #2 is in the upper left of this photo but the scale wasn't quite right. Attempt #3 is on scale and has a 2 watt replacement in a dog bone 1/4 watt form.

the paper wax capacitors in this set are all going to be either re-stuffed or re-created. Lou was kind enough to send me a bunch of potter capacitors from another AW23 and Norman has offered to send along some more once I know what I need. At this point I am experimenting with using paper fireworks tubes to re-create the potter caps. I will post more on this as I find out how it is going. The potter caps are a serious PIA to re-stuff and my results have been mixed. I think these capacitors were originally potted in natural beeswax so have some inbound. Will post photos once I get some form of outcome.

EDIT: oh forgot to mention. There are 5 domino type capacitors in this receiver that are 5000 pF values. These make me really suspicious. That is a huge value for a true mica cap. I am probably going to have to go all "Radio Fixer" on at least one of them. Gary has some pretty good examples of what is inside these large pF value caps back a few pages in this thread. Anyhow there are 2 Sangamos and 3 Cornell Dubliners.

Cheers

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Dec Tue 05, 2017 10:24 am 
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The resistors are looking very good. Tough having to do a lot for the rest of the chassis. What you need is a multiple mould: would that be very hard to make? Say 3 or 4 at a time?

Maybe the carbon granules settle out? But some would come nearer the correct value if you put power in them I think. Just for interest you could try that: just a potential divider across a supply and measure the volts as it heats up. But to leave them? Maybe for non-critical position you could? See if the volts are close enough after power up and change them then if not.

The cap could still be mica at 5000pF but you have to suspect it? Is it the right value and does it have any leakage. Those EMI (RCA) ones are 2300pF and if you look back have about 10 pieces of mica and corresponding number of plates. All mainly used to decouple heaters but these radios, the best from EMI, went up to about 60mHz to receive TV sound.

But slow steady progress and I hope you are enjoying doing it. The end result will be worth it, something to be proud of.

Gary


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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Dec Tue 05, 2017 7:00 pm 
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Hi Gary,

I originally did make a multiple location mold. The material i am using sets in 3-5 minutes and cures really hard in 1 hour. I pull 1 from the mold after 2 minutes and a second after 5 minutes. Really isn't a problem and I have already molded the bulk of what will be needed. Right now I am in process of testing the paint. It is really really thick stuff and am going to thin it out a bit to see if I can get a more even and uniform coat.

The large mica caps are measuring 10% - 15% high with a digital capacitance meter. Sometimes high measurements are leakage with this type of meter.
The progress is slow but really fun to learn some new skills. Have been also molding capacitors with some success.

Cheers

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Dec Sat 09, 2017 9:03 am 
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BOM Incoming
Attachment:
BOM incoming.jpg
BOM incoming.jpg [ 125.85 KiB | Viewed 408 times ]


Cheers

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Dec Sat 09, 2017 9:45 am 
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Impressive piece of work! You really have got into the spirit of this. Going to look great.

Gary


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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Dec Sat 09, 2017 5:22 pm 
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Thanks Gary.

Ridiculous amount of work to figure out. This radio is special enough to deserve it. Should look cool when done. A little bit more than half of the caps are re-stuffed potters and the others are replicas.

Cheers.

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Dec Sun 10, 2017 3:16 am 
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Wow, great job. Nice to see those old caps find a new home.


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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Dec Sun 17, 2017 9:18 pm 
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Lou deGonzague wrote:
Wow, great job. Nice to see those old caps find a new home.


Thanks Lou. Really appreciative that you sent over the old caps. I reverse engineered the graphic file to create the sleeve (or close at least) for the .05 600 volt caps.

Working with the old caps gave me a twinge of nerdy modeling nostalgia. So anyhow modeling nostalgia now sated!
Attachment:
Modeling nerd nostalgia sm.jpg
Modeling nerd nostalgia sm.jpg [ 70.84 KiB | Viewed 249 times ]


Looking forward to actually working on the receiver now.

