All done...
 

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. :thmbsp:

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?

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! :)

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...

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

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!

Come in Houston....
 

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?

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!

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...

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

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.

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...

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.

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.

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.

6AQ5 screen grid voltage
 

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.

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

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 :)

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!

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.

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.

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.

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.

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?

Greetings from FixitLand!

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'

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.

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

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?

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.

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

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 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.

Hooray! :banana: *party* :banana:

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.

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?

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.

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*)

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.

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.

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.

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.

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!