ovation fuzz from the late 60's/// help me resurrect it?

[Rob Strand]

no, its not channel one, cuz ya can switch the preamp cards, and the problem stays with the socket, not the cards.
the only thing after them is... you guessed it... that mixer stage.

From what I can see, the channel 1 jack connects to the mixer.  Yes?  It's the red wire on pin 3.

The mixer isn't one of the "amplicards" so if it fails it's "allowed" to fail.

i will get at it tomorrow.
immune system is wiped out from one medical issue, which has led to a mrsa infection that is really kicking my ass.

thanks bro... more soon

Take it easy.  Plenty of time to sort this stuff out.

I was thinking about the small caps on the Fuzz circuit.   The MOSRITE/FUZZRITE (originally from an amp) had small caps and a blend control.

[pinkjimiphoton]

rob,
if you're still up, pour yerself a libation and grab some popcorn, i gotta LOTTA info coming.
but first, that solder bridge you thought you saw is indeed a solder bridge, its actually i think shorting the center of the voltage divider to the center of the transistor... i assume the b?... right to freekin ground.

also, fuzz pot is 50k. didn't say a or b on it,  so... assuming its linear by the way it seems to act.

[pinkjimiphoton]

ok, i also traced out the wires to the mixer from the rest of the circuit, all the ones that go in along the bottom, viewed from the back.... it looks like they tried to get more gain out of it by tacking on those resistors, not realizing they'd shorted the dang transistor. i may be wrong.. gotta look in better light.

anyways, from left to right across the bottom

orange wire goes to channel 2 output switch, switch connects to tip when open, so normally closed.

dark brown wire goes to channel 2 tip

blue wire goes to reverb slot # 10, from left to right viewing the board from the component side

light brown goes to channel two volume #3 if looking from the back with the lugs up, on the right

black wire goes to the tuner on switch

three black wires on this one, to parallel out ground, reverb #12 socket and reverb pot ground, and both channels vol 1

red wire channel one pin 6 with card sideways, components to left and + voltage

purple reverb socket pin 8

[Rob Strand]

if you're still up, pour yerself a libation and grab some popcorn, i gotta LOTTA info coming.
but first, that solder bridge you thought you saw is indeed a solder bridge, its actually i think shorting the center of the voltage divider to the center of the transistor... i assume the b?... right to freekin ground.

OK, so it is shorted.   It's basically shorting out the emitter resistor on the transistor.   You can see the "" in that area of the circuit on my schematic.

I really don't now if it's part of the factory mods with those resistors, someone elses mods, or a mistake.    Shorting out the resistor will add more gain to the transistor *but* it will also change the bias points.   It is possible those resistors on the back of the board attempt to trim the bias point with the shorted resistor in place.   I really don't know what it *should* be.   BTW what is the transistor part number of the Mixer board?. I can work out  the expected bias voltages and you could check that by measurement on the circuit.

I don't know if I should be scared or what on the next drop 

[pinkjimiphoton]

if you're still up, pour yerself a libation and grab some popcorn, i gotta LOTTA info coming.
but first, that solder bridge you thought you saw is indeed a solder bridge, its actually i think shorting the center of the voltage divider to the center of the transistor... i assume the b?... right to freekin ground.

OK, so it is shorted.   It's basically shorting out the emitter resistor on the transistor.   You can see the "" in that area of the circuit on my schematic.

I really don't now if it's part of the factory mods with those resistors, someone elses mods, or a mistake.    Shorting out the resistor will add more gain to the transistor *but* it will also change the bias points.   It is possible those resistors on the back of the board attempt to trim the bias point with the shorted resistor in place.   I really don't know what it *should* be.   BTW what is the transistor part number of the Mixer board?. I can work out  the expected bias voltages and you could check that by measurement on the circuit.

