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Function Gen with Tubes - Exact 240 - Now with Manual and Schem
Posted by FlyingHacker on 06 Oct, 2015 19:59 -
Hi,
I recently acquired an Exact 240 from a garage sale. This is a function generator that will output Square, Sine, and Triangle waves up to a whopping 10kHz! This guy has tubes, and a few transistors.
I got her to power up, did it slowly with a Variac. She actually works quite well. The sine wave needs a hint more shape tweaking, but I think it can get quite nice.
I am posting in the repair forum because (a) it is pretty cool vintage tech, and (b) I am looking for advice regarding the cardboard tube capacitors. I have ordered a schematic from Tucker (who else? Do they own every manual on the planet?)
My understanding is that those cardboard caps are just waiting to fail. Advice I have found online suggests gutting the tubes and installing modern electrolytics inside the tubes for aesthetics.
Yes, the previous owner worked at McDonnell Douglas aircraft. This unit has a cal note from Aug 1972! I assume it was made in the '60s.
UPDATE: Manual here: The date is from 1962:
http://meanmutha.com/Exact_240_Function_Generator_Manual_w_Schem.pdf
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A look at the top inside. You can see all the adjustments are clearly marked. Who needs a manual!
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Alternate angle.
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Directly soldered on the front switches.
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The questionable cardboard tube caps, and valve goodness.
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Point to point connections on the "main" board.
And a view of the underside.
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Very cool, I look forward to updates.
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Very nice construction, point to point wiring and the time it must have taken to do the wiring harness. Those questionable cardboard tube caps need to be replaced. I made the mistake once and left them in a tube amp, as it tested good and one day shorted and fill the room with smoke.
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Very nice construction, point to point wiring and the time it must have taken to do the wiring harness. Those questionable cardboard tube caps need to be replaced. I made the mistake once and left them in a tube amp, as it tested good and one day shorted and fill the room with smoke.
Would your suggestion be to clean out the existing tubes and put electrolytics inside the tubes? How hard is this to do? Do you typically have to melt wax out, or will the pull out?
I am waiting on the schematics because at least some of them do not appear to have any markings. I want to make sure I get the right values. -
Hi,
I recently acquired an Exact 240 from a garage sale. This is a function generator that will output Square, Sine, and Triangle waves up to a whopping 10kHz! This guy has tubes, and a few transistors.
I got her to power up, did it slowly with a Variac. She actually works quite well. The sine wave needs a hint more shape tweaking, but I think it can get quite nice.
I am posting in the repair forum because (a) it is pretty cool vintage tech, and (b) I am looking for advice regarding the cardboard tube capacitors. I have ordered a schematic from Tucker (who else? Do they own every manual on the planet?)
My understanding is that those cardboard caps are just waiting to fail. Advice I have found online suggests gutting the tubes and installing modern electrolytics inside the tubes for aesthetics.
Yes, the previous owner worked at McDonnell Douglas aircraft. This unit has a cal note from Aug 1972! I assume it was made in the '60s.
I would guess,a little bit earlier,maybe mid to late '50s.
The transistor types will give a good clue as to its vintage. -
I am thinking of trying to reseal the fixed up tubes with beeswax. Supposedly this is what most of these capacitors were originally sealed with. I also bet it would be easier to get out that, say, hot snot should someone every need to replace them again in another forty or fifty years.
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Yes, the previous owner worked at McDonnell Douglas aircraft. This unit has a cal note from Aug 1972! I assume it was made in the '60s.
I would guess,a little bit earlier,maybe mid to late '50s.
The transistor types will give a good clue as to its vintage.
I think the only transistors in the unit are the two power transistors you can see in the photos of the cardboard tubes. From the part number it looks like they are germanium. 2n554 and 2n176. -
They definitely sound like transistors from the period I referred to,or maybe very early 1960s.
10 years are a long time,& by the end of the 1960s,transistors were mainly Silicon,& many of the type numbers familiar today were already with us.
PNPs were pretty much reserved for special purposes like complimentary symmetry audio amplifiers by then,too. -
Very nice construction, point to point wiring and the time it must have taken to do the wiring harness. Those questionable cardboard tube caps need to be replaced. I made the mistake once and left them in a tube amp, as it tested good and one day shorted and fill the room with smoke.
Would your suggestion be to clean out the existing tubes and put electrolytics inside the tubes? How hard is this to do? Do you typically have to melt wax out, or will the pull out?
I am waiting on the schematics because at least some of them do not appear to have any markings. I want to make sure I get the right values.
You have to desolder them, sometimes you have to untwist the locking tabs and once capacitor is out, then heat it and remove the cardboard cover, or drill the inside out, then you stuff it with the new capacitor and you can use hot glue is fine. -
Thanks, OldSchoolTechCorner. I found some videos on YouTube using the term "restuff".
I will know more when I get the schematic, but most of these caps seem to have three or four leads, rather than two. Not sure how they are set up. Is there sometimes more than one capacitor inside a single tube?
