Fleetwood model 4068... Advice?

[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?

[Reece]

Those sets were made to run without fans, as long as the back is ventilated, but there's nothing wrong adding one: heat is the enemy of all electronics.

[Rodzilla]

you can also get similar small muffin fans that run straight off the AC line...with an ac rated one,you wouldn't need an adapter,you could just add a plug and connect it to the phono outlet...tho often these type of small fans tend to make more noise than they need too running full speed and can use some kind of speed control to slow them down and cut out some of the noise

[VintagePC]

Thanks for the advice.

This is a nice quiet one that came from a dead PC power supply.
I've got it on a variable wall wart (Set to about 10V) to keep the noise down.

I've also got a pass-thru power plug I can drop in between the brick and phono outlet, so all's good to go! (I'd otherwise use the TV chassis outlet, but this one is switched so the fan is only on when the marshmallow toasters are running...)

Basically, now all that's left to do is figure out the radio and why it's not working...

[electroking]

electroking will help you with the radio! But it will have to wait one more day,
I have a rush job to finish tonight. Bye for now.

[VintagePC]

Hooray! *party*

Seriously, welcome back, and take your time. It's been quiet without you... even though I've been enjoying CDs on my tube set (and contemplating roasting some marshmallows on it...). Really makes you realize how much some speakers/sets sound like you have your head in a tin can.

I'm pretty tempted to put a decal on the front of this set that says "electroking"... Much nicer model name than "stereo Hi-Fi", don't you think?

Edit:
Also, did a quick test at wall voltage; the HV goes up to 303V... 6% over 285, so that should be all right - this is with the radio tubes off the HV.

[VintagePC]

I've whipped up a spark-gap transmitter with an old relay and .1 uF cap, so we have something to test the oscillation with even if there isn't any AM rolling in- as is usual during the day (with the exception of a faint local one).

Edit: Some more news... both good and bad. The good news is the set oscillates; I can hear the buzz from the spark-xmitter when I bring it close to the antenna. The bad news is no stations... Anything that needs to be checked, or is it adjustment/alignment time?

[electroking]

Hello again,

Well, a spark gap gizmo will indeed produce noise that can be received across the
band, but it is a crude instrument at best...

I won't start writing an essay about the theory of superheterodyne receivers right
away (but I can do it if you tell me it would help). Instead, let's determine if
the oscillator is running. When tuned to the bottom of the band (550 kHz or so),
the oscillator should be running at 995 kHz, and at the other end of the band
(1600 kHz), it should run at 2055 kHz. These figures are just 455 kHz about the
desired carrier frequency.

If you have two working AM radios in the house, you should be able to do some
testing. Just one working radio is actually needed for the test, but two will help
demonstrate the procedure. If you set one of the radios at the bottom of the
AM band, its LO will run at about 995 kHz as stated above, and if you set the second
radio around the latter frequency, you should be able to find a spot where you're
just picking up the radiation from the first radio. You have to try it to hear the
effect. Now as you increase the tuning of the 'source' radio, you can track its
radiation by also increasing the tuning of the 'observing' radio.

The trick to check your receiver is to use the same 'observing' radio as above
(it will work fine if it is a small portable unit with a speaker), placing it near the
oscillator tube (6BE6) in your Fleetwood. You should be able to crudely
determine whether the oscillator is running.

Let me know what happens next. Good evening.

P.S.: the method described, using a small AM radio as a probe, will not work for LO
frequencies above the upper limit of the probe (1600 or 1700 kHz), corresponding
to carrier frequencies of about 1150 to 1250 kHz. There is no guarantee that an
oscillator that works at a lower frequency does not quit at some point in the
frequency range.

[VintagePC]

Thanks for the details; I had assumed the SG was very crude, but better than nothing; of course now I know better.

No need for the essay (unless you want to- I'd still read it out of interest); I've had a look at the wiki article, which does an interesting job of explaining how it works.

I've got a couple of AM sets around here; I'll try the test and we'll see what happens. It'll have to be tomorrow though; it's getting late and I have part-time work tomorrow morning. ( earning some spare dosh to fund such projects as this, of course... *grin*)

[VintagePC]

Okay, the results are in...

I practiced for a bit on my bookshelf stereo using an old walkman. It picks it up quite well, and I was able to locate the noise (Kind of like a pulsing hum, for lack of better description) and track it up and down the dial.
(Also found that the bookshelf system makes a LOT of noise.. I could even locate the frequency of the VFT display; it would pulse whenever a new segment lit up or went out.)

However, on the tube set, no luck. (took the shield off the 6BE6 for this test). I tried sweeping the frequencies on both the tube set and the walkman. I picked up some noise, but nothing that changed on adjusting the set dial.

[electroking]

Hello,

Removing the shield on the pentagrid converter was definitely a good idea to
enhance oscillator radiation. You now have to see why it's not oscillating.

Check the continuity of the IF transformer primary (1st IF transformer) that
feeds the plate of the 6BE6. Then, under power, measure the plate and
gride voltages on the tube and compare with schematic. Testing with the new
tube might also be a good idea. There is also the possibiliy that the coil was
not installed correctly after you rewound it. Oscillation depends on feedback
from grid (pin 1) to cathode through the circuit formed by C4, coil and C2+C2A.

Also, as I told you before, the oscillator is more likely to work at a lower frequency
if something is marginal in the circuit, so just leave the tuning at the low end
of the band while searching for oscillations with the test receiver. You may also
want to disconnect the coil from the variable capacitor and make sure the
capacitor itself is not shorted (bent plate). You have to disconnect the coil
to do this, otherwise you would be just checking the continuity of the coil.

The article on 'pentagrid converter' in wikipedia might make good reading if
you haven't already looked up. Bye for now.

[VintagePC]

Hey,

I re-soldered the coil's ground connection since I wasn't happy with it; but it didn't seem to make a difference.
I re-checked the installation with my original photos and drawing, and I think I got it right.

Voltages are also within tolerance (~90V, <10% over 85) and the primary IF is not open-circuit.

Tried the other tube, and I got some static out of the speakers, but couldn't locate the frequency either. I should also note that with this tube, the IF voltage dropped down to about 69v... Odd?

C2/C2A are not shorted.

[Reece]

Are you sure that you rewound the coil in the same direction as the old one was and that the coil ends were connected to the terminals same as originally? Oscillation depends on the two coils being in the correct phase WRT each other. A test (and possible correction) would be to reverse the connections to ONE of the coils and see if it oscillates. It may be way off but should tune in something on the dial or make a squawk somewhere in the broadcast band on another radio.

[electroking]

Also, the new tube caused a drop in plate voltage, indicating that it was drawing
more current than the old one. I would stick with the new tube, assuming that
the old one was tired. The exact plate voltages on the schematic may be
based on reception of a station, which would cause a certain level of AVC (negative
voltage across R7, providing a gain reduction of the two tubes by biasing their
control grids). Good luck, correct operation may not be far off!