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ARP 3620 Keyboard S&H - IMF3958 Dual FET Substitution?
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Broadwave



Joined: Feb 16, 2007
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PostPosted: Fri Sep 13, 2013 3:03 am    Post subject: ARP 3620 Keyboard S&H - IMF3958 Dual FET Substitution? Reply with quote  Mark this post and the followings unread

I'm having a tiny problem with the S&H section of the ARP3620 Keyboard clone I'm building - although it's working (more or less) I'm currently testing the board using two unmatched 2N5459 FETs in place of the elusive IMF3958 (or the expensive 2N3958)

I started thinking if the Dual FET business was really necessary - why not just take it out along with the LM1458 op-amp and replace the whole shebang with an LM442 or similar?

Do you think it would work? I can't see any reason why not... I just don't want to start tearing into my beautifully crafted PCB Wink

I've attached the original circuit along with what I THINK may work.

Any comments would be gratefully accepted.


Original S&H.jpg
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Original ARP 3620 Circuit
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Original S&H.jpg



Modified S&H.jpg
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THeff



Joined: Sep 01, 2006
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PostPosted: Fri Sep 13, 2013 9:05 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Andy,

This is petty much what Yves is doing on his Dual Gated Slew module.

http://www.yusynth.net/Modular/index_en.html

-Tim
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Broadwave



Joined: Feb 16, 2007
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PostPosted: Fri Sep 13, 2013 9:16 am    Post subject: Reply with quote  Mark this post and the followings unread

THeff wrote:
Hi Andy,

This is petty much what Yves is doing on his Dual Gated Slew module.

http://www.yusynth.net/Modular/index_en.html

-Tim


Thanks for the pointer Tim, looks like I'm heading in the right direction... I'll give it a try - can't do any harm Smile
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steffensen



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PostPosted: Sun Aug 17, 2014 2:47 am    Post subject: Reply with quote  Mark this post and the followings unread

AndyR1960,
Did this work out? Smile
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Broadwave



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PostPosted: Tue Aug 19, 2014 2:28 am    Post subject: Reply with quote  Mark this post and the followings unread

steffensen wrote:
AndyR1960,
Did this work out? Smile


I never got round to trying it Wink

I discovered that the original problem I had (voltage droop on the second voice output) was due to a missing cap and not the FETs Embarassed

The two 2N5459's work fine. I also tried it without matching them (but they were from the same batch), and it still works with no problems, so I wouldn't get too hung up about sourcing IMF3958/2N3958.
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steffensen



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PostPosted: Tue Aug 19, 2014 6:54 am    Post subject: Reply with quote  Mark this post and the followings unread

Cool, thanks for the info.
I too had great results by using 2x 2N5459's instead of the expensive 2N3958 on my 4027-1 clones.
Not all pairs work out ideally though, but those from the same tape always seem to do however!
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nisios



Joined: Sep 02, 2006
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Location: Lisboa - Portugal

PostPosted: Wed Oct 22, 2014 5:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'm having the same problem.

I'm repairing an Arp 2600 and its drifting one semitone in 15 minutes.
Tryed cleaning everything, changing the switch fets and nothing resulted.

I already socketed and changed the op-amps so now i think I will go ahead and bypass the guard fets.

What do you guys think is the best op-amp for this task? 442 as mentioned or is there anything even better?

I also noticed that the power supply line (after changing filter capacitors) drops a little bit in the 3620 board whenever i press a note. You can hear a slight pitch difference.

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Broadwave



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PostPosted: Thu Oct 23, 2014 1:19 am    Post subject: Reply with quote  Mark this post and the followings unread

nisios wrote:
I'm having the same problem.
I already socketed and changed the op-amps so now i think I will go ahead and bypass the guard fets.


Try replacing the big .47 (Polycarbonate or good quality Polyester) S&H cap - sounds like it may not be holding the keyboard voltage. It maybe going a bit leaky due to age.
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nisios



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PostPosted: Thu Oct 23, 2014 2:41 am    Post subject: Reply with quote  Mark this post and the followings unread

Already did that. I replaced it with Wima box type. Its tiny compared to the original and the leads are very close wich is not good sign to keep super high impedance, but, it did nothing bor better or for worse.
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nisios



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PostPosted: Thu Oct 23, 2014 2:46 am    Post subject: Reply with quote  Mark this post and the followings unread

I will try change them to bigger MKP just in case.

Andy, what is the cad software you use to make your schematics, they look great.

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Broadwave



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PostPosted: Thu Oct 23, 2014 3:14 am    Post subject: Reply with quote  Mark this post and the followings unread

nisios wrote:

Andy, what is the cad software you use to make your schematics, they look great.


It's Adobe Illustrator, with a small library I created, I just drag and drop and keep my fingers crossed.

I also use it for my PCB layouts... I must be some kind of masochist Wink

nisios wrote:

I'm repairing an Arp 2600 and its drifting one semitone in 15 minutes.


Actually, that's not bad at all.

Just make sure you clean up well after any soldering - that area of the PCB is particularly sensitive. I now use HydroFlux solder, the flux is water soluble and after washing the PCB, leaves no residue - great stuff.

Last edited by Broadwave on Thu Oct 23, 2014 3:33 am; edited 1 time in total
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nisios



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PostPosted: Thu Oct 23, 2014 3:27 am    Post subject: Reply with quote  Mark this post and the followings unread

heheh, i think you are....

