ARP A2803-003-1

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Don T
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ARP A2803-003-1

Post by Don T » Mon Oct 27, 2014 6:00 am

Ok, I'm hoping someone can help me with this:

I'm building a couple ARP 4027-1 VCO modules, using the same board layout as the original. Here's the question I have:

What JFET was actually used in the 4027-1 module? (I have an original 4027, not 4027-1, module, but it is no help). The part number listed in the schematic is: A2803-003-1, as shown here:

Image

Now, in the ARP Little Brother Service Manual, it lists a JFET with that part number, and in parenthesis underneath it says: 2N5459. So far, so good, however the JFETs on a 4027-1 board are NOT 2N5459, unless the pinout has changed since the 1970's. See the pic below:

Image

The JFETs in the photo have the pinout shown in the pic, SDG. A 2N5459 is DSG. Yes, it's simple enough to turn a 2N5459 45º and bend the source forward and the gate backward, but I'm curious to know what the original part is.

So, if someone knows, or if they can go read the part number off a 4027-1 module they have in hand, I would be most grateful! And yes, I have looked online everywhere I know to look, but I have not been able to find a photo of a 4027-1 that has a readable part number on either of the two JFETs.

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LED-man
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Post by LED-man » Mon Oct 27, 2014 8:39 am

in the TTSH works a 4027-1 for Q3 and Q4 is a 2N5459.

drain to ground.
for Q3 Gain is connected to R7
for Q4 Source to CA3046/CA3086
( same as in your schematic picture)
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Reality Checkpoint
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Post by Reality Checkpoint » Mon Oct 27, 2014 8:50 am

In my searches I too found the 2N5459 as being the part for Q3 and Q4.

These were also used in the ARP 3620, and Steffensen has used them in his Oscillarp. He also mentioned probable replacements in his build documents (perhaps worth experimenting with?)

"Possible replacements for the 2N5459's would be:
2N5457, 2N5458, 2N5485 or BF245A (pinout reversed)."

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daverj
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Post by daverj » Mon Oct 27, 2014 11:25 am

From looking at Arp part number sheets, it looks like the A28xx numbers were internally assigned to parts that were hand selected. (A2801-008 were op amps, A2801-003-1 were FETs, etc...) So that number doesn't specify a manufacturer part number. And without finding some kind of internal document defining what was used for the hand selected parts you can only go by what people have seen printed on the parts in actual machines.

Unfortunately, without finding that internal document you also don't know what parameter they were hand matching those parts with. So even finding the part number doesn't make it the same as what they actually used.

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Broadwave
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Post by Broadwave » Mon Oct 27, 2014 2:00 pm

I have the original blueprints and schematics of the 4027-1, and both FET's are definitely marked up as 2N5459 and the pinout, as per your pic, is correct.

I think you'll find that the drain and source in this case are interchangeable, it doesn't matter which way round they go.

Hope this helps.

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Post by Piedwagtail » Mon Oct 27, 2014 8:59 pm

(off-topic as it were)

Does the 4027 schematic floating about on the net resemble the sub-module?
I'm tempted to copy it but it's pretty advanced in comparison to the 4027-1.

Robert

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Post by Don T » Mon Oct 27, 2014 9:14 pm

Broadwave wrote:I have the original blueprints and schematics of the 4027-1, and both FET's are definitely marked up as 2N5459 and the pinout, as per your pic, is correct.

I think you'll find that the drain and source in this case are interchangeable, it doesn't matter which way round they go.

Hope this helps.
That's it! That's also what I get for trying to think too much, I had forgotten that some JFETs are symmetrical :doh:
LED-man wrote:in the TTSH works a 4027-1 for Q3 and Q4 is a 2N5459.

drain to ground.
for Q3 Gain is connected to R7
for Q4 Source to CA3046/CA3086
( same as in your schematic picture)
I was aware of the TTSH parts list, but the board layout of the TTSH 4027-1 isn't exactly the same as the original. (The drains go to +15V by the way)
daverj wrote:From looking at Arp part number sheets, it looks like the A28xx numbers were internally assigned to parts that were hand selected. (A2801-008 were op amps, A2801-003-1 were FETs, etc...) So that number doesn't specify a manufacturer part number. And without finding some kind of internal document defining what was used for the hand selected parts you can only go by what people have seen printed on the parts in actual machines.

Unfortunately, without finding that internal document you also don't know what parameter they were hand matching those parts with. So even finding the part number doesn't make it the same as what they actually used.
True enough, and I knew the JFETs were selected parts. The Odyssey service manual designates two different selected JFETs, one is painted orange, the other is yellow (can't remember which is which off the top of my head). I've seen photos of the 4027 that shows it uses the orange ones. I'd like to know what made them 'orange' other than the pigment used in the paint!

My Mark III Axxe has all orange 2N5459 JFETs, maybe one day when I get a ton of extra free time ( :lol: ) I'll pull them out and check them for Vgs and Idss and see which parameter matches more closely.

Thanks everyone!

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Post by Broadwave » Tue Oct 28, 2014 1:44 am

Piedwagtail wrote:(off-topic as it were)
Does the 4027 schematic floating about on the net resemble the sub-module?
I'm tempted to copy it but it's pretty advanced in comparison to the 4027-1.
Robert
The 4027 schematic is the 3rd revision of the 2600's VCO module - the previous two were the 4011 & 4017.

