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Curtis Mathes Power Amplifier - Dead! Please Help

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MrFixIt

New Member
Hello,

I have a curtis mathes KA250 Stereo Amplifier (I think panasonic SU-370 is the same) that I need to fix. It was my fathers that he would never get rid of. So for x-mas i want to fix it for him.

The stereo has no sign of life. If I press power I get nothing.

can anyone get me a service manual?

One thing I thought was weird was that even if the power button was off I could read 120v from the primary (red and blue wires) on the transformer. Not sure how that is possible considering it was switched. Also if there is power going into the transformer then why wouldnt the light come on since it is connected directly to the transformer? I need help on what to test... I have a digital multimeter. How could i test the transformer to see if it is good or bad. I can order a transformer but whats to say when I put a new one in it wont blow up again (if thats the problem). I can take close up pics of anything if need be and I can use my multi-meter and also capture video or images.

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Top of switch board


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Bottom of switch board


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Primary connections on transformer reads 120v whether the power switch is on or off


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Transformer - P/N#SLT5M406


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Overview of Mainboard


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On the board there is one spot that looks like it got hot.


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Another shot of mainboard


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These are the three main wires that goto the main board. the black one comes directly from the transformer and the other two come from the speaker impedance switch. the other black wire on my thumb was grounded to the casing. with the power on i can turn my multimeter to Volts AC and test between the back and brown and get no reading and also between the black and grey and get no reading and also between the grey and brown and get no reading but when I test between the black ground that is on my thumb and the other wires i get about 5v ac.


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Light - two wires go directly into the transformer, doesnt light up whether the switch is on or off


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Four Wires that goto a switch for impedance.
Orange,White, Light Blue, and Blue
Between the white and the light blue there is a black wire that goes to the main board.



Please help me troubleshoot and fix this unit I am a bit lost. can you help guide me in the right direction... thanks!
 
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user_88

Member
The blue ceramic disk, next to the power switch looks like it might be a MOV ... used to protect from lightning, etc. It may be shorted out. De-solder it and see if you get the switch to work .... measure 120v or no voltage, according to the switch position.

.... Be careful with live circuits ....
 
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MrFixIt

New Member
re

I tested power from the wall to the chassis and it is good. Here are some more photos... of me testing on the transformer... when i test resistance in ohms on the primary... i get O.L and when i get .3 ohm from wires 1-2 and .7 from 1-3 and 1.1 from 1-4 and 1.3 from 1-5 on the secondary. I am also reading 120v whether the power switch is on or off... how is this possible? I tested the switch and it is good.

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MrFixIt

New Member
The blue ceramic disk, next to the power switch looks like it might be a MOV ... used to protect from lightning, etc. It may be shorted out. De-solder it and see if you get the switch to work .... measure 120v or no voltage, according to the switch position.

.... Be careful with live circuits ....

I de-soldered the blue "MOV" part# kv103p and it now switches correctly. before it would read 120v whether the switch was on or off... now it read approx 3v when switch is off and 120v when switch is on.

the power light still doesnt come on.
 

user_88

Member
Sorry about the editing and re-editing of my previous message ....

With some sincerity, I would recommend that you de-solder the blue plastic/ceramic disk, next to the power switch .... in you photo.
This part could easily be 'blown' and causing the voltage to the transformer to not be affected by the switch. It won't hurt anything to just remove it ... It is only intended to protect the circuit from line transients ... and won't be necessary for diagnostics ... while you are trying to find out what the main problem is.
 

user_88

Member
OK .... roger on the blue part being removed from the circuit ....

Can you measure the voltage of the two wires going into the blue plastic neon light?
If that is in fact a neon light, the two wires should have a significant AC voltage, maybe 80 or 90 VAC.
... Just thinking that the neon light might be defective, if you do measure that there is voltage there.
If you do not measure AC voltage going into the light bulb, then there is a defect someplace else.

I see that you have previously tried to measure the voltage between the gray and brown wires ... going into the neon light.

