Pre amp output shorted?

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What I have done by now:
- changed relay to Takamisawa -> no function
- changed the driving transistor (a BC237) to a replacement type BC 547 -> no function
- changed the diode and cap besides the relay -> no function

I am unsure about the diode, because I can only decypher 1N4, but nothing more detailed, so I replaced that with a BZX79C24 24V 500mW (Zener diode). May be, this is the wrong type?

I am tempted to bridge the relay and connect the audio signal directly. Would that cause damage to anything, provided I always switch on the preamp before I switch on the power amp and vice versa?

Ben
 
Relay change didnt work :(

I have just been rethinking and i cant tie in the shorted outputs with the headphone socket output not working , unless there is a second relay . so bypassing the relay youve been working on might make no difference.

Refering back to an earlier post what voltages are you getting at the 18V points.
 
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Good evening

I've got some interesting news:
I replaced the wrong zener diode with the 1N4148 you suggested AND the relay now switches to ON after a few seconds. I can measure, that both output channels are lifted from ground, when the relay switches ON.

If I insert the headphone jack, the relay switches to OFF (muting the output) and when I remove the headphones, the relay switches back ON.

So far so good!

BUT on the phones I have only the right channel, the left channel is dead and silent, no noise no hum, no signal. So may be, I am back at the stage, where the disaster began, when I had the problem with the left channel going dead, for a couple of days, before the pre went completely silent. I need to verify that, with the pre connected to a power amp, but cannot do so today, as it is already a bit too late.

I think, I'll write a quick mail to Canton and ask, whether they would be willing to dig in their archives for a circuit diagram, though I am not hopeful, as they just bought the pre and relabelled it...

Thank you so far!

Ben
 
Hi

Some progress :)

So is the original diode you took out faulty ? ,
and you get proper sound through the right channel ...not just noise.

Interesting about the insertion of headphone jack mutes the output to the amp as it makes the relay operation circuitry more involved than i had imagined ( the hazard of trying to work without a schematic ) .

I tried again yesterday to hunt down a schematic , using MBL also , but still no joy . A site i have used before , that has numerous other manufactures schematics , doesnt list one Canton or MBL amp :( are they that scarce because they seem like quality amps .

PS.. can you get the original diode in bright light possibly against a white backgroud to try and read the lettering to confirm it is a 1N4148 .
 
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Hi

Some progress :)

So is the original diode you took out faulty ? ,
and you get proper sound through the right channel ...not just noise.

Interesting about the insertion of headphone jack mutes the output to the amp as it makes the relay operation circuitry more involved than i had imagined ( the hazard of trying to work without a schematic ) .

I tried again yesterday to hunt down a schematic , using MBL also , but still no joy . A site i have used before , that has numerous other manufactures schematics , doesnt list one Canton or MBL amp :( are they that scarce because they seem like quality amps .

The right channel is fine, no noise, no distortion.
Yes, the headphone circuitry seems a bit more sophisticated than expected.
I also had no luck finding anything about these amps so far. May be Canton reacts to my request email, but this is not really probable, though some people have had favourable experience with their service dpt. MBL is one of those smaller, but quite reputable (MBL - Unique High End Audio) companies over here. The products are superb and that is the reason, why I think, the preamp repair is worth every effort. If I don't get it running again, I would send it in for (supposedly expensive) repair as a last resort.

What I can do is, fetch my oscilloscope and try to trace the signal through the right channel and in parallel through the left channel, to see, whether I can detect the point, where the signal path is broken. There are so many transistors on the board, that this will take some time (not the least, because I have never done that before...) That's an involving learning excercise for me.

Ben
 
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Realising im asking questions so will just type

When connecting the scope probe to an earth connected piece of equipment you may or may not need to attach the crocodile clip to chassis ( not need to if scope is earth connected ). If you take the sprung hook end off the probe be aware that the metal band near the point is the earth side of the scope ( the same point as the crocodile clip ).

Happy hunting .
 
I have just had a thought........based on the fact that inserting the headphone jack causes the amp output to be muted ( the relay has and has not operated ) and also the fault on the left channel ( has worked has not worked ) .....is there a piece of loose metal floating about inside the headphone socket or are there some failed solder joints at the headphone socket .
 
I have just had a thought........based on the fact that inserting the headphone jack causes the amp output to be muted ( the relay has and has not operated ) and also the fault on the left channel ( has worked has not worked ) .....is there a piece of loose metal floating about inside the headphone socket or are there some failed solder joints at the headphone socket .

