Ideas on fixing my Ross Phaser

[Ponchus]

Weird, I posted this same topic last week...someone answered with a suggestion to spray contact cleaner on the pots. Didn't work. Figured I'd update the post to see if there were any other suggestions, and my user ID and password no longer worked. I re-registered and my oroginal post is completely gone...weird

Anyway, I have a Ross phaser from 1980 (a black Taiwanese model). I just picked it up on eBay, and it looks mint. However, it doesn't seem to be working properly. There are 2 knobs: Rate and Depth. The rate knob doesn't seem to do anything until the halfway mark (12 o'clock). From 12:00 to just under Max (which is roughly 5:00), it does increase the rate, as it should. At 4:00 or so, it jumps into a bad warble. So basically, my rate is limited to between 12:00 to 4:00 (or perhaps slightly higher). As soon as I hit that 4:00 point, the rate increases drastically into an unmusical mess.

Besides the rate knob, the depth knob is weird. It doesn't seem to be truly increasing the rate until higher settings, which don't sound that great.

Most importantly, though, is the sound of the phase itself. I've never played on a perfectly working Ross Phaser, but I trust Ross's quality and I seriously doubt that their phaser is supposed to sound like this. This phaser has a really deep sound that (for lack of a better description) sucks tone, then releases, sucks tone then releases, etc. It seems as if the guitar tone gets softer then louder, softer then louder, with an increased bassy presence. It's not a musical sound, it's not a pleasant phase. It's hard to describe (I should post sound clips, but that's a huge hassle right now due to not having an amp in the house at the moment).

In my first post, someone suggested that the pots might have gone bad. Does anyone agree with this? I did spray them with contact solution to no avail. But still, if the pots went bad, wouldn't the result be scratchiness when I adjusted the rate and depth? This isn't happening at all. Also, would a bad pot affect the tone of the phase itself? Because when the rate is between 12 and 4 o'clock, it IS in fact phasing. I just don't like the sound at all and I have a gut feeling that it's due to something not working well...

BTW, I'm a newbie, so please respond in layman's terms :-)

Thanks for any help

[Mark Hammer]

I'm the one who suggested the contact cleaner. I'm not exactly sure what else to suggest.  The performance, as you've described it, is both systematic and erratic at the same time.  Reminds me very much of two table radios we have at home, both of which have a small zone a bit above "off" where the volume pot is scratchy and unpredictable.  Above that range everything works fine.  Both radios are pretty old (One I bought in 1982 or so) so the resistive element is obviously worn in those places where the pot wiper most often travelled.

Your thought about "scratchiness" is correct, except that scratchiness would occur if the audio signal passed through the pot.  In the case of the Depth control it does, but not in the case of the Rate control.  What you would expect in that latter case is erratic changes in speed.

I'm wondering if there is something wrong with the LM13600 that forms the LFO.  That would be the one common link I can think of that could a) result in bizarre sweep behaviour, and b) by making the unit sweep where it shouldn't, render degrees of regeneration barely audible.

BTW, have you found ANY pcb traces lifted from the board, or possibly cracked?  Consider gently reflowing the solder on the board just to make sure there are no hairline fractures of the traces getting in the way of normal functioning.

If I did not do so before, here is alink that will give you the lowdown on Ross Phasers: http://www.home-wrecker.com/phaserguide.html

[Ponchus]

Mark,
Thanks for your help (again!). I opened up the phaser to see if I can visually see any cracked connections and whatnot (sorry, I'm a newb here)...anyway, I do see something of interest, possibly...I don't have a digital camera, so I'll try to describe this best as I can...

I'm looking at the underside of the circuitboard (the side where I DON'T see the parts, I only see the soldering connections)...on this side, the shafts of the pots stick through (the pots are still connected to the board, I haven't removed them or anything). Around the hole that the pots stick through, printed on the board, there is a ring of what I guess is copper (plus there's a nut and a washer holding the pot to the board). This copperish ring is a different color than the rest of the printing on the board, BTW. This copper ring has some sections that look thin, and in one or two places possibly disconnected. Is that of any significance?

[petemoore]

  Check the continuity across the questionable area, just set DMM to beep mode [diode checker it should beep when the DMM leads touch together, perfect for checking connections].
  I'd check that out first.
  Next thing I'd do, especially if you fix a trace [I'd use schematic reference before adding connections unless it's obviously clear where connections are intended to be], is Beep Mode V+ to V_, if there's a small resistor seriesed at V+ to circuit, go on the circuit side of that resistor, make sure there are no power supply shorts before powering up.
  Also, just a hunch, the elecrolytics, for some reason...ya never know, could be they dried up or something.

[MartyMart]

I had a similar problem with my Line 6 Pod, the "drive" pot was acting "strange"
within various presets, it would "jump" and max out, then would do nothing at
all.
It was intermittant and I was thinking "poo software problem" !!
Took it apart and found the drive pot solder conections to be "dry"
IE: they looked "dull' and slightly "cracked" - BUT it was subtle to say the least !!
Added a touch of fresh solder and "BINGO" problem solved.
I'd spoken to Line 6 and they sent me two free pots anyway !
So It's fixed and I have spares.
It may well be that simple for you ?

Regards,
Marty.

