MXR Phase 45 Fault - original script logo from 1975

[sohosteve]

Yeah, the tantalum caps die with age. That's pretty common.

FOAM- used to wrap the circuit board... it ROTS.
After it starts to rot, it eats into the circuit tracks and components.
LOOK for tiny spots of green corrosion on the circuit tracks and components.

The foam will actually turn into a sticky corrosive liquid goo. (with age)
All of this residue must be carefully removed.
We suspect that this foam is formaldehyde based. It can eventually destroy the unit.

Ok.
The foam is long gone but the board looks nice and clean with no sign of rotting components or tracks
I'll order some replacement caps - could someone please tell me what value they are?
I'm not a electronics guy though I can handle a soldering iron and solder sucker to swap out components.
Thanks

[52 Bill]

It was already mentioned above in Mark Hammer's earlier response, 10uF.

[Audiotexan]

Not sure if I missed it or not, but I didn't see where you addressed DrGonz78's astute observations of bad solder joints.
These alone will give issues since voltages/currents are not being properly delivered to where they need to go.

I'm including a pic highlighting the target areas (special focus on the black notation areas). =)


Reflow the solder around these joints and retest for proper operation.

Also, the fact that after being plugged in for half an hour it slowly starts to work makes me think that it is more likely that it is a capacitor issue.

This is the nature of the beast with 'cold solder joints', and also highly suggestive to not needing parts (though not guarenteed).

Cold solder joints heat up and expand as voltage/current is applied and start to complete the connection. When they cool they shrink. So if they're already on the verge of not having a connection (ie: the two areas with black notations), then as they heat up, they come *closer* to making a proper connection. And you get some form of operation. It may not be proper, but if it gets you closer to "proper" then that's telling.

If it was on my bench, I would not order any parts until those joints are addressed!

Only until you know that all points of contact (or in this case 'suspect points') are 100% good, you cannot begin to nail down a failed component(s). Once your joints are cured, only then can you see where you're at. =)

Regards,
Audiotexan

[sohosteve]

Thanks to both Audiotexan and 52 Bill for your fantastic help and advice.
52 Bill - yes sorry - so he did! I should really put my brain in gear before being allowed anywhere near a computer keyboard! ;-)

Audiotexan - I didn't follow that up as in 'real life' the solder joints look fine - but I shall bow to your greater knowledge, take your advice and report back with my findings.

You guys rock!! :-)

[sohosteve]

@ Audiotexan - Actually, having just looked at your highlighted version of my picture I can see exactly what you mean!
Jeez!
I need glasses!!! 8-/

[sohosteve]

Actually I think that's fixed it!!
The only problem now is that the pot seems to have totally died!
I had to put some Caig Deoxit 5 in as it seemed very scratchy (physically and audibly) however now it only phases when the pot is up to full speed and does nothing at (signal but no phase sweep) when moved to a slower setting.
Any idea what value the pot is that I need?
I have a few spare pots so could easily try one out - I have 1Meg Lg, a CTS 500 (but that's no load pot so suspect that won't work when fully clockwise), and a 250k though I'm not sure if the second two are Log or Lin - what's my best bet?

Not sure if I missed it or not, but I didn't see where you addressed DrGonz78's astute observations of bad solder joints.
These alone will give issues since voltages/currents are not being properly delivered to where they need to go.

I'm including a pic highlighting the target areas (special focus on the black notation areas). =)
[ATTACH=CONFIG]29346[/ATTACH]

Reflow the solder around these joints and retest for proper operation.



This is the nature of the beast with 'cold solder joints', and also highly suggestive to not needing parts (though not guarenteed).

Cold solder joints heat up and expand as voltage/current is applied and start to complete the connection. When they cool they shrink. So if they're already on the verge of not having a connection (ie: the two areas with black notations), then as they heat up, they come *closer* to making a proper connection. And you get some form of operation. It may not be proper, but if it gets you closer to "proper" then that's telling.

If it was on my bench, I would not order any parts until those joints are addressed!

Only until you know that all points of contact (or in this case 'suspect points') are 100% good, you cannot begin to nail down a failed component(s). Once your joints are cured, only then can you see where you're at. =)

Regards,
Audiotexan

[Audiotexan]

The only problem now is that the pot seems to have totally died!I had to put some Caig Deoxit 5 in as it seemed very scratchy (physically and audibly) however now it only phases when the pot is up to full speed and does nothing at (signal but no phase sweep) when moved to a slower setting.
Any idea what value the pot is that I need?

