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Louie, Technician
Category: Home Theater-Stereo
Satisfied Customers: 12277
Experience:  Have been in the business 25+ yrs., down to component level.
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Boston MCS 95 subwoofer, attached to Denon AVR 591 surround

Customer Question

Boston MCS 95 subwoofer, attached to Denon AVR 591 surround amplifier. Unit will not power up (LED off). AC power at PC board connector, fusible link good, switching diode changed as suggested in other help forum (quadruple checked polarity) (PCB is burned around diode and resistance is elevated across it). Still will not power up, nothing else visibly wrong.
Submitted: 1 year ago.
Category: Home Theater-Stereo
Expert:  Louie replied 1 year ago.

Hi and welcome to JustAnswer,

Thank you for the question and your patience. Should you still need assistance for information purposes only...

Initially, which diode was replaced please? ...and which is the "...fusible link..."?

Customer: replied 1 year ago.
Diode = per PCB silkscreen "ZD3" just to the right of the diode. Located to the right of the aluminum heat sink in the center of the main PCB, just to the right of the small upright PCB w/two capacitors and one diode on it, just to the left of two larger capacitors.The fusible link is located on the main PCB just to the right of the power connector on the bottom edge of the board, labeled - F1, T2.5A/250v, just to the left of the large rectangular capacitor on the bottom edge of the PCB.
Expert:  Louie replied 1 year ago.

Appreciate the additional information.

A review:

  • • ZD3 is a zener diode, what is the value/specs of the replacement?
  • • is F1 a glass type fuse? If yes, is the small wire straight or slightly curved?
  • • is there a connector labeled as J1? If yes, what is the resistance reading between its 2 pins?
  • • determine the resistance of TH1 out-of-circuit.
Customer: replied 1 year ago.
Per the other help source, replace with 1N914-1N4148 from Radio Shack--that is exactly how the package was marked
Not a standard glass type fuse, just small wire
I do not see a connector labeled J1, I do see a connector "CN1" (the only "J" numbers I see relate to small bare wire jumpers)
Do not see anything labeled TH1
Expert:  Louie replied 1 year ago.

Please post back with clear picture(s) of the circuit board with the part label readable. Click HERE.

Customer: replied 1 year ago.
Here are some pictures that might help.
Expert:  Louie replied 1 year ago.


And yes, it would appear that instead of a fuse, a jumper wire is in place.

The thermistor TH1 is usually located almost between the 2 large capacitors C2 & C6 and the bridge diode BR1.

From the schematics/Service Manual, the zener diode ZD3 should be 18V / 0.5W.

Next test would be for carefully testing for the presence/consistency of voltage between the 2 outermost pins of the bridge diode BR1.

Customer: replied 1 year ago.
Voltage on 2 outermost pins of BR1=334-335 vdc.
Expert:  Louie replied 1 year ago.

Carefully follow/trace/re-measure this voltage at:

  • + leg of C2 and - leg of C6;
  • Drain of Q1 and Source of Q5;
  • • anode & cathode of D1;
  • • both legs of resistor R7.

Similarly, carefully measure the voltages at:

  • • pins 1 & 3 of CN2 using its pin 2 as common ground/GND/reference;
  • • pins 1, 2 & 4 of CN3 using its pin 3 as common ground/GND/reference.

Incidentally, were you able to confirm/determine the absence/resistance of TH1?

Expert:  Louie replied 1 year ago.

Hi again,

Any update on the Boston MCS 95 subwoofer?

Customer: replied 1 year ago.
I apologize for not responding sooner but the response link is broken. Just figured out the fix. The response link has redundant information in it not allowing a connection.Right now I am back on the road and have not completed the last checks that you requested. I should be back home shortly before Christmas and will try to get back on the Sub. Temporarily have connected another sub but it is substandard for that system.
Expert:  Louie replied 1 year ago.

I understand and appreciate the post back; no prob when able.