Pre amp output shorted?

[SimpleMind]

My trusted Canton EC-P1 preamp seemingly dies a week ago. It started with the intermittend loss of the right channel. When I put some pressure on the rca connector borad from the outside, the channel came back to life. After a couple more days, the Canton went dead fully, no signal, neither on the rca-outputs nor through the headphone jack.

After I opend up the preamp I found two things: firstly, the rca connectors are mounted on their own massive metal board and connected via wires to the main pcb. That rules out a broken solder joint, which was my first assumption. The second dicovery was, that somebody had already exchanged the voltage regulators (7818 and 7918) in the past, applying only limited soldering skills.

Unfortunately I cannot find and service manual or pcb layout anywhere for this preamp, not for the identical MBL 4005 and my own electronic skills are limited.

I did some preliminary measurements and found the following results:
- the DC +18V and -18V seem okay (actually I get around +/-17.5V on my multimeter)
- the hot center pins of the preamp outputs have no resistance, measured against ground. With other words, both connectors, the shield and the center pin are connected to ground.

I don't think that is the way it should be or am I mistaken?

Assuming, that the center pin of the output sockets should not be connected to ground, where should I go on with my search for faults?

I would be very thankful for any help, as this is a very good preamp and buying something in the same league is out of my reach currently.

Ben

[epicyclic]

You say the centre pins measure zero resistance to ground , does this alter when the volume control is moved . Have you checked the board fully for poorly soldered joints . Does giving the preamp a sharp knock bring it back to life.

[mjf]

can you take a foto of the inside.......to have an overview?
foto vom innenleben,mit den ausgangsbuchsen,um eine übersicht zu bekommen.

[Juergen Knoop]

mute relays involved?

[SimpleMind]

Thank you so much for your responses! I try to answer the questions first:

Jürgen: yes, there is an old Siemens mute relay (Siemens V23042-A1005-B101 - 24V) in the output signal path, right in front of the output connectors. I already thought that relay to be suspicious (or perhaps a zener diode adjacent to the relay?)
This relais type seems to be out of production and I only found a Siemens V23042-A2305-B101 so far, but am unsure, whether this could be a replacement or not, the small diagram printed on top certainly looks very much alike.

mjf: I will take images today evening and try to upload them here. I have to read up, how I can do that.

epicyclic: I try the volume control thing and the knocking. Maybe, I should try knocking on the mute relay Jürgen suggested?

The pcb looks good from the top, but the backside looks as whether the layers start to fall apart. It is a fiberglass board, by the way and may be the wrinkles are normal for that kind of pcb?

For measurements I have a multimeter which can also measure diodes and transistors and caps. I also have a nice oscilloscope, but am only starting to learn how to use it. I feel red in my face, when I have to admit, that I only start to understand electronics and the lack of a circuit diagram makes things even worse for me...

So, thank you very much for your help!

Ben

[epicyclic]

If the canton has ouput muting then you could easily see zero resistance to earth on the hots when the power is off . If you still see it with the power on this could point to the muting not coming off , maybe the relay or the driving circuitry at fault .

[SimpleMind]

Originally Posted by epicyclic

If the canton has ouput muting then you could easily see zero resistance to earth on the hots when the power is off . If you still see it with the power on this could point to the muting not coming off , maybe the relay or the driving circuitry at fault .

Thank you. I'll do the measurements in the evening, as I am currently sitting at the desk of my workplace... I'll report the results and post the pcb images.

Ben

[SimpleMind]

Originally Posted by epicyclic

If the canton has ouput muting then you could easily see zero resistance to earth on the hots when the power is off . If you still see it with the power on this could point to the muting not coming off , maybe the relay or the driving circuitry at fault .

Hi epicyclic

You are seemingly pointing into the right direction. I switched on the preamp and heared a loud click from the relay. Then the measurements showed, that the center connectors of the output were lifted from ground - no short anymore.

That happened only once. I switched off the amp, connected a source and a headphone, switched on again, but the relay did not do anything. Accordingly the outputs were grounded and I got no signal. Several repetitive on/off cycles did not move the relay anymore to come back to work.

So, yes it is either the relay itself (which is bad, as I found only one source for that old model and they command a premium of more than 70 Euros for the relay) or it is the driving circuit - whatever this is.

I also made some images of the pcb, which I will try to upload now.

Thanks so far!

Ben

[epicyclic]

The diode and capacitor combo , just to the right of the relay , are directly across the relay coil . With the power on CAREFULLY measure for a voltage across this combo . If you see a good voltage turn the power off ,let the power drain away , then see if you can measure the resistance of the relay coil across the combo ...red lead to black band of diode .......... otherwise you could just see the diode junction .

[SimpleMind]

Good evening epicyclic

I have lately taken some measurements and the idea about the fault gets even unclearer. So I guess my nickname is quite on the spot...

- voltage reading taken at the base of the v-regulators is 27.5V and -27.5 respectively. As they are 7818 and 7918 types, I would expect a reading near 18V (as I got in the past)

- voltage reading at the binding posts (overview image, located in the triangle, quite obvious on the pcb) is 35V, though the binding posts are marked with "18V"
May be my measurements are completely off? Or is this kind of open circuit voltage?

- voltage reading (preamp switched on) across the diode/cap combo is consistent with the readings at the v-regulators at ±27V.

I have to ask one more stupid question: which "red lead" do you mean? resistance between the relay diode's black band and the red lead to the output RCA connector? There I get a reading of 8.5kOhms. Exactly the same as I get between the diode and the red lead on the power supply connector. (If I measure from the other side of the diode I get 5.3kOhms, by the way.)

If my voltage readings look good to the educated eye, it would still boil down to either the relay itself or the diode/cap combo, I guess.

It would be really great, if I could find the schematics of the pre, but so far no luck with this.

Thanks again for your help!
Ben