[Joeztech]

I was replacing various portions of B+ wiring on the 40-201 chassis and decided to take advantage of a lull in the work by checking resistance from various B+ points to ground to verify my work. I discovered that on the B+ line going to the IF and Mixer-Oscillator circuit had a short to ground. I had replaced R40 because it had increased in value more than 20% already. I disconnected the lead going to the two tubes and the short to ground was still there. I found that C40, a 0.1mFd @ 400VDC had shorted. I had ordered a 0.1mFd @ 600VDC to replace it if it was bad, so I had one on hand and replaced the offending part.
   
   
It is always a good idea to check for potential shorts before applying power to a chassis. After this capacitor was replaced there are no longer any shorts to ground on the B+ lines in the chassis. I will make one more check with the speaker assembly connected to make sure there are none in the field core.

I did not have a 4.7 ohm resistor to use in a negative feedback circuit to the volume and tone control circuits, but I did have some 10 ohm 5% resistors on hand. I connected two in parallel to make a 5 ohm resistor. I had a 68 ohm resistor already on hand and used it and the two 10 ohm resistors in parallel to create a feedback circuit similar to Philco sets in the late 1940s. The audio output transformer used in this set was a Stancor A-3823 replacement which did not have the needed feedback tap on the secondary windings. The addition of this feedback network will restore negative feedback and keep the distortion to a minimum like the designers intended.

I also removed the existing phonograph jack and control switch on the back of the chassis. I did some checks with my ohm meter and had discovered that the switch, which interrupted the cathode circuit of the mixer-oscillator and IF amplifier was intermittently open. Rather than try to find a new switch to use in the circuit, I decided to rewire those circuits to be like they were originally. I might still add a phonograph jack, but it will be an RCA type jack and not the 1/4" size phone jack that was used.

Joe

[morzh]

Joe

Are you telling us that you DID NOT replace the paper caps? This is a mistake and I think we've been repeating this here at this forum ad nauseum - DO replace ALL caps.
Especially (if you feel lazy to replace all tube caps) those that are there to decouple power voltages (B+ and such), as those if shorted will cause burns. But it does pay to replace ALL.

[Eric Adams]

I agree with Morzh. Why waste your time, and possibly do some damage, before a complete recap?

[Joeztech]

No, I am not saying that I am not replacing all the paper capacitors. I am saying that I discovered this in the process of replacing them. This one would have been a deal breaker if I had applied power. Checking for shorts to ground in the B+ circuitry is always a good thing to do after or during repair and this was part of that process. The other thing I am finding as I continue replacing capacitors is that the resistors have changed in value anywhere from 19% to 57%! That was in the phase splitter circuit alone! There is no telling how many more are off value at this point. It appears that most all of the capacitors except for mica types will wind up being replaced. Many of the paper capacitors have already been replaced in the past, but I do not trust the replacements. This example goes to show just how bad these old paper capacitors can get.

I restored negative feedback to the audio circuits by adding a 5 ohm and 68 ohm resistor string at the output transformer secondary and running a new wire back to the low end of the volume control and tone control center lug like originally done by Philco. I did not have a 4.7 ohm resistor like Philco used in their late 1940 series of radios, but had lots of 10 ohm resistors on hand. I bridged two in parallel to create a 5 ohm resistor. I removed the switch and phonograph jack from the radio as it turned out, the switch was very intermittent and because of the way it was constructed it could not be cleaned internally. I might still add a phonograph input, but it will be an RCA type jack instead of the1/4" phone jack type that was used here.

Cheers!

Joe

[codefox1]

Please replace all the paper capacitors and all the electolytics. For the fewcents apiece, probably best to replace all the carbon resistors as well for help in alignment and long term dependability. Stanngely enough the tubes are amongst the most hardy components. Best!

[Joeztech]

All the old paper type capacitors have now been replaced and most of the resistors as well. There were a few exceptions. Some resistors were 20% types and bridged across an inductor. I left those alone as they are just there for reducing the Q of the circuit to broaden response. Some of the bypass capacitors in the front end and IF strip could be well served by .01 disc ceramic capacitors which would have shorter leads and less inductance added to the circuit. However, it is not critical since the IF is only 455kHz. In the RF it could be a potential issue as the SW high band operates up to 18mHz which could be an issue depending on other circumstances. All in all, I have to say the radio works quite well as it is. The last order of capacitors from tubesandmore also took care of replacing the new electrolytic that had shorted intermittently. The new one has a 450VDC rating while the one first used was only a 300VDC unit. It proved not to be able to handle warm-up voltage surges.

Joe