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High voltage readings - Tom45acp - 05-08-2020

I'm wrapping up repairs to a model 91 with a 225 chassis. There is a problem with plate voltages for the RF(44), detector oscillator(36) and the IF (44) tubes. The voltages are 265 to 272 volts, which exceeds the maximum plate voltage shown in the tube manuals. Measurements taken were at 115 volts on the line, plugged directly into a wall receptacle at 124 or 125 volts drives the plates up to 285 to 290 volts.Needless to say, the radio is very loud when the volume is turned up. Other than these voltages, the radio is working well.

Output of the power supply is 332 volts at the input to the filtering section and 286 at the output of the filter.

I have several solutions for lowering the voltage but I'd rather not put in parts that weren't in the original design if I can help it.

Any suggestions would be appreciated.

Thanks

Tom

RE: High voltage readings - gregb - 05-08-2020

Check all the sections of the large voltage divider resistor and see what you find.

Gregb

RE: High voltage readings - Tom45acp - 05-08-2020

This is a 2 section resistor, one section is 90 ohms with 5.6 volts across it and the second section is 210 ohms with 20 volts across it.

RE: High voltage readings - Radioroslyn - 05-08-2020

You might want to have a look at your cathode resistors, it could be that you have a tube that isn't drawing it's fair share of current.
GL

RE: High voltage readings - Tom45acp - 05-09-2020

The cathode resistors are well within tolerance.

The total plate current for the two 44 tubes is 9mA. If need be, I can get a reading on the plate current for the 36 tube.

One of the differences in the 225 chassis is that part 21 shows up on the schematics that are available (but do not include a 225 chassis) as 10k ohm. As this radio came from the factory, there was a 15k ohm installed. I don't know if this matters.

One of the things that has puzzled me is how the cathodes of both 44 tubes are shown as having different voltages when they are both wired together. I'm assuming this is an error.

RE: High voltage readings - Steve Davis - 05-09-2020

Tom, in the early 91, resistor 21 was 15K it was later changed to 10K to make the oscillator more reliable. I would change your 15K to 10K for that reason. Changing will have almost no effect on your B+ voltage though.

Steve

RE: High voltage readings - morzh - 05-09-2020

Tom

Did you measure the voltages relative to the Cathodes of the tubes or to the Chassis?

Also keep in mind today's readings are higher as back then the voltages were measured by a lower impedance meter which would load the circuit and lower the voltage.
Today's meters are very high impedance.

RE: High voltage readings - Tom45acp - 05-10-2020

Steve

Thanks for the explanation of why the resistor was changed from 15k to 10 k. I did see the change in  a service bulletin but I didn't want to make the change without knowing why it was made. 



Morzh

Plate voltages were taken to the chassis and to the cathode and there is not much difference between them.

You mentioned the lower input impedance of the older test equipment and that caused a Homer Simpson "Doh" moment. I put a 229 kOhm resistor across the input of my meter and the measured voltages were still high, but only exceeded the maximum allowable plate voltage by 4 to 7 volts, much better. I'm still concerned that this may be an issue when this radio is plugged into the wall receptacle where the line voltage will run the plate voltage back up to 275 volts or so.

RE: High voltage readings - morzh - 05-10-2020

Another question: is your radio the console with two speakers?
If not is this chassis supposed to drive two speakers and only drives one?
Or if it drives two are both field coils ok?

Last, is the 1st filter condenser of proper capacutance or higher

RE: High voltage readings - Tom45acp - 05-11-2020

This radio is the console type and one of the two speakers was missing. I replaced it with a modern speaker and inserted an appropriately sized resistor to make up for the missing field coil. I'm guessing that the original speakers field coil went bad since there was a speaker installed that hardly had anything left of the cone and did not have the voice coil connected, just the field coil. I also had to replace the audio output transformer and will probably rewind the original next winter.

Both filter caps are 10mfd. The originals had previously been replaced with this size. I recall coming across something that mentioned going from 6mfd to 8mfd and possibly another reference to going up to 10mfd. I decided to go with 10mfd since that is what was installed as a replacement at some time in the past.

RE: High voltage readings - morzh - 05-11-2020

Tom

You see why I asked then.
The 1st field coil in this radio is a filter/dropout, the second is a load. Insufficient load will increase the voltage.
If I read the parts list correctly, both fields are 3200 ohms, so your voltage roughly divides by two after them. I do not know if they are the same though.
Make sure you check the voltage at the specified Mains voltage from Variac.

Also about that cap.
The first cap is specified as 6uF for a single speaker and 8uF for the dual.
So, two things.

1. You do not have to have that extra resistor instead of the missing field coild. The reason the cap was increased to 8uF was to compensate for that extra load, but you do not have it if you do not parallel that resistor, and so you can revert to 6uF.
2. If you have 10uF you increase the voltage further so it very well might be the reason behind that 4 to 7V you are having.

So, what I would do:

Take off that resistor and make the first (#50) cap 6uF. This way you also do not dissipate a few extra watts and wearing the xfmr and rectifier out.
If you want 8uF, keep that resistor.

RE: High voltage readings - Tom45acp - 05-11-2020

Morzh,

I sure appreciate you taking the time to help me out.

I can see how the lower value capacitor would lower the voltage. Since the lower value cap would discharge to a lower voltage before recharging, that would effectively lower the output voltage. I only have one 8MFD on hand so I'll wire it in tomorrow and check the results. I have the feeling that I'll be ordering another 8 or two 6MFD.

I've looked over the schematic and I can't see how the resistor I put in is in parallel with the field coil for the surviving original speaker. See if the following makes sense. What I see is a 3 element voltage divider. After passing thru the field coil, a B+ voltage is taken to the plates of a couple of 37 tubes, passes thru a 13kOhm resistor then on to the screen grids of the 36 & 44 tubes. Also after the field coil, the B+ goes thru the second field coil (resistor in my case) then to the chassis then the 85 ohm section of the candohm and back to the center tap of the high voltage winding. If there is no field coil or resistor, I can't predict what this would do to the biasing, but I don't think it would be good. On the other hand, if this was a one speaker set,the four part candohm would be the voltage divider and lack of a field coil would be a loss of load that would raise the voltage.

RE: High voltage readings - morzh - 05-11-2020

Yes,

The second speaker field forms a divider with the first field. In case both are 3200 ohms it will half the voltage. I do not know the spec of the first field, only the second is shown as 3200 ohms.
Part number of the field assembly for the dual speaker might help, you can try to find it.
I kind of doubt they intended divide the voltage by half but...

If both values are correct (and they even increased the first cap to 8uF to compensate for that divider drop) I don't see how the resultant voltage should exceed the specified one much.

RE: High voltage readings - Tom45acp - 05-12-2020

Somewhere, I saw a chart of philco speakers and it showed the DC value of their field coils. I'll try to find it again to check on the field coil value for the missing speaker.

I think that lowering the value of the filter caps will get the voltage down to where it needs to be. I think I may go all the way down to 6mfd if needed. When I first started working on this, there were plate voltages around 290 volts, so a lot of progress has been made. I only need to wring out a few more volts then decide how to handle the increase that will come from a higher line voltage.

Thanks again for the help. With your suggestions and some thinking about them, I've got a better understanding of the power supply.

RE: High voltage readings - RodB - 05-12-2020

I'm not entirely sure how this affects the B+ but the choke coil has some inductive reactance to the pulsating DC, adding to the resistance. Therefore, if the choke coil is 1000 ohms you can use a 1200 to 1500 ohm resistor to replace it. You should be able to use the replacement resistor to tweek the B+ without much adverse effect.