Purple MC76 build troubleshooting...

[SSLtech]

Kris,

Been a long time since I built mine, but here's a few shots to give you some inspiration...

That resistor does get RIGHTEOUSLY hot... that's normal. Best ones I saw used those 25Watt gold-finned power resistors with the screw-mounting holes, mounted on the back of the chassis (which is VERY handily nearby!) so they run nice and cool.

Just keep at it.

I socketed BOTH FETs and it paid dividends.

The calibration definitely calls for patience, and remember that the tougher the strugle, the greater the sense of achievement, the sweeter-tasting the celebratory beer.

Keith

 

[DrFrankencopter]

Hi Keith, thanks for the photos. It's interesting to see how many different versions of the MC76 PCB are out there. Mine is different even from yours. Mine has a trimmer pot in the stereo link section, and the power supply section is more towards the side of the PCB.

I ran audio from my iPhone into the unit today, and I can definitely report that it is a compressor, and that the ratio switched appear to work. It sounds a little murky, but that might be the iPhone having a hard time driving the input transformer. I get no activity from the GR meter though.

Cheers

Kris

 

[DrFrankencopter]

Interesting problem...

- the Gain reduction meter works...but backwards. It registers gain reduction by deflecting the meter to the right....

I've tried swapping the GR meter leads from the main PCB to the meter PCB, but it keeps working backwards. Also, it seems that the meter zero pot is backwards too. On a working unit, turning the meter zero adjust CCW should bring the meter lower (toward -20 dB), right? Mine works the other way....

Has anyone seen that before?

Cheers

Kris

 

[DrFrankencopter]

Made some serious progress. Did a ton of probing around the meter circuit and ultimately found that Q12 had a dead short between emitter and collector. The short wasn't at the transistor, it was at R74 and R44. The pads on the MC76 PCB were really close together in this area, and I had to do a little bit of cutting to clean it up.

The meter zero's & nulls now.

Now my only problem is that 20:1 doesn't seem to compress (maybe the threshold is too high...I haven't put in the T&C resistor yet, as it's not on the Hairball Audio ratio PCB. I was planning on hanging it off the switchbank).

Oh, and attack seems a little slower than I expected, maybe I've got the wrong value of cap off the attack pot.

That sweet tasting celebratory beer is getting closer.

Cheers

Kris

 

[Delay]

Hey Kris,
i also build a MC76 and also have some problems.
On my Unit i can´t set the Q Bias. The Input is to low when i get 4dB in and the Q Bias Adjustment doesen´t affect on the Meter. I check all Powers but i seem to be ok. When i switch to GR Mode the Meter can set to 0dB but doesn´t affect on the Input Signal.

Maybe i Crashed some of the Transitors. As i first Powerd up the Unit i put the Big C26 & C25 the wrong way. The Resitor R82 was getting so hot that it burns. I put some new C25&26 and a new R82 in, now everythig seems alright, but maybe i kill some other Parts with this fould.

This effect Kris that the Meter is the wrong way i also have on 2 other Rev F Boards, so i will check the Q12 thangs for this idee

Best Regard from germany
Sebastian

 

[Delay]

I alos have some problems with my MC76.
The Gain reduction meter runs otherwise. If the Kompressor reduce 10dB the Gain Reduktion meter says +10dB. So the Voltage on the Meter goes up with more kompresson. I checkt everything.
On thing i found out, when i put the Ratio 20:1 and 12:1 Buttom together the Meter runs like it shoot.

Any ideas for this Problem?i checkt other Ratio Boards, always the same, and the samer Ratio boards runs on other 1176 perfekt.

Also Q6 is getting very hot, i am not sure if its normal. I can´t touch it after 3 Second running the unit.

Best Regards Sebastian

 

[SSLtech]

Kris,

Is your board like THIS one?

-Keith

 

[smilan]

I alos have some problems with my MC76.
The Gain reduction meter runs otherwise. If the Kompressor reduce 10dB the Gain Reduktion meter says +10dB. So the Voltage on the Meter goes up with more kompresson. I checkt everything.
On thing i found out, when i put the Ratio 20:1 and 12:1 Buttom together the Meter runs like it shoot.

Any ideas for this Problem?i checkt other Ratio Boards, always the same, and the samer Ratio boards runs on other 1176 perfekt.

Also Q6 is getting very hot, i am not sure if its normal. I can´t touch it after 3 Second running the unit.

Best Regards Sebastian

Hi Sebastian, I'm having the same problems with my MC76 board.
Did you managed to solve those issues?