Pioneer SR-60 Reverb - Modifications Help

Hello,
I have a Pioneer SR-60 unit and although it says "Reverberation Amplifier" it's actually a Delay, it uses a MN3008 BBD delay chip.



I would like to have a MIX knob, so I could have 100% WET when needed, as in stock mode there's no provision to change the MIX between Wet and Dry.

It would be great also to be able to change the Delay Time and the Feedback level.

here is the schematic for the unit:






https://i.imgur.com/JVhDN9V.jpg

The schematic is a bit confusing to me because of all the switching going on,
I need help from you guys.
How do you think I could implement a MIX Knob?

Thank you so much

This is the datasheet for the MN3008:

It seems to me that the mixing resistors for the Dry and Wet are R39 (WET) and R41 (DRY),
does it seem correct?

Could I use a POT instead of those 2 resistors to change from fully Dry to 100% Wet?

Thank you

Hello

Whoops said:

It seems to me that the mixing resistors for the Dry and Wet are R39 (WET) and R41 (DRY),
does it seem correct?

Click to expand...

Seem correct to me

I suppose you can't replace them as is by pot as this is summing, and resistors are part of the amp gain setting.
Maybe try a blender pot before them, like VR1 but at both input ?

Best
Zam

zamproject said:

Hello

Seem correct to me

I suppose you can't replace them as is by pot as this is summing, and resistors are part of the amp gain setting.
Maybe try a blender pot before them, like VR1 but at both input ?

Best
Zam

Click to expand...

There is already a Reverb pot, so you need a dry pot betwee, C13 and the input of IC4/1§
You need to lift resistor R41 and connect it to the wiper.  You must maintain the signal going to C21 because it controls the gain of the BBD output. It's a noise-gate that mutes the output of the BBD when there's no signal.

Thank you.

To make it simpler I could just install a Kill Switch to be honest.
Would this work?
Thanks

Whoops said:

Thank you.

To make it simpler I could just install a Kill Switch to be honest.
Would this work?
Thanks

Click to expand...

I don't see why not...
Yoy may have to add a bleed resistor to ground in case there are clicks.

abbey road d enfer said:

I don't see why not...
Yoy may have to add a bleed resistor to ground in case there are clicks.

Click to expand...

Thank you so much I will try that out.

How about the Delay time and feedback? Any idea on how could those  be changed?

Whoops said:

Thank you so much I will try that out.

How about the Delay time and feedback? Any idea on how could those  be changed?

Click to expand...

Delay time can be adjusted only by changing te frequency of the master oscillator; C49, R131/132/133 around IC2. i don't remember the formula for frequency, it's about 40 years I haven't dealt with this. I suspect the actual values are such that it gives the longest delay. That means you can increase the frequency for shorter delays, but decreasing frequency would result in aliasing and noise.
Regarding FB, I can only suspect it goes via R32 (between Q4/6 and IC3b.
Yes, this switching is impossible to follow.

abbey road d enfer said:

Yes, this switching is impossible to follow.

Click to expand...

yes all the switching going on in the schematic really confuses me.

abbey road d enfer said:

I suspect the actual values are such that it gives the longest delay.

Click to expand...

Probably yes, it sounds pretty short but the chip has a max delay time of 102ms so it's just slap back territory.

Can you understand what happens in the circuit when the Reverb/Echo/Duet switch is changed from reverb to echo?

In terms of sound when I change that in the machine I can't listen any diference

Whoops said:

Probably yes, it sounds pretty short but the chip has a max delay time of 102ms so it's just slap back territory.

Can you understand what happens in the circuit when the Reverb/Echo/Duet switch is changed from reverb to echo?

In terms of sound when I change that in the machine I can't listen any diference

Click to expand...

The difference is how the noise-gate works. I believe one position is a hard noise-gate, the other is a softer expander. Hard to say.

Whoops said:

To make it simpler I could just install a Kill Switch to be honest.

Click to expand...

