mc 250 with hum

muscmp

Active Member
i broke down one of my two mc250 amps due to a hum. i have not run any voltages yet. i'm enclosing some pics. though old, 1967, the can caps and axials all seem fine. i don't see any burn spots anywhere.

any recommendations?
thanks,
mikeB

mc1.jpg mc2.jpg mc3.jpg mc4.jpg mc5.jpg mc6.jpg mc7.jpg mc8.jpg
 
A DVM on AC could show the effectiveness and balance of the power supply elyticaps for starters. If the amp is passing a signal, but with him, check for hum with the level controls fully down to find whether fault is internal or external to the amp.
 
A DVM on AC could show the effectiveness and balance of the power supply elyticaps for starters. If the amp is passing a signal, but with him, check for hum with the level controls fully down to find whether fault is internal or external to the amp.

great info, thanks
 
Third pic down then..have a look..cap on the lower left...looks like a resister....lead on the right hand side...
 
You are looking for the for a balanced reading between the large elyticap can and neutral. If not, one may be going out.
 
The caps further downstream should absorb any residual ripple if all else is good. This is a simple but non standard procedure, so readings are relative.
 
A DVM, Oscilloscope, and in-circuit ESR checker are handy for basic checks, many hobbyists might have only a DVM.
 
A DVM, Oscilloscope, and in-circuit ESR checker are handy for basic checks, many hobbyists might have only a DVM.

i have a dvm. i'm used to working on tube guitar amps so this is a different animal to me. i did just recently buy a used heathkit oscope but i have not done any beginners practicing. i believe it is a 40ms scope and i have a 10x probe.
i hope to do my testing tomorrow. i've built a load box with 4 and 8 ohm resistors and cheapo 6" speakers when i need to hear something.
thanks,
 
i have a dvm. i'm used to working on tube guitar amps so this is a different animal to me. i did just recently buy a used heathkit oscope but i have not done any beginners practicing. i believe it is a 40ms scope and i have a 10x probe.
i hope to do my testing tomorrow. i've built a load box with 4 and 8 ohm resistors and cheapo 6" speakers when i need to hear something.
thanks,
The o'scope can show you if the power supply is sufficiency "clean", and the residual hum and noise level.
 
yep, agree with Pio1980, you need to measure the power supply caps. If you don't have an ESR meter,. you can use a DVM set to AC mv and look for AC ripple on the meter. Anything over a few mv would suggest the caps are shot.
 
yep, agree with Pio1980, you need to measure the power supply caps. If you don't have an ESR meter,. you can use a DVM set to AC mv and look for AC ripple on the meter. Anything over a few mv would suggest the caps are shot.
DC imbalance as well, check both.
 
i've been out of it with a cold so i'm finally getting some voltage readings. i've enclosed pdf file that includes the schematic

mc 250 voltages

Line voltage 113

C3 and C4, 100uf/12v input section both 15.68v

C11 and 12, 330uf/3v left and right output sections, .009v left and .058v right

C27 and 28, 9300uf/45v power supply section 41.4 and 41.6v

C29A, B, C and D 80/200,80/200,150/150 and 50/150 power supply section A105.3, B112.3,

C84.5, D98.2

C31A200/100 and B 500/75, power supply section, A-42.2, B-57.8

C32 and C33, 500/16, power supply section, C32 15.64, C33 -11.44

thanks,
mikeB
 

Attachments

  • MC250sm_10L01_21L97.pdf
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Me2
Early McIntosh MC250.
Okay, this thing is messing with my mind. First year model. Bring it up on a variac get a loud 120Hz buzz through both channels on stereo setting at low voltage with vol controls at min which increases to serious jagged waveform at about 30V AC. All wiring to caps appears correct and grounds test as ground. Removed and tested filter caps on nice LC101 tester. Replaced both multisection caps with new as test bad, large 9300uF caps test only 400uA leakage and ESR of .18 ohm and within 5% of each other so using them. After reforming caps, installed and same problem. Scope confirms serious 120Hz on positive rails at bridges (negative rails are smooth). Tried swapping big caps, no difference scope is showing all hum on positive rail. Check bridge diodes with Fluke 77 says OK. Remove power from all but power amp, no change. Remove power from each channel independantly, no change. All four heatsinks have 2 insulators (nylon and paper) each side of main chassis.
 
DC imbalance as well, check both.
can you elaborate on what you mean by a DC imbalance - between positive and negative rails with reference to ground? As I bring mine up on variac the offsets are not the same even if I switch the 9300uF cans.
 
can you elaborate on what you mean by a DC imbalance - between positive and negative rails with reference to ground? As I bring mine up on variac the offsets are not the same even if I switch the 9300uF cans.
DC and AC voltages should be essentially identical between them, a balanced condition.
 
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