Luckily, these tubes all look like they are bolted in with flathead machine screws and nuts. So they should be pretty easy to remove once I desolder them. I will do one at a time and take pictures to make sure I get things right.
Thanks. -
Thanks, OldSchoolTechCorner. I found some videos on YouTube using the term "restuff".
I will know more when I get the schematic, but most of these caps seem to have three or four leads, rather than two. Not sure how they are set up. Is there sometimes more than one capacitor inside a single tube?
Luckily, these tubes all look like they are bolted in with flathead machine screws and nuts. So they should be pretty easy to remove once I desolder them. I will do one at a time and take pictures to make sure I get things right.
Thanks.
You need a schematic as could be more then one capacitor, in those cans. -
Looks to be built in or after 1966 the two transistors in the first batch of pictures have clearly visible date codes.
I love it, looks great It would be cool to see the schematic.
I am betting the power supply has a series pass tube regulator for the high voltage.
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Oooooh, sweet
Quite a low frequency range on that, too. Would be a perfect complement to an analog X-Y plotter, perhaps?
Those caps are electrolytic, of course; the cardboard sleeve is usually used for insulation because the case is "hot". Sometimes the markings are stamped on them; other times, they're just a blank guard, and you will find the ratings on the metal can inside.
Looks like regulated heaters! Those TO-3's are 3A 40V PNP germaniums. Lots of diodes underneath, maybe two regulated pairs or something? Also guessing 6080 and 12B4 for main pass tubes, possibly something like plate-and-screen supplies, or +/- supplies. Bipolar would make sense given the low frequency range (the output certainly cannot be transformer coupled! That 12BY7 must be an output follower or something), and would necessitate the insulated capacitors, or one or two of them at least.
My vintage rackmount porn, in trade:
http://seventransistorlabs.com/Images/Gaussian_Noise1.jpg
http://seventransistorlabs.com/Images/Gaussian_Noise2.jpg
http://seventransistorlabs.com/Images/Gaussian_Noise3.jpg
http://seventransistorlabs.com/Images/Gaussian_Noise4.jpg
http://seventransistorlabs.com/Images/Gaussian_Noise5.jpg
http://seventransistorlabs.com/Images/Gaussian_Noise6.jpg
http://seventransistorlabs.com/Images/Gaussian_Noise7.jpg
I don't have use for it at the moment, but it seems to work. First time I powered it up, the caps got awfully warm. Didn't have a variac at the time...
Tim -
Oooooh, sweet
Way too much boat anchor goodness in this thread.
Quite a low frequency range on that, too. Would be a perfect complement to an analog X-Y plotter, perhaps?
Those caps are electrolytic, of course; the cardboard sleeve is usually used for insulation because the case is "hot". Sometimes the markings are stamped on them; other times, they're just a blank guard, and you will find the ratings on the metal can inside.
Looks like regulated heaters! Those TO-3's are 3A 40V PNP germaniums. Lots of diodes underneath, maybe two regulated pairs or something? Also guessing 6080 and 12B4 for main pass tubes, possibly something like plate-and-screen supplies, or +/- supplies. Bipolar would make sense given the low frequency range (the output certainly cannot be transformer coupled! That 12BY7 must be an output follower or something), and would necessitate the insulated capacitors, or one or two of them at least.
My vintage rackmount porn, in trade:
http://seventransistorlabs.com/Images/Gaussian_Noise1.jpg
http://seventransistorlabs.com/Images/Gaussian_Noise2.jpg
http://seventransistorlabs.com/Images/Gaussian_Noise3.jpg
http://seventransistorlabs.com/Images/Gaussian_Noise4.jpg
http://seventransistorlabs.com/Images/Gaussian_Noise5.jpg
http://seventransistorlabs.com/Images/Gaussian_Noise6.jpg
http://seventransistorlabs.com/Images/Gaussian_Noise7.jpg
I don't have use for it at the moment, but it seems to work. First time I powered it up, the caps got awfully warm. Didn't have a variac at the time...
Tim
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Oooooh, sweet
Quite a low frequency range on that, too. Would be a perfect complement to an analog X-Y plotter, perhaps?
It is pretty low frequency, which is kinda cool. How were you thinking it would tie in with a plotter? I wish I had a plotter... Or maybe I should build one.QuoteThose caps are electrolytic, of course; the cardboard sleeve is usually used for insulation because the case is "hot". Sometimes the markings are stamped on them; other times, they're just a blank guard, and you will find the ratings on the metal can inside.
Looks like regulated heaters! Those TO-3's are 3A 40V PNP germaniums. Lots of diodes underneath, maybe two regulated pairs or something? Also guessing 6080 and 12B4 for main pass tubes, possibly something like plate-and-screen supplies, or +/- supplies. Bipolar would make sense given the low frequency range (the output certainly cannot be transformer coupled! That 12BY7 must be an output follower or something), and would necessitate the insulated capacitors, or one or two of them at least.