[cjlectronics]

I've seen the problem you descirbe on many pedals I've repaired.  A few things can cause this problem but there is one thing you can do to help solve these symptoms.  First the causes of this problem:
1. Most of the time the wiper arm inside the pot will lift ever so slightly when you turn the pot toward one end.  This could have been cause by sudden shock, (ie dropping it on the pot which bent the wiper or just wear and tear over the years.)
2. Dirt and corrosion have built up on the wiper so it makes a poor connection as the wiper slides over the carbon element
3. The carbon element has been worn away at the spot where the wiper travels and thus the only resistance seen is the rated resistance of the pot. (Like Mark suggested)

Most often the above symptoms can be cured by removing the pot from the circuit board and peeling the pot open to expose the element and the wiper arm.  This is not difficult as the backshell is held on by 4 ears that fold over the front face of the pot.  Peel the ears back and the backshell will come off.  Then using a delicate hand gently clean the carbon element with a soft cloth and a mild cleaner.  You can spray the pot/cleaner you used earlier on this cloth and use it to clean the element.  Do the same for the wiper arm but be careful not to bend it too much.  You should also be able to see the wiper arm work throughout the portion of the carbon element that is in question.  Look for dirt and corrosion on the wiper and also for any wear on the element. 

To put it back together reverse the steps you did to take it off but just remember to put the wiper in the middle of its travel so the stops get aligned right.

You can also buy a new pot but sometimes an exact fit, form, and function drop in replacement is not always available. 


Something else to note on Ross phasers that I have seen.  I had a Ross Distortion Phaser come in for repair recently, the client mentioned the distortion didn't work and the phaser sounded OK,  The distortion section was a bad op amp...easy to fix!  The phaser portion was not working anything like I know Ross phasers should sound like. It was just weak and not deep with warmth and groove.  What struck me as odd was that all of the TOA op amps were replaced with LM13700.  I knew that the original design called for LM13600 but i read in a few threads that the LM13700 was a drop in replacement.  Well, for Ross phasers DO NOT BELIEVE IT!  I replaced all of the TOA's with the LM13600's and that pedal came alive!!!  Just like the Ross phasers I'm familiar with.

Good luck,
CJ

[Ponchus]

hey all, thanks for the great responses. I'm going to hit up Radio Shack today and see if I can buy a replacement pot as a test.

Now I'm a total newbie at this, so I didn't really understand some of Pete's suggestions  I don't own a DMM, but maybe I should invest in one. Not sure how to test continuity just yet, or how to use a DMM. Guess I better start learning though if I ever intend on building my own pedal...

However, regarding my last post, I'm still unsure if the copper ring surrounding the pot is possibly a cause for concern. If anyone can comment on that, that would help big time. In the meanwhile, I'll clean or replace both pots, I'll touch up all the soldering connections on the pedal, and I'll post an update...

BTW, in response to CJ's post, I do have LM13600 in there...not 13700, thanks for the heads-up...

Hey, worst case scenario, do you guys have any suggestions on who in the NYC area would be trustworthy to fix this for me?

[Ponchus]

Hi all.
So I touched up most, if not all, of the solder joints, and removed the pots. I couldn't find replacement pots at Radioshack, so I swapped them (put the Rate pot in the Depth slot, and vice versa)...pedal sounds exactly the same, no difference. So I'm still out of luck...

How do I test the chips (13600s) in this thing to see if any are bad?

[Mark Hammer]

Hi all.
So I touched up most, if not all, of the solder joints, and removed the pots. I couldn't find replacement pots at Radioshack, so I swapped them (put the Rate pot in the Depth slot, and vice versa)...pedal sounds exactly the same, no difference. So I'm still out of luck...

How do I test the chips (13600s) in this thing to see if any are bad?

Since it is a 4-stage phaser, you can "test" the chips by pretending it is a 2-stage phaser.  In other words, remove the LM13600's, bypass the first two or last two phase shift stages, and rotate/swap chips.  Unfortunately:

1) This assumes they are socketed already and don't need to undergo the risk of desoldering.

2) You know how and where to bypass the two relevant stages.

3) There is ONLY one chip that has gone deviant.

Though you may have swapped the pots (inventive! I like that), you still need to verify that they both work.  So, if you can bear to get in there once more, AND you have a meter, measure the resitance on each side of the wiper as you rotate the control.  Thre should not be any sudden or unpredictable jumps.  If you want to get right in there, you might consider removing the backs off the pots and checking the status of the wiper and resistive/conductive element.  No telling what sorts of grime may have been built up over the 20 odd years this pedal has been around.

[Ponchus]

Hi again,
Unfortunately:
1) The LM13600's are soldered to the board and will need to be desoldered
2) I don't know how to bypass (or even find) the 2 relevant stages

So I guess I'm pretty screwed...I gotta start reading up on some serious engineering books, to get up to speed...any suggestions for the layperson?

[Mark Hammer]

You're not screwed, merely limited in terms of the more "exotic" types of tests to be performed.

If you have an audio probe, you can plug your guitar into the pedal, plug the probe into a suitable amp and monitor the status of the audio signal at different points, or simply measure the AC voltage with your meter.  Both are useful, though it's tricky to make sure you're holding the probe in the right spot while you look at the meter (both need your eyes).  Somewhat easier to look at the board and schem, and listen as you contact/probe different test points. 

One thing that might help in identifying (by ear) what is going wrong is to lift one end of the 27k resistor connecting the input and output opamps (only one dual op-amp on the board and only one 27k resistor straddling two pins).  This will isolate the phase shift signal (its normally what you do to turn a phaser into a vibrato by using ONLY the phase shift signal) so that you can perhaps better describe what is happenign with the phase shift.  If you do this and it is behaving normally (i.e., the problem is not in the phase shift but in the sweeping) then you should hear a slight pitch shifting.

BTW You still need to confirm that both pots are fully functional.