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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Jan Wed 03, 2018 7:08 pm 
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The 0.05 mfd capacitors will bypass any stray RF currents in the filament circuit near the oscillator to ground but will not bypass a significant degree of 60-cycle filament current. It would be best to keep the cathode connection to ground as close to the tube socket as possible. Later chassis had a solder lug riveted to the chassis at the cathode lug. I have a vague recollection that Scott Radio Laboratories recommended that the cathode of the oscillator tube on early sets be grounded at the cathode lug of the tube socket to minimize oscillator problems.

Who's an electrical engineer? My professional engineering practice specializes in flood studies!

Norman

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Jan Wed 03, 2018 7:46 pm 
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Norman. Great info. I will just ground right next to the cathode. Strange that this little tidbit of information filtered to the last guy who worked on this but not the info about changing the .1 400V caps to .05 600V. I imagine that in 1995 when this fellow worked on the receive there was not the advantage of a robust data set easily available.

Flood studies - that must be interesting.

Point of engineering comment: Some of the concepts that experienced restorers and electrical tinkerers have as a fundamental, new restorers and tinkerers don't know.

For instance it wasn't until a studied up and read quite a bit that I understood some of the following:

AC can pass through a capacitor but DC cannot (unless it is leaking).
AC can induce across a transformer, coil, or choke but DC cannot. To pass a transformer the positive to negative swing of the AC is required to create a magnetic field.
Capacative reactance to frequency was a new one for me on this particular receiver.
The charges and functioning of vacuum tubes is fascinating all on its own. Rider's "Inside the Vacuum Tube" was a really good source on this.

It wasn't that I hadn't read these fundamentals before but more that I hadn't really thought about how they are put in practice in one of these receivers. This Scott is quite the education.

Cheers.

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Jan Thu 04, 2018 2:48 am 
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Thanks for the help and explanations.
Attachment:
Oscillator after re-work.jpg
Oscillator after re-work.jpg [ 103.89 KiB | Viewed 674 times ]


Cheers.

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Jan Thu 04, 2018 9:38 am 
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Oh! such neat work :) Going to be a show piece under there.

Gary


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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Jan Thu 04, 2018 5:25 pm 
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Radio Fixer wrote:
Oh! such neat work :) Going to be a show piece under there.

Gary


Thanks Gary, appreciate the encouragement.

The funny thing is that once I am done with this I am going to bolt a steel shielding plate over the entire bottom side.

Have been having a fever for the last couple of days yet my thermometer kept reading 95 F. I thought I might be turning into a vampire or something. My lovely wife brought home a new thermometer and was actually running 101 F.

Anyhow, going to keep the shop closed for the day and the ibuprofen coursing through my fever addled brain. Hopefully I will feel well enough to get a bit more done on this beast.

Cheers.

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Jan Thu 04, 2018 5:50 pm 
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Yep! Been there but I still think its worth it for the personal satisfaction.

"The funny thing is that once I am done with this I am going to bolt a steel shielding plate over the entire bottom side."

The plating on the tube bases looks superb still so that's a bonus.

Hope you feel better soon, missed it so far !

Gary


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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Jan Thu 04, 2018 11:38 pm 
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Temp under control.

So next question is about the sensitivity and selectivity control pots. I could pull them out of the receiver and clean them up IF it is possible to get into the pot itself. Are these serviceable and do they need servicing usually?
Attachment:
sensitivey - selectivity.jpg
sensitivey - selectivity.jpg [ 89.58 KiB | Viewed 618 times ]

Another option would be to wait until the system is energized and see if they actually need to be cleaned.

Thanks in advance

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Jan Thu 04, 2018 11:43 pm 
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You should be able to pop the cap off the sensitivity control for inspection and cleaning. Removing the cap from the potentiometer attached to the selectivity-fidelity control is another story. The shaft coupler will have to be moved to remove that cap. Based on my past experiences, the set screws on the shaft coupler will be very difficult to loosen. In addition to being very tight, they are coated with lacquer.

Norman

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Jan Thu 04, 2018 11:52 pm 
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Great Norman and thank you for all of the quick responses. I will try swabbing the coupler with lacquer thinner for a bit before attempting to break the threads loose. For re-assembly some removable thread locker should work.

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Jan Sat 06, 2018 7:30 pm 
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Update on the selectivity and sensitivity control pots. I could not remove either of them. For now I am going to continue on and test them once the receiver is energized. If they are noisy once I test them then I will have to figure out how to pull them out of the chassis.

The sensitivity control spins with the front nut. I used a penetrating oil to no avail. If I have to remove this later I will try to come up with something similar to a strap wrench like one would use on an oil filter. This might work well enough to keep the body of the control from spinning.

Norman was not kidding at all about the tightness of the screws holding the selectivity control shaft in. After dabbing quite a bit of lacquer thinner on the screws I was still not able to break them loose. I tried using a heat gun to warm the entire area but still no luck. The problem here is that the torque on the screws feels like enough torque to either bend the shaft or distort the couple on this assembly. Rather than damage anything I am deferring until later and only if necessary.