Everytime i see pretty schematics its the same story.

Why dont cad programming folks use some nice symbols.....and the best programs are always the worst looking ones....

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Elettronico



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PostPosted: Thu Jun 28, 2018 5:34 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi all!
I have an older ARP 3604P keyboard that doesn't work properly. The S&H circuit is a little bit different from yours; it is more like a "track & hold" circuit, with an high-impedance buffer for the capacitor. This unity-gain amplifier was sold by ARP as a "closed" module and so the schematic isn't available on the user manual. I would like to repair (or rebuild) it. Do you think that the schematic proposed is good for that purpose? I mean, the operational amplifer Z3B with the double FET Q5 and related components.
Many thanks.
Best regards,
Alessio
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Broadwave



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PostPosted: Sat Jun 30, 2018 12:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Elettronico wrote:
Hi all!
I have an older ARP 3604P keyboard that doesn't work properly. The S&H circuit is a little bit different from yours; it is more like a "track & hold" circuit, with an high-impedance buffer for the capacitor. This unity-gain amplifier was sold by ARP as a "closed" module and so the schematic isn't available on the user manual. I would like to repair (or rebuild) it. Do you think that the schematic proposed is good for that purpose? I mean, the operational amplifer Z3B with the double FET Q5 and related components.
Many thanks.
Best regards,
Alessio


I've just had a look at the schematic, and I can't see why a standard FET Op-Amp (TL071) wouldn't work in place of A1 - just ignore the R19 offset resistor between pins 5 & 9. It's probably worth replacing C6 & C7 while you're at it.

It looks like most simple Keyboard S&H circuits I've encountered.


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Elettronico



Joined: Jun 28, 2018
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PostPosted: Mon Jul 02, 2018 5:15 am    Post subject: Reply with quote  Mark this post and the followings unread

Broadwave wrote:

I've just had a look at the schematic, and I can't see why a standard FET Op-Amp (TL071) wouldn't work in place of A1.


Hi Broadwave, thanks for your reply.
I will try but I guess that the bias current of the OpAmp inputs is not negligible.

Broadwave wrote:

It's probably worth replacing C6 & C7

Yes, of course. I have two questions about if. What kind of capacitor do you suggest me to use? I'm thinking that MKP could be a good choice. Do you agree?

Another question: while C6 (0.1uF) is the hold capacitor, what is che purpose of C7 (0.47uF)? It looks like a low-pass RC filter on output, coupled with R17 and R20.

Many thanks for your help.
Best regards,
Alessio
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piedwagtail



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PostPosted: Wed Jul 04, 2018 12:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

CA3140 is the input impedance king.

C7 is as you say a lp filter in off , probably to stop the pitch zinging back abruptly when you switch off the portamento. When on it is the majority influence portamento cap. Double duty. I'd blame Dennis Colin for that sort of intelligence.

Try everything and spread your results - no one knows that much about electronics.

R
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Grumble



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PostPosted: Wed Jul 04, 2018 11:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

with the switch in position drawn, A1 (with R17, C7, R20 and C8 ) is a bandpass filter ranging from about 70Hz - 400Hz
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Elettronico



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PostPosted: Thu Jul 05, 2018 12:46 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for your replies, guys.

piedwagtail wrote:
CA3140 is the input impedance king.

Good to know. I will try with different opamps to see what happens.
Maybe also the LF4xx could be good.

piedwagtail wrote:
C7 is as you say a lp filter in off , probably to stop the pitch zinging back abruptly when you switch off the portamento.

OK, it's clear.

Grumble wrote:
with the switch in position drawn, A1 (with R17, C7, R20 and C8 ) is a bandpass filter ranging from about 70Hz - 400Hz

I'm thinking that in this configuration (with the negative feedback directly from the output) it shouldn't be a BPF, 'cause in that case the DC (control voltage) from the keyboard won't pass.

Thanks all, I will share my experimental results.
Best regards,
Alessio
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piedwagtail



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PostPosted: Fri Jul 06, 2018 3:28 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:

OK, it's clear.


Awaiting experimental results; one careful snip and a jumper to earth should yield data!

R
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Elettronico



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PostPosted: Thu Mar 26, 2020 11:38 am    Post subject: Reply with quote  Mark this post and the followings unread

piedwagtail wrote:
CA3140 is the input impedance king.

C7 is as you say a lp filter in off , probably to stop the pitch zinging back abruptly when you switch off the portamento. When on it is the majority influence portamento cap. Double duty. I'd blame Dennis Colin for that sort of intelligence.


Dear guys, I left this project for some month and now I'm working on.
CA3140 is very good in replacing the old "high impedance amplifier" A1; it works fine. About the hold capacitors C6 and C7, I'm trying with different types; the best results are with MKP capacitors, beacuse of their good linearity and very low "dielectric absorption" (... of you are interested into this topic, some useful info can be found in the LF398 S&H chip datasheet, in the section of suggested capacitor).

About the network composed by R17 (10k) and C7, its purpose is very simple.
When portamento is on, C7 works as an added "hold" capacitor, increasing the time constant of the circuit composed by the 500K potentiometer; when portamento is off, the voltage on C7 is kept at the output level by R17 (10k) in order to avoid any charge exchange between C6 and C7 when portamento is turned on; in this way, the voltage at the output doesn't change if you turn portamento on.

I will let you know further developments.

Regards,
Alessio
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