Problems starting up a dead Scott Philharmonic

[Ken Carr]

I am fishig for a few suggestions as how to proceed with my attempt to revive a Scott Philharmonic.

My photo section here on the group shows photographs that may be helpful.

 

Background:
Rather than just plugging in the set as found, I elected to replace all of the capacitors in the power supply amplifier first and then try to slowly bring it up before moving on to the receiver.I also tested all power supply tubes and they all came back as in fine shape.

I did the power up using a variac and an inline light bulb. All components were connected (P.S., Receiver, speaker). The result was that the light bulb glowed (brighter as I increased power) and the radio showed no sign of life, not even glowing filaments in the power supply.

Next I attempted to get the resistance for the input side of the power transformer. It came back as zero ohms. I did not disconnect the transformer leads when doing this. I also did a check on the filament leads from the rectifiers (power off) and got the same result at the output side. Of course I may be measuring something other than the P.S.

I then tried a few different hookups:

-All tubes in place, connect receiver to power supply only by the 4-prong on/off switch. I went up as high as about 50% voltage with no result.

- I the removed both 5U4 rectifier tubes and powered up again. The light bulb glowed but much weaker. Also the filaments lit on the four amplifier tubes.

- Then I removed both rectifiers and connected the 4-prong on/off switch AND the chassis plug to the power supply. Nothing came alive including the amplifier tubes. Again current was making the light bulb glow, a little brighter than in the last test.

- Finally I put the rectifier tubes back in place and made all connections (chassis and switch) except the speaker (did plug in a headphone set). Nothing happened except lighting up that bulb again.

 

Also, the upper left section of the receiver chassis (where the chassis plug exits) has a large 2,000 ohm power resistor gray in attached photo) hanging and attached to nothing. There are a couple of wires there that are attached to big solder blobs that are not anchored anywhere. Something was 'repaired'. If anyone has a good photograph or two of this area so I could see the original wiring, that would be helpful.

Thanks.

[Norman S Braithwaite]

How many watts is the light bulb?  Given the high current requirement of the Philharmonic, I would not be surprised if a 200-watt light bulb would glow at full line voltage.

Norman

[Ken Carr]

I was using a 40 watt bulb. I just tried it again with a 60 w and a 72 w. The results were about the same. I don't have any 100 w bulbs on hand. Maybe I should upgrade my series light tester by putting in a few more sockets?

Does it sound like I might have a shorted power supply?
 
Norman S Braithwaite said:

How many watts is the light bulb?  Given the high current requirement of the Philharmonic, I would not be surprised if a 200-watt light bulb would glow at full line voltage.

Norman

 

[Norman S Braithwaite]

I don't think that the power transformer has a short but I can't tell for sure yet.  You can test the transformer by removing all tubes in the power amplifier, disconnecting the tuner chassis except for the four pin plug, and turning the set on with a low power light bulb in series with the AC power line.  If the bulb does not illuminate then the transformer does not have a short but if it lights to full brightness then the transformer is likely to have a short.  The problem with using a series light bulb when firing up the full chassis is that the light bulb must have a power rating much greater than the full chassis before the chassis will come to life.  Except for non-linear load characteristics of the chassis, if the light bulb rating is equal to the chassis rating then the AC line voltage will be divided equally between the light bulb and the chassis (60-volts on the light bulb and 60-volts on the chassis).  The chassis will barely operate with 60-volts.  Again, except for non-linear load characteristics of the chassis, if the light bulb rating were 2-times that of the chassis, the light bulb would see 40-volts and the chassis would see 80-volts.  At 80-volts the chassis may start to come alive.

Norman

[Ken Carr]

Norman,

I put in the 40w bulb and pulled all P.S. tubes and disconnected the receiver.

The bulb did not light at all until I got up to 75% line voltage.

With a 60w bulb it did not light until 85% line voltage.

 

In each case it was that dim orange glow. Progress?
 
Norman S Braithwaite said:

I don't think that the power transformer has a short but I can't tell for sure yet.  You can test the transformer by removing all tubes in the power amplifier, disconnecting the tuner chassis except for the four pin plug, and turning the set on with a low power light bulb in series with the AC power line.  If the bulb does not illuminate then the transformer does not have a short but if it lights to full brightness then the transformer is likely to have a short.  The problem with using a series light bulb when firing up the full chassis is that the light bulb must have a power rating much greater than the full chassis before the chassis will come to life.  Except for non-linear load characteristics of the chassis, if the light bulb rating is equal to the chassis rating then the AC line voltage will be divided equally between the light bulb and the chassis (60-volts on the light bulb and 60-volts on the chassis).  The chassis will barely operate with 60-volts.  Again, except for non-linear load characteristics of the chassis, if the light bulb rating were 2-times that of the chassis, the light bulb would see 40-volts and the chassis would see 80-volts.  At 80-volts the chassis may start to come alive.

Norman

[David C. Poland]

Hi Ken. ..... Looks promising.
- The resistance of each of the primary and secondary windings of the power transformer is too low to measure with a meter.
- Have you checked AC voltages of the power transformer without load? pull rectifiers, disconnect the receiver. See if you get about 300 more or less volts AC each half of high voltage winding TO THE CENTER TAP. Careful not to just put meter end to end of the high voltage winding because you will see 800 or so volts AC if your meter can handle that much AC voltage. BE CAREFUL.
- With the 5U4 rectifiers pulled out, but receiver cable plugged in to amp - the receivers tube filaments and dial lights should come on, and if so, indicates the power transformer primary is ok and the 6 volt secondary is OK. You already said the 5U4 filaments glow in an earlier test. indicating the 5 volt secondary is OK. Only with the 5U4's installed will you get high voltage DC..