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Jan Sat 06, 2018 7:55 pm 
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Progress update.
Right next to the selectivity control is this resistor network.
Attachment:
Resistor network small.jpg
Resistor network small.jpg [ 92.65 KiB | Viewed 555 times ]

The two 500K resistors are on a different part of the schematic. Anyhow all of these resistors were between 40% and 80% high in value.

I clipped the resistors out and then used a solder sucker on each terminal followed by a solder wick. Once the solder was gone I pulled out the remaining resistor leads and cleaned the terminals with a toothbrush and denatured alcohol.
Attachment:
Resistor network terminals.jpg
Resistor network terminals.jpg [ 88.76 KiB | Viewed 555 times ]


Finally the resistor network was ready for re-install with new components.
Attachment:
Resistor network complete small.jpg
Resistor network complete small.jpg [ 77.07 KiB | Viewed 555 times ]


The molded resistors are sort of a mixed bag. I may end up making another 1.5K as that particular resistor didn't turn out that well. Will decide if it bothers me enough to make another soon.

Note that the 500K resistors have not been replaced yet. That leads me to the next piece of work...

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Jan Sat 06, 2018 8:10 pm 
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Next up RF diode choke assembly. This assembly sits directly next to the 6B7 tube socket. Here is the schematic snippet showing the choke (it is inside of the dashed line).
Attachment:
RF diode and 6B7.jpg
RF diode and 6B7.jpg [ 48.9 KiB | Viewed 554 times ]


I like to start out by making a connection map when possible for my notebook.
Attachment:
RF diode choke small.jpg
RF diode choke small.jpg [ 85.31 KiB | Viewed 554 times ]

All of these connections can certainly be figured out by just looking at the schematic but I like to be careful. If I get distracted and don't get back to working on the receiver for a couple of days the map helps. Another benefit is it seems to help me compare the schematic connections against what is actually there. The previous person who serviced this unit may or may not have connected things correctly and I like to verify.

Here is a photo of the underside of the RF diode choke assembly.
Attachment:
RF diode choke underside small.jpg
RF diode choke underside small.jpg [ 94.44 KiB | Viewed 554 times ]


Right. So that is the current state of things. I will certainly post some more photos once the re-work of this area is complete.

Cheers.

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Jan Sun 07, 2018 10:06 am 
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Location: Nr London, England, SS1 3PT
Yes! with you on working on a layout diag as well as a schema. Can pick up changes as well.

Must say there is a lot to the chassis but you seem to have plenty of room to work without much layering. Thought of starting on this Philips 753A motor tuned but its gone back in the waiting pile :) So much is hard to get at and certainly some of the switch wafers (note each held by its own bracket with no studding running through. Its a very solid way of mounting. The mid wafer, at least, to has to come out as the band-pass coil has the usual 240 and 480 Ohm open windings. The whole of that coil is bypassed at the moment but I couldn't leave it like that.

In the whole chassis pic there is a switch from both sides. One is waveband and the other is bandwidth, p/u etc. The left one is waveband and note how the switch wafers sit right above there appropriate coil.

It does work sort of work and the motor part is clean and perfect.


Attachment:
Close up WEB.jpg
Close up WEB.jpg [ 135.25 KiB | Viewed 516 times ]

Attachment:
Bottom view WEB.jpg
Bottom view WEB.jpg [ 169.9 KiB | Viewed 516 times ]


Just posted for comparison not to steal your thread.

Gary


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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Jan Sun 07, 2018 6:18 pm 
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Radio Fixer wrote:
Yes! with you on working on a layout diag as well as a schema. Can pick up changes as well....
...Must say there is a lot to the chassis but you seem to have plenty of room to work without much layering.

Absolutely. There are a bunch of items that are not the same as the original schematic. I opted to replace with what was actually installed in the chassis. You are right about there being plenty of room. As I said earlier somewhere this chassis design is the product of a group of organized minds.

Radio Fixer wrote:
Thought of starting on this Philips 753A motor tuned but its gone back in the waiting pile :) So much is hard to get at and certainly some of the switch wafers (note each held by its own bracket with no studding running through. Its a very solid way of mounting. The mid wafer, at least, to has to come out as the band-pass coil has the usual 240 and 480 Ohm open windings. The whole of that coil is bypassed at the moment but I couldn't leave it like that.

In the whole chassis pic there is a switch from both sides. One is waveband and the other is bandwidth, p/u etc. The left one is waveband and note how the switch wafers sit right above there appropriate coil.
Gary


That is a mess. Interesting design. Are you going to try and rewind the coils?

