Low voltage in Philharmonic power supply

[Troy Taylor]

Hello-

I am in the midst of restoring a pointer dial Philharmonic that I bought this past summer from a forum member.  It is in great condition and had had very little previous work done to it, so there were no unpleasant surprises or shoddy workmanship to deal with.

All the caps have been replaced now, including the 'hidden' ones, and the set is now playing (with the occasional hiccup I still need to find and remedy) and sounds quite good!  But there are a couple of things that I think are not right.

Most noticeably, the 10H choke gets warmer than both the 30H choke and the power transformer.  The 10H is not ever too hot to touch, but it gets hot enough that you cannot leave your thumb on it indefinitely, which you can easily do with the other choke and power transformer.  This heating up is gradual and takes more than an hour of continuous operation to get that hot.

I went through the voltage checks for the power supply listed on pages 14-48 and 14-49 in RIDERS and found that all of the measured voltages are quite low.  Please note that test instructions include 110VAC supply with the HI-LOW switch DOWN, and I used a variac to ensure that the input voltage was as stipulated.  These are the results:

TEST 1: Correct voltage = 430VDC.  Measured = 330VDC

TEST 2: Correct voltage = 410VDC, Measured = 286VDC

TEST 3: Correct voltage = 375VDC, Measured = 300VDC

TEST 4: somehow I neglected to measure this one

TEST 5: Correct voltage = 125VDC, Measured = 101VDC

TEST 6: Correct voltage = -28VDC, Measured = -23.7VDC

TEST 7: Correct voltage = -3VDC, Measured = -4.0VDC

TEST 8: Correct voltage = 300VDC, Measured = 244VDC

The filament voltages measure correct. 

The 390-0-390 AC center tap transformer measures 331-0-331

So my main questions are:

Could there be something wrong with the power supply circuit that is dragging all these voltages down?  Or might I have a substitute transformer with a lower nominal voltage?  It certainly looks original and operates cool to the touch, so if it is actually not the right transformer it was otherwise well adapted to fit and handle the current requirements.

Would these low voltages somehow contribute to the 10H choke getting hotter than the other components?  Or could a faulty choke cause these voltage problems?

Any suggestions or advice would be much appreciated!

Thank you and best regards-

Troy Taylor
Edmonds, WA

[mike hadley]

Troy,

The 10H choke is in the supply to the 6L6 output tubes, the 30H choke runs the radio through the field coil,

are your 6L6 tubes ok, have you checked the values of the biasing resistors for the tubes?

check the audio output transformer for possible problems, winding resistance and insulation etc.

You didn't say if you changed the caps in the power supply?

Just as a measure of worst case, my Philharmonic is running on 50hz as I'm in the uk,

the chassis has it's original transformer,

all of the smoothing the caps are polypropylene, and the resistors are new.

After about four hours of playing at reasonable volume the power transformer was about 100f, both chokes were cool to the touch

[Scott Seickel]

What Mike said.  I would also look at the 100uf/50v capacitor and even if it is new, maybe swap a new one.  Do you have a different pedestal speaker that you can try?  It is possible to have shorted turns in the output transformer which is in the speaker.  Check all the 6L6's and swap them if you have extras.  

If you have the chrome covers on the power transformer and it looks like an original, then it is.  This is not a power transformer that was available in the aftermarket. 

[Karl Sherwin]

Troy, I agree with Mike's suggestions, especially the 6L6 bias.

BTW, is that Riders available for download? I'll be working on my Philly soon and would like to have as much service info as possible.

[Karl Sherwin]

Silly me, I found the Riders info on the Radio & Broadcast library...

[mike hadley]

Karl

Look at the top of this page, Scott info and archive, see Philharmonic folder, lots of information there

[Karl Sherwin]

Thanks Mike, I had that info from this site, but didn't have the entire Riders Vol 14 info, now I do!

[David C. Poland]

Hi ,Troy,

I powered up my pointer Philharmonic about 75 minutes ago. The power transformer is mildly warm to touch on top.

Both amp's chokes are mildly warm. Nowhere near hot, just warm, no problem leaving a finger on either 10 seconds or more.

After checking the suggestions above, and if you think you have a defective 10 Henry choke, let me know.

[Troy Taylor]

Hello-

Thank you all for the suggestions and advice.  I went through the power supply/amp chassis again and checked everything.  The biggest problem I discovered was the 10H and 30H chokes were miswired opposite of the schematic.  So the 10H was supplying the receiver chassis, and the 30H was supplying the 6L6 output tubes.  I do not know if I made this mistake, or if it was made by the last guy who worked on this chassis.

When I got it, the 30uF, 30uF, 40uF, and 60uF caps had all been replaced with four 40uF caps.  I replaced these with the correct values, but I do not know if I made the wiring mistake at this time, or if it was already there and I didn't catch it.  Regardless, I corrected it, and now the chokes both operate at low enough temperatures that, even after several hours of operation, I can comfortably hold my thumb on both of them.

All the capacitors are new and I could not find any problems with any of them.  All of the resistors measured within 10%, but I replaced all four of the 150 ohms anyway, since two were right at +10% and the other two were a -10%.  I used 1% replacements, so now they are all very close together.

The voltages still measure low after these changes; minimal difference from what I posted earlier.  Not sure where else to look for a potential problem, or if these lower voltages even _are_ a problem.  The set is working great, playing Super Bowl LV right now.

Is anyone else listening to the game on a Scott?

Thank you all and best regards-

Troy Taylor

Edmonds, WA

[Troy Taylor]

I should also mention that the power transformer is running about 118F; noticeably warmer than either of the chokes, which are both lukewarm at best.  Set has been operating continuously for over 3 hours now.

The 6L6 tubes test "OK", but are near the marginal line on my Jackson 648.  They all test very nearly alike.  I have not measured the resistances in the transformers yet.

-Troy

[David C. Poland]

Troy - Would appreciate a voltage reading on the 6J7 plate (pin 3) -  front far left, 2nd back, Scratch Suppressor.

See if it is somewhere 160 volts? 

[mike hadley]

Hi Troy,

If there was a problem with the power transformer you would know about it by now, if there is a shorted turn it's usually followed by smoke, dont't forget that the 6L6 tubes are running in class A and will be pulling a fair bit of current, plus there's 53 watts of heater's and 30 watts for the rectifiers, then there is the dropper dissipating 12 watts, and that's without the radio itself pulling current.

After 3 hours your power transformer is ok, I would try a new set of 6L6's and go from there.

[Troy Taylor]

Hello-

As I continue to listen to this wonderful old set, I have a new question.  How do verify that the output tubes in the amp are properly biased?  And, if they are not, how do I correct the problem?  I am presently using a matched set of 5932s; a ruggedized 6L6 built by Sylvania.

Listening to this set now as I type.  Sounds great and I love it.  Will be even better when I get the cabinet built.

Thank you!

Troy

Edmonds, WA