Philco 37-650 Plate voltage

[Rick D]

Hello to all
I have been working on a Philco 37-650 ,all caps changed now but I
have a problem with the sound,more so after a few hours of operation.
It is like it has too much base it makes the speaker rattle on music
with bass in it,i checked the voltage on the 6F6 tubes and got 358 volts
on the plate,seems high to me so I checked the #52 resistor and found
it was 4.25K and should be 3.5K.Do you think this could be the problem
with the sound and should I change this resistor,looking for any help
to get this set to work proper,thanks so much Rick
Link to schematic https://www.nostalgiaair.org/pagesbymodel/219/M0013219.pdf
PS-I tried three different speakers on the set same thing

[Peter Heembrock]

Hi Rick,

358V is a bit high I think and it is close to the maximum ratings for the 6F6 tubes…have the 6F6 tubes been tested and if so are they weak?

Also a good idea to measure all of the dogbone type resistors in that set since they tend to drift up in value over time and can upset bias voltages etc. I am currently working on a 37-650 as well ( just started ).

Important note here is to also check the candohm resistor ( item number 58 ) and I would recommend replacing it with discrete power resistors since these also drift in value and do eventually fail.

The basic rule is that if the bias voltages are not correct then the set will never work properly no matter what you do.

Peter H

[Jean Marcotte]

Rick,

358V is way too high for that set. According to voltage data on page 7-67, the plate voltage is 250V. You shoud have 265V across filter cap 57. You probably have around 375V there. Don’t let power on too long on that set until you find the trouble. Check the DC resistance of you field coil. It should be 460 ohms. If it is significantly lower, it is your problem. If it is OK then check the resistance of the primary winding of the power transo. It should be 2 ohms. Anything significantly lower, then you have a partly shorted primary. You can also check filament voltage when warm. Anything above the normal 6.3V confirms this.

Jean

[Rick D]

Hello Jean
First of all thanks for your reply,I checked the voltage across #57 cap and got 377 volts and on the
field coil resistance I got 413 ohms and on the primary winding I got 3.5 ohms,and on the heater voltage
on start up 6.9 volts and then down to 6.8 volts.I have run this set several times for about three to
four hours at a time and find that after about two hours the problem starts,it,s ok when it is just
talk on the radio,but when any music comes on with some volume the bass seems to make the speaker
sound if it is rubbing or torn,like there was something leaning on the cone and vibrating,if you
turn it down it,s ok,I have tried three different speakers,good ones and it,s the same,I have felt
the power transformer and that is just a bit warm,and the field coil on the speaker is cool,any
ideas on what to try would be great,sounds like I have to get the plate voltage down some how,
only thing I don’t know how or what to do about it.I have put a lot of work into this set,all new
caps,and those black tubs aren,t much fun,thanks for any help all the very best Rick

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[Rick D]

Hello Again
One thing I found well checking was #52 resistor on the schematic is 4.25K,should
be 3.5K,do think this could be causing the problem,all the best Rick

[Jean Marcotte]

Rick, resistor 52 is only dropping voltage on the screen grid of one of the 6F6. It cannot by any means increase the voltage on the plate by over 40%. The problem lies in the power supply. Do a last check. Measure the voltage on the screen grid (pin 4) of one of the 6K7 (they are tied together) or across resistor 62 if it is easier. You should read 100V. Then we will see.

Jean

[Rick D]

Hello Jean
On the one 6K7 #4 pin I got 266 at the very start and in a second it dropped down
to 103 volts on the second 6K7 in the tuner section of the radio same thing but
at 104 volts,Jean I installed a new output transformer in this set,I was thinking
this might effect the set so I tried different caps on spots #51 & 48 ,I pulled
the two .003UF caps and tried several .01UF to .005UF,the .005UF is in the set now.
,I had the set running for about
1& 1/2 hours ,it worked fine up till then but the speaker noise started again.
I sprayed the volume control with de-oxit when I checked the 6K7 voltage,maybe 5 min. till
I started the set up again it sounded fine ,maybe the 5 min. gave it time
to cool ?,as I said even after 3 to 4 hours running the power transformer
is just a bit warm and the field coil is cool,I will let it run for
another two hours and see what happens,if you have something you think I
could try just let me know,thanks again all the very best Rick

[Jean Marcotte]

Rick,

I am totally lost now. I expected you to get around 140V on the screen grids but you have normal voltage there. The filaments voltage is a bit high but not as much as I expected. The screen grid voltage is taken before the field coil directly from the rectifier tube. If the voltage is normal there, how can it be 100 volts higher on the other side of the field coil ? Are you sure about that 358 volts reading on the plates ?

Jean

[Rick D]

Hi Jean
Sorry I had a problem logging on,I rechecked the voltage and it is the same,the set will run
for about an hour and a half befor the speaker acts up.I know I have to get the voltage down
but I don,t know how,I am not that good a schematics but I have to try,maybe the problem is
around the 8UF cap,I see that the center of the old can cap has no wire on it but the other
terminal still has a wire thats splits off one to the candum resistor and one that goes to
two resistors that go to the 6F6 tubes one to each one on pin #5,any help I would be very
thankful for,all the best Rick

[Steve Dow]

Hi Rick,
I have been reading this thread, and noticed the symptom takes about an hour to develop. It
seems to me distortion of the audio is occurring.
The grid bias for the 6F6 tubes is developed across resistor 58 (19 and 109 ohms). The other thing
about this, is the three filter capacitors. The polarity of connection should be checked,
as the + of 59 connects to the – of 59A and 57. Also the 19 ohm section of that
pair ( part 58) should be checked as well as the 109 ohm part. The mid tap
of this pair provides bias to the 6K5 audio amp, as well as a suppressor bias to the
1st RF amp. The AVC, and the operating point of the 6K5, is controlled too, both can
affect the sound.

