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ARP Axxe Repair Thread (was Cleaning Vintage (Arp) Sliders)
MUFF WIGGLER Forum Index -> Music Tech DIY Goto page Previous  1, 2, 3, 4, 5, 6, 7  Next [all]
Author ARP Axxe Repair Thread (was Cleaning Vintage (Arp) Sliders)
valis
.. The cap in question..





You can see that I replaced an LM301, which got my noise circuit functioning properly again..
megaohm
valis wrote:
.. The cap in question..






I'd be saddened if I opened up an Axxe or Odyssey and didn't see the familiar big yellow cap.
cry

That's purely from an aesthetic point of view.

Sound or function wise... hmmm.....

p.
emdot_ambient
Well, I've got the parts ordered. I'm going to try replacing the LM1458 first, then the cap...er...um...maybe the other way around.

I just hope one of those works...not really wanting to try and find a 2N3958, they end up being something like $13 or $14 apiece.
emdot_ambient
Wait a minute...have I misread the schematics? Here's a close up of part of the schematics from the service manual...what's that dotted line symbol that I have circled in red?

I've never seen that before, and the connection from above doesn't seem to connect to C13, but rather to the dotted line thingy.

seriously, i just don't get it
Scott Stites
Not having seen the PCB, I'd venture to say that circle is a guard ring. Page 7 of the LF398 datasheet mentions a similar hookup (output of the SH is connected to the guard ring), which seems to be how this configuration is hooked up as well.

http://www.national.com/ds/LF/LF198.pdf
emdot_ambient
Interesting. Never heard of that. Hmm...lemme look...
emdot_ambient
FIXED!

It was just the capacitor. Finally got around to opening it up and replacing the cap.

Replaced it with this one: http://www.mouser.com/Search/ProductDetail.aspx?R=930C1P47K-Fvirtualke y59850000virtualkey598-930C1P47K-F

Even though it's only like 19mm long and 9 in diameter, compared to the original that was something like 40mm X 20mm.

Works perfectly fine now. thumbs up
emdot_ambient
I still have to wonder if there's any advantage or disadvantage to using a higher volt cap, though. Like would it likely hold its tune longer?

seriously, i just don't get it

I'm actually doing a test now to determine how quickly this "CV memory" actually lasts without pitch degradation.
emdot_ambient
Question...does anyone have wiring diagrams for the CV/Gate/Trigger inputs and outputs on the AXXE?

I'm pretty sure the one I'm working on has had them re-wired. When you plug an input into the CV/Gate/Trig it's supposed to pass the signal to the output so you can chain synths in a Master-Slave1-Slave2-SlaveN fashion.

But this one doesn't do that. With no jack in the CV input, the CV output is the 0-3 volts of the keyboard (does not include the transpose). Which is fine. But plug a CV into the input and nothing at all comes out of the Output.

The service manual doesn't show how the jacks are to be wired and I can't find it anywhere on the internet.

The Pedal input's a bit confusing, too. I'm not sure I understand what kind of jack it is just from looking at it. I'd really like to replace all the jacks. They look and feel rather rough.
valis
I do not but if it comes down to it I can draw you a diagram the next time I open up my axxe..
emdot_ambient
Thanks. I'll check my second Axxe. I bought that one after its first owner had only had it a month or so. I know I had to change some of the jacks on it before...hopefully I didn't screw that up!
valis
Good luck! Remember, I have a different revision Axxe so I'm not sure how different the internal wiring would look..
chromium
emdot_ambient wrote:
I still have to wonder if there's any advantage or disadvantage to using a higher volt cap, though. Like would it likely hold its tune longer?

seriously, i just don't get it

I'm actually doing a test now to determine how quickly this "CV memory" actually lasts without pitch degradation.



For hold-type applications, you might look for caps with low "dielectric absorption" ratings. The various poly varieties are usually good choices. I was experimenting with different cap types in a track and hold circuit, and settled on a polystyrene cap.

Nice chronicle of the keyboard refurb, BTW! I did this on my Multimoog a few months ago. Learned the hard way about lube! eek! (bought those dry bushings from the Ebay guy). Man what a difference, though. No more snaggletooth keys, and it no longer sounds like someone is tap dancing next to me when I play it hihi

One other little tid-bit that was interesting to me (and you mentioned this) was the different tension springs for the black and white keys. I never liked the way the keyboard on my Pro-one felt, and when I dug into that one just recently I saw that the springs were all haphazardly installed! I counted them out, and sure enough there was the right number of each tension for both the black and white keys - so after I did the bushings, I re-installed the springs correctly. Huge improvement - no wonder the action was funky all this time! Strange thing is that it was made ~1983, and I bought it like that used around '86. Wonder if it was just a factory goof? In any case, I'm really happy with it now w00t
kindredlost
This thread is awesome.