One thing that happens occasionally is that the glass fuse .... pictured above ... will look like it is okay, but will actually be defective. .... Just to be sure, could you remove the fuse from its holder and examine it carefully ... Check it with your continuity checker. If there is anything that doesn't look right, you should replace it.
Another test for the fuse is to see if there is any voltage drop across the fuse holder .... using your meter. Any measured AC voltage across the fuse holder indicates a bad fuse or a bad fuse holder.
 
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MrFixIt

New Member
I hooked up multi-meter 1 probe to each side of the bulb.... no ac voltage reading. the leads for the bulb go directly into the transformer...
 

MrFixIt

New Member
OK ....
Have you verified that the fuse and the fuse holder are good?

Yes both fuse holder and fuse is good. I checked continuity between them... no sign of voltage with multimeter.

I am getting 120v to the primary winding on the transformer.
 

user_88

Member
If the fuse is good, then as the next step, I would suggest that you de-solder the two wires on the primary side of the transformer .... That would be the two large red and blue wires.

You can't test the continuity of the primary winding of the transformer unless it is isolated from the circuit.

Once you have removed the red and blue wires from the primary side, you can use the meter continuity test to see if the transformer primary winding is intact, or possibly shorted to the case or some other winding.

Initially, just check the continuity of the two main primary winding connectors.

If the blue MOV was blown, then it looks like lightning or something might have been the initial cause of the malfunction.
 
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MrFixIt

New Member
ok I de-soldered the blue and red wire and checked for continuity and there was none. I switched my multimeter to ohm and read O.L
 

user_88

Member
That would indicate a bad transformer .... Apparently, the single MOV part was not enough to absorb the excessive joules from the causal event.

I would try to replace the transformer, if you could find the correct part.

I would not be concerned about damaging the new transformer, since there is evidence that an electrical surge of some sort was the cause of the problem.

If there was some indication that the malfunction was caused by excessive power usage ... playing the unit with too much volume, then there could be other problems. However, at this point, it looks like a new transformer would be sufficient to get the unit back in operation.

At any rate, you will have to have a new/working transformer in order to proceed .... to find out if anything else is defective on the secondary side.
 

user_88

Member
... Before you remove the defective transformer, take the time to draw a sketch of the different secondary wires and their location.

... Label the wires if there is any color duplication or other identification difficulties.
 

MrFixIt

New Member
No worries on misplacing the wires since they are all marked where they go....

One more thing.... if I put the blue wire on the other unused connector... I get voltage from the secondary winding...

see pic...


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MrFixIt

New Member
Where do I get these parts? How can I tell the voltage of the lamp... is it neon?

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ke5frf

New Member
Getting the transformer might prove to be the most difficult thing. This appears to be an old unit. The manufacturer will not likely help you or stock the part.

I'm sure a lot of help will come here on this site, but a dedicated "Audiophile" website will probably prove fruitful as the hobbyists on those sites have broad experience with restoring and maintaining old equipment. They will give you tips in abundance.
 

user_88

Member
This company might have the MOV :
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.... That looks like an unused winding on the transformer board .... blue wire reference.

The neon light might work ... if the voltage from the transformer is present ...

Finding the transformer will probably be the main obstacle to getting the amp working.
 
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ke5frf

New Member
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What do you know, the transformer might be easier to get than I imagined.
Be aware that while it sounds like you are on the right track, transformer failure is pretty uncommon and measurements can be confusing. You really need to know exactly how the transformer is wired and tapped to have any certainty in your troubleshooting.
 

MrFixIt

New Member
Success!!!

Ok. so the unit is fixed! Here is what I did...

I unsoldered the blue AC switch wire and hooked it up to the other contact point or extra winding on the primary side of the transformer. After I done this the unit powered up. Next I re-soldered the blue ceramic disc as the unit switches just fine with it hooked up... next I needed to replace the lamp... so the only thing i had laying around that was similar was a x-mas light bulb. Next I was having trouble with the sound comming on... it turned out to be a cold solder joint on the speaker relay. Next the volume knob on the unit was messed up it was only partial and you had to play with it to get full volume. I took a paperclip and scraped the contact points for the switch and now it works good.

I am now happy to report that the unit is on and sounds great! Thanks for everyones help!

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