The Fine Things are Always Hand Made
For several years I've been collecting repair info/tips on the Philharmonic. Now that I finally have one, I was going through these notes & files and came across an evaluation of the Philharmonic by Richard Majestic. He goes into length of the problem of the IF stages being overloaded and a fix (100K resistors in the grid circuits) to solve that problem. I've seen 1, possibly 2 responses to that fix, the gist is that Richard's conclusions were incorrect, but no detail as to why. Is this overloading indeed a problem when restoring the set with modern capacitors? I'm interested to know why Richard's conclusion is incorrect, if indeed that was the case. Thanks, Karl
Agree. Norman B offered comments of problems with Majestic's analysis of some years ago. I recall one problem was Richard's misunderstanding of some cap values in the B+ circuit.
The Philharmonic has 3 AVC circuits which are really sensitivity controls - in the RF and IF stages.
With tremendous sensitivity, it is necessary to deal with potential local 50,000 watt station and yet receive distant weak stations beyond the capability of most radios to begin to receive. The AVC circuits are designed to avoid audio distortion and preserve sound quality for high fidelity AM stations. Rider's Hour a Day book worth studying: Automatic Volume Control.
The reported issue was oscillation when the IF was approaching peak performance. The problem was most likely failure to remove the oscillator tube while conducting the IF alignment. The oscillator tube must be removed when aligning the IF amplifier.
I will comment little as those sets were not on my bench and there is very little documentation on the procedures he used for restoration and testing other than the scant details in his article. That said, the article often sounds like a hit piece on EH Scott and the Philharmonic stating that many of the ideas in the Philharmonic were taken from other manufacturers. He also repeatedly states that Scott used the cheapest capacitors made and they become leaky and often short out- which is blatant BS. I challenge anyone to find a wax capacitor that has not become leaky after 80 years, and I have yet to find one that has shorted. Keep in mind that wax capacitors were the standard back in the 30's. I have seen plenty of C-D, Micamold and other big name capacitors that were original equipment in the Philharmonic. The author is on this site, maybe he will comment.
From personal experience with my own Philharmonic, with the local oscillator tube in circuit and the set wired for agc alignment, there is so much gain around the I.F amp stage that if you are not careful you can get the whole if strip to start ringing.
However with the local oscillator disabled as per Normans suggestion, once aligned properly either with a wobbulator or using the meter and signal generator method, you will find that there are not many radios from the thirties that will perform as well as a Philharmonic.
A properly repaired and aligned radio will not require any further modification to the grid connections, could you imagine what would have happened to the reputation of EH Scott if he got it wrong?
Scott has it so right about wax capacitors, I have not found a good one so far, the best place fro them is in the trash.
You will find that it's not unusual to have a radio of this age that been visited buy someone in the past who has "got it going" and has fitted poor quality or even old faulty parts behind,
you will find that putting the chassis back to factory original, a quick tube check, changing the wax caps and checking resistor values, will usually rectify most faults.
From the number of Philharmonics I have done, I find that it is very important to use AVC tubes (6B8) that are high performing if not NOS. After the alignment, I run the test input from a microvolt up to 1 volt at the antenna terminals while tracking the 3 stages of AVC. If all goes well, the RF AVC engages as the IF AVC hits maximum. The RF amplifier grid voltages continue negative until I'm near 1 volt. They are typically between -75 and -90 volts when overload is reached.
I have never had a set break into oscillation if these stages are properly rebuilt.
I have never aligned an IF strip with the oscillator enabled on any radio. I always thought that disabling it was standard procedure? Normally, I just jump across the osc variable cap, in the case of the Philharmonic, it needs to be done from the bottom, or the tube removed.
Lately I have been thinking about taking one of my all original cap Philharmonics and thoroughly going thru it (minor repairs, alignment, etc), basically get it performing the best it can with the original wax capacitors. Then I would document it's performance with the original wax capacitors. I would measure the current draw from the supply lines to document the current with "leaky" capacitors. Then I would measure a whole battery of performance items like sensitivity, selectivity, distortion, noise, etc. After that was concluded, I would replace the wax capacitors with modern film capacitors and then repeat for comparison. Any thoughts on this being worthwhile?
Scott, that is a great idea. Tube type radios have a lot of tolerance for leaky capacitors in low impedance circuits. Although I would still do a full recap of a chassis being restored regardless of the outcome of your study, we would all be able to learn from it and may be more comfortable firing up a receiver to check for fatal problems before conducting a full recap.
Yes agreed Scott, killing the local oscillator is a must, as my philharmonic had been badly messed with, I was using a signal generator connect to the antenna input to try to get anything out of the radio, as it was so far out of adjustment it wouldn't even receive a local am station that puts out 400Kw some 25 miles away.
Also as Bill states it can be worth checking the agc line, that's how I found a very tired 6h6,
What you mention about old caps would be an interesting exercise, however you would still have to weed out the really bad ones first
Thanks everyone for their replies, much appreciated!
Scott, I have encountered 1 shorted wax cap, it was in a '41 Zenith Chairside (w/ the old FM band), the cap was an IF tube B+ bypass.
Scott, I also think your idea to document the differences in performance stock vs new caps would be really interesting.