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?


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The volume range expander will only work correctly with orchestral and operatic performances where there are extended periods of loud and soft passages.  However, the eye tube should have some variation in degree of opening associated with the expander control position and should modulate according to the loudness of the program being reproduced.  The expander operates independently from the RF and IF amplifiers therefore RF and IF alignment may be conducted regardless of expander operation.



In the Philharmonic Technical document, there is a procedure for adjustment of the system. If you have the voltages right on the tube pins and the audio levels can be adjusted according to the document, then the system is working. The control should work in conjunction with the loudness so that you need a bit of volume to make the effect.

It was designed as mentioned for the crescendos of classical music with dynamics however music with a pronounced drum beat should be able to be adjusted so that the drums increase in level while the rest of the sound stays lower or the same.

If it's hard to tell, you can always plot output voltage as the volume is turned up with a test tone. Expander off will be somewhat linear as level is plotted against V.C. position. Expander on, the plot should curve upward.

My sensitivity controls work smoothly across it's range. You may have a spot where the wiper opens. It probably won't affect operation. For the tendency towards what seems to be oscillation, Tap & bump testing is important to be sure everything is tight, making good connection and non microphonic. Make sure all inner shields are fitting snug and outer shields also before evaluating. Good luck

Joe, on expander operation. When clicked on, action is progressive as you advance the expander control. Probably would use it advanced to half to 75% rather than fully advanced. Sounds like your pot has a problem at the low end.
At minimum, the feature almost seems to reduce the volume dynamic range. Near fully advanced, the room volume really ramps up dramatically on loud music crescendos. There is some noticeable lag on voice material - as when the announcer starts the percussive ad for the Chesterfield cigarettes. The attack of the first syllable of the work Chesterfield is softened but the 2nd syllable gets really loud. Hence, Normans's comment of the intended use for music. Music tends to have more stable volume level from second to second.
I used to use my Philly back when it was my hi-fi with a Fisher preamp and AR turntable for classical music. As with voice material, you can hear the lag effect on some kinds of rock & roll music. On short wave, the expander action lag at moderate setting can seem to soften some of the loud bursts of static.

I am making some progress.

Was able to get the sensitivity control apart and after cleaning and some adjusting of the wiper I was able to eliminate the dead spot.  Prior to putting it back, I did some checks and found that my set for the sensitivity circuit is wired like the schematic in the EH Scott book rather than Rider version.  There are quite a few differences.

I think the expander is working but for some reason, when the sensitivity hit its dead spot, it drove the expander eye tube wide open and it won't reset.  I'll trouble shoot that tomorrow and hopefully do the alignment. then too.


Are you saying the eye tube ONLY went to full display, or that the expander circuit went to full expanded volume and could not be scaled back to a more moderate action with the control?

Perhaps the sensitivity pot still has a problem, or the contact surface broke?

The sensitivity control setting will affect expander action, because advancing the sensitivity increases the signal (audio) strength presented by the 2nd detector to the audio stages ... and the expander circuit is part of the audio section ahead of the 3rd audio. Read the owner manual on the sensitivity, selectivity and the expander controls. A person using the volume expander is probably doing so on a high quality signal (local or strong regional) and would have dialed back the sensitivity for sake of silent tuning between strong stations, unless DXing.

Consider, by the mid 1930's, big cities like Chicago, New York ... had several of 50,000 watt regional stations in the 1930's putting out a pretty quality strong signal. And it was the several big cities like Chicago that had the experimental high fidelity stations just above 1500 KC. For these locals, a moderate sensitivity setting is more in keeping with a pleasing radio experience, unless DXing.

Yes indeed the sensitivity does affect the expander.  I'm going to reinstall the "fixed" sensitivity control later this morning to see how that improves the expander function.

Prior to fixing the sensitivity control, when I placed it on the dead spot (which maxed the sensitivity) the expander eye tube went full open when I turned the expander function on. Then, when I adjusted the expander control, the eye tube remained wide open even thought the voltage at Grid #3 of the expander push/pulls varied from -4 to -11 volts.  That was puzzling since the eye tube grid is connected to the same point and I would have expected its opening to vary,  I was using the 9 knob schematic though so this may not be correct.  I will pull out the Hobbs schematic for the 7 knob version to further troubleshoot. 

