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ARP 3620 Keyboard S&H - IMF3958 Dual FET Substitution?
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AndyR1960



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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Modified circuit
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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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AndyR1960



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



Joined: Jul 11, 2012
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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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AndyR1960



Joined: Feb 16, 2007
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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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AndyR1960



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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



Joined: Sep 02, 2006
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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



Joined: Sep 02, 2006
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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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AndyR1960



Joined: Feb 16, 2007
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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 AndyR1960 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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