zenith: 8S463 5808 voltage levels

I'm working on a 8S463 with a 5808 chassis. I recaped the chassis, and it powers up, and I alignment, but I'm getting a hum in the audio. 

I looked at the schematic and I see the voltage levels on the schematics. I have the line voltage set to 112V, but none of the voltages line up with the schematics. My transformer output voltage is about 20% higher than they're shown on the schematic.

I'm pulling my hair out, any ideas?

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Dear Tom,

I assume you also changed all the filter caps too. If so and you still hear the hum, I've two failures in mind to look for. You say that the transformer output is 20% higher than rated. Does it mean you have at the high voltage side over 300V on each of the two windings? If so, does the transformer heat up even without the tubes are inserted? In this case the transformer might have a short circuit between the windings.

The other possibility is that one of the tubes has a short circuit between the heater and the cathode. Just take the tubes out one by one and see when the hum stops. For the 6F6 you have to change to a new one.

Good luck,

Rolf

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I have to set the line voltage to 101.7VAC to get 265VAC on the output of the transformer.. B+ is 205 VDC, and B-  is -0.7V.

I tried pulling tubes to see where the hum was. I pulled the preamp out, and I didn't get any hum out of the output tube. But when I put the preamp back in, I don't get any sound. Maybe I screwed up the output tube. 

The tuning eye still works when I go through the dial.

This is my 1st recap so I have a steep learning curve.

Thanks

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Hello folks,

I have this same unit and am having trouble aligning he radio.  The instructions for alignment say the first step is the 1st Det. Grid using a 1/2 mfd dummy antenna.  Is this connection to the top cap of the I.F. 6K7G or is it to one of the pins on the 6A8G Converter?  I tried the top cap of the 6K7G and was only able to align trimmers C & D and could get no response on trimmers A & B.  Is that because this top cap goes directly to the 1st IF can?

Any input and help will be appreciated!

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In old american documentation, the mixer tube is often mentioned as the 1st detector. So in this case, you should connect the signal generator to the top cap of the 6A8G.

In practice, it usually works well to connect the signal generator to the radio's antenna connector and set the tuning control to a frequency reasonably near the IF - i. e. at the low frequency end of the MW band in case of a radio with an IF around 450 kHz.

Regarding Tom Mitchell's voltage measurement problems in earlier posts in this thread, please note that many cheap digital multimeters will not measure AC voltages correctly if the waveform is distorted (high overtone levels), such as can happen on the AC mains supply in some areas due to other loads drawing distorted current. In case of an old radio, remember that the heater circuits need an AC voltage with correct RMS value, regardless of what the waveform looks like and what the peak voltage is. While the rectifier for the anode voltages needs an AC voltage with a correct peak value, regardless of the RMS value.

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Hello Torbjorn,

Thank you so much for your input and clarification.  I connected my Heathkit signal generator to the top cap of the 6A8G Converter as suggested and am getting no 455KHZ signal tone to the speaker.  I also input a 400HZ audio only signal into the top cap and was able to get that signal to the speaker but any adjustments to the A,B,C or D alignment screws has no impact on the signal.

I also did the "poor man's" test of touching the top cap on the various tubes.  I get a very loud response from the TC of the 6F5G tube but no response from the 6K7G or 6A8G caps.

Any ideas on isolating the problem will be appreciated!

Herb

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Check the DC voltages around the tubes, particularly the IF stage (6K7G). Check that the IF coils are ok (no open circuit).  If possible, try changing tubes or try your tubes in another working radio.

A simple test to see if the mixer and IF tubes have emission and seem to be in reasonable working order:  Measure the screen grid voltage.  Connect a negative voltage to the AGC line (for example, a 9 V battery with - to the AGC line and + to chassis), then the screen grid voltages should rise a bit (hard to tell exactly how much, that depends among others on whether they are fed just by a series resistor or via a, possibly relatively low-ohmic, voltage divider. If both tubes have their screen grids fed by the same network, repeat the test with first one, then the other of them removed from the radio.

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Very helpful information.  On the model information, the last file is the ideal socket voltages for this unit.  I tried to upload it but the file was too big.

I'm a bit of a neophyte when it comes to vacuum tubes and radios so bear with me.  I assume the AGC line is the negative DC voltage line, correct?

Also, what is the best way to test the IF coils for an open circuit?  

As for voltage levels, I am getting 322 VAC on pins 3 & 5 of the 6X5G instead of 265 VAC.  On the 6K7G I am only getting 54 VDC instead of 90 on G2 (pin 4) and getting 3 VAC on both the TC (G1) and pin 1 instead of 0 V.  On the 6A8G I am getting 23 VDC at pin 4 or G3 instead of -58 VDC.  I was also getting -1.8 VDC on the TC instead of 0 V.

Finally, I hooked up a 9 V battery to pin 4 of the 6A8G and it was reading -7.5 VDC instead of the +23 VDC without the battery attached.

As always, I have an open mind to learn and fix this baby!

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