Cheers

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Dec Sun 17, 2017 11:48 pm 
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Parts listing for this receiver and some measured values below:
Attachment:
AW 23 BOM.jpg
AW 23 BOM.jpg [ 74.56 KiB | Viewed 235 times ]

Note this listing does not include capacitors 437, 293, or 295 which are located on the coil wheel.

So as I mentioned earlier most of the resistors in this receiver have doubled in value.

Some of the mica capacitors might be good (stk. #46 and #49 are close). Some of the larger mica capacitors might also be good. A bunch of the mica capacitors are way off so I am suspicious of them all now of course. I have replacements for any of them that leak on hand.

So the next step I guess is to go ahead and replace the "special rubber covered low loss wire".
I am going to be using Cal Test 20 AWG silicone jacket test lead wire. The outside diameter is .106" / 2.7mm.

The next size up that I found was 18 AWG and had an outside diameter of .144" / 3.66mm.
The larger size 18 AWG does NOT fit through the eyelets in the receiver tube shields.

Cheers

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Dec Thu 21, 2017 3:43 pm 
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Braithwaite wrote:
The IF amplifier has nine tuned circuits. When adjusting from a sharp bandpass to a broad (hi-fi) bandpass, three tuned circuits remain centered, three add capacitance moving down in frequency, and three subtract capacitance moving up in frequency. Near the broad bandpass position the top of the bandpass characteristic is more or less flat with mild dips between the sets of tuned circuits. When opening up the bandpass characteristic (adjusting from selective to high fidelity), the energy conveyed by the IF amplifier is increased. If I understand correctly, the energy conveyed is proportional to the area under the bandpass characteristic. For a bandpass that is variable from 2-khz to 32-khz (audio bandwidth is half the IF bandpass), there is a tremendous range of energy reaching the detector, far more than the AVC circuits can address. Hence, EH Scott Radio Laboratories included a variable resistor on the selectivity-fidelity control to adjust bias in a manner that compensates for the variation in energy reaching the detector while adjusting the IF bandpass characteristic. As such, the volume of a tuned station remains nearly constant when adjusting the bandpass characteristic. What one should hear is a pure change in tone rather than a change in volume and tone. That is elegant circuit design!

Incidentally, the audio bandwidth varies between 1-kc and 16-kc. Marvin confused the low IF bandwidth with the high audio bandwidth.

Fun with the variable selectivity... Some night, with the AW-23 bandpass control set to the selective position, tune to a distant station located 10-kc off a strong local. Then slowly turn the bandpass control toward high fidelity and hear what happens.

Norman


Ok thought to re-post this quote from Norman as it is a good explanation of how the selectivity-fidelity control works.
Somewhere back on page 2 or so Dave Poland mentioned not to remove the selectivity-fidelity control shaft while trying to change out all of the rubber coated wire, good advice. Luckily the shaft doesn't need to be removed to accomplish this.
Attachment:
Band Width Control 1.jpg
Band Width Control 1.jpg [ 105.24 KiB | Viewed 159 times ]

Here is an image showing both the selectivity-fidelity control shaft and also the locations and connections on the schematic.
Again, the engineering on this set is great! Each of the highlighted leads on the schematic is attached to the control shaft at a terminal lug on each individual air trimmer (see image below).
Attachment:
Band Width Control 2.jpg
Band Width Control 2.jpg [ 67.74 KiB | Viewed 159 times ]

The arrow is pointing to the solder joint where air trimmer 6 is connected and the red circle shows an unused terminal connection point. 18 AWG test lead wire does fit through the chassis for these connections.

So today the plan is to get all of this test lead wire connected through the chassis to both the top side coils and pass it back through and down from the coils to the tube sockets as required. Wish me luck!

Cheers.

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Dec Thu 21, 2017 4:50 pm 
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Really appreciate you posting such great detail. Next best thing to actually working on one.

You wont need much luck, seems to me you are right on top of it now.

thanks Gary


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