As some of you know I have a zenith with a volume control that doea not increase volume its stuck at about half volume maybe less and all I hear is some faint hum as i go all the way up I took it apart and Im not sure but is there supposed to be a wiper disk attached to the little metal part that rotates in this . there is a black disk in the bottom half of the control and in the top part here the single piece side of the metal thing throws the on off switch in the bottom . Is the other side supposed to be holding a small disk and can i make one if so

This sounds like a reason to contact Mark Oppat. He's the ultimate antique volume-control/power-switch guy.

http://www.oldradioparts.net/

Fred
owner
Classic Audio Repair

Ha ha. I just ordered one from him about an hour ago. Your right. Hes got it

ok I double checked my volume control and it reads 2 meg ohms across the top taps on each side , the top of one to the top of the other and it drops smoothly down to 2k so that seems to be working any ideas why the volume does not go up it is at half voluime to start and when i turn it up i hear the hum of all that power going to the speaker but no increase in volume to the speaker is there anyone who can confirm the connections to the volume control for me I never took any wires off i got it this way. its a zenith chassis 1005 model 10s 470 schematic

Having looked at the circuit if you are getting an increase in hum as you turn the control, there is some chance that it is working. Other wise there could well be a miswire by whoever had it before & why it was possibly abandoned.

Check that the centre of the wiper actually goes where it is supposed to and also check that there has not been a decimal error in one of the caps. You may have to resort to unwiring the pot to get a proper apraisal, of it.

Too big (or leaking) on a cap to ground in the right place, I would expect to kill signal, as would a too small coupling cap. There are a lot of switches there to cause issues. Have you , tested the output tubes, or tried feeding a signal into the phase inverter grid to see if they are both working?

Marc

I haven't tested tubes I don't have any equipment to send signals either one of the 6v6 tubes if you wiggle causes a lot of crackling. Which tubes if you know do the controlling of the output to the speaker I was going to order a couple new tubes. The wiper goes right where it is supposed to go also and the tone controls function. I was wondering if maybe the fact that I have made rectifiers with diodes and resistors in series that I don't have the proper volts too high or too low . Could that be causing it not to work right. I have around 330 vdc from the rectifier

This has one of these highly useful circuits with no voltages on it. The critical voltage is that which appears at the screen of the 6V6 and I do wonder at the logic of using two 6X5, when there were better. The 6V6 can go to 315V or so but at that voltage the screen must have less voltage than the plate.

If the circuit had voltages on it, it would be a matter of simplicity, working out what series resistance (if any would be required) I would tend to get the voltage back to say 250... 285 max. on pin 4 (screen) of 6V6. You seemto be hamstrung by a lack of basic equipment, owing to a recent score, I have too many of some items eg I have three signal generators. One of which (Sanwa xxx-301) I need circuit as some one cut a wire and I would like to confirm what & why and in the meantime... no switch on?.

Most of those signal generators for alignment (even the cheapies) have an audio output, which you could benefit from. If you have other sets that take 6V6 or even 6F6(bias wrong) you could put the 6V6 in the known good set to get an idea. There are rare occassions when a CRO is useful for signal tracing or looking for distortion (This is why they are not a priority item unless you do a lot of repairs... I do) this may be one of them.

I would still have a serious look at every wire & component I note that there is a mod (different model?) on the schematic for different tubes. Carefully analyse the circuit to ensure that the correct tube for that wiring is in the hole marked 6Q7..... it does happen and I have seen it. I have one in the repair line ????

Marc

Ok thanks. Ill try some different resistance on the rectifiers and lowercthe voltage. Right now I have a 200 ohm resistor on each diodes ill try 300. Ill check the voltage first

You can also just use one big one in series with the field. This is the PIB. If the set had voltages on it a back bias or the fields voltage drop would tell you the current it should draw and you would be able to get a good value for the resistance and it's wattage.

Try 300 to 330 Ohm, do not put caps on the diode side of the resistors yet, as that will act as a choke input filter and keep the voltage down as well.

Marc

I tested pin 4 of 6v6 tubes. 268 volts on both. I ordered some new tubes. The 6j5 and 6k5 and 6q5 and some new resistors to alter volts

I'm not sure what your describing here. Right now I have two diodes . Each one passes through a 200 ohm resistor then they come together and go to the pos of the first electrolytic cap. And into the speaker field coil . I used a 33 uf cap also instead of a 20. Should I put a 20 back in. I havea 22 uf. Should I just merge the two diodes and then pass through 300 or 330 ohm resistor singly. I guess there are different ways to do it but same results. How would you do it?

Having resistors in series with the diodes will dampen any surge a bit. You definately need to loose a few volts. An issue that can arise here is the voltage rating of the filter caps. Diodes will behave like an 80 and similar filament rectifiers, by developing voltage before the heater tubes have time to get into conduction. This will mean that the filter caps and anything on B+ should be 500V or better. The diodes should not have hassles with the capacity indicated.

The comment re the position of the resistors. Resistors are often used in place of a choke, or field. Chokes are actually exellent filters.
There are two common types of filtering used in tube radio's. At very high voltages with some rectifier tubes, they specify "choke input filter". In this combination B+ goes into the choke and there is only a smoothing cap after the choke. This produces slightly less voltage.

The "capacitor input filter" has a capacitor either side of the choke and will produce a greater voltage. The resistor that you are using will behave as a choke input filter, as there is no capacitor, at the junction of the diode & resistor..

One should be able to find further info on the WEB,

I would save one resistor by joining the output of both diodes and then connectiog to the field, pretty much in a similar fashion to the 6X5's.

Marc

just to update i ordered three tubes and installed them the volume control works now a 6j5 a6q7 and a 6k7 also i replaced the two 10 watt diodes i was using and installed 2 1n 4007 diodes and 400 ohm 5 watt resistor in an old 6x5 tube Im really happy Im not sure how the tubes fixed the volume control or if it was the diode replacement but its working and its got such powerful bass wow thanks for all the help you guys are awesome

I would suggest thet the answer is quite simple, and subtsituting the tubes has proved it. One, or all of those tubes (more likely one) is a dud and has for one reason, or another lost emmission.

You can now, as you seem to be doing, enjoy it.

Marc