- To power up the entire radio - the speaker must be plugged in because the speaker field coil is part of the high voltage circuit.
- I assume you have reasonable resistance readings of the speaker field coil (it has a tap) and of the output transformer primary in the speaker.
- The purpose of the light bulb tester is protecting the radio and avoiding fireworks (or wasting the radio's fuse) if the power transformer is shorted.
- As Norm states, for a high tube count radio, a 40 watts bulb is way too restrictive - 200 or 300 watts bulb is more realistic for a Philharmonic.
- As for your light bulb tester set up. Get a screw in adapter in place of the light bulb so you can plug in a big floor lamp having several lights. Be sure the floor lamp is "off" ahead of time. The more light bulbs "on" in the lamp the lower the resistance ( because the light bulb filaments are in parallel) and so the higher the voltage seen by the radio. I use one of my vintage floor lamps with 3 candelabras and the 3 way mogul bulb as a way to progressively present more and more AC voltage to the receiver's power transformer until I am confident the transformer can handle 120 volts AC. I once had one that shorted at about 50 volts AC. After passing this test sucessfully, I just use a Variac as restoration progresses..
- With an AC meter, monitor the AC voltage on the power transformer primary. If things seem OK, start switching on the floor lamp 3 way switch one by one.. Each time, note any dial light glow, filaments start to glow brighter as you progress to the floor lamp being full "on". Also have a DC meter somewhere in the high voltage circuit AFTER the speaker field coil, to veriify the field coil is good. The high voltage B+ reading action takes a few seconds to start building, and then builds high to a peak as the filter caps charge, and then voltage declines somewhat after a peak. If all goes well, at near full voltage seen by the radio, the voltage regulator VR-150 should fire up with a purple glow.

- And the radio needs an antenna - 10 feet or so is enough. Turn the dial and try for a station or static , before you advance the floor lamp to the next setting.

[Ken Carr]

Norman and David, thank you both for the suggestions.

I did purchase a screw in adapter for the light bulb tester. It has receptacles on it too. This allowed me to plug in a second lamp with about a 60w bulb.

With all tubes in their place on the power supply / amp  (PS) I was able to apply about 95% power with the variac before the bulbs lit. Also, all 6 tube filaments lit up (the two 5U4 rectifiers and the four 6L6 amplifiers).

I have an old candelabra lamp (it has ten 40 watt bulbs) that I am going to wire with a plug on the end. Then I can plug it in the light bulb tester and screw in bulbs, one at a time to increase voltage. I will also make the measurements that David suggested.

Please be assured that I will be back with more questions. I appreciate both of you sticking with me. Thanks again.

I have attached a photo of the testing setup. The gooseneck lamp and the bulb in socket working together allowed me to get power to all of the PS.

 

[David C. Poland]

KEN

Do NOT power up the amp with rectifiers inserted unless there is also a proper load - namely the speaker and receiver are both plugged into the amp.
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Without a load, the plate voltage will peak higher, possibly high enough to damage the power transformer or exceed the max of the first filter condenser which is ahead of the speaker field coil. That is why earlier suggestions stated the 5U4's be pull out when doing preliminary amp tests for gross shorts and to verify the 6 and 5 volt windings were Ok. And why the speaker and receiver be plugged into the amp when the 5U4's are inserted. Remember - the power transformer wire is over 70 years old and the insulation coatings on the windings is not as robust now.

[Ken Carr]

David- Very well, thanks for the caution.

[Ken Carr]

David - I have not checked the power supply voltages yet since I am not sure of the identification of each terminal (obviously I'm a novice at this).

I did apply a 9-volt battery to the speaker voice coil and was able to get static. It appears that is fine. I am not sure which speaker terminals go to the field coil.

I applied partial power gradually with up to 3 light bulbs (rectifiers pulled) and was able to get the receiver dial lamps and and tube filaments to glow dully.

Once I work out transformer and speaker terminals I will do some voltage tests. So far I don't believe I have released any smoke from the set.

 

[David C. Poland]

Ken, with the circuit diagram ( Scott Documents site) you should be able to trace the speaker cable wires to the plug pins. You can check voltages from the terminal strip under the terminal strip cover on the speaker. Diagrams show the prong side of the cable plug.

Suggest you color code the amp diagram. Use color pencils. For example, red for the high voltage from the plates of the 5U4's, to both chokes (look like transformers but have only one winding). continue with red from the one choke onward through the speaker field coil and back to the amp past the filter cap and on to the receiver plug, then continue on the receiver diagram as far as the voltage divider. From the other choke pick, say, orange, onward to the speaker output transformer center tap through the transformer and back to the amp to the plates of the 6L6's. From the field coil center tap, pick, say, yellow and follow it.

Have you acquired a tube manual yet? any of the late 1940's or 1950's manuals would have most of the radio tubes you may encounter.

[Ken Carr]

David, thanks for the tips. I do have tube manuals and have used them to look up voltages. Eventually I may find a good original Rider's since most of the reproduced drawings are not clear enough to read the small print easily.

The voltage checks from the speaker terminals should be easy. I already did resistance checks and they came out exactly as printed on the terminals for the voice coil and field coil.