I did a refurb on my Axxe which is similar to Valis' (black face with gold graphics and the tune knob).

Not all Axxe internal circuitry is the same of course and this was a major problem at first for me. I bought & downloaded a few manuals from online dealers and internet sources. It takes a close look to determine all the model changes.

I had less success with the key bushing fix. I ordered off eBay and at first they helped quite a bit but after a few weeks the keys began sticking again. I may have to try the lubricant. I was under the impression it was lithium lubricant but that may not be the case.

I had a power supply problem at first. A couple of caps later the thing came up fine, but now it is something else. My guess is it is related to capacitors again somewhere in the circuit. The synth powers up properly but there is no sound all of the sudden.

The board bath idea was nothing new for me. For years I have been a service tech at a CNC machine shop which has large boards that were made in the 60's with pre-CMOS technology for the most part. Lots of TTL logic circuits and 90% discrete circuitry. We regularly bathe our boards in distilled water with a mild non-detergent soap (no oily additives) and a soft brush. I usually use compressed shop air at ~80 p.s.i. then gently heat with the hair dryer to remove the last bit of moisture. My main concern is the edge connectors and dip switches. The distilled water really isn't even enough of a conductor to worry about. It s the contaminents in the water that do the damage. We have a $4,000 water distillery in our shop so I have all the pure distilled water I need on tap.

Some of the boards have the resin protectant and this does little to help, but it makes them look cleaner for any desoldering that needs to be done.

The CMOS and later era of integrated circuit boards are where I get nervous. Especially socket IC's. Lots of places for corrosion and poor connections.

The lube spray works pretty well too, but the slider pots have to be very clean before there is a real improvement.

Those slide toggle switches are notoriously flakey after they get old. Mostly it is due to not enough use. Even though they are semi-enclosed the dirt and grime gets the best of them and the contacts get filthy.

I think the cv and gate inputs may be switched. I'll have to bust open my Axxe when I get home tonight to check.

My Axxe was one I bought new in 1980. I had it for a few years and sold it off in a fit of madness. The friend that ended up with it gave it back to me. When I went to pick it up it was in a garage, buried under a sofa couch with a TV set on top of it! It broke my heart as this was the first synthesizer I ever owned. I cordially thanked the person for being thoughtful enough to return it as a gift. What else could be nicer?

-David
chromium
kindredlost wrote:
...but after a few weeks the keys began sticking again. I may have to try the lubricant. I was under the impression it was lithium lubricant but that may not be the case.


That was my Multimoog experience in a nutshell. Some keys took a second to pop back up. The lubricant will get rid of that problem.

Just make sure whatever lube you use is silicone-based (or maybe even teflon-based) and *not* petroleum-based. The latter will have a degenerative effect on the rubber bushings.
emdot_ambient
I made the offer to Valis and I'll extend it to whoever wants it...I've got a huge tube of the lube that was originally used for these Pratt-Read keyboards. It's EXPENSIVE and you really can't buy just a little.

All it takes for the bushings on an Axxe or Minimoog sized keyboard is a tablespoon or two. I'd be more than happy to send some free to whoever wants it...squeeze a bit in a plastic baggy and mail it off. All you'd need to do is dump the dry bushings into the bag and mush it all around until they're coated.

I don't even need paid for postage as long as postage isn't more than a few bucks. Obviously I won't do this for 100 people, but if someone here needs it and doesn't want to hunt the correct stuff down, PM me.
emdot_ambient
In case you want the dirty details...the proper lubricant for the bushings used on Pratt-Read keyboards is Dow Corning 7 Release Compound.

And when I said "EXPENSIVE" I meant like $11 to $15 for a 5.3 oz tube when all I really needed was like a tablespoon or so. Not hugely expensive, but if you only have a small keyboard to fix up, you end up buying way more than you need.