I don't know if I should be scared or what on the next drop 

it looks like they added them resistors to try and get it to bias, none of the transistors are marked that i can see. all of them are big buttons... the resistor to the left, looking from the back of the board is definitely bridged to ground.

asssssssss load of pics coming. crummy cheap phone pics, but if anything is interesting or helps make sense of this, hey, cool!

stay tuned. may take two photo dumps to get 'em all lol

[pinkjimiphoton]

pics of all the cards, they all suck, of course, but just to give ya a feel for some of that ol "vintage prawn" thang... here goes.. lets see how many pics they'll let me add at once

"e tuner (actually G tuner) " board





preamp channel 2, slightly differnt caps but the same board





reverb board





channel one board





tremolo board





solder bridge



gutz










100uF @50v (x 2)



10,000uF @ 150 <i think>



switches







power in and footswitch jack



output jacks... channel 2 only to the left




actual model #. the case its in says 6005



crappy front panel shots










holy cow, information overload!!!!

i will slap it back together enough to get voltages but i'm betting that bridge is the key to the whole mess.
thanks for the hang, man...
will check back in the ...umm, well, ...
tomorrow 

peace out!

[Rob Strand]

Awesome.

I think I've got brain fry now ...  Bzzzzt.


BTW, I checked the circuit biasing assuming the transistors are generic like 2N3904's.
I've assumed the emitter resistor is 22k.  I can't read it so well.
Results:
1) resistor on back of board + shorted emitter resistor --->  looks OK for supply > 20V
2) resistor on back of board + without shorted emitter resistor --->  biasing doesn't look so good!
3) no resistors on back of board + without shorted emitter resistor ---> looks OK for supply > 20V

Case (1), ie. as it is now, has a lot more gain than (3), ie no mods.

So the mods are probably OK and the bridge is probably intentional.

What doesn't make sense is why the gain is so low.  Maybe the transistor is stuffed!

[pinkjimiphoton]

bzzzzzzt indeed!!!!

yeah, i will try and take a peek today to confirm,  but i'm thinking same thing, snuffed transistor.
in fact, i'm wondering if them resistors weren't a dodge to try and get it to pass signal with a roasted q.

i dunno about a 3904, its a really BIG button, about as big around as a sharpie.

i COULD desolder it and take it out and measure it. i am surprised there's no markings on it. but it appears to be the same transistor used on the tremolo card and elsewhere. if i have to, i can sacrifice one of the ones on the trem card to get the unit working, then replace with something more modern once i have some specs to deal with.

i think its fried too, and thats why there's such a discrepancy in volume. i'm also wondering if this may be the same unit i had all those years ago, as i seem to remember blowing it up in the studio and the guy running the place having to fix it.
memory is funny. once it starts, and the details start coming back... ya never know.

haha back then i had this modded big muff. the volume pot broke, so i replaced all three with 1 meg pots from radio shack. it was unstable and loud and in some setting unuseable. but boy, i loved that thing

maybe thats how it got cooked..... the world may never know how many licks it takes to blow up an ovation!! lol

[Rob Strand]

i'm wondering if them resistors weren't a dodge to try and get it to pass signal with a roasted q.

I thought that at first too.  It looks like a factory mod *except* the solder bridge.   If it's a factory mod the aim would be to increase the level a bit.

i dunno about a 3904, its a really BIG button, about as big around as a sharpie.

I only used that to get an idea of the bias voltages.  The gains of the big buttons aren't that different to lower gain generic transistors like the 2N3904's.  So I'm confident my numbers are fairly representative of the

The bigger buttons are the TO-105 package, some common NPN's were (there are others),
   2N3566 to 2N3568
   2N3641 to 2N3642
   2N3569

IIRC they are more like 2N2222's.

i COULD desolder it and take it out and measure it.

Upto you.  If you measure +V and the collector voltage of the mixer transistor it should show up any issues.

transistor used on the tremolo card

Honestly, to see the transistor is bung I'd be just putting in any old transistor.   If the +V rails is high, like 50V, you might need to consider the transistor voltage.   

i think its fried too, and thats why there's such a discrepancy in volume. i'm also wondering if this may be the same unit i had all those years ago, as i seem to remember blowing it up in the studio and the guy running the place having to fix it.
memory is funny. once it starts, and the details start coming back... ya never know.

