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Help with MA5100

Fridge

Fridge

Member
I was given a MA5100 amp by a friend to clean up and refurbish. I am a newby at trouble shooting and can use some help. I recapped all electrolytics and some other caps in the audio path. The unit is only putting out about 4 wpc. I tested voltages throughout including all transistors. All voltages are fairly close to the schematic many are a bit high but pretty much not more than 10% high. The only issue I can find is that resistors R231 and R232 are severely overheating....discolored - burned the pcb - backside is browned. Both L and R channels are performing identically. I am trying to avoid the mistakes I have made in the past by shot gunning parts and in that process ruining the stereo...I have ruined a few, but have had more successes. What volume I do have is nice, full and clean...If any one can offer some suggestions to set me on the right path to solve this problem I would greatly appreciate it. Thanks!
 
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By the way, I do have a scope and signal generator but don't really know how to use them properly-could use some help there too :)
 
When both channels are malfunctioning the first thought is the power supply but I assume you have the proper voltages from the main filter cps.

How are you measuring the voltages out of the amp to determine you have 4 watts output?

What are you using for a dummy load?

Do you have probes for your scope?

If you are seeing clipping on your scope is the amp clipping semetrically?
 
Filter cap volts are 39.3 vs 36 on schematic. The voltages are consistent across the board either equally high or some are dead on. I am using a pair or 8ohm 100w resistors in a set I built with a fan for my dummy load....I am pretty much guessing at the wattage as at full blast there is no clipping so I calculate the watts at full blast..no where near clipping amplification going out. I do have probes for scope. I am getting a clean sine wave on speaker switches for both channel but then nothing on the preamp board? I am getting about the same level of output with both speakers and head phones. I cleaned switches up but that did not help with the output?
 
I repaired on of those about 3 years ago. My unit had a "speaker protection" circuit to avoid sending over-voltage to the speakers. It was defective. It was a small aluminum mounted board near the middle of the amp. My audio signal was being clamped by that protection circuit. I would try bypassing it. I did and it cured my problem. The unit's power came back and I left it that way. Worked for me.
 
R231and232 are expected to get hot.

This is not really a DIY forum but....

Have you ever used your signal generator and osciliscope on a known functioning unit?

A scope is nothing more than a frequency visual voltage display, so if you have not done it already you should hook up you generator to your scope and observe how the display can be adjusted to show you voltages at different frequencies.

With a known functioning unit you should be able to explore the input and output of every stage to determine the voltage gain from each stage.. By first learning the voltage output of your generator you can learn to read the vertical divisions of your scope to determine voltages.

I would not trust a DVM to measure AC voltage unless the frequency as somewhere between 40 and 100 hz.

A wild guess for your problem would be the speaker switch. With your probe you should be able to observe any differences before and after the switch.
 
What source are you using when you measure the power? Does that source provide sufficient input voltage to drive the amp to full output?
 
John Caruso said:
I repaired on of those about 3 years ago. My unit had a "speaker protection" circuit to avoid sending over-voltage to the speakers. It was defective. It was a small aluminum mounted board near the middle of the amp. My audio signal was being clamped by that protection circuit. I would try bypassing it. I did and it cured my problem. The unit's power came back and I left it that way. Worked for me.
Click to expand...

That sounds promising. I will check the output monitor circuit, I have read where others have had that issue.

As far power source, plugged into standard power strip with circuit breaker. I did check it on DBT and no problem there.

I thought the problem may be in the speaker switch as input is solid to switch but not picking up signal on preamp board. I am new to understanding schematics,I am not certain the exact location to check the signal comes out of the switch?

Anyway, I do appreciate everyone's help here.

Thank all
 
If the signal is going into the switch but not out of the switch the switch has to be suspect.

The preamp board is way upstream of the switch so you must not have your test equipment set sensitive enough to register the small preamp level signal.

The amp uses a crowbar protection circuit that is designed to blow the power supply fuses to protect the speakers if a output would short. It works for each channel individually.
 
Thanks c_dk, It must be frustrating walking DIY's through simple trouble shooting issues. I just realized that I totally misspoke about the Speaker Switch signal. I meant to say the Selector Switch-set to Aux- that the signal feeds into from the jack. My apologies-wow no wonder. I will set the sensitivity way up on the scope and try to trace the signal again.
 
TSmith8605 said:
What source are you using when you measure the power? Does that source provide sufficient input voltage to drive the amp to full output?
Click to expand...

I was not clear enough. Sorry. Try again: What SIGNAL source are you using when you measure the power? Does thatSIGNAL source provide sufficient input voltage to drive the amp to full output? If you're using an oscillator/tone generator as input, what is the signal voltage level from the oscillator/tone generator?
 
I re-cleaned every and plugged into dbt no short visible but as I cranked up the volume the bulb lit up, turned volume down bulb dimmed. Once again the sound is solid and clear.
 
The bulb acts as a limiter which will limit the units power output. As the unit draws more power it will light up brighter and brighter and will tax a unit's regulator circuits (if it has them) so should be used with that fore knowledge.

By limiting the power draw from the AC mains it will limit the amp's output.

The dim bulb is a great down and dirty test piece but will greatly limit how much you can probe a units performance.

I much prefer using a variac to control the available AC power and a DVM hooked up in the appropriate place to monitor current draw.

Using ohm's law I believe your scope is showing 2 squared/8 = 1/2 watt
 
the unit was taken off the DBT and plugged directly into mains power. I have no idea what the problem is with this amp?
 
I would gently suggest that, given what you have told us about you experience and your statement "I have no idea what the problem is with this amp" you should hand the amp over to someone else at this point.
 
Fridge, I'm not suggesting you "give up easy", I'm suggesting you hand the amp over to someone with the knowledge and experience necessary to effect proper repairs without damaging it by mistake. Additionally, whomever you hand it over to may let you watch and learn how and what they do. The MA5100 was my first McIntosh so I am partial to see it sing properly again.
 
Ramjet, I totally understand, just I am frustrated with myself. I feel like the problem is relatively simple and easy to solve. Good honest techs are hard to find these days, I have had a couple of bad experiences in the past. Maybe I will contact Mark at Blueglow to see if he is available he is close by where I live, but I don't think he is taking anything new on right now. I have also obtained a Thorens TD124 along with this MAC and I was hoping I could muddle through and repair and refurbish this set as a sense of pride!
I meant no offence to you and do respect your thoughts.


Thank You

BTW: I am being super careful in my probing around not to ruin this unit.
 
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