Fleetwood model 4068... Advice?

[VintagePC]

I've replaced the out of spec 33Ks, and one out of spec 470.

The 2.7 reads 2.6, so should be all right... but there's still 16V on the pins.

I did check pin 5 as you suggested, and it is 265v bang on.

[electroking]

Check the voltage on the first grid (pin 7). This should be ZERO. Any positive
reading here indicates some leakage from the plate of the previous stage (through
a defective capacitor C14 or C19). Another thing to check is to make sure that
the speakers are connected to the correct taps as found when you got the set.

The present conditions are giving you about 8.5 W plate dissipation, while the
tubes are rated for 12, so I would say that if sound through the speakers is
acceptable, you could leave things as they stand, and just go around the
four audio tubes and measure all the specified voltages. This will give us a
global picture of the operating conditions of the audio stages. You might
also try substituting the brand new tubes (6AQ5s first, then 6AU6s) and see
what this does to the voltages. Worn tubes can do funny things...

[VintagePC]

There's about 100mv on one pin 7, and 50 on the other.

The speakers are connected correctly- I made a diagram before I disconnected anything. (and I verified that they are indeed 8-ohm speakers)

Both C14 and C19 were originally replaced, and I doubt _both_ of them would be bad... but it's possible.

The only other voltages on the schematics is the 1.1 on the 6AU6s;
here I measure about 0.8 on one tube and 0.5 on the other. All of the HV seems to check out, so I'll try doing the tube roll. (any relation to the grid plate shuffle?... Sorry, couldn't resist.)

Also, PM to follow.

[VintagePC]

No difference with the tube changes... so I think that if we're within operating specs, I have myself a working set... I should note that there's some speaker hum when I measure the voltages on pin 7... I assume that's normal, since the meter is tying the pin to chassis ground.

Edit: I did find that changing the function switch to BC (as said on the schematic) lowers the voltage to about 14V... within the 20% tolerance.

However, I checked some of the other voltages. Pin 6 of the 6AQ5 should have 230V, but there is 265. Also, the 42V and 56V on the 6AU6s actually measure at about 10 and 20V. Wierd.

I doubt it's the cause, but I did note on the schematic that R35 (filter section of power in) masures only 228K as opposed to 270K.

[electroking]

Bingo!

Will look at your PM later, just found out something important (from what I just
read)! When in phono mode, the power supply does not feed the two radio tubes,
so current through R18 is lower, and so the screen voltage on the output tubes
does get higher, increasing cathode current. Also touching the control grid
does induce hum as your whole body, meter, etc. act as a coupling capacitor
to feed hum to the amplifier stage. Doing this to the grid of the previous
stage (6AU6) should induce even louder hum as you now have two tubes
amplifying.

The funny voltages on the 6AU6 tubes: do they remain funny when you set
the switch to radio? If the two channels have different readings, something
is wrong with components... Bye for now.

[electroking]

You mentioned that the voltage on pin 6 of the 6AQ5 tubes was 265 instead
of 230. Something might be open in the network formed by R21, R24 and
R13, which would explain the wrong voltages measured on the 6AU6 tubes.
Those parts are nearly 50 years old (just like me), don't forget, but
fortunately there are not too many of them. Good luck.

[VintagePC]

Recieved and understood I'll re-check the voltages with the switch in radio mode.

[VintagePC]

Well, I've ended up replacing just about every resistor in the set (save 5 or 6). The tube voltages around the 6AQ5 seem to be better now (at 12V in radio mode), but there's still some weird stuff going on around the 6AU6... I get about 30V on pin 5, and 20V on pin 6. This does change when I adkust the volume knob. However, the 1.1V line is now at about 0.9V, much better than originally.

Also, the tubes in the radio part are way out;
There's 89V on the 6BE6. (I think the schematic says 85) and there's well over 40 on the 6DC8. Could be okay if the schematic shows 56, but it looks more like a 36 to me. Haven't tried to tune anything.

There are also a multitude of disc caps and resistors attached to the volume knob, of which I can't find any trace on the schematic. I haven't checked those yet. A wild guess says they may be to filter interference, but I don't know. Pics available on request.

The set's probably been too damp, as these are carbon composite resistors.

I should also note that the replacements are all salvaged resistors, and while I've checked them to make sure they're within 10%, I have no idea how they behave under load.

I'm inclined to go and buy new ones and just replace all of them for this reason, and also because the leads on these are much shorter than what was in the set; so I've had to do some clipping and solder to the old leads... which doesn't look pretty, but it works in a pinch.

Reese- hope you have some advice (even if it's just to go buy new resistors) since Electroking is away for a few days

[electroking]

Last message before I get on my trip!

The voltages on 6BE6 and 6DC8 look fine to me: 89 is only 5% over 85, and
40 is only 11% over 36 (correct as I see it), so no big problems here. Salvaged
resistors are OK if they measure OK. I would suggest at this point that you
let it cook for a few hours, and come back and do some measurements. Just
have it play some background music from a CD player or similar source so you
know no catastrophe is happening. Back in a few days!

[VintagePC]

All right then... have a good one!

Ah, I see how you calculated the voltage %s... I was thinking 36+(20%x36)=43.2V max, as opposed to (40-36)/36=11%

I'll check the price for new resistors just for kicks; if it's not much I'll just replace them all; I know the sloppiness of the current job is going to nag at me, and some of the replacements get a tad warm for my liking...

Edit: 100th Post! YaY!

Also... resistor prices: I can get all of them for $7 (excl shipping.)... so I'll probably just toss them in in the next order (digi-key) This is for carbon film 1/2 watts, with the exception of the 2watt ones needed in some spots- these are metal oxide film.

[VintagePC]

Also, is there anything I need to worry about before running this set off wall voltage? right now, I have it on the variac at ~95%, but the set was designed for 117V...I've read talk of dropping resistors, but this is only when replacing a metal rectifier.

[VintagePC]

Ok... been running the set for a while with a CD player. Sound is good (even on the built-in speakers), but there are a few things:

1. Left channel is fainter than the right. Likely a dirty balance or volume pot.

2. There's a faint hum with no audio and the volume on max. This goes away at lower volumes, so it's not of major concern.

3. Power transformer gets hot- still touchable, but not comfortable, kind of like a fresh cup of coffee. Voltages are steady and looking good after ~2h of running.

4. Some sharp audio (e.g. guitar) had a bit of distortion to it- It went away when I fiddled with the tone pot, so I think it is again, a dirty pot.

What is the suggested approach? replace the pots or clean them?

I don't have any deOxit on hand, are there good substitutes? It already got much better just from working the controls for a bit with the set off.

Edit: Just re-probed the radio tube voltages. They start high and eventually come down and stabilize. The 6BE6 is indeed at about 88V, but the 6DC8 is at about 55... well outside a 20% tolerance, if the schematic indeed shows 35V. It's too fuzzy to make out; can someone see whether it is "36" or "56" V? on the 6DC8? This datasheet suggests the tube can handle it (http://tubedata.itchurch.org/sheets/020/e/EBF89.pdf) but it may indicate part of the reason the AM isn't tuning.

Also did a quick check on sweeping the dial, but no stations came in. Could be that there's nothing rolling in yet, but I should at least be able to find the local AM at 1450... but it's possible that rewinding the oscillator bumped it off the top edge of the range.

[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'