[AVSorrell]

Hi Group -

Working on my Philco model 212 (early) had have done the normal things - tested transformer windings for continuity, tested tubes, replaced capacitors and resistors with modern ones. When I powered up, I have tunable hum from the speaker. Measuring voltages, all the power transformer voltages are in spec as noted in Service Bulletin #54.

For the tubes all filament voltages are normal. However for the tube plates voltage several are too high and some too low. Rectifier # 80 tube plates are 750 vac as they should be. But for all the # 24 tubes (1st RF, 1st Dect., 1st I.F., 2nd I.F., ) the plate voltages are all about 223vdc. when they should be ranging from 180-190 vdc. For the # 27 tube Osc. I have -1 vdc for the plate voltage when it should have 45 vdc. for the # 27 Detector Amp and # 27 1st A.F. plate voltages I am generally within spec. for the two 2nd A.F. #45 tubes, plate voltage are high too 321 vdc when they should be 255 vdc.

Ideas are what could be affecting the plate voltages like this? -- Andy

[MrFixr55]

I had this once, due to a very weak 45 in a Gloritone 26.  B+ was about 310 at the Output transformer.  Replaced the 45 and all was well.  However, I don't know what you mean by tunable hum. However, after originally posting this, I looked at the schematic and noted that you have 2 45s in push pull.  Since it is possible but not likely that both 45 tubes are low emission, we want to look elsewhere. This is likely a type A biased output stage.  These draw a lot of current even under no signal conditions.  So we need to look for something else that can cause low current.

Assuming that the output transformer does not have an open leg, the other high probability failure is in the Cathode Bias resistor for the 2 45s.  This is R59 on your schematic.  It is likely part of a tapped wirewound resistor.  Actually, there are several parts of that resistor that can be bad.  Replace it with a string of 5 watt resistors.  R57 which is likely part of this tapped wirewound resistor will also cause very high screen voltage, as this resistor forms a voltage divider with the speaker field.  However, if this is open, the field strength will be very low, but there could also be hum.

In my office / shop, I can hardly operate an AM radio due to the computer, fax machine, LED lights and unshielded AC wiring.  It does not matter whether the radio has a loop or is hooked up to my outside long wire.  I have to turn off the computer, the monitor and the lights.  Even the ceiling light which is on a dimmer., and there is still some noise.
The symptom is a hum, stronger on some parts of the band than others.  If I take the radio outside, it is an entirely different story.

[RodB]

Andy,

This happened to me when I mistakenly grounded the negative side of the filter caps to the chassis when they should have been connected to the hv center tap for floating B-.

[morzh]

Using modern DMM measuring voltages will show higher values vs shown in the documents.

[MrFixr55]

This is true but this is a real big difference. In my case with the Gloritone, there was a 50V difference between bad output tube and good output tube with the same DMM, and the measurements after the new tube the values were within 2o% of the stated voltages.

[AVSorrell]

Okay. I strongly suspect that I have a faulty P-P input transformer Philco part # 3537. I saw in morzh's post from 2014 that he used hammond 124B for his similar 111. I happen to have a have a Hammond 125E from another project I have not used. Before I seek out a 124B, is there a chance the 125 E may work as a replacement for the PP input transformer? -- Andy

[morzh]

Andy, no it will not work. 125 are output xfmrs and 124 are interstage (driver) ones; they have very different turns ratio. It won't work at all.

https://www.tubesandmore.com/products/tr...tage-10-ma

AES has this for $17. I have not looked if this one is exactly what's needed, but there is a good chance it will fit well, and Hammonds are way more expensive, 124B is $60 or so. Andm yes it is the same 1:3 ratio that Hammond is, so if Hammond worked, this one should to, just check the plate current of the tube that drives it. The AES one can take 10mA.

[MrFixr55]

Hi Andy,

Is the Primary or the Secondary of that interstage open?

If it is the primary, there is a "bridging" technique that cn be used to "R-C" couple the plate of the driver to one leg of the secondary on that interstage transformer. I believe Ron Ramirez wrote an article on this. A LOT cheaper than replacing the transformer.

[morzh]

It is typically the primary, for it is the one being the heat-dissipating load of a tube. The secondaries are rarely open, and if they do, it is likely corrosion and not burn-out.

There is also using the secondary as an auto-transformer kindof, which is what Ron has in his article.