MC77 (v46) Q6 Getting Very Hot (over 130°C)
- Thread starter smilan
- Start date
Help Support GroupDIY Audio Forum:
Probably not...
But why are you measuring temperature? does the unit not works or has any problem?
But why are you measuring temperature? does the unit not works or has any problem?
Well, the unit works well, but it acts quite differently compared to my Hairball Audio Rev. A.
The main difference is the needle of the MC77 moves slower compared to the needle of the Rev. A (as you can see in this short video:
*Upper unit is the Hairball Audio Rev. A
Lower unit is the MC77
Also the gain of the Rev. A unit is significantly hotter than the gain of the MC77 unit.
The sound characteristics are pretty similar to each other when I'm setting it for the same gain reduction, attack and release.
I have no idea if there should be differences between the units due to different transformers, FETs and circuit differences or if one of the units (or both) are faulty?
The transformers on the MC77 are:
CM-3303 at the input
CM-96731 at the output.
I didn't check the transformers on the Hairball Audio Rev. A unit (the unit is inside my rack so I was too lazy to take it out, but I can do that
The reason I measured the temperature the first time was that I smelled that something was too hot when I first turned on the MC77.
I would also think that transistor is too hot.
But the differences in gain and the needle movement might be because the circuits are slightly different, I don’t know…
I don’t know what are the differences between the MC77 and the 1176 RevA
But the differences in gain and the needle movement might be because the circuits are slightly different, I don’t know…
I don’t know what are the differences between the MC77 and the 1176 RevA
Rob Flinn
Well-known member
How are you measuring the gain of each unit ? There are quite a few factors that might effect this. How th Q bias is set, the taper of the pots, the type of fets used etc. From what I seem to remember the different revisions of the 1176 don't have exactly the same gain anyway therefore you may be comparing apples to oranges
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I sent a 1kHz / 0db (0.775ACV) from my DAW to each compressor.
With the output pot at mid position, attack pot fully CCW (off) and release pot fully CW, with meter on +4 I set the input knob on each unit so the meter will show 0VU (on both units).
I monitored the outputs on the DAW that show -19DBFS.
To achieve the same output I needed to raise the input knob of the MC77 much higher compared to the input knob of the REV.A.
Rob Flinn
Well-known member
Therefore all of what I said in my previous post are variables that might apply. I have a pair of MC77 that I made but I can't check how hot Q6 gets for you because they in France at the moment & I'm in the U.K. Q6 will get quite hot because it's class A, but you may have an issue, which if you do will likely be in that area, check the resistor values round Q6 & the output transformer wiring. I have a pair of MC77 that I made but I can't check how hot Q6 gets for you because they are at a house I own in France at the moment. I have attached a pdf from the hairball/mnats rev D build documents. It has test voltages on it which should be pretty close to what you get for the MC77. I don't know if the prts numbers are the same, but you should be able to figure it it. If you voltages are within 10% then it's likley operating normally.
I think the different ballistics of the meters it's just because the cheap "vu meters" made nowadays generally are not very consistent.
The difference in gain could be clarified by following the audio path with an oscilloscope and compare the two units.
The difference in gain could be clarified by following the audio path with an oscilloscope and compare the two units.