At this point it's the only thing that makes sense.

If the temolo is only an oscillator board and the gain control is done elsewhere, it crossed my mind that it is possible the gain control element is stuffed and making the gain low.    Given we don't know intricacies of the wee beasty it is a possibility.    At this point I'm happy to pursue the simple theory that the transistor is stuffed 

the world may never know how many licks it takes to blow up an ovation!! lol

   Soon those DSP/modelling amps will have note counters in there that expire and blow-up the amps.

[pinkjimiphoton]

slept the damn day away again, if its warm enough downstairs i may go have a tinker and see what happens. i don't wanna leave it apart so long i forget how to reassemble it like i did with the maestro rhtym n sound thats been sitting, working, but disassembled pretty much for the last 5 years

i am sure i have some old TIP style npn's that should handle 50v ok, but first i need to get some voltage measurements happening on all the cards, etc.

[Rob Strand]

I had a look at all those pics.  I've worked out a bit more but I've hit the same old brick-wall.   I don't know how the modules connect between themselves.

For the Fuzz, I've managed to get this,



The blue input wire and yellow output wire stay connected.   The switch on the fuzz pot kills the first transistor by disconnecting the emitter.   The weird thing is the orange wire on pin 8 seems to go somewhere as well.   The orange wire has DC on it so it could upset something else.

For the tremolo:
- The board contains the LFO (sine-wave) and it has two funny looking devices which appear to be Opto's.
- The LFO is a phase-shift oscillator.
- From what I can see the output of the LFO (pin 5) feeds two transistors, one for each channel.  The output of those transistors drives the 2x Opto's.  The output is one the collectors.  One side of the opto's resistance connects to the emitter of the driving transistor - an odd connection.
- The "tremolo outputs" are more or less variable resistors that short the signal to ground.  These appear on pins 3 and 7 (counting the key hole).
- So... the signal doesn't pass *through* the tremolo board.
- The wiring for the tremolo has two yellow wire on one side and two white wire on the other side.
- I can see the tremolo switch wiring but I suspect the two "tremolo outputs" go to the tremolo switch.
  The switch are either in series with the "tremolo outputs", or, they short the "tremolo outputs" to ground.
- Anyway, as expected, pulling the board out will simply remove the "tremolo outputs" and the variable resistors to ground.

Where the tremolo is in the signal path I don't know.   You might be able to see where it goes from the two wires I mentioned.   The important thing is it is unlikely to cause the problem you are seeing.

For the mixer I'm not sure if,
- the blue wire on pin 8 goes to the fuzz board
- the orange wire on pin 11 goes to the CH2 output jack
- the brown wire on pin 7 goes to the CH2 output jack
- Also not sure where the purple wire pin 1 goes.  I think maybe to the channel 1

BTW, the Channel 1 and Channel 2 preamp boards seem to be slightly different.

[Rob Strand]

slept the damn day away again, if its warm enough downstairs i may go have a tinker and see what happens. i don't wanna leave it apart so long i forget how to reassemble it like i did with the maestro rhtym n sound thats been sitting, working, but disassembled pretty much for the last 5 years

If you need to rest you should do it.   I'm sure no one will mess with it in your electronics dungeon over the next week.

[Rob Strand]

Oh I forgot to mention the reverb.

It's the usual thing where you have a reverb tank drive circuit and recovery preamp.

From what I can see it is entirely mono.  No combining of channels 1 and 2, and no mixing.

[pinkjimiphoton]

I had a look at all those pics.  I've worked out a bit more but I've hit the same old brick-wall.   I don't know how the modules connect between themselves.

