[Joeztech]

The original Philco audio output transformer in this set was replaced with a Stancor A-3823. This transformer has a center tapped primary and two plate leads for the output tubes. With leads disconnected from the tube sockets and the spike suppression capacitors, I measure from CT to one plate lead 240 ohms. From CT to the other plate lead I measure 450 ohms. That is hardly balanced and it appears that this transformer has problems. Also on the secondary only two connections out of 6 possible taps are used. There are secondary taps labeled from 1 to 6 and the no. 2 tap is grounded to the chassis. The no. 5 tap is connected to the lead going to the speaker voice coil. According to the schematic provided by Philco, there is supposed to be a third secondary tap that connects back to the volume control low end and the center contact of the tone control. This is apparently part of a feedback circuit. In this set this lead to the tone control and the volume control was simply grounded when the transformer was replaced, so whatever feedback that Philco intended has been defeated.

Has anyone out there seen this happen on another 40-195, 40-200 or 40-201?

I have no information on the Stancor transformer to get any idea how the secondary should really be connected, but I suspect that tap no. 1 should have been grounded and tap 2 should have gone to the feedback through the volume and tone controls.

It appears I will need to obtain a new replacement transformer since the two primary circuits are not balanced. One side must have some shorted turns.

Is there a recommended replacement for the output transformer that is currently available? If not, then I will have to do more research before ordering a replacement.

Joe

[morzh]

Joe

I only can tell you that if the centertapped winding's halves resistances are not order of magnitude different in resistance, this is normal as the resistance is not your parameter, the unductance is. Your numbers of turns in both halves should be equal but one being on top and another under it, the sheer length of teh wire for the outer half is larger hence the difference in resistance.

[Joeztech]

morzh;

Well, I compared the differences against the H-K A300 which uses 7408/6V6GTs in a similar Williamson push-pull circuit and although the windings do differ, they are only different by about 7%. In this unit if you take an average of the two resistances and compare the two readings one is 1300% different and the other is 69% different. Unless there is something else going on I can't see why there would be that much different. Maybe 10 or 15%, but 1300% or 69%? It just does not seem reasonable. I might connect it to my Heathkit impedance bridge and see what I measure there using a 1kHz tone.

I looked at my calculations on the Stancor and realized that I did the math wrong. However the last portion of the winding is about 87% higher resistance than the first portion of the winding.

Joe

[Ron Ramirez]

Allow me to address the feedback tap issue...

Another collector came up with a way to get around the feedback tap problem. It involves connecting a 1/2 watt resistor to the high side of the output transformer secondary; the other end of this resistor connects into the volume/tone control circuit as the original tap did. You will have to experiment with different values for this resistor to find the "sweet spot" for your set; it will vary, depending upon the individual radio, between 22 and 100 ohms. Some work well with 68 ohms; others need less; still others, more.

Philco actually used the resistor for inverse feedback in some of their postwar console models; this is where my friend got the idea.

[morzh]

Joe, you said (my understanding): from CT to one plate is 240 Ohms and to another plate - 450 Ohms. The average is 335 Ohms. I hardly could call the difference between 335 Ohms and either 240 or 450 Ohms "1300 %". It is roughly +/- 34%.
Am I missing something?

[Ron Ramirez]

A correction to my earlier post: It takes a resistor between 22 and 100 ohms, plus another resistor of 4.7 to 6.8 ohms.

Look at the schematics of the 46-1209, 46-1213 or 46-1226 to see how it should be hooked up.

[Joeztech]

My apologies to morzh and everyone. By the time I did the math my brain was about fried at the end of the day. The missing feedback is bound to be a problem. Ron, thanks for those suggestions. I will look up the schematics you mentioned for parts values. Some negative feedback needs to be there to reduce distortion.

[Mondial]

Is the CT clearly marked on the replacement output transformer? It almost seems like the 450 ohm reading is across the full primary and the 240 ohms is from one plate to CT. Have you tried all the possibilities as there is no guarantee that the transformer was wired in correctly by the previous person.

According to an old data sheet for the A-3823, brown and blue are the full primary and red is the CT.

[Image: http://amradio.freeiz.com/tubestuff/thoardson.jpg]

[Joeztech]

Mondial;

Thanks for that Thorardson-Stancor page. It helps explain more about how these transformers are to be used. Based on the chart, I believe in this set it is set for a speaker impedance around 3.2 ohms. Did Philco ever state what the impedance of their speakers were?

Joe

[Joeztech]

I decided to add the negative feedback circuit similar to what Philco used in their late 1940 sets. That approach involves adding a 4.7ohm resistor and a 68 ohm resistor in series across the secondary of the audio output transformer. I did not have a 4.7 ohm resistor, but I did have over 100 10 ohm resistors, so I paralleled two of them to make a 5 ohm resistor. I soldered a 68 ohm resistor in series with the 5 ohm resistor across the secondary of the output transformer. I added a terminal strip alongside the transformer to hold these two resistors at their junction. Then I added a yellow wire back to the terminal strip where the black wire from the volume-tone control circuits came from the top of the chassis to a terminal strip underneath. At the same time, I removed the wire that was added that grounded that point. Now the negative feedback will be restored and keep distortion to a minimum as the designers intended.

Joe
   

[codefox1]

Aye. Happy listening.

[Mondial]

Joe, have you checked the phasing of your negative feedback connection?

In another posting you mentioned oscillations at certain volume control settings. With that replacement output transformer, you have a 50% chance of applying positive feedback as there are no phase markings on the transformer leads.

You can try exchanging the two primary plate leads and see how it affects the oscillation and the operation of the bass and volume controls. If you have a dual trace scope, checking the phase of the feedback signal compared to the volume control input signal should be easy enough.

[Joeztech]

Mondial;

Thanks for that input. I do have a dual trace oscilloscope and two probes, so checking the phase should be easy to do. Also if I ground the feedback and it eliminates the oscillation, then I could conclude the feedback is the wrong phase and reverse the connections at the transformer secondary and try again. It goes to show that using OEM parts simplifies repairs in many ways. The photo above shows many paper capacitors still there. Most have now been replaced except for a few that I need to order.

Joe

[Joeztech]

Mondial;

You were exactly right, the phasing was positive instead of negative. Once I reversed the speaker connections and the feedback connections at the transformer secondary the oscillation went away. Just reversing the speaker leads eliminated the mid-frequency feedback oscillation but then I had low frequency motor-boating oscillation that varied with the tone control. Then reversing the connections of the end points of the resistive feedback network eliminated the motor-boating. The key was that just grounding the feedback lead to the volume/tone control stopped all oscillations. Now the radio is silent when it should be and I can go on with troubleshooting and alignment!

Joe

[Joeztech]

I ran into problems doing the alignment. With a 455kHz signal fed into terminal 1 of the antenna terminal strip as the instructions suggest I had no trouble getting the 1st. IF transformer primary and secondary peaked. I was using my oscilloscope to look at the RF waveform. However I ran into trouble at the 2nd. IF transformer. The plate circuit of this one peaks in a normal manner, but when I go to the secondary at the detector tube, there is a very strong signal there with the secondary trimmer nearly all the way closed. It is as though there is an oscillation happening and the detector tube does not seem to indicate any audio being detected. I had this transformer out and replaced wires in it, however there could still be a problem there. I suspect a possible open capacitor or a loose wire. I need to recheck my connections to the detector tube too. I will do that first as it is easier than taking the IF transformer out again.

Joe