EH Scott Radio Enthusiasts

The Fine Things are Always Hand Made

I've recapped my dial pointer Philly and it plays good on all bands, but I have a few nagging issues that I would like to take care of before I start the alignment.  I want to make sure these are real problems and not "operator error". 

1.  When I rotate the sensitivity control I get increasing gain as expected, but there is a very narrow spot just off the CCW position that increases gain to the  max.  I've shot the pot with cleaner but no change.  I assume this is not normal, correct?

2.  When I turn the expander control on, the expander eye tube opens up indicating maximum expansion but I can't hear any difference.  Once I turn the expander on, the eye tube remains open no matter what other actions I take (tune off station, rotate the expander pot, rotate the selectivity control, etc.)  I've rechecked the expander wiring and all looks fine.  How is this feature supposed to work and is there a good way to test it?

3.  Every once in a while, the set seems to almost want to go into an oscillation, but it doesn't.  Not sure what this means.

Tomorrow I am going to recheck all supply and tube voltages.  Any other ideas on problems 1-3 above?

Thanks,
Joe

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Joe,

Cambric is a varnished type of cloth used for electrical insulation and where high puncture resistance is needed. I buy sheets of it and it always comes in handy. Also look up "fish paper", they both can be used in tight places where you don't want the possibility of a short. Of course, tape also would work in a spot like the bass choke can but electrical tape doesn't offer much protection and doesn't like hot locations.

BTW, I always recommend measuring every resistor in any set I work on. Takes a little longer but saves time and aggravation in the long run.

I think I found my feedback problem.  The bass control is a dual pot, one at 1 Meg (it's okay) and the other at 10K (its open).  I disassembled the 10K pot, cleaned it but no luck.  So I need a replacement.  Anyone know where I can get one? 

I have the eye tube socket apart and will replace the open 1 meg later today. Once I get a replacement Bass control I'll do the alignment and I should be good to go.  (Just received the replacement dial glass from Radiodaze yesterday.)

Hopefully someone has a tip on finding a replacement bass control.  See picture.

Joe

Attachments:

If I can't find a replacement pot I may just use a fixed 5K resistor to ground.  Not sure varying a 10K in series with a 100K resistor makes much of difference anyway.  Thoughts?

Mark Oppat is the go-to guy for replacement pots.

VRE eye tube works fine now (it was the bad 1 Meg resistor)

VRE system works

I reinstalled the Bass control with the 10K section replaced with a 3.9 K to ground.  No more tendency to oscillate, but I have a new problem--loss of volume which I had plenty of before.  Also, I hear no difference in bass as I adjust the bass control.  Does the bass boost design normally provide good bass? 

I am getting good signal strength through the IFs (the tuning eye snaps shut), but the volume is low even with volume control full open.  I don't think my bass mod should have cost me volume.  I'll get out my scope this week and see where I am losing audio gain.

Any thoughts on the bass--is it normally good or fair on the Philly?

I'll get with Mark O on a replacement pot.

Thanks

Joe

Joe,

Both sections of the control contribute to the bass level. Even with the 10K section jumped, you should have some change. I suspect that you might have a wire loose somewhere from the control to the bass choke can.

Good luck.

Cambric is a woven fabric that can be used to electrically insulate or at least separate. Not sure what it was made from. Withstands some heat. Has some thickness. Might be used to separate layers of transformer windings or to line a box.

Well, I am back in business.  Have full volume now.  After much searching I found two caps touching (these are ones I installed and had moved around as I was troubleshooting).  All good now except my low growl has come back when I turn the bass control up.  I have the 10K pot replaced with a fixed 3.9K resistor on the bass control but I don't think that is the source of this problem.

Can anyone explain how this bass control system works?  (the bass chokes and bass control pots).  If I can figure that out maybe I can find the source of the growl.

BTW, my set has a number of resistors that are different from Scott's newly drawn schematic and also different from the schematic in the Hobbs book.  They all look to be original resistors so I didn't change them out.  I wonder if any of them represent changes to get rid of the bass growl that I have?

I am currently going thru a unmolested dial pointer like yours noting all the changes from the Scott published schematic and making an "as built" drawing.  These Scott schematics suck, but they were a good starting point.  If you need me to verify anything, let me know.  I expect my "as built" drawing to be done in a couple of weeks.   Maybe posting a YouTube video so we can hear the "growl" may help. 

Scott,

Great idea, I'll see if I can figure out how to upload a video from my Samsung S4 to youtube to give everyone an idea of the sound.  I'm still trying to figure out how this bass control circuit actually works.  I've yet to get any bass improvement out of it.

BTW, while I was troublshooting my loss of volume I found a number of resistors that differed from your most recent schematic.  In some cases they matched what is shown on the schematic in Hobbs, but in some cases they were different from both your schematic and Hobb's.  I wonder how many wiring versions there really are?  I think my set is one of the earlier versions.  Maybe some of these differences are production changes to fix operating problems?

Your schematics have been very helpful during this troubleshooting effort. I'll send my markup when I get done so you can compare to the as-built your are creating.

Scott,

If you have time would you check your bass control for me?  I need to verify how both pots (1Meg and the 10K) are wired in the circuit.  As you rotate the bass control from off to full on (clockwise) is resistance increasing or decreasing for each pot as wired in the circuit?

My 10K pot was bad and now I'm not sure which way it was wired in.

I'm still trying to figure out how the bass circuit works.  Maybe Norman knows?

Thanks

Joe

Joe,

Both sections increase in resistance as the bass is increased (clockwise).

With the 1 meg section at minimum, all frequencies are shunted equally to audio (AC) ground. As the 1 meg increases, the bass choke adds impedance to the circuit at low frequencies, increasing the bass level while the mids and highs are still shunted by the overall "C" value. While this is happening, the nominal midrange level actually drops a bit. This is compensated by the 10K section which relaxes it's attenuation on the 2nd audio grid (slightly) and normalizes the effect.

Hope that helps

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