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Jan Sun 07, 2018 6:32 pm 
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Attachment:
RF diode choke underside complete.jpg
RF diode choke underside complete.jpg [ 91.97 KiB | Viewed 492 times ]

Here is the RF diode choke underside after completion. There is a small domino capacitor in the bottom here that should be 75 pF. Mine measured 70 pF so I left it in there. In order to measure the capacitance one of the legs of the cap has to be lifted.

Attachment:
RF diode choke installed.jpg
RF diode choke installed.jpg [ 75.67 KiB | Viewed 492 times ]

RF diode choke re-installed. This part was a pain in the butt to fit correctly but I did eventually manage to get all of the leads just so.

While working on the RF diode choke it makes sense to sort out the 6A7 converter as well.
Attachment:
6A7 - 6B7 schem.jpg
6A7 - 6B7 schem.jpg [ 70.82 KiB | Viewed 492 times ]

So there is the schematic snippet for both tubes.

Attachment:
6A7 - 6B7 sockets.jpg
6A7 - 6B7 sockets.jpg [ 76.07 KiB | Viewed 492 times ]


So that leaves the RF choke and the RF tube socket to handle. Hopefully later today.

Cheers.

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Jan Sun 07, 2018 7:04 pm 
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Its all very neat and I like the way the caps are supported through the can.

No! my coil winder cant do wave winding but I know an expert who does them. The pic is one, same function and high Ohms, very fine wire, from another Philips. He had to do all 3 windings, includlung the tuned one in the middle, as the coil winder works from left to right and that way he gets the spacing right. Everything was pretty spot on and it trimmed up with just a slight change to a padder. The coils were built up with quick drying lacquer to stop them collapsing. Bet it takes experience to get so correct.

Asked him how much did I owe him and he said £30 so passed him a couple of 20's and walked away with the coil very happy.

Attachment:
Bandpass coil rewound  WEB.jpg
Bandpass coil rewound WEB.jpg [ 44.76 KiB | Viewed 490 times ]


Gary


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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Jan Mon 08, 2018 5:33 am 
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Great price for the work.

Next up RF choke and RF socket
Attachment:
RF and choke.jpg
RF and choke.jpg [ 58.57 KiB | Viewed 464 times ]


Here is the choke in place
Attachment:
RF choke connections.jpg
RF choke connections.jpg [ 50.35 KiB | Viewed 464 times ]

Finally something fairly simple to deal with.

After removing the choke here is what the underside looks like.
Attachment:
RF choke underside.jpg
RF choke underside.jpg [ 67.53 KiB | Viewed 464 times ]


After re-work
Attachment:
RF choke underside after re-work.jpg
RF choke underside after re-work.jpg [ 69.92 KiB | Viewed 464 times ]


The Mica capacitor here tested at 96 pF and was supposed to read 100 pF so I left it in place.
Attachment:
RF choke installed.jpg
RF choke installed.jpg [ 73.27 KiB | Viewed 464 times ]


So there you have it. Those are the 3 diode assemblies. While I was working on them I put together a notebook page like so.
Attachment:
Diode assemblies.jpg
Diode assemblies.jpg [ 116.34 KiB | Viewed 464 times ]


There are a few details left to deal with. Mostly final cleanup. I will need to pull out all of the tubes and test them pretty soon.

Cheers

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Jan Wed 10, 2018 5:18 am 
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Hi all,

Top side progress.
Attachment:
Top side IF re-assembled.jpg
Top side IF re-assembled.jpg [ 106.75 KiB | Viewed 414 times ]


Putting this receiver back together is a real pleasure, great build quality!

The leads off of the 4 IF coils are a bit of a trick. the lossless wire needs to be fed through the IF cover eyelet. The thinner 20 gauge wire fits but the 18 gauge will not fit through the eyelet.

I used really long leads off of the grid caps, maybe 10 inch. There is plenty of room inside of the IF cover can to coil the wire. The lead length allows for the can to be off to one side of the receiver while inspecting the IF coil.

The 4th IF 6D6 was substituted with a 77 tube. All of the other tubes were in great shape. I am using a Hickock 600a tester and the tubes all tested stronger than required.

Cheers

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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Jan Wed 10, 2018 10:19 am 
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Location: Nr London, England, SS1 3PT
Um! Looking very pretty and an end in sight. I'll get that Philips chassis boxed up for sending over to you, you are now the Man :D

Not sure about the 10" grid caps? Wouldn't have done that myself. Thought the idea of the cap at the top was to keep the grid lead away from all the other connections down in the base. See if Norman thinks that's OK.

Gary


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 Post subject: Re: EH Scott Imperial High Fidelity Receiver (new acquisitio
PostPosted: Jan Wed 10, 2018 1:23 pm 
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Wow, it is a thing of beauty! :D

I was surprised to see those two neon tubes in the lower right.
Granted, I am unfamiliar with the Scott AW 23, so I'm learning a lot about it in your thread.
I suppose they are for regulation of the oscillator voltage.
Pretty fascinating stuff! Also wonderful work you are doing. Just a 1st class restoration! :D

-Steve

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