MC2105..right channel warmer than left

[paulram]

Ever since I have had this amp the right channel seems to run warmer than the left. This I observed just placing my hand on top of the rear ventilated cover. Recently I did a recap and having the cover off I was able to touch the output heatsinks and confirmed that the right output channel outer heatsink ( with transistors Q124 , 120 .116 ) was indeed much warmer than the rest. In addition the heatsinked transistor on the driver board ( Q112) was also warmer than the other heatsinked driver board transistors. This is the transistor that also heats the diode , D104, beneath it. I swapped Driver boards and the situation followed the board. So now the left inboard output heatsink ( with Q123 , 119 ,115 ) got warmer than the rest. Is this transistor Q112 ( driver ??? ) going bad? If so what is the equivalent replacement? Any further thoughts?

 

[petehall347]

have you checked the bias current ?

 

[paulram]

mmmmm.I am not the smartest in that area...( still learning alot here...) lets see ...looking at the schematic I see the base voltage as 1.2v and the emitter as .6v..which is the bias ? neither ?...BTW its an NPN trans.

 

[petehall347]

you measure the bias across the emitter resistors .they are generally the big ones near the heatsink .they connect between the emitters on the outputs transistors ..the big ones on the heatsink .. set your meter to mv dc and check all 4 ..thats if it has 4 transistor output ..there will be one resistor for each pair of outputs .. be very carefull not to slip with your probes and cause a short .best to use clip on leads or insulated probes . i would expect to see around 10mv or so across each resistor

 

[paulram]

Since the heating issue followed the driver board when swapped around would it be more appropriate to address the driver board first ? Just curious!

 

[petehall347]

its the drivers that determine the bias .check the voltage across the resistors ...have you any pictures ? i can then point it out to you .. have you got the link for service manual ?

 

[paulram]

Hi
I did some voltage checks and determined that the affected channel had a higher value than what is shown in the service schematic. I then did a resistance check of the 3.6k 5% and found it to be low and out of tolerance. As an experiment I swapped these resistors around and found that the heating followed the low value resistor. It certainly doesn't hurt to change these , which I will do shortly.
Some readings were Base voltage of transistor should be 1.2 and indicated 1.39 , Emitter should be .6v and indicated .78 Resistor should be 3.6k and indicated 3.1k. Keeping my fingers crossed and hoping thats all...cheers

 

[paulram]

I installed two new resistors and the voltage measurements are now pretty close to specs , both base voltages are spot on at 1.2 volts. The right channel emitter now reads .6 volts and the left reads .66 volts. Overall though the situation is much improved. Now I have a question...did I use the appropriate resistors? They are ohmite power wire wound vitreous enamel resistors rated at 495 volts 3.6k 5watts, 5% . The originals appeared to be sand resistors. Comments please?

 

[Kevin Gong]

Paulram
You should be fine. This resistor is use for voltage drop from 80V line to the 1.2V bias voltage. and you got the right voltage reading and rating, so it's good. The only resistors that are touchy are the .33-5w under all output transistors. Those are design to blow like a fuse when the output transistors shorts and save the PS. Better to get those from McIntosh.
Kevin

 

[paulram]

re resistors

Thanks Kevin. You know whats funny , now the other side is a tad warmer even though the voltages after the 3.6k resistors are spot on at 1.2 v for both sides.I am happy the way it is though .Thanks for the advice regarding the "fuse" resistors. And by the way when I purchase the 3.6k resistors from Mouser I had no idea that heat sinks were available for them..perhaps in the near future I will add these...regards

 

[Kevin Gong]

Is this the same amp that you posted back in Nov 8? If it is, I responded to your post with info about re-capping and what each caps are for. The info should help you to decide what mod to do with the amp. Check it out.
Kevin

 

[paulram]

Kevin
Somehow I missed you reply to my nov.8 post. Yes it is the same amp...I have done pretty much a total recap of this amp and it is performing rather well. I did not use the film as u suggested ( because I did not see your Reply)..I used Nich kz there and for the 330 on the driver board I had some silmics leftovers 6.3v so I paralled to 380 IIRC.