And for all you chemists/material science geeks:
INGREDIENTS CONTAIN: 63148-82-9 Polydimethylsiloxane. 7631-86-9 Silica, amorphous. 70131-67-8 Dimethyl siloxane, hydroxy-terminated.
hmmm.....
kindredlost
I did some more work on refurb of the Arp Axxe. Em_Dot sent me a bit of the Dow Corning lubricant. It helped immediately, then later I saw the sticky keys again. very frustrating

I think I may have the wrong bushings. seriously, i just don't get it Does anyone have a reliable source for the proper ones?

I forgot to look at the cv/gate jacks to see if they are switched or not. I tried to get the thing powered up again and found the -15 v whacked and putting out +5 v. I replaced all the caps and the supply is putting out +15v and -16.4v. I can't get the negative side any closer to -15v than that so I have a couple of power transistors to replace tonight. If that doesn't get it any closer I may have to look into the regulator chip. I don't think it is the pot for that side, but I'm no EE.

Does anyone have a good idea of a replacement power supply for the Axxe? Just in case.

-David
emdot_ambient
When you replaced the bushings did you take the keys all the way off and clean the chassis, keys, key tops (if any) and all that? It's clear that the older Axxes had a different keyboard build than the newer ones like I have. For starters they have metal keys with plastic key tops, and the newer ones have all plastic keys, no separate top that attaches to metal keys with a screw.

I ask because on Minimoogs it's possible to change the bushings without pulling off the keys. I don't recommend doing that because you may not be able to get rid of all the old bushing gunk. Also, the Minimoog's Pratt-Read keyboard has another part similar to these bushings that could need replaced or at least cleaned and re-lubricated...it's called the bellcrank cap. You can see them in this shot (the yellowish caps on top of those white things: http://www.flickr.com/photos/31603983@N05/3454217306/in/photostream/

I don't know if the older AXXEs used this kind of Pratt-Read keyboard or not...my newer ones don't. If they DO...that's another thing that might cause key sticking.

Here's another site that has some info on Pratt-Read keyboards (though it's talking about ones used for organs, I think it's still valid): http://www.users.cloud9.net/~pastark/sonote56.htm

Quote:
A message on our Web Page reads in part: "The keyboard keys will get very stiff to push after a day or two of inactivity. The longer the stiffer. I run up and down the keyboard a few times and then they are smoother again. More and more are getting clackity..." I also ran into sticky keys, and found it to be the grease in the channels. After many years, in some keyboards, the grease has hardened, sometimes the bellcrank elbow caps (or bushings) are worn. This is a relatively simple problem to correct. Remove the key caps. Remove the key return springs. Then remove the key channels. Be careful not to loose the bellcrank caps, they may stay on the bellcrank, or they may stick in the grease in the channel. Take all the bellcrank caps and clean off the old grease. Clean all the old grease out of the key channels. Now replace the caps and grease them liberally. Replace the key channels, springs and key caps. If a key does not feel right, check to make sure the bellcrank cap did not slip off. Note that the white (natural) keys use a silver colored key return spring, the black (sharp) keys use a blue colored key return spring. I phoned Mr. Woody Comstalk, president of Pratt Read & Co. to ask him if he remembered what grease was used on the keyboards his company manufactured. He did not remember. To the rescue came Ray DeVault of DEVTRONIX, the same man who rescued Schober owners who had not completed the purchase of kits when Schober folded. He gave me the name of an outfit that sells the grease and the bellcrank caps. ORGAN SERVICE CORP. PO Box 372, Marengo, IN 47140-0372, (800) 457-4408. The grease is D-C #7, item 642-002-00, 5.3oz. $15.95, The bellcrank caps (also called bellcrank bushings or key bushings) are item number 72320-264, $.20 each.
emdot_ambient
As for power supply...no clue what the amps are on that thing. Other than that I would think that any +15/-15 VDC linear supply would work if it fits in the case.
Synthbuilder
kindredlost wrote:
... then later I saw the sticky keys again. very frustrating


I replaced the bushings on a MkIII Odyssey a couple of months ago. I did the first octave and put the key tops back but I noticed a stickiness in the key actions that I never normally get. The bushes are pre lubed and come from Rich at Arcsound. They've always been very good so I looked into why they didn't work so well this time around.

I don't think it's the bushes. Well, at least the bushes will work just fine with just about every other P&R keyboard you may have. The thing is on this ARP the tangs that the bushes slide onto had been pressed too flat with a circular punch. A lot of P&R tangs have this flattening of the top but this ARP had wider and flatter tangs than normal. Thus when the bushes went on they were forced to be wider than they should be. This caused the sticking of the keys. Perhaps P&R used thinner bushes in later years and maybe had to flatten tangs accordingly?