Should have an update later today



This is getting interesting.  First of all, I rechecked the normal tube voltages are all are in spec (at least the ones I could get to).  The sensitivity problem was resolved with the fix on the sensitivity control.  I took the expander control out, gave it a good cleaning, and checked all the wiring and it matches the schematic in the Hobb's book.  With a strong station tuned, I believe the VRE is working as I get increased gain when I turn the expander control about 75%. I also checked the bias voltages to the expander rectifier with the expander both off and on.  Voltages are -8V with expander off and -12V with expander on, both right in the middle of the specs shown on the Hobb's schematic. But I am getting weird eye tube action:

1.  Power up with expander off, eye tube is almost fully closed (I think that is correct position)

2. Turn expander "just on", eye tube tightly closed (again, I think that is correct position)

3.  Slowly rotate expander control on up to about 50% position, eye tube modulates slightly open and closed as I modulate control from 0% to ~50% (again, I think this is as it should be)

4.  Rotate expander control to full open, volume increases noticeably, eye position goes full open (this is as I expected it, however the eye tube appears to be stuck in the fully open position)  No matter what I do with the expander control, even turning it off, the eye tube remains fully open.

5.  Turn expander off, power down set, and power set back up, eye tube resets to step 1 above, slightly open, and the cycle will repeat.

6.  Rotation of the expander control does change the voltage to the #3 Grids indicating that the VRE function is working even when the eye tube is "stuck" open.  Interestingly, the grid of the eye tube and the #3 grids are directly connected.

So,  I am a bit puzzled.  It appears the 6E5 has a problem but I don't have a good spare right now.

Has anyone seen this problem before?



The first thing that comes to mind is the possibility of part of the expander drive ckt going into oscillation. You may be able to see it with a scope. Most of that system is bypassed by that capacitor pack with the 7 / 0.5 uf caps. Make sure that assembly measures correctly and is well grounded. If a test bypass with another 0.5 cap from eye tube grid point to ground changes things, you have a big clue. Good luck.

Hi Bill,
I did replace the 7 X .5 mf caps but will give that a try. I checked the voltages at the pin socket with the eye tube out and the grid pin voltage varies as it should. I wonder if an open 1 meg resistor in the eye tube socket would cause it to "lock" open.

I just noticed a low frequency oscillation that wasn't there when I had the chassis on its side when I worked on it. When I set it in its normal position the low frequency oscillation emerged. If I put a slight lateral stress on the bass choke can, the oscillation goes away. So I will be getting back into the can this weekend. When I recapped under that can I used .022 mf for all 4. I may need to add .05mf to two of them.


I tried several different 6E5s and they all behaved a bit differently.  So, I did what I should have done from the beginning and checked the 1Meg resistor. Sure enough it is open or at least intermittent.  I've replaced several of these in the past on other sets, but never on a Scott.  Is there a trick with the Scott eye tube sockets?

Also, I found that my low frequency oscillation appears when I turn up the bass.  When I recapped, I used the same value of caps for all 4 around the bass choke (.022 mf).  The schematic calls for 2 of them to be .02 and two to be .025.  I don't think that difference would cause any oscillations, but could be wrong.  Anyone ever experience this problem?  I will trouble shoot the bass in detail tomorrow.



No trick but all of those eye tube socket resistors drift high. Don't think it's the cause of the problem, but it does sound as though something is not right in the bass choke can. I lined the inside with Cambric for isolation.

The tech document shows how to measure the 60 Hz dip in the response. If the dip is at other than 60, you'll need to adjust those caps. Still shouldn't cause an oscillation. Loose wire or short somewhere more likely.

The 1 Meg resistor is definitely open. I have B+ on pin 4 of the socket but nothing on pin 2. Have B+ on both pins on the tuning eye.
What is Cambric?

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