McIntosh MC 250 - Burnt Wire/Diodes

[zkhan2]

Schematic

• zkhan2
• Jul 5, 2021

Burn_Closeup

• zkhan2
• Jul 5, 2021

I recently hooked up my MC 250 to a MC 112. It worked wonderfully for 20-minutes and then I noticed a slow chimney of smoke coming out from the heatsink and the smell of burnt electronics. I quickly unplugged the unit.

I opened it up and saw that a white wire was burned black and that area of the chassis had black soot on it under the resistor.

It turns out that the white wire connects to R70 (400 Ohm 5W 5%) over to the rectifier between D17/D19. The part of the wire towards the diodes is fine. The value of the resister is spot on. What could have caused this? Would a bad diode(s) cause this?

The Service Information booklet does not show the value of the diodes. I am assuming all the rectifier diodes are 1N4820 (1.5A 400V). Should I just go ahead and replace all the diodes D17 - D20 and D13 - D16?

I have included two pictures for your reference.

I am just above a novice when it comes to repair. I have ordered all the electrolytic caps to do a restoration on the unit. But want to make sure I am using the right diodes for the two rectifiers.

Again, I am most interested in your opinions on what could have caused this.

Specifications:

McIntosh MC 250.
Serial No. 23L29
Factory Service Contract Date Sent: January 14, 1970 from factory
Service Information booklet: "Starting With Serial No. 21L98

Thank you all for your input.

 

[avionic]

Lets see a bottom shot of C31A & B...

 

[zkhan2]

Lets see a bottom shot of C31A & B...

Here is a photo of C31. Half circle is 200MFD and Triangle is 500MFD. I do have C31 and C29 ordered from Hayseed Hamfest.

 

[avionic]

At least it hasn´t started making a mess ..yet.

 

[zkhan2]

At least it hasn´t started making a mess ..yet.

Just replaced the large caps. I'll try to finish the rest of the caps later this week.

IMG_3367

• zkhan2
• Jul 17, 2021

Removed the Aero M blue heat shrink and the Hayseed Hamfest stickers and put dry transfer...

IMG_3366

• zkhan2
• Jul 17, 2021

Removed the Aero M blue heat shrink and the Hayseed Hamfest stickers and put dry transfer...

IMG_3365

• zkhan2
• Jul 17, 2021

Removed the Aero M blue heat shrink and the Hayseed Hamfest stickers and put dry transfer...

 

[zkhan2]

The burning issues has been isolated to the 47 Ohm resistor connected to the Q26 transistor. I ordered four resistors to replace them all. I was able to source all the TO-66/TO-3 that I need. I hope the following reference I created is correct:

McIntosh MC250 Transistor Matrix

• zkhan2
• Jul 19, 2021
• 3

Matrix showing obsolete MC250 part numbers with the replacement part numbers. (Resistor is 10%...