Solution: removed the bushes again and filed down all the tangs so that they were of the same thickness from the stem to the ends. With a decent needle file it took me no longer than 30 minutes to do the whole keyboard. Once the bushes went back on the keyboard was perfect.

Tony
Synthbuilder
kindredlost wrote:
Does anyone have a good idea of a replacement power supply for the Axxe?


No need to replace it. It'll be something simple. Try replacing the two 10K resistors, R8 and R12, first.

Also, pull out the output power connector to the PSU and test the voltages with it off load. Do you now get +/-15V? If so, it could be the power device Q5.

I don't need to tell you that the ARP power supply has way too many exposed parts running at high voltage. So do be careful.

Tony
kindredlost
Quote:
Solution: removed the bushes again and filed down all the tangs so that they were of the same thickness from the stem to the ends. With a decent needle file it took me no longer than 30 minutes to do the whole keyboard. Once the bushes went back on the keyboard was perfect.


I'd thought about that but considered it a last solution. I make metal parts for a living and believe me it's much easier to remove steel than replace it. I'll definitly do this. The sticking is while the keys are UP. Some are slugginsh no matter how much lube I give them. Slow action.

The resistors were my next fix. I already replaced the Q5 as a first try. With or without a load there is still a minimum of -16.2 volts off the negative side.

The manual for calibrating the supply says to check positive polarity and set it first, then reverse the leads to ground and -15v to check for +15v again. I think this could be sue to an older method using a VU Meter style voltmeter. Anyway, the resistor idea sounds likely because of the need to reduce the -15v side obviously isn't working. That is what they balance through Q3 (if I read the circuit right).

At a hazard of straining the goodwill here, I do have another general query about the octave rocker switch calibration. According to the manual you can only adjust the upper setting to get it in tune with the middle setting. I can get this fine but then the lower range is sharp by just less than a quarter tone. I can get both high and low ranges the same but I cannot figure out how to make all three ranges closer. Is there another pot to calibrate or am I creeping into the main board repair region? eek!

Thanks emdot_ambient and Synthbuilder for all your help recently. Guinness ftw!

-David
kindredlost
Complete success!

The resistor change made the -15v power side calibrate in nicely. Right in the middle of the pot.

The bushing pegs were 0.14" wide and I filed them down to 0.13" and straightened all of them with a square and straight edge before reassembly. This proved to free up the sluggish key action very nicely.

Once the power supply was right the calibration of octaves went smoothly. The thing plays like new now. I forgot how good the Arp Axxe sounds. It really is a classic sounding synth and the very best of the mono-oscillator synths. Very stable oscillator.

I may go ahead with a midi kit by Synhouse now. Has anyone done this retrofit? It looks pretty easy.

http://www.synhouse.com/axxe1.html

-David
emdot_ambient
I've considered that MIDI retrofit also. Only thing I'm a bit skeptical of is my ability to do the holes in the case without messing up the synth visually.

Now on my issues...

1) I checked the power supplies on both my Axxes and was able to calibrate the one I've refurbished to +/-15V no problem. The one I have yet to work on, however...I got the -15V set but when I checked the +V I found that, though it's reading +14.99V....adjusting the trimmer has NO effect at all. As in, turn the dial as you like up or down and nothing happens. Do they even make a trimmer like that anymore?

2) I've done some measuring on the CV out jack of the one that's been (somewhat) refurbished and I'm not getting the 0V to +3V I'm supposed to off the keyboard. I'm actually getting +0.01 to +2.97V.

I'm wondering if it's something as simple as a resistor in the Keyboard Current Source circuit. That circuit's supposed to feed the keyboard with its High (+3V) and Low (0V) CV, which are then passed through 36 100 Ohm resistors (resistor arrays actually? There's no schematics for the actual keyboard circuit in the servicing manual) to get the correct CV off each note.

Further tests show something that's probably significant, but I'm not sure exactly what it means: High C has a voltage of +2.97V, which is +.03V too low. The next lower C, which should show +2V is actually showing +1.98V...+.02V too low. One C lower, which should be +1V is showing +.99V....+.01 volts too low. Bottom C, however, is +.01V too high.

It's just kind of odd and probably not randome that I'm seeing a voltage error of +.03, +.02, and +.01 on the different octaves.

It's a very simple circuit....?

Actually the resistor value on the left is a bit unclear on the manual's schematics. They're both 1% reistors, too.
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