Good evening epicyclic,

that's an idea, which also dawned on me. I have to desolder and remove the jack to see, whether there is something broken or some metal inside. It is a possibility as I have already found one or two old solder pearls from a past repair (which the seller did not mention).

I have another good news: Canton sent me the circuit diagram for the preamp via email. I have asked now, whether I am allowed to share the diagram here. I don't want to violate their copyright after they have been that helpful, so I wait for their answer. Nevertheless I could send you the diagram directly via email, if you are interested. It sure is an interesting layout.

Ben
 
Hi Mind

Yeah very interested to see schematic . Could you send it via the private message facility on here .

Hello epicyclic

I even got Canton's permission to upload the schematics here, with some quite understandeable caveats:

Canton Elektronik GmbH + Co. KG (see Lautsprecher | CANTON - German loudspeaker tradition - Deutsche Lautsprecher Tradition seit 1972 (de)) retains the Copyright for the circuit diagrams and has not given the diagrams into the Public Domain. So my message to all here, interested in the diagram: handle it respectfully and watch the Copyright.

I must say, I am very positively surprised by the service Canton provides to a customer, who uses a 20 years old product - that's very nice.

Some explanantions:
- the schematics have some short notes in German. and I'll give a summary here in English:
. generally they show only one channel. On the board all components with a leading "1" desginate the left channel (R1xx, C1xx, IC1xx etc.) whereas all components with a leading "2" designate the right channel.

- in the schematics the simple word "out" sumarizes three output connector sets (2 x RCA and 1 X symetric XLR)
- the part HIFI4:CTNPLAN1 is the phono section
- the part HIFI4:CTNPLAN2 is line section and power supply
- here "Kopfhöreranschluß" = headphone connector

So, that will give us a much better idea about the inner workings of the preamp.

Ben
 

Attachments

  • Canton EC-P1 preamp parts list.pdf
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  • Canton EC-P1 preamp diagrams.pdf
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Before you pull the relays make sure the relays are actually getting power. It is more likely the coil drive circuit has failed as you never get 2 relays failing at the same time in the same way. It is far more likely that the drive circuit is not activating the coils

As for4the voltage regulators, look up the pinout on the internet. What would be the base lead if they were a transistor is the input lead and the readings you have on the +7818 are normal. The 7818 is I-G-O and the 7918 is G-I-O. G= ground, I= input and O = output.

It sounds like you ar einover your head here and if you want to keep this item, you need to have someone local who is more of an expert look at it.

DO NOT change the relays until you are sure they are getting a drive signal first.
 
Before you pull the relays make sure the relays are actually getting power. It is more likely the coil drive circuit has failed as you never get 2 relays failing at the same time in the same way. It is far more likely that the drive circuit is not activating the coils

As for4the voltage regulators, look up the pinout on the internet. What would be the base lead if they were a transistor is the input lead and the readings you have on the +7818 are normal. The 7818 is I-G-O and the 7918 is G-I-O. G= ground, I= input and O = output.

It sounds like you ar einover your head here and if you want to keep this item, you need to have someone local who is more of an expert look at it.

DO NOT change the relays until you are sure they are getting a drive signal first.

Hi Dan

Thank you for your very wise and considerate opinion and warnings. But epicyclic got me on something with his remark, that he would look more closely on the headphone stage.

First I replaced the relay, complete with its driver transistor and ofcourse (as mentioned above) replaced my wrongly chosen zener diode with the correct type. The relay than switched correctly as it should, but the left channel was still dead.

So I got my 2-channel analogue oscilloscope and simply tranced the signal on both channels in parallel. The circuit diagram I received from Canton was obviously the one missing part, I needed. So I found, that the left channel was lost on the main pcb, just at the soldering points, where a lead goes to the volume and balance control pcb. That was it: two of the five leads had broken away from their solder points, but you couldn't see it, as they have very thick insulation, which somehow still was glued down to the pcb and was thus hiding the broken leads.

I resoldered the two leads and now everything is fine and back in working order!

Thanks so much for all of your help and encouragement. Next is my old Denon DRM-800 Tape, where one channel is way too muted, than another old Pioneer CTF-1000, which needs new belts and a replacement switch, than the dead display of and older HK receiver. And than I want to start building new speakers. So, lots of fun over the spring and summer to come.

Ben
 
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