I don't know much about tubes to be honest. I'm in my forties, and transistors had pretty much taken over when I was growing up. My dad had a few tube items, but he didn't want me messing with them because they were fragile and high voltage.
When you say regulated heaters do mean some sort of heating element for frequency control, like an ovenized oscillator?QuoteMy vintage rackmount porn, in trade:
http://seventransistorlabs.com/Images/Gaussian_Noise1.jpg
http://seventransistorlabs.com/Images/Gaussian_Noise2.jpg
http://seventransistorlabs.com/Images/Gaussian_Noise3.jpg
http://seventransistorlabs.com/Images/Gaussian_Noise4.jpg
http://seventransistorlabs.com/Images/Gaussian_Noise5.jpg
http://seventransistorlabs.com/Images/Gaussian_Noise6.jpg
http://seventransistorlabs.com/Images/Gaussian_Noise7.jpg
I don't have use for it at the moment, but it seems to work. First time I powered it up, the caps got awfully warm. Didn't have a variac at the time...
That looks like fun. Love the graphics on the front panel.
I don't have a huge use for this Exact 240 either, but it seemed like fun to clean up. Perhaps if I need a real slow oscillator for something, maybe amplitude modulation of some sound waves or something. -
After a little research in tubes I am guessing you mean one of the tubes looks like it has two heaters in it? Or that would be the purpose of the two germanium power transistors... .??
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Looks to be built in or after 1966 the two transistors in the first batch of pictures have clearly visible date codes.
I love it, looks great It would be cool to see the schematic.
I am betting the power supply has a series pass tube regulator for the high voltage.
So they do!
I must admit I didn't go & look at the pix offsite,so couldn't blow it up big enough.
When I did I could see them too.
They could be replacements----OK,so I'm reaching here!
I was going by the general look of the thing,plus the fact they used Germanium transistors.
As a specialist device they wouldn't sell that many,so a 1966 one might not look any different to a 1959 one.
Back in 1965,my then work had a self -tuning ISB transmitter which used PNPs as part of the motor drive circuit for the autotune,(it was already a few years old).
It was s/n (1),& we basically did the "debugging".
It kept killing the PNPs.
Ultimately,a Supervising Tech & a couple of Senior Techs redesigned the circuit to use Silicon NPNs---no more problems.
The EE said "well done" & went on his way! -
Very nice piece you have there. I would also restuff those caps to keep the original look.
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Short intro to tubes:
- They're more-or-less low transconductance, low current density, depletion mode N channel FETs. No such thing as P-channel, which makes things difficult from time to time.
- Operating voltages are >10x higher than for most SS (solid state) circuits. So 100V supplies are kind of entry level, and 200-300V supplies are most common (with 500+ for power amplifiers and such; the largest ~MW size transmitter tubes are circa 30A and 30kV).
- Likewise, currents are lower, like 1-10mA for signal tubes, up to 100mA for outputs. The biggest common tubes are line output / sweep tubes, used for switching the deflection and flyback inductors in TVs; they go up to maybe 1.5A peak.
- Triodes are the only amplifying device that has a most nearly constant-voltage output characteristic. All others have the output characteristic (Id vs. Vds, Vg as parameter; Ic vs. Vce, Ib as parameter; Ia vs. Vak, Vgk as parameter) which rises sharply at low voltages, then levels off to a constant current ("linear") region.
- In a tetrode or pentode, the screen grid acts as the triode plate (while not drawing 100% of the current), in effect setting what the plate current will be.
You can also think of the screen as a second grid, where the current flow is due to the sum voltages on both grids. Difference being the screen is mu times less sensitive.
The last weirdness is, the cathode doesn't do jack until you heat it up.
So when I say heaters, I mean literal heaters, they make the cathode hot, so it can emit electrons! Archaic, right?!
The heater is insulated from the cathode, so it can only be so-and-so voltage away from it; it's possible they have two heater supplies for two sets of heaters, one around 0V and one around -200V or so.
Tim -
"FETs with a fire in 'em!"
"it's possible they have two heater supplies for two sets of heaters, one around 0V and one around -200V or so."
Possible,but not common.
The heater/filament insulation is pretty good,really,& normally there wouldn't be much of a problem.
Perhaps if a tube was used in association with the cathode of an Oscilloscope CRT,that may be needed.
Some older Vacuum tube rectifiers have "directly heated" cathodes,(as do high power Transmitting tubes).
In that case,there is no insulation between "cathode" & "heater" as they are one & the same.
Rectifiers with directly heated cathodes are used with a special power transformer heater winding which needs to be capable of operating at the full positive HT Voltage.
Most "indirectly heated" tubes operate with heater voltages of 6.3volts-----even the "12volt" series of tubes
have centre taps to allow operation on this voltage.
Some tubes for use in series filament chains have higher heater voltages,& tubes used in old portable radios used 1.5 volt directly heated tubes.
High power Transmitting tubes usually have filament/heater voltages in the region of 6 to 7.5 volts,at lots of amps!