MC2100 Can't Pass A Dim Bulb Test! Help Please...

[bhuskins]

I have an MC2100 that has a blown main fuse. Initial inspection shows no burnt components or burnt boards other than 1 of the 2 thermistors was a bit chard and about to fall apart. I replaced both of the thermistors with 4 Ω 4A NTC ones that installed easily. I pulled both output boards and removed the power leads to the input board. I also pulled all 12 output transistors...all of which surprisingly tested just fine. I was expecting to find one or more bad but they are all OK. With everything removed I performed a 100w dim bulb test and it lights up bright and stays that way. Now I assume the issue either has to be the bridge rectifier (which this one has the dual L bracket style), one or more of the legs of the 4 way power supply filter or one or both of the main filter caps. Being that I can't safely power up the unit yet without the dim bulb in place (shining brightly), how can I determine what is causing the issue?

 

[petehall347]

check the bridge first for being shorted and move forwards ...or if bridge ok for sanities sake remove bridge .power up and check AC from transformer .

 

[bhuskins]

So I pulled the bridge off the main caps and all that's running to the bridge are the 2 leads coming from the transformer. I still light up the DBT...does that prove the bridge is the issue and I'll just replace with a modern one?

 

[damacman]

You're trying to determine if the power xfrmr or the bridge is at fault ... Process of elimination.

 

[ConradH]

I'd pull the transformer leads and make sure it, or something related to it, isn't bad. IMO, troubleshooting of this sort isn't done under power. The name of the game is to confirm that all the important components are working, using a DVM or other test equipment, so confidence is high when power is applied. Can you test the bridge with a DVM on a diode check range? Also, what's the wattage of the dim bulb? I've had transformers with enough loss that they'd probably light a very low wattage bulb.

 

[bhuskins]

Well with the bridge removed...no bright light...plug in straight to get an ac reading off the leads...reads about 56vac so I'd assume that's about right...don't see that on the schematic but I'd suspect the bridge is very likely the issue