Forgot to say that R41 is for the left side and R42 is the right side counterpart. So both connections have to be lifted

abbey road d enfer said:

The difference is how the noise-gate works. I believe one position is a hard noise-gate, the other is a softer expander. Hard to say.

Click to expand...

Both sound quite similar

Whoops said:

Both sound quite similar

Click to expand...

I don't doubt it. I would think the difference is only how the repeats tail out.

abbey road d enfer said:

Regarding FB, I can only suspect it goes via R32 (between Q4/6 and IC3b.

Click to expand...

I tried messing around with R32. R32 doesn't seem to be the Feedback of the Delay, might it be the amount of Right channel that gets sent to the Left channel Delay?

Tried a 470K Pot  instead of R32 (220K) it only changes the volume of the delay but Feedback/number of repeats stays the same

Whoops said:

I tried messing around with R32. R32 doesn't seem to be the Feedback of the Delay, might it be the amount of Right channel that gets sent to the Left channel Delay?

Tried a 470K Pot  instead of R32 (220K) it only changes the volume of the delay but Feedback/number of repeats stays the same

Click to expand...

Sorry, FB is via R122 (110k).

​Hi,
I spent some time today trying out the suggestions and spend a little bit of time tweaking the values so that the control range provided by the potentiometers was nice,
These were the values I liked:



For the FEEDBACK, I liked a 51K resistor with a 200K Log pot in series, it gives a good range between few repeats and it goes into Oscillation in the full rotation.

On the DELAY TIME, this one is tricky. A 100K or 200K Pot instead of R132 works nicely for slowing the delay time, but there's problems...
The problem is probably what you  said Abbey :
"That means you can increase the frequency for shorter delays, but decreasing frequency would result in aliasing and noise."

With longer delay times there's a constant High Frequency whine pitch note that goes down in pitch and it's quite present.
I suspect that that high pitch note or noise is also there on the stock delay time but at an higher pitch above the hearing range.
Slowing down the delay brings that note into the audible range and it's quite irritating. (after the pot reaches around 70K)

Is there any solution for this? Or at least a way to attenuate?
I don't have any noise besides that constant high Pitch sound

Thank you so much for your help, I had great fun today trying your suggestions​

Whoops said:

With longer delay times there's a constant High Frequency whine pitch note that goes down in pitch and it's quite present.
I suspect that that high pitch note or noise is also there on the stock delay time but at an higher pitch above the hearing range.
Slowing down the delay brings that note into the audible range and it's quite irritating. (after the pot reaches around 70K)

Is there any solution for this? Or at least a way to attenuate?
I don't have any noise besides that constant high Pitch sound

Click to expand...

Have you checked the actual frequency of the oscillator? What you're hearing is probably the clock itself. Remember that BBD's operate at low clock frequency.
Clock residuals can be optimized by balancing the two resistors that come from the two different phases of the BBD. That's R90 & 92.
I've simulated the anti-alias filter: it's a 6-th order with its -3 dB point at 2.8kHz.

abbey road d enfer said:

Have you checked the actual frequency of the oscillator? What you're hearing is probably the clock itself. Remember that BBD's operate at low clock frequency.
Clock residuals can be optimized by balancing the two resistors that come from the two different phases of the BBD. That's R90 & 92.
I've simulated the anti-alias filter: it's a 6-th order with its -3 dB point at 2.8kHz.

Click to expand...

I think thats correct, it's the clock frequency getting into the audio band, I'm going over the limit of the manufacturer recommendations for the delay time use of this BBD chip.
I will use mainly in faster delay times, if I need to go slower I can try to EQ the FX return on the DAW, no worries.

I will use this for mixdown doing a send from the DAW to the unit, I like the fast delay it gives and I know it was designed to be like that, I'm doing the mods just to give it a little bit more flexibility and useful sounds as in stock mode it just gives 1 sound (maybe 2 with the Duet mode although that one is weak) and it's a one trick Pony.
At the moment I have flexibility to get some more sounds out of it including the stock one which is an improvement and I will leave it be.

Thank you so much for the help