Metaxas CP-1 - Impossible repair? Crackling & Popping on one channel.

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Hello,
i got my hands on a CP-1 from MAS Metaxas.
never heard of it? me neither,
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its so rare, you wont even find it on the internet, except in 2 (now 4..5) places.
seems to be from the 80s, probably 86.. 87 or so.
in their "how it all began story" pdf mentioned on page 7 http://www.metaxas.com/HIFI_2015.pdf
on some site mentioning it in "DAS OHR" magazine and the 2 or 3 posts i made.

i found a picture of a later revision, still CP-1 but industrially made PCB instead of self-etched. in low res, due to original got deleted and using google image cache.
ncl5GQe.jpg






it has a technical problem i cant seem to fix, atleast not easily.
one channel loudly pops and crackles when its cold, after warming up it goes away.

im listening to it since an hour now, and havent heard a single crack / noise anymore.



the popping is so loud, it basically swings almost from rail to rail. and then it starts to create noise after settling. hooking it up directly to a high power amp will likely damage it or the speakers attached.


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its not mechanical issue, i tapped and hammered on it, but couldnt reproduce it, it comes and goes on its own.


i replaced all capacitors with new panasonic (FC & FC-A) ones and a x-class foil cap in the supply and renewed the thermal compound, but the issue still persist.
(attached pictures show stats before replacing components.)


i narrowed it down to be one of the potted Gain cells which control the output gain. before that, everything seems to work even the phono stage.

The signal before that is clean and the powersupply doesnt glitch when it happens. so it must come from one of the gain cells.


i confirmed that by swapping left and right, and the issues moved with it.
(its the one with the ripped off sticker in the attached pictures)

gFdFJCl.jpeg



The potting doesnt look like epoxy, more like very hard tar.. bakelite? shellac? cracked corners look like glass / obsidian, its very hard,

i have no clue.
i tried melting it with a soldering iron, well it stinks badly but didnt get soft.


GjTbMBU.jpeg


The faulty cell is 2°C hotter after a hour of listening, and as i mentioned, at this temperature the crackling and popping is gone.
(ignore the hotspot top left, its the resistors for the front LED; they are both dropping 46V no idea why one is hotter, powersupply is perfectly 23.8V -24.1V on both channels.)

i suspect it to be a discrete opamp to be inside, but i havent got a xray machine (yet :p)

it has 5 connections, +-24V, signal in and out and capacitor feedback of some sort. (2 tantalums in series, in paralell with a foil cap)


also since theres no info about it anywhere, its very unlikely to find a schematic.


anyone got a idea what to do?
can anyone can tell me something about this rare unit?



rebuilding both cells either descrete or with opamps is the last resort.

its definately not something to throw away.



i also contacted the inventor, kostas metaxas if he can tell me more about this unit but havent heard from him yet.
 

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i tought, since it runs fine when hot. a pre-heating circuit thats switches the outputs on when the modules reach their 55°C operating temperature

:D


OCOA - oven controlled output amplifier


this unit doesnt even have a power switch, so its running all the time anyways.




The modules in the bright cardboard are the phono stages, the ones in the dark resin are the output stages, they get their signal right after the volume pot (which gets it from the selector/phonostage etc.) and the output is directly coupled to the output jacks. so it just seems to be a simple amplifier.
 
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Don't bother trying to remove the potting. It would be a mess, and you probably would destroy the module beyond repair in the process. As I see it, you only have two practical options, that will exactly keep the unit's current sound character:

1) Keep contacting Metaxas to ask them if they have a new old stock replacement module they are willing sell you.

2) Leave the unit constantly powered, unless away from home for an extended period of time. Many audiophiles leave their equipment powered this way.

Option 2 will likely end up as the best solution.
 
1. i allready did, he answered me with "its 40 years old, he cant help me"


2. thats a good solution, i just need some protection that disconnects the output when its below its operating temperature, otherwise someone who just hooks it up and forgets the warmup period might damage his equipment.



i dont plan to keep it forever, i got enough stuff coming in and piling up, but selling it in this condition only results in problems.
 
i made a circuit that compares the temperature from a thermistor in a voltage divider and switches the outputs via reed relays when it reaches 45°C.
i built it for both modules, but i figured, just monitoring the faulty one and switch on both is better. so half the circuitry wasnt necessary.


i could tap off the led and even build a small heating circuit to accelerate it.


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Administrator
Joined 2007
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My thoughts (fwiw :)) is that trying to keep the unit running in a faulty state is madness.

What if it gets worse ?

What if it goes on to damage partnering equipment in the future if the fault suddenly becomes permanent ?

It is possible the other module isn't far behind this one in terms of failing and/or the performance of the modules has also subtly deteriorated over time.

I would be looking at making two new custom 'modules' whether that be IC or discrete.
 
replacing the modules makes this preamp just a switch box with a phono stage.

i mean, yea i could do that, nobody knows this device so nobody knows how its supposed to sound in the first place.


and i would need to find a good sounding opamp that accepts +-24V rails, most common ones (ne5532, lt1115) only go to +-22V (absolute max) or a opa132 which only goes to +-18V (abs. max). so i might need to go with discrete



the problem with damaging equipment down the chain is a big concern, i usually would hook it up to some capable amps, if it decides to glitch, it would definately wake up our neighbours and might release some blue smoke somewhere.


after it warmed up i listened for hours, without a single glitch. but thats the current state, further aging could make it worse as you said.
 
i hooked it up to my scope, yellow is the channel with the faulty module.


thats what the channel looks like after turning it on while cold (5 seconds/div, 2V/div)
it went over 20V peak.

after it reached operating temp, it went smooth like the other working channel (blue)


(after the second image, i helped a bit with a heat gun)

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Reminds me to NEVER buy ANYTHING from THAT company!

Let's be fair, the unit is four decades old. While Metaxas has seemingly zero market presence in the States, their products have been well reviewed in the American audiophile press.

In addition, they long ago released, what was possibly the first fully discrete solid-state DIY pre-amp design (phono, line stage power supply) from an legit high-end commercial gear vendor. Which may still be floating around on the web.
 
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@russc of course, when i hook up something directly it sounds different than with this preamp in line, if better or worse...well i wont comment on that, it would result in a war like allways.



trashing is no option, atleast not yet. this thing isnt your normal consumer crap, and since i got plenty of time anyways.






what a mess, i just depotted a module and tried to reverse engineer it.
might have gotten far enough to see whats going on, but resistor markings are mostly gone and i cant identify 2 transistors.


at about 250..300°C the potting became very brittle. had to use hot air since it wont stay brittle.
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thats what i have gotten so far, anyone recognize?
maybe anyone know what those transistors Q1 and Q4 could be. not even sure if q4 is a PNP. its marked 2Nxx62 (xx missing..) could be 2n5962 npn or 2n5462 jfet





im trying to get some component values from the ones that arent broken yet.


edit:


found all resistor values, D1 is just a diode no zener, D2 is unknown, probably the same? idk


second stage transistors are really p and n jfets (m333 02B, 2Nxx62)
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mkc

Member
Joined 2002
Paid Member
Hi gamerpaddy,

I think there are a slight mistake in the drawing. My quess would be that J2 and Q1 are both jfets and of same type and so are J1 and Q4. They are configured as differential pairs. Drain from Q1 are connected to emitter of Q2 and drain from Q4 are connected to emitter on Q3.

Please let me know if you are unsure of what I mean.

Cheers,
Mogens
 
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