For the Fuzz, I've managed to get this,



The blue input wire and yellow output wire stay connected.   The switch on the fuzz pot kills the first transistor by disconnecting the emitter.   The weird thing is the orange wire on pin 8 seems to go somewhere as well.   The orange wire has DC on it so it could upset something else.

i will try and trace the socket to where it goes tonite. i think the description coming off the mixer board is in this post:

https://www.diystompboxes.com/smfforum/index.php?topic=121835.msg1147111#msg1147111

For the tremolo:
- The board contains the LFO (sine-wave) and it has two funny looking devices which appear to be Opto's.
- The LFO is a phase-shift oscillator.
- From what I can see the output of the LFO (pin 5) feeds two transistors, one for each channel.  The output of those transistors drives the 2x Opto's.  The output is one the collectors.  One side of the opto's resistance connects to the emitter of the driving transistor - an odd connection.
- The "tremolo outputs" are more or less variable resistors that short the signal to ground.  These appear on pins 3 and 7 (counting the key hole).
- So... the signal doesn't pass *through* the tremolo board.
- The wiring for the tremolo has two yellow wire on one side and two white wire on the other side.
- I can see the tremolo switch wiring but I suspect the two "tremolo outputs" go to the tremolo switch.
  The switch are either in series with the "tremolo outputs", or, they short the "tremolo outputs" to ground.
- Anyway, as expected, pulling the board out will simply remove the "tremolo outputs" and the variable resistors to ground.

i believe the trem switches just short to ground, and when they do, it lets the trem circuit short each channel to ground off and on. i will check and write the signal path down.
i gotta look closer at that board, i guess, i haven't cuz i've been focused on the problem.
some of the guys at the diy-audio forum think i should start by removing that bridge on the mixer board.

Where the tremolo is in the signal path I don't know.   You might be able to see where it goes from to two wires I mentioned.   The important thing is it is unlikely to cause the problem you are seeing.

the trem is the last card before the mixer... circuit flow is channels one and two in parallel, then fuzz, then reverb, then oscillator/tuner and finally trem... then the mixer and outputs.

For the mixer I'm not sure if,
- the blue wire on pin 8 goes to the fuzz board

see the listing of the wires, https://www.diystompboxes.com/smfforum/index.php?topic=121835.msg1147111#msg1147111
the blue wire does go to the fuzz board, pin 10

- the orange wire on pin 11 goes to the CH2 output jack
- the brown wire on pin 7 goes to the CH2 output jack

yes, you got it. thats exactly right

- Also not sure where the purple wire pin 1 goes.  I think maybe to the channel 1

purple wire goes to reverb socket pin #8

BTW, the Channel 1 and Channel 2 preamp boards seem to be slightly different.

yeah, i think they used slightly different caps etc to make it a bit "hotter"...
but either preamp board will work pretty much identically in either preamp socket.

remember, their stuff was pretty vegematic, they'd use exactly the same thing with a bunch of different names for different applications.

i mean, this same preamp was sold for lead, rhythm, bass and keys with slightly different applications.
pretty sure the board are all generic, so some parts/pins/features may or may not even be included, depending on the version
of the circuit.

these guys were as bad as me and fuzzboxes, lol... "hey, lets use the same circuit again and see how many variants we can come up with" lol

the fuzz looks pretty promising. i'm betting adding a gain to it and playing with some values may end up with a decent sounding unit. hell, i intend to improve what is there stock, if possible.

almost ready to check downstairs and see if i'm up for it. it will tell me if it wants me to touch it, i am sure.

damn peyote buttons. things been talking to me that shouldn't be for 40 some years now. lol

^^^^ i may be kidding, or may be not... lol

bad humor i know, can't help it, i was born a mostly white child in the fertile muddy waters of the willimantic river delta...

yeah, the reverb seems to have switches that either send either channel, both, or none to the reverb.

[Rob Strand]

some of the guys at the diy-audio forum think i should start by removing that bridge on the mixer board.

