Marantz model 140 voltage problem

[Isaiah Hood]

Well well well.

After several years of owning my Model 140 it has kicked the bucket. I'm currently trying to repair it, and I hope those better with schematics can help me. A little back story. . . I was sleeping in my dorm room enjoying the best sleep that one can get in the military when I was rudely woken up by a VERY loud noise coming from my Klipsch floor-standing speakers. Soon after waking up I came to grips with what was going on, and I pulled the plug on my amplifier. Here I am a year and a half later trying to fix what's wrong with the amplifier. I started by replacing the output transistors because I thought the voltage reading I was getting was caused by this; it was not. I should have +47v coming from the collector of the output transistor and --47v coming from the emitter of the opposite mosfet. The positive voltage is present and the same on both channels; however, the negative voltage rail is much lower than it should be... sitting at about --30v/--31v on both power amplifiers. As of right now, I don't know what I should be looking at replacing. I was thinking the bridge rectifier, but with little experience with them, I am unsure if this is what creates the negative voltage rail in the first place. I am seriously open to hearing what you guys think. If you're thinking there is a short on either of the amplifier boards, note that the readings do not change with they both disconnected and I probe the pins.

Cheers!!!!!
Sorry for the rant.

 

[sregor]

Sounds like open filter cap. When one goes open, there is a large sawtooth on the supply, and voltage drops to about half to two thirds (somewhat depends on meter) If you have a filter cap floating around (anything over 1000 uF of sufficently high voltage, you can jump it across the suspect cap (mind polarity!) and see if problem goes away or decreases. Sometimes (if the caps have screw type connectors) it can be loose connection to that cap. My 2 cents.

 

[AE Bob]

To test for an open filter capacitor, you can measure the AC voltage across the capacitor with a multimeter. If you see several volts AC across the capacitor it is open (likely disconnected internally).

 

[Isaiah Hood]

Sounds like open filter cap. When one goes open, there is a large sawtooth on the supply, and voltage drops to about half to two thirds (somewhat depends on meter) If you have a filter cap floating around (anything over 1000 uF of sufficiently high voltage, you can jump it across the suspect cap (mind polarity!) and see if problem goes away or decreases. Sometimes (if the caps have screw type connectors) it can be loose connection to that cap. My 2 cents.

Thank you for the response! I do have one question though. If the capacitor is the problem shouldn’t I have the full —47v on the bridge rectifier or would I have to disconnect the filter capacitor to get an accurate measurement?

 

[AE Bob]

The voltage will read low with an open filter capacitor because the voltage across the capacitor is not the peak voltage of the DC pulses coming from the bridge. It is closer to the or average voltage of the DC pulses coming from the bridge. The filter capacitor normally charges to the peak voltage of the pulses, and "fills in" between pulses, causing the voltage to be higher (peak). When the cap is open this does not occur. Placing another capacitor (even one of somewhat lesser value) in parallel with the open capacitor will bring the voltage back to the peak value. If jumping a cap in parallel brings the voltage back it is a sure thing the main cap is open.

 

[Isaiah Hood]

To test for an open filter capacitor, you can measure the AC voltage across the capacitor with a multimeter. If you see several volts AC across the capacitor, it is open (likely disconnected internally).

When doing this test, do I probe both the positive and negative of the capacitor or do I test each terminal individually while the black probe is on the chassis ground?

Thank you for your response!

 

[AE Bob]

You just measure the AC voltage between the + and - terminals of the capacitor. It should be very low. You can compare it to the AC voltage on the other capacitor for reference.

 

[Isaiah Hood]

You just measure the AC voltage between the + and - terminals of the capacitor. It should be very low. You can compare it to the AC voltage on the other capacitor for reference.

Bob,
I just got home and measured the AC voltage on the capacitor and it measured 13.3v. In addition, I measured each capacitor in DC, and sure enough after unplugging the unit, one side held a charge and decreased within tolerances and the other side went straight to 0v.
I guess now all I need to figure out is what's the best option to go about replacement because these large dual capacitors are no longer manufactured. Would you happen to know anyone who has done this?

 

[AE Bob]

The best option is to gut and re-stuff the capacitor cans with new caps. New devices of the same value are small enough to easily go inside the old cans..