Well, that might have been enough to slide any loose carbon around inside (worst case chunks of the track came up)...but hopefully, just working the pot back and forth 20-30 times, should clear anything off the track.

Judging by an old Ampage schem I have on file, the pot should be a 500k. However, it the copy I have wasn't noted as to taper. So I can't say whether it's audio or linear. But at least for the moment, you can toss a 500k in there to verify operation. I'm sure others can fill in the taper

PS: If anyone has non-homebrew schems for either or both of the phase45's (block/script), I'd appreciate a copy!

[sohosteve]

Nope!
It's sadly the worse case scenario - I eventually opened up the pot and found that most of the carbon has totally melted down leaving just a small section of track - where it is working at full speed no doubt!
I did find one clue from on my extensive Google searches where someone said that the pot is a 470k Reverse Log - whatever that means!

Ok. So I guess the 500 may be close enough - though it is the 'No Load' pot - I guess that may work as long as I don't turn it all the way up?
I'll try that anyway.
Regardless thanks for all your help and have a great weekend!

[sohosteve]

This looks quite interesting Phase 90/45 and phaser/univibe mods

PS: If anyone has non-homebrew schems for either or both of the phase45's (block/script), I'd appreciate a copy![/QUOTE]

[sohosteve]

Oops! Sorry - non homebrew!
Try here https://www.generalguitargadgets.com...6-mxr-phase-45

PS: If anyone has non-homebrew schems for either or both of the phase45's (block/script), I'd appreciate a copy![/QUOTE][/QUOTE]

[Audiotexan]

Nope!
It's sadly the worse case scenario - I eventually opened up the pot and found that most of the carbon has totally melted down leaving just a small section of track - where it is working at full speed no doubt!

Well, at least it's an easy fix. Hopefully that pot's the final issue for you!

I did find one clue from on my extensive Google searches where someone said that the pot is a 470k Reverse Log - whatever that means!

'Reverse log' is the taper we were referencing earlier. Basically, taper means how it reacts/interacts (or 'feels') in the circuit as you adjust from minimum to maximum.

The 500k will electrically get you by, but due to the change in taper, you'll notice that it doesn't give you the same sound at the same spots you previously used it at (other than absolute minimum, or absolute maximum).

Hope you have a great weekend as well! And thanks for trying to track down the schem...the thought's appreciated! *grins* (though I was trying to imply factory. Mea culpa. lol)

[sohosteve]

Hey there
I've not had much of a chance to finish fixing the old Phase 45 - hence the huge delay in the follow up...
Anyway, I tried the touching up the solder joints and sluicing out the trim pot with switch cleaner - no improvement - the same fault - no depth to the sweep of the effect until it's been used for ten minutes or so and then the effect gradually becomes more noticeable until it appears to be functioning properly.
So I decided to try replacing the Tantalum Caps and ordered some 10uF as recommended.
However, I have realized that these probably have a polarity.
There are no obvious(to a numpty like me) markings on the ones on the board beyond the three colours and I wondered if I could seek some more of your generous assistance.
(I did actually attempt to research this online but I wasn't sure from what I did find, if the colours denoted the cap value or the blue stripe meant that leg was the negative side!)

I've added some new, super high def photos to hopefully ease the process....

[Mick Bailey]

Take a look where the + battery lead connects to one of the capacitors and this will tell you the polarity of these. Alternatively, power up the unit and use yourr DMM to determine the polarity. Or take a look at the board; all the -ve legs of those caps connect to ground (or if you prefer, battery negative).

[g1]

I wonder if there is not a small dot near one of the legs indicating polarity. Maybe on those caps underside that we can't see.

[Mick Bailey]

On some older tantalum caps if there's a dot in the coding scheme, when this faces you the RH leg is positive. I think most modern caps follow similar - when you read the value the RH leg is positive, also often reinforced with a bar, row of +++ symbols and a longer connecting leg.

When I look at the track side of the board I can see the the three caps all connect to the track supplied by the black lead. This suggests that the side of the cap with the green stripe is positive.