After looking at the biasing I'm pretty sure you either have no mods at all (no added resistors or bridge), or, you have all the added resistors with the bridge.   The resistors + no bridge made the least sense.  The biasing was quite bad.  The DC voltage on the collector (and +rail voltage) will  tell you if it is ok as it is now.

i will try and trace the socket to where it goes tonite. i think the description coming off the mixer board is in this post:

https://www.diystompboxes.com/smfforum/index.php?topic=121835.msg1147111#msg1147111

Ah excellent - thanks!   I don't know how I missed that.  Maybe back then I couldn't piece it together.

the trem is the last card before the mixer... circuit flow is channels one and two in parallel, then fuzz, then reverb, then oscillator/tuner and finally trem... then the mixer and outputs.

see the listing of the wires, https://www.diystompboxes.com/smfforum/index.php?topic=121835.msg1147111#msg1147111
the blue wire does go to the fuzz board, pin 10

Ok thanks.  I read over all that stuff again and see if it puts anything into perspective.

The bottom line is it's all pointing to the mixer board!!

remember, their stuff was pretty vegematic, they'd use exactly the same thing with a bunch of different names for different applications.

I can see why companies do that.  It means all you stuff produce has a consistent sound.  If you come-up with an improvement then you should roll it out on all models.  No point staying with the old.

the fuzz looks pretty promising. i'm betting adding a gain to it and playing with some values may end up with a decent sounding unit. hell, i intend to improve what is there stock, if possible.

I have a feeling it will be running on a higher supply voltage and might not translate to 9V so well.

damn peyote buttons. things been talking to me that shouldn't be for 40 some years now. lol

^^^^ i may be kidding, or may be not... lol

bad humor i know, can't help it, i was born a mostly white child in the fertile muddy waters of the willimantic river delta...

[pinkjimiphoton]

yeah, definitely the mixer.
i'm packing a couple guitars up that sold on ebay... yay... we can eat this month
then i'm gonna put it mostly back together so i can fire it up and get some voltages.
may be tomorrow before i get to it, i feel pretty lazy at the moment

i'm betting the fuzz was something else they stole and adapted. if i know dino, he can make it run on 9volts. may have to change some resistor values, but once we know where the shit on the socket goes, and some voltages, we'll be able to figure out a lot more.

personally, i think using the same thing for all their units was either lazyness or productivity, i mean, the bass preamp version also had reverb and fuzz. who the hell uses reverb on bass? ok, chris squire, but...

i figure it was just easier to milk the same designs, i mean, they are gonna work for whatever ya throw into them, so what the hell... one designm many products, right?

[Rob Strand]

yeah, definitely the mixer.
i'm packing a couple guitars up that sold on ebay... yay... we can eat this month
then i'm gonna put it mostly back together so i can fire it up and get some voltages.
may be tomorrow before i get to it, i feel pretty lazy at the moment

No problem.

i'm betting the fuzz was something else they stole and adapted. if i know dino, he can make it run on 9volts. may have to change some resistor values, but once we know where the shit on the socket goes, and some voltages, we'll be able to figure out a lot more.

Probably get close.   You lose a bit of gain at low supply voltages.

i figure it was just easier to milk the same designs, i mean, they are gonna work for whatever ya throw into them, so what the hell... one designm many products, right?

Sure.  Cheaper to make as well.

I've updated the mixer schematic.  I makes sense a lot of sense to me from a how it works perspective.  I'm still not 100% on where pin 7 and 9 end-up.   I got a bit mixed up with the brown wires since there three (dark/light look the same on the pic to me).



Still don't understand where pin 11 (blue), pin 2 (yellow) and pin 8 (orange) on the fuzz end up. 
No big deal.  The way I've redrawn the mixer certainly puts more emphasis on the fault being in the mixer.

[Rob Strand]

Schematic title fix and bias checks

[pinkjimiphoton]

just got the last of the stuff packed for monday, and pics taken for the next batch of ebay stuff.
tomorrow i'll slap her together and get some voltages. all will be revealed.

yeah, in some cases, they have multiple wires connecting. it makes it even weirder!

what i'll do is start with preamp card one, and trace where the wires go to, and try and work my way thru the whole mess. and get voltages after... better to trace stuff before i re-assemble it.

it LOOKS worse than it probably is cuz its got all the p2p stuff all over... we'll probably end up with enough info some crazy sucker could clone it if they were stupid enough like me wanted to.

i'll also do my best to get the correct values on all the resistors etc on the mixer.

for shits n giggles i may pull the q and try something else with it... i got one of them little chinese tester things, works great for pinouts n gain. if its working, i'll put it back, but if not... that will be our problem, and to me, the most likely candidate as everything else appears to be working. i doubt the resistors or caps have died.

so the 39 and 33k resistors in parallel should be, like, 17.875... call it 18k..  the 22k on the back is in parallel with the 100k, so it works out to 18032.xxx k... so we could call that 18k too, right?

why wouldn't they just use like, a 22k resistor and be done with it? should be close enough i'd imagine.

i definitely wanna compare it with and without that solder blob, too,,, and with one end of both those resistors lifted to see what it does.

right now i'm leaning towards a shorted transistor, thats not actually operating but still passing signal. i had a blown kustom like that years ago i used as a preamp so i could get reverb outta my marshall jcm800 combo,  the power amp was blown, but you could still pass signal thru it.

i'm wondering if them resistors are whats letting the signal bleed thru the whole mess... i bet they fudged it when built, and them resistors are a dodge... or they messed up the design... would seem to me, summing two different preamps together passively would be problematic. maybe the q on the mixer is a gain stage for the mixed channels, or perhaps a buffer to keep the 1 and 2 channels isolated?

i think i picked up a lousy week to give up caffiene... and sugar... and alcohol <<especially alcohol>> and gluten.
that said, i've lost 20 lbs in the last week. so its probably a good thing, but i got the brain fog setting in from no carbs, lol

the damn infection thing isn't helping either. whomever invented this shit should be flogged!!!

left hand ring finger in the first knuckle is where its worst. holy shit. can barely bend it. good thing i'm off til thursday!!
with luck it will be better and i'll dodge a week or two of intravenous antibiotics, cuz man, hospitals scare the piss outta me.
last place ya wanna be if you're sick!!!

anyways, thats it for tonite. thanks for everything rob!!
 

[Rob Strand]

what i'll do is start with preamp card one, and trace where the wires go to, and try and work my way thru the whole mess. and get voltages after... better to trace stuff before i re-assemble it.

It's up to you at the end of the day.  These things all always turn out to be more work than you think.   If you can get the connections I could trace the boards.  The channel preamps have 17 connections which involves too much guessing without the wiring.

it LOOKS worse than it probably is cuz its got all the p2p stuff all over... we'll probably end up with enough info some crazy sucker could clone it if they were stupid enough like me wanted to.

Funny you should say that.  I was thinking both of those things the other day.

for shits n giggles i may pull the q and try something else with it...

Should be fine.   The only thing you might need to be careful about is the transistor voltage.  I don't know what the supply rail is on that thing.  Some transistors might poop out.

so the 39 and 33k resistors in parallel should be, like, 17.875... call it 18k..  the 22k on the back is in parallel with the 100k, so it works out to 18032.xxx k... so we could call that 18k too, right?

why wouldn't they just use like, a 22k resistor and be done with it? should be close enough i'd imagine.

That's why I think it's a factory mod.   They built the boards and the gain was probably too low so they tweaked the design and instead of pulling parts they just solder extra parts one.    To me this makes sense because the part values before and after the mods seem to show correct bias points.
(The mods also drop the the output impedance.)

think i picked up a lousy week to give up caffiene... and sugar... and alcohol <<especially alcohol>> and gluten.

Maybe too much to slam on yourself all at once.   Dumping caffeine alone can screw you up for a week or more.

that said, i've lost 20 lbs in the last week. so its probably a good thing, but i got the brain fog setting in from no carbs, lol

20lbs is one week is crazy.   The "safe" weight loss rate is something like 1kg/wk (2lbs/wk). 

the damn infection thing isn't helping either. whomever invented this shit should be flogged!!!

I freakin hate getting sick or anything like that.    Good luck with it.