Fleetwood model 4068... Advice?

[electroking]

Originally Posted by VintagePC

Still twiddling my thumbs.

...
The only issue I could see is with the set's floating ground- Would it cause an issue if I plugged in an audio source with a hard-wired ground?

Anyway, once I've recapped, what would be the best way to power up the set? I don't have a variac (alas) but I'd thought to pull all the tubes, check the transformer output voltages, and then put the tubes back in one by one, testing voltages (indicated on the schematic) as I go.

No problem with ground floating ground, it will just cease to be floating
when connected to a grounded source. This is not really a hot chassis,
the power transformer does provide isolation. You can ground the chassis
to earth if you make sure you have the correct power line polarity as
discussed before, end even with incorrect polarity, the leakage will
be small and tolerable as long as the line filter components (one
resistor and two capacitors) are OK.

Do not plug the tubes one by one! Instead, install all the tubes
except the rectifier. It would be best to start up with a variac
(do a bit of research with this word), or with an incandescent
bulb in series, but in a pinch the line fuse will do. With power
applied, tube heaters should light up (not very bright, but visible
in the dark). Let the set run (while watching) for maybe 30 minutes.
The power transformer should get mildly warm, but not any hotter.
Without rectifier, no DC will be generated, so no voltage measurements
to be done at this point.

You may use your old tubes for this test (hope you did not discard
them), many or all are likely to be good. Bad tubes would be
detected later by measurements and substitutions.

Now with power off, install the rectifier tube. The time for the
full test is approaching. Securely hook up your DC voltmeter to C23 and
ground. Turn on power, and watch the meter. The situation
is now the same as before, so no sparks are expected, but as
the rectifier warms up, the correct DC voltage (275) should appear.
If the value is substantially lower, it means you have a short somewhere
down the road from the rectifier, HIT THE POWER SWITCH. Otherwise,
listen for some hum in the speaker, etc. You may start doing the
voltage measurements you suggested.

This should get you going for a while. Good night.

[VintagePC]

Yes, I did keep the tubes that were in there I'm not tossing anything until I know for sure it's bad.

Regarding the variac... I might be able to borrow one from work... I'll have to ask. If not, then I should have enough stuff to make a dim-bulb tester.

[VintagePC]

Good news- I've sourced a variac that I can borrow.

With this, I've read that the procedure is to bring the set up in steps (typically 10v steps/30 min?) Can someone confirm or provide a link to a good guide for doing this? I've found a few, but those all discuss re-forming the original filter caps and precautions if you're bringing the set up with the original paper caps- I'll have replaced all of those by then, so I don't know if the process differs much.

There's also something about a point around 40-50V where things get near the minimum operating voltage to run, and that it should be left there for longer.

Thanks!

[electroking]

Re-forming applies to the power supply electrolytics. If they are new and
correctly wired, there should be no problem with those capacitors.

Without the rectifier, you can turn up the variac as slowly as you wish, but
when you do the test with the rectifier in place, dont run the unit under
say 80% of nominal voltage for more than a few minutes. If it seems
to be idling OK at 80%, you can get it up to 100%. This radio is much simpler
than a TV, where for instance lack of horizontal drive could fry the sweep tube.

Also, in addition to what I wrote to you last time, you should do the initial testing
with rectifier with the function switch in PHONO or STEREO, rather than
RADIO (or BC). This will keep the two radio tubes off the high voltage.
Of course, this test should be done with the speakers (or load resistors) connected.
After checking the high voltage at C23, the next measurements should be
the cathode voltage at the output tubes (pin 2 of each 6AQ5, 12 volts) and
at each of the driver tubes (pin 7 of each 6AU6, 1.1 volts). Correct values
at these four points will almost certainly guarantee that the tubes are correctly
biased.

Once you get the audio section working, we'll have a look at the radio part.
Good luck.

[VintagePC]

Thanks for the info- good thing I asked too- since sometimes being to cautious is just as dangerous as not.

My capacitors arrived today... So I'm going to have me a restuffing party!

[VintagePC]

Just spent an afternoon recapping... what an experience.

Nothing quite like trying to figure out how the leads are attached when they all run in to a giant glob of solder.

Anyway, pictures and more info posted in the build log.

I'm waiting to get that variac before I test it... that should be next Monday.

[VintagePC]

I've also thought about refinishing the cabinet. There are some nice pieces of solid maple, but the rest seems to be made of hardboard. It's all coated with some horrid brown paint-on coating that is peeling like crazy.

I'd thought to strip it and put on maple veneer (and varnish it) instead. Can anyone with cabinet work experience advise on the best way to do this?

[VintagePC]

Okay... some good news, some bad. The good news is I've borrowed a variac. The bad news is that I borrowed it as-is, and unfortunately, it seems not to do anything.

Looks like I'll have to fix it before I can fire up the radio. I've already done the background research and have figured out that it's either going to be a simple fix, or not at all. Fingers crossed!

[VintagePC]

Some even better news... The variac just had a funky switch, and is working now!

I've done the preliminary non-rectifier tests, and all seems well so far- transformer didn't even get warm, all of the tubes lit up, and both the dial lights lit.

I kept the voltmeter across the filament line, and it's bang on at 6.3v AC.

(the occasional twitch to 6.4, but I expect that's because the wall voltage is 120V as opposed to 117 the set wants.)

I also checked the chassis to ground voltage, there's about 33v AC, so no major shock hazard.

I also probed the HV out from the transformer, that's at about 500V AC.
It seems a little on the high side, but I still live in a silicon world where rectifiers are that efficient

It's getting dark now, so I'll wait until tomorrow to put in the rectifier and do the secondary tests. Pictures of that warm tube glow coming soon!

Edit: Pics posted in the build log. Here's one to get you started...

[Reece]

The HV from the xfrmr. will drop when the rectifier is in and the tubes start drawing current.

[VintagePC]

That's true; I've seen it to some extent with low-voltage unregulated AC wall warts, but I didn't realize it was that extreme at higher voltages... but it makes sense if you think about it.

Cheers!

[VintagePC]

We have liftoff... REPEAT, WE HAVE LIFTOFF!

I get audio from the speakers using a test CD (has tracks for checking left-right separation, phasing, etc.) and player hooked up to the Phono input. Pots could do with a little cleaning, but that's not an issue unless you're moving them.

High voltage is okay (~280 V across C23 with variac at 95%).

However, I didn't run the set for very long- I checked the other test points and get 17V on the 6AQ5s, and about 0.8V DC on the 6AU6s.

That's well out of the 20% voltage tolerance, but I checked the two 470 ohm resistors on the AQ5s. one was a bit high- 500-something ohms-, and the other within 10%; so that's not the cause (both have 17V on them).

Thoughts?

[electroking]

Hello again, and congratulations for keeping the smoke where it belongs (that is
inside the components)!

The bias on the output tubes is determined by R33, R34 and R18 (as well as
the two cathode resistors of course). Once you have checked those five resistors
and made sure they are within tolerance (+/- 10 % I assume), the tubes should
work properly. You may also want to check the plate voltage at pin 5. Your
present results (cathode voltage higher than expected) mean that the cathode
current is a bit high. Also make sure your cathode bypass capacitors are correctly
wired (+ to cathode, - to ground). Good luck!

[VintagePC]

Yep... at least one of the 33K ones is on the high side. What wattage should the replacements be? I only have some smaller jobbies on hand (2mm dia. x 6mm long)... but I'm not done searching yet.

Edit: found some 1/2, possibly 1 watt ones...

[electroking]

1/2 W should be fine, screen voltage is close to plate voltage.