Subjectively, although a simpler design, the later 4027-1 is the better VCO - No messing around with Hi Freq tracking and, in my experience, more stable.

All my VCO builds are now based on the 4027-1, can't fault them.
Don T wrote:My Mark III Axxe has all orange 2N5459 JFETs, maybe one day when I get a ton of extra free time ( :lol: ) I'll pull them out and check them for Vgs and Idss and see which parameter matches more closely.
I never knew what the difference was either, but I'm not sure if measuring the specs of a 36 year old FET is going to help much - I certainly don't measure the same as I did in '78 :wink:

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daverj
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Post by daverj » Tue Oct 28, 2014 12:05 pm

I don't know the circuit, and this is just a wild guess, but most of the times I've seen a requirement for FETs to be matched it was the Vgs that was matched.

And it's possible that a given circuit might not have needed yellow vs orange ones, but maybe just needed all of them to be the same color (ie: all in the same circuit matched each other). So it's possible one unit might have had yellow and another of the same board might have had orange (or another color).

But that's just a guess. It's also possible that a given circuit required Vgs below a certain level and therefore that board did always use the same color.

I wonder what happened to all of those types of internal documents like that? Probably rotting in a dump or a basement somewhere.

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Post by fuzzbass » Wed Oct 29, 2014 6:56 am

I have no specific info about how or why ARP selected FETs, but forty years ago the variance among a batch of FETs was going to be greater than you will see with recently manufactured parts, so long as they come from a trusted source.

That said, I noticed a lot of TTSH troubleshooting threads homed in on a bad FET. Not sure if the problems were sourcing or handling. If it is a concern, consider socketing the part. FETs are by nature very sensitive, so take all precautions with static discharge and soldering temperature/duration. Don't solder all three legs in one quick pass. Obtain spares.
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Post by camelneck » Mon Sep 03, 2018 4:12 am

Broadwave wrote:
Piedwagtail wrote:(off-topic as it were)
Does the 4027 schematic floating about on the net resemble the sub-module?
I'm tempted to copy it but it's pretty advanced in comparison to the 4027-1.
Robert
The 4027 schematic is the 3rd revision of the 2600's VCO module - the previous two were the 4011 & 4017.

Subjectively, although a simpler design, the later 4027-1 is the better VCO - No messing around with Hi Freq tracking and, in my experience, more stable.

All my VCO builds are now based on the 4027-1, can't fault them.
Don T wrote:My Mark III Axxe has all orange 2N5459 JFETs, maybe one day when I get a ton of extra free time ( :lol: ) I'll pull them out and check them for Vgs and Idss and see which parameter matches more closely.
I never knew what the difference was either, but I'm not sure if measuring the specs of a 36 year old FET is going to help much - I certainly don't measure the same as I did in '78 :wink:


The 4027-1 VCO's are a much simpler design, but they don't sound as good as the 4011 or the 4027 especially in the high frequency range. If you don't believe me ask Phil Cirroco who is an expert on ARPs.

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Post by loki » Mon Sep 03, 2018 2:13 pm

Broadwave wrote:
The 4027 schematic is the 3rd revision of the 2600's VCO module - the previous two were the 4011 & 4017.
I would love to see schematics for the 4011 and 4017. Do you have them by any chance?

The 4027-1 is clearly a cleaned up version of the 4027. It would be interesting to see whether there was a clear line of development from one to the next or did someone start with a clean sheet of paper at some point? Was the 4011 derived from the VCO of the ARP 2500?

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Post by sduck » Mon Sep 03, 2018 9:40 pm

I hope the last 2 posters noticed that this is a 4 year old thread - a few of the old posters aren't here anymore.
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Broadwave
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Post by Broadwave » Tue Sep 04, 2018 1:53 am

loki wrote:
Broadwave wrote:
The 4027 schematic is the 3rd revision of the 2600's VCO module - the previous two were the 4011 & 4017.
I would love to see schematics for the 4011 and 4017. Do you have them by any chance?
In all my years of repairing ARP synths, I've never seen the schematics for either the 4011 or 4017 :despair:

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Post by devinw1 » Wed Sep 05, 2018 10:26 am

A2803-003-1 is definitely the JFET 2N5459. The odyssey uses them and has em on the BOM. The "B" on the end is when they were binning/screening them for various purposes.

Image

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Post by devinw1 » Wed Sep 05, 2018 10:27 am

Broadwave wrote:
loki wrote:
Broadwave wrote:
The 4027 schematic is the 3rd revision of the 2600's VCO module - the previous two were the 4011 & 4017.
I would love to see schematics for the 4011 and 4017. Do you have them by any chance?
In all my years of repairing ARP synths, I've never seen the schematics for either the 4011 or 4017 :despair:
4017:
http://www.arptech.synth.net/pdf/4017.pdf

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Post by loki » Wed Sep 05, 2018 4:24 pm

This has been out there for a long time. It is not a schematic of the 4017 but of the circuitry that surrounds the module to make it functional.

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Post by devinw1 » Thu Sep 06, 2018 9:41 am

loki wrote:
This has been out there for a long time. It is not a schematic of the 4017 but of the circuitry that surrounds the module to make it functional.
Ahah, you are correct! Gotta wonder if some old Arp guy has those prints in his basement somewhere haha.

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