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:To Doug Criner. I am attempting to get this unit running and am having B+ problems. After a short time, B+ starts creeping downward. In the post of 2/24/08, you (at one time) provided a modified schematic of the stacked B+ runs. That part of the post has been taken down, and my note to the webmaster did not help. Is it possible to send another off line?
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: : : :  : : : : : : : :Sir Jim. . . . . : : :I don't be the Right Honorable and Esteemed Doug Criner . . . Esq . . . .buts . . . : : :The very last entry on that thread was mine with its sole schematics, and now missing the EXPIRED mark up schematic, but I see that the voltages are still there, on the other diagram. : : :Seems like the servicing party was Tim , before. : : :With your info of decreasing B+ voltage , lets work with that initially, as that is probably going to be a situation of the power supply being gradually LOADED down after warm up. : : :Initially . . . monitor that B+ again, just after coming up from being stone cold. : : : Then move ye olde metering to the 1st grid of the 6F6 AF output tube and see what value its NEGATIVE voltage biasing level is. : : :If that NEGATIVE voltage then gradually diminishes with time, the current consumption of the power supply will increase accordingly /proportionatively and pull / load down the B+ to create the effect that you are experiencing : : :Naturaly the suspects would be the coupling capacitor feeding leaked B+ into the output stages 1st grid circuitry from the plate of the earlier 6F5 audio stage . That in turn pulls down the negative bias. : : :Research and see where we go from there. : : :Less likely considerations would be a gassy 6F6 or it having grid emission, or a weak rectifier tube, on its last legs. : : : : : : : : : : :ASIDE: : : :Referencing to the OLDE string is : : : :http://www.nostalgiaair.org/Forums/Messages/897/M0059897.htm :73's de Edd : : : : : Life is short, make fun of it.. : : : :  : : : |
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::To Doug Criner. I am attempting to get this unit running and am having B+ problems. After a short time, B+ starts creeping downward. In the post of 2/24/08, you (at one time) provided a modified schematic of the stacked B+ runs. That part of the post has been taken down, and my note to the webmaster did not help. Is it possible to send another off line?
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Edd,
SOme new developments...
The very first thing I did when starting work on this Zenith was to check all 8 tubes and recap the power supply C22,23 & 24. B+ was low then, but I sort of expected that as all wax caps were OEM. I could pick up strong stations. So I started recapping backwards sarting at the 6F6. At some time, I lost reception, but get good snaps and crackles so the 1st & 2nd audio must be up. When I discovered the socket voltage chart, I could not make sense out of my readings compared to it.
For example, the plates of the 6X5 are 314 vac vs. 265.The cathode is 295 vs 277vdc. The plate of 6F6 (3) is 235vdc and G2 (4) is 245vdc vs 220vdc. G1 is a whopping -17.
Although I observed polarity (or so I thought) when recapping the 'lytics, performance seemed to degrade after C24 was replaced.
My question is that because I had to wire up caps to approximate the values to within range of about 4 mfd., for example , C24 is no longer 16 mfd, but 20 as is 22 & 23. 22 is now 10 mfd and 23 is now 20. Because they have not smoked, I figure that polarity is correct, but just for the record, where should the + side be connected?
On the socket voltages, is the DC readings referenced to chassis ground?
Since I posted here, I have replaced the 6X5G with a NIB 6X5GT, discovered that the 6A8 was shorted and the 6F5 was weak. All replacements checked OK on tester.
Whatever I did seems to have corrected the degrading B+ that I firt wrote about. It is stable, but seems high. I hate to continue recapping until I (we) dope out whether there is a problem or not.
Many thanks for the help,
Jim K.
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: : : : : : : : : : : :Sir Jim. . . . . : : :I don't be the Right Honorable and Esteemed Doug Criner . . . Esq . . . .buts . . . : : :The very last entry on that thread was mine with its sole schematics, and now missing the EXPIRED mark up schematic, but I see that the voltages are still there, on the other diagram. : : :Seems like the servicing party was Tim , before. : : :With your info of decreasing B+ voltage , lets work with that initially, as that is probably going to be a situation of the power supply being gradually LOADED down after warm up. : : :Initially . . . monitor that B+ again, just after coming up from being stone cold. : : : Then move ye olde metering to the 1st grid of the 6F6 AF output tube and see what value its NEGATIVE voltage biasing level is. : : :If that NEGATIVE voltage then gradually diminishes with time, the current consumption of the power supply will increase accordingly /proportionatively and pull / load down the B+ to create the effect that you are experiencing : : :Naturaly the suspects would be the coupling capacitor feeding leaked B+ into the output stages 1st grid circuitry from the plate of the earlier 6F5 audio stage . That in turn pulls down the negative bias. : : :Research and see where we go from there. : : :Less likely considerations would be a gassy 6F6 or it having grid emission, or a weak rectifier tube, on its last legs. : : : : : : : : : : :ASIDE: : : :Referencing to the OLDE string is : : : :http://www.nostalgiaair.org/Forums/Messages/897/M0059897.htm :73's de Edd : : : : : Life is short, make fun of it.. : : : : : : : |
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::To Doug Criner. I am attempting to get this unit running and am having B+ problems. After a short time, B+ starts creeping downward. In the post of 2/24/08, you (at one time) provided a modified schematic of the stacked B+ runs. That part of the post has been taken down, and my note to the webmaster did not help. Is it possible to send another off line?
::
:
:
Edd,
SOme new developments...
The very first thing I did when starting work on this Zenith was to check all 8 tubes and recap the power supply C22,23 & 24. B+ was low then, but I sort of expected that as all wax caps were OEM. I could pick up strong stations. So I started recapping backwards sarting at the 6F6. At some time, I lost reception, but get good snaps and crackles so the 1st & 2nd audio must be up. When I discovered the socket voltage chart, I could not make sense out of my readings compared to it.
For example, the plates of the 6X5 are 314 vac vs. 265.The cathode is 295 vs 277vdc. The plate of 6F6 (3) is 235vdc and G2 (4) is 245vdc vs 220vdc. G1 is a whopping -17.
Although I observed polarity (or so I thought) when recapping the 'lytics, performance seemed to degrade after C24 was replaced.
My question is that because I had to wire up caps to approximate the values to within range of about 4 mfd., for example , C24 is no longer 16 mfd, but 20 as is 22 & 23. 22 is now 10 mfd and 23 is now 20. Because they have not smoked, I figure that polarity is correct, but just for the record, where should the + side be connected?
On the socket voltages, is the DC readings referenced to chassis ground?
Since I posted here, I have replaced the 6X5G with a NIB 6X5GT, discovered that the 6A8 was shorted and the 6F5 was weak. All replacements checked OK on tester.
Whatever I did seems to have corrected the degrading B+ that I firt wrote about. It is stable, but seems high. I hate to continue recapping until I (we) dope out whether there is a problem or not.
Many thanks for the help,
Jim K.
I just picked one of these up a few weeks ago. That got me interested in your posting.
Your substitutions on the Electrolytics are OK. As for polarity, the positive goes to the place electrically closest to the 6X5GT. The negative goes to the place electrically closest to the center tap of the transformer.
I have noticed already that this radio has a candohm resistor in it. I recommend replacement of them. They can commonly short to ground. A possible cause of the problem you are having is that one of these starting to open. That can happen, too.
I am sure you thought of reviewing all of your capacitor replacements. A wiring error can cause this.
Best Regards,
Bill Grimm
With B+ drifting down, I would tend to suspect a leaking capacitor or other short somewhere in the power supply section or the plate supply circuit. Have you recapped the set? Do the heater voltages droop too, or just the B+?