Vox historians: help reading minutiae on 1960 Vox AC15 OA/031 schematic

[dchang0]

Wow, the stuff surrounding V.7 is impossible to read.

Here's the screenshot:



Everything unreadable is outlined in red.

At the very left, just under SW1

VIB/TREM ?????? I think this says VIB/TREM SPEED, but that last word looks like it starts with a D. Could be VIB/TREM DEPTH

Just under the V.7 label, it looks like:

?0V, 1?0V? No idea what this is supposed to be.

Just under the left half of V.7:

2.?V
2.?V?

To the top right of the right half of V.7:

200V Not sure if this is 200V, but it seems right.

To the bottom right of the right half of V.7:

1?.5V
1?V?

No idea what that should be.

At the very bottom:

DEPTH PRESET?


Any help is much appreciated!

[dchang0]

Final set of questions

On the power transformer:



The Center Tap says:

120??
0V

What is that 120??

And right under the iron core, there appear to be two characters--what are those?

Thanks very much for all your help so far.

Here is the FIRST DRAFT of the final relettered schematic. IT HAS NOT BEEN DOUBLE-CHECKED!!!!

If you see any errors, please let me know.



(Resolution is limited to 150dpi due to 1M max attachment size.)

[g1]

For the vib., I think speed on the left and depth preset on right.
For the OT, I think 120mA (current rating of secondary HV widing), and E S at the bottom, for "electrostatic shield" wire.

[dchang0]

Makes perfect sense. I'll update the schematic. Thanks!

Here's the latest draft:

[Alex R]

For the vib., I think speed on the left and depth preset on right.
For the OT, I think 120mA (current rating of secondary HV widing), and E S at the bottom, for "electrostatic shield" wire.

yes, and under the '120mA' that's '0v' on the centre tap.

Under the 'V7' label - that's going to be a reference to the variable voltage there (I guess). Can't read it though. It might give the range of voltages, or a voltage with the footswitch off.

[dchang0]

I'm wondering if the voltages for each valve are either:

1) voltages calculated at different points inside the valve itself

2) allowable bias voltage ranges

3) maximum or minimum safe values (highly unlikely)

or as you suggest

4) voltage range depending on potentiometer settings

If we can figure out what these labels are supposed to mean, that can help us figure out what they are supposed to be...

[Alex R]

They're expected voltages at a particular connection to the valve, the plate in this case. Cathode voltages are also labelled sometimes, as they are on this diagram. It's handy for repairers to know what to expect at that particular circuit node. They're just written next to the connection. But v7 is an oscillator, which means the plate voltage will vary as it oscillates. Sorry not to make that clearer.

eta it looks like the ac30 vib/trem circuit. Here's a link to what looks like a contemporary schematic for the AC30 (desn;t matter that it's the bass model, differences are elsewhere and tiny. Most if not all values in the vib/trem area are going to be the same.

http://www.drtube.com/schematics/vox/ac30bass.gif

eta just says '120v' on the oscillator plate on that one - maybe leave it at that ?

[g1]

I think he's asking about spots where there are 2 voltages listed for the same point, in areas aside from the oscillator ?

[dchang0]

Ah, now I get you. Good point--the voltages WOULD vary in the oscillator.

Thanks for the AC30 reference. I'll see if I can use any AC30 docs to help decipher the original AC15 schem.

(I could just go lazy on it, but as you say, it's handy for repairers. And part of this is just nitpickiness for historical accuracy.)

[dchang0]

I think he's asking about spots where there are 2 voltages listed for the same point, in areas aside from the oscillator ?

Looking closely at the schem, there don't seem to be any dual-voltages outside of the oscillator circuit, so that lends credence to Alex R's hypothesis. I'm going to run with it and track down the AC30 schems that might have similar/same values...

Thanks for both your help!

[dchang0]

Latest draft schem.

I took Alex R's suggestion of cribbing off of the AC30 schem, and while some of the voltages aren't the same, they are close enough to help make educated guesses.

Thus, pin 6 on V.5 is probably 180V (since on the AC30 it is 200V). It could still be 190V but probably isn't 170V nor 160V.

I totally overlooked 56V at pin 8 on V.2. That's now in there.

I also somehow accidentally deleted the drawing number OA/031! So that's back in place.

On pins 8 and 3 of V.6, I educated-guessed 3.5V-4V, since the AC30 says 4V at that point. Could be 3.6V or 3.8V, but my bet is that they wouldn't have bothered to write 3.anything if it had been too close to 4V, since they simply went with 4V on the AC30 drawing.

I'll dig around some more and deduce the other values as best I can, then come back when I'm stumped.

Thanks!

[g1]

Looking closely at the schem, there don't seem to be any dual-voltages outside of the oscillator circuit, so that lends credence to Alex R's hypothesis. I'm going to run with it and track down the AC30 schems that might have similar/same values...

I see dual voltages on both V6 and V7, but perhaps they are all only when trem is engaged.

The attached pdf may help, the tubes are numbered differently, I'm not sure what all else is different.
It's courtesy of Steve Luckey (from here: 1960 Vox AC-15)
and he has more cool stuff here: Index: Amp projects and more...

[dchang0]

WOW, thanks! I've heard of sluckey's take on the AC15 but never seen the documents. The parts list is super-helpful to me right now.

Sadly he took the voltage measurements with the oscillator disabled, so they are close enough to confirm the ranges to within 5-20V but not precise enough to help decipher the letterings. The same is true of the various JMI AC30 schems--all close enough to confirm but not close enough to pinpoint.

That's okay, I've decided to commit to the numbers that SEEM to be right, visually, based on the tracer's handwriting (his 9s lean to the left while his 8s lean to the right, etc.). The first complete draft, v0.5, is here:



From here on out, it'll be just making corrections as people discover them. Thanks again, all of you, for your help!

I can't seem to attach a PDF--oh well. The schematic appears to be readable enough on a crappy printer even at 140dpi on US Letter paper with the tiny print. I'll leave it as-is.

[dchang0]

Hey, g1--you just gave me the one schematic that has all the small, hard-to-read voltages right on it (and to think I had this schematic all along but assumed it was for a different model of AC-15).

On the "Vox AC-15 No. V-1-5" schematic, I was able to find that the voltage at pin 6 on the EF86 is actually 90V, not 80V as I had thought nor 50V.

It also confirmed perfectly all the voltages in the oscillator section, down to the decimal point. I got many of them correct--190V at pin 6 on V.5, 80-74V on pin 6 of V.6, 2.2-2.3V at pin 3 on V.7, and 80-130V at pin 1 on V.7. But it corrected the voltage at pin 8 on V.7 from 13.5-18V to 18.5V-18V and 3.5-4V at pin 8 of V.6 to 3.9-4V.

So, now that every one of the small unreadable voltages is accounted for, I am releasing version 1.0 of the drawing and calling it done! THANK YOU, g1!

[Alf]

Thanks a lot for making this schematic legible. Great job.

Alf