The other thing that might help is to connect your meter between chassis ground and
the end of the 109 ohm section of part 58 that goes to yellow green wire on the power transformer.

While the radio is playing see if that voltage changes as the sound gets distorted. You
could also measure the midpoint of that resistor pair (part 58) as well.

Some causes could be gassy 6F6 tubes, or leaky capacitors to grids, but I think you already changed those.

That output stage in the radio depends on the screen of the top 6F6 providing drive for the grid
of the bottom 6F6. This makes resistor 52 quite critical.

.

[Rick D]

Hello
Sorry Steve I did not get back to you right away but I wanted to check the wiring
with the schematic.I found a wire going from the neg tab on the old not in use #57
8UF cap going to the bias resistor #58 on the 109 ohm side and this also had a wire
spliced in going to the same tab as the #52 resistor, this spot also has two resistors one to each
6F6 tube on pin #5.I could not find any reason for this to be here when looking
on the schematic,so I cut it and tried the radio,set worked fine and so I checked the
voltage on the 6F6 plate,it was around 250 volts,thought I had the problem
fixed,I covered the cut ends and laid the set down and tried again,would not work.
I put the wire back in place,the radio sounded like it did when I ran it for 4 hours,
so I checked the voltage,everything was the same except I had no filament
voltage at all.With no voltage on the filament the set still works and the
tubes light up,can this happen?,of course the sound is not that great,the
speaker rattles with any volume and bass.Do you think that it was the
transformer all this time and acted up after 3 hours of operation.
Is there any way I could put in a small transformer to take care of the
filament voltage and leave the original in to take care of the rest of
the radio,I don,t think I could change the whole unit and it has the
5Y4 tube socket built right into the top cover of the transformer.
Any thoughts because I think I have had it with this set,I have
put in so much time and I am getting no where,if I could find
a replacement chassis I would throw this one out the window.
Seems the more I do the worse it gets,all the best Rick

[Steve Dow]

The wire on the negative terminal of the not in use capacitor 57 is bad news because the insulating washer keeping it
from the chassis probably long ago deteriorated and if it shorted to ground, would kill the bias.

I’m not clear on the filament issue. If the tube are lighted there is voltage (6.3).

If the Philco transformer works for a while and then the 6.3 vanishes, the soldered splice
inside the transformer which joins the solid copper winding to the flexible wires which
exit the bottom may have loosened.

That project , in itself , (taking the transformer apart and dealing with old brittle
insulation) is major. If you get it wrong, the radio is trash. In that was the case.
use of another transformer would do, just add up the filament amps needed and some for the
pilot lights.

The phase of the 6.3 volt windings voltage should be what is with the existing now.
The radio uses hum bucking, and this might make a difference.

[Rick D]

Hello Steve
I removed that wire from the #57 cap,I did check the voltage on the filament again,got no reading tried both meters
same thing,but as I said the tubes light up?,the sound is not that great.I am thinking now that maybe the
filament part of the transformer was always on it,s way out,before it would run for almost two hours
befor it acted up.The high voltage on the 6F6 tube plates 375 volts,could that be because I put in a new
output transformer from hammond a 124 D,it is 152ohms with 75 ohms centre tap,when I look at the schematic
the original was 700 ohms with a centre tap of 370 and 330,just can,t figure why the voltage is so high.
On the 6k5 there is a 99K resistor in there to take it down to217 volts on the plate,they both come from
the same source.I understand Dan is a friend of yours,tell him Rick said hello,Steve thanks for your
help all the very best Rick

[Steve Dow]

The Highest voltage in the radio is on the filament of the 5Y4.
It is connected to higher voltage side of the field coil, at the terminal
strip on the speaker assembly.( Blue white tr wire.)

The center tap of the output transformer should be 315 volts to chassis.
This voltage is on the + of capacitor 57.

The lower voltage side of the field coil is connected to the green white tr wire
going to the speaker assembly. That connection is where the center tap of the output
transformer is connected. That goes to the + terminal of capacitor 57.

If the center tap of the output transformer is connected to
the blue white tr wire, the plates will get (based on some calculations)
about 40 volts more than normal. This will alter the operating points.

Look at the wiring at the speaker terminal strip.

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[Rick D]

Hello Steve
Thanks for the reply,I did check the wires and they are
on correct,I checked the voltage on the 6F6 plate again
it was 315 volts today,it has been as high as 358 volts.
I have check that candohm resistor item #58 19 and 109 ohms
many times even put the heat gun on it to see if it would
change,no way but I think as Peter said I should just
change it,the voltage going up one day and down the next
day from 315 volts to 358 volts,I don,t know for sure
but I think this resistor could do that,I don,t think
that the transformer would do that ,do you fellows,
Anyway guys thanks for all the input and help,
will let you know how I make out,all the best Rick

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