legacy Euro Overdrive switch issue

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Pav
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legacy Euro Overdrive switch issue

Post by Pav » Mon Mar 23, 2020 12:58 pm

Hi Tony,
Sorry to see you are selling up your own 5U system.

I built the 3U overdrive a year or two back and never thought the gain was working or was too subtle for me to notice.
When i later built the 5U overdrive, I thought - THATS how its supposed to sound.

Well to cut to the chase, Today I changed the DIY panel to a Shaeffer one to match the design of the new panels.

SO I was testing again, and discovered perhaps I was right about the "no gain" gain control.
With balance knob fully cw,
I was switching between Overdrive and Distortion and getting the click documented in the build guide, as you transition.
If i switched to overdrive very slowly, I noticed I got the gain control back occassionally, albeit temporarily.
If I balance the on-off switch position to "middle" (as if a 3 way switch), the gain control works fine.

I could jump to the conclusion its a dodgy switch, but prefer to think it maybe its something else on the pcb,
and something that should be connecting isnt except when the switch transitions between on and off.
So i thought i would check with you.

I appreciate its probably been a long time since you looked at the circuit.
Thanks in advance.
rgds Pav

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Re: legacy Euro Overdrive switch issue

Post by Synthbuilder » Mon Mar 23, 2020 3:55 pm

Have you got a 'scope?

If so have a look at the signal on pin 1 as you switch from overdrive to distortion. There should be a difference between the two. In distortion mode the signal should be hard clipped to around +/-3V with the drive control at maximum. If not the switch could be stuffed.

Tony

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Re: legacy Euro Overdrive switch issue

Post by Synthbuilder » Mon Mar 23, 2020 4:00 pm

Pav wrote:
Mon Mar 23, 2020 12:58 pm
Sorry to see you are selling up your own 5U system.
No need to be sorry. I need the space to allow myself the room to develop new projects. That includes my growing Oakley Eurorack system. I do rather miss having control knobs that I can actually turn easily, but at least I can get a lot of modules in there in a relatively small amount of space.

Tony

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Re: legacy Euro Overdrive switch issue

Post by Pav » Tue Mar 24, 2020 7:51 am

Im lucky to have a bedroom dedicated to audio kit - but the 5U oakly is currently on the hall landing as i have been trying to identify why im picking up the dreaded computer noise in my audio paths, and ive ripped up every cable and lead under my main desk. That and earth hum on mixer channels connected to my eurorack or keyboard, when neither turned on. ANyway thats another long and tedious topic.

Results of tests with Scope. (off = full ccw, on= full cw)

Input triangle 10v pp from e-dvco.
timbre always off.

1. Gain Off , Switch =OD, Balance off result output = input
2. "" "" Balance on result output = 5vpp //attenuates 50%
3. Gain On "" "" NO CHANGE 5vpp clean triangle
4. Gain Off Switch = Distortion Balance on result output 3.4vpp clipped
5. Gain On "" "" result output 4vpp, shape nearly square wave, hollow sound

The switch in the middle position just shoots the pp voltage back to 10 and is merging the clean and distorted modes
hence that switch appears to be ok.

Its test 3 that surprises me - no "clean" overdrive, which if it were a guitar pedal, i would expect.
that and the balance attenuation.
rgds Pav

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Re: legacy Euro Overdrive switch issue

Post by Synthbuilder » Tue Mar 24, 2020 10:22 am

Sorry, I meant to say measure the signal on pin 1 of U1. That way you get the signal from the Overdrive's main gain stage only.

But yes, test 3, appears to show a problem. I wouldn't expect to see a triangle wave in either mode when the gain pot is turned up.

Tony

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Re: legacy Euro Overdrive switch issue

Post by Pav » Wed Mar 25, 2020 6:25 am

with overdrive switched , i see @2.8v pp shark fin wave on p1 U1 .

I would appreciate schematic being resent, look like i didnt store it.

so ill use my own numbering.to now check signals on switch pins with oscope
if i hold the panel side towards me, lets label the switch pins from left to right 1,2,3 and inner and outer.

distortion mode: gain working -
pin1 inner sees the distorted output signal. pin3 sees a signal too looks like a clipped triangle( sinewave ) ripple
none of the outer pins has a signal.

overdrive mode: gain not working
pin3 inner sees the signal we see at u1p1.
pin 1 inner has no signal
there is a clean triangle on pin 1 outer , nothing on pin3 outer.


hope that makes sense.
rgds Pav

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Re: legacy Euro Overdrive switch issue

Post by Synthbuilder » Wed Mar 25, 2020 9:17 am

Pav wrote:
Wed Mar 25, 2020 6:25 am
overdrive mode: gain not working...
So the gain pot does nothing? If so, D1 or D2 are shorted.

Tony

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Re: legacy Euro Overdrive switch issue

Post by Pav » Wed Mar 25, 2020 5:20 pm

well I pulled up the lead of D1 to run a diode test of dmm on both D1 & D2 and they passed.
As i was using a germanium diode -one of the the two suggested, i swapped back to 1n4148 ... sadly it did not solve the puzzle.

peculiar - not a lot else to check.
rgds Pav

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Re: legacy Euro Overdrive switch issue

Post by Synthbuilder » Thu Mar 26, 2020 3:41 am

Pav wrote:
Wed Mar 25, 2020 5:20 pm
peculiar - not a lot else to check.
Indeed.

Measure the resistance across pins 1 and 2 of U1 with a multimeter. Make sure the module is powered down and not connected to anything. What's the resistance in overdrive and distortion modes with the gain pot set full?

If it is close to zero in overdrive mode you have a short somewhere near those diodes.

Tony

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Re: legacy Euro Overdrive switch issue

Post by Pav » Thu Mar 26, 2020 8:12 am

odrive resistance 20k, distortion resistance 30k

i have measured every resistor on the board is correct value and they are.
i have swapped out u1 with a tlo72 - not that.
tested socket u1 for shorts - no joy.

I next decided to put a filter and vca in front of the dvco triangle... in odrive mode
and i could hear a tiny change that could not be seen in the visual trace on scope
but measured at 0.16v voltage change when turning gain to full in odrive mode.
Resistance test of 100k log pot itself was 2-98k...so pot is working.

- seems that just leaves capacitors c2 and c5 .correctly marked values. both have a significant 120k resistance and pass continuity
in situ.

reflowed everything connected to u1...no joy

so it would appear there is the tinest of changes in odrive mode, not zero.
rgds Pav

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Re: legacy Euro Overdrive switch issue

Post by Synthbuilder » Thu Mar 26, 2020 8:39 am

Try shorting out C2. No need to remove it, just solder a wire link across its pins on the underside. Anything different?

Might be worth doing a comparison of signals around U1 with the 5U version. It's the same circuit.

Tony

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Re: legacy Euro Overdrive switch issue

Post by Pav » Thu Mar 26, 2020 4:03 pm

Tony ,shorted c2 as suggested..triangle looks more sine with vpp 7.2v but pp does not change with gain control.
I dont have the 5U installed and running at the moment - but will try comparing the overdrives when i can.

ok i removed both diodes D1 and D2 from the circuit.
Gain control works ! but the peak to peak reading was a huge 21 volts. with gain up full.
- if i understand correctly the diodes are clamping the voltage when in circuit. (and in distortion the d3 and d4 come into play)
put fresh diodes back - and im afraid im back to square one with a static 6v ptp ever so slightly bent to the right triangle.

looking at the schematic - this all seems correct behavior - but when they are in circuit - the million dollar question still is
why is gain pot totally neutralized in this mode.
rgds Pav

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Re: legacy Euro Overdrive switch issue

Post by Synthbuilder » Fri Mar 27, 2020 9:17 am

Actually, thinking about this some more. I think you may have a working circuit. What signal are you putting into your Overdrive? If you are running a full +/-5V triangle wave you'd probably not see that much change at pin1, U1, when you alter the gain. It should change shape but not a great deal. That's because the unit is not expecting to see such a large simple signal at its input. The Overdrive is expected to sit after a filter rather than straight from a VCO.

But what puzzled me is the original comments that your 3U build sounded different your 5U build.

Try this, use a 1V p-p (+/-0.5V) 440Hz triangle wave into the Overdrive. Now measure the output at pin 1, U1. At low gain the output should be around 0.7V p-p in either distortion or overdrive modes. Turning the gain up you should see the output rise in distortion mode to around 6V p-p. In overdrive mode the signal will not rise anywhere near as much but should change shape, becoming a squared off waveform at around 1V p-p. Both the 5U and 3U versions should behave identically in this respect.

Tony

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Re: legacy Euro Overdrive switch issue

Post by Pav » Mon Mar 30, 2020 4:44 pm

Apologies I moved on to finishing vcas . II'll get back to that test later. I have problems with the vca...replace pot board u3 3 times trying to get linear mode and any cv to work, oh and outA ha no output,but B does. I did reverse the u3 at one point,could have done some damage.
I'll open up a new thread if I cannot make progress tomorrow.
rgds Pav

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