Fleetwood model 4068... Advice?

[Reece]

You'll get varying responses, but a few drops of WD-40 dripped in where the terminals come out of the pots, and working the pots back and forth many times, has worked for me lots of times.

[VintagePC]

Thanks, tried that and it has reduced the noise considerably.

I think I may also have located part of the balance problem... There's some weird stuff going on with the volume control that's not in the schematic. It may also be the result of differing resistors in each channel- while they are all within tolerance, one example is the 100K ones; one channel has a 100.6K, the other is about 107K... Should they be as closely matched as possible?

The pot has a 4th lug which appears to be some sort of center tap- resistance goes from 300K to 0 and back up again as the knob is turned.

The schematic just shows the wiper going straight to the 6AU6, but here, the wiper goes through a 220K to ground, as well as to the 6AU6. One channel is shielded, the other isn't, so that may well be the source of some hum.

In addition, the audio input from the function switch runs through a cap to the 4th lead of the pot, then to a 47K resistor, through another cap, and to ground. The same circuit exists on both channels.

Does anyone have an idea what purpose this might serve? The mounting for the volume pot is different than the others, so I wonder if someone changed it and added a hack to make it fit/convert a linear to a log pot? My meter doesn't go to 3 megs, but afaict, it is a 3 meg pot.

Does someone have an idea what this might be?

[rojoknox]

Greetings from FixitLand!

Originally Posted by VintagePC

The pot has a 4th lug which appears to be some sort of center tap- resistance goes from 300K to 0 and back up again as the knob is turned.

The schematic just shows the wiper going straight to the 6AU6, but here, the wiper goes through a 220K to ground, as well as to the 6AU6. One channel is shielded, the other isn't, so that may well be the source of some hum.

In addition, the audio input from the function switch runs through a cap to the 4th lead of the pot, then to a 47K resistor, through another cap, and to ground. The same circuit exists on both channels.

Does anyone have an idea what purpose this might serve? The mounting for the volume pot is different than the others, so I wonder if someone changed it and added a hack to make it fit/convert a linear to a log pot? My meter doesn't go to 3 megs, but afaict, it is a 3 meg pot.

Does someone have an idea what this might be?

The fourth lug on the pot usually means it has a loudness tap. Components connect to it in such a manner that bass is accentuated at lower volume levels, making the sound (supposedly) more "full-bodied." Anyway, as you note, the schematic doesn't show this. No harm in leaving it as is, at least for the time being. It could've been a factory circuit change.

Having one channel shielded (I assume from the function switch to the 6AU6 grid) and the other not shielded is definitely a hum issue. Does the hum you hear come from the unshielded channel? Or both, equally? If the latter, we're talking a power-supply issue most likely.

From the volume-control wiper to the 6AU6 grid, you say you have a 220K resistor to ground as well. That's a good idea, actually; should the wiper go open-circuit, without that 220K resistor the grid would float and cause the tube to operate erratically at best (go into runaway at worst).

Take care,
--
J. E. Knox 'The Victor Freak'

[VintagePC]

Having one channel shielded (I assume from the function switch to the 6AU6 grid) and the other not shielded is definitely a hum issue. Does the hum you hear come from the unshielded channel? Or both, equally? If the latter, we're talking a power-supply issue most likely.

From the volume-control wiper to the 6AU6 grid, you say you have a 220K resistor to ground as well. That's a good idea, actually; should the wiper go open-circuit, without that 220K resistor the grid would float and cause the tube to operate erratically at best (go into runaway at worst).

Haven't checked for sure, but I do get the distinct impression that the hum is coming from one speaker, not both.

I've got plenty of shielded wire floating around in my parts box, so I'll replace it anyway. The hum is only noticable with the volume at or near max, so the power supply is doing a very nice job.

Regarding the volume control, the 220K does indeed serve to make the pot a log-pot instead of linear. Doing it this way is far cheaper, since log pots are more expensive. I'll have to replace them, since the readings at the same setting are different for each channel... and probably contributes to the balance issue. (and it's practically impossible to get an accurate reading of resistance with things in parallel and the uncertainty of a variable control!)

Edit: Yup, the 220s were out of spec. Replaced them and now the balance is equal on both sides.

You mentioned the loudness tap; this explains why at about 1/2 the control, the bass suddenly jumps up in volume... Suggestions to fix? The 47K resistors seem to be within spec.

[VintagePC]

I've had another thought cross my mind (stop groaning! )

The set gets nice and toasty just sitting on top of the cabinet; I don't know if I like the idea of enclosing it. I have plenty of PC fans in the parts bin; I thought to mount one just to get some air circulation over the tubes/power transformer.

I figure the best approach would be to tap the switched "phono motor" outlet with a small 12V wall wart to power the fan, and rig the fan to blow across the tubes and out the back (or vice versa- in the back and over the tubes).

Good idea?

[VintagePC]

...would be to reverse the connections to ONE of the coils and see if it oscillates.

D'oh! WE HAVE A WINNER!

Swapped the coil leads, and almost immediately found the sweet spot using the handheld. Followed it up and down the dial for a bit, then tried to find a station (plenty of squeaks and noise at this time...).

Found one, but it required cranking the volume about 1/2way and shoving the 'ANT' spade connector in to a 10 foot USB cable (which is currently masquerading as a radio antenna).

This was with the old tube, mind you; I did another check on the voltages while it's on a station, and it's now sitting at 106V... so as you suggest, the tube could indeed be weak. Will pop the other one in after it cools down to see if there's a noticeable difference. (Edit: Doesn't sound like a major difference to me... and the voltage on the new tube also reads 100 now)

I would have been willing to bet that I'd rewound the coil properly; I remember putting it on the end of a pen and pulling the broken wire off so that it unwound in an overhand fashion... and the current one is wired in the same way. (it will only go on the pen one way b/c of the lugs on one end.). Next step is to figure out if we match up with the dial; that'll have to wait until sundown when more AM starts coming in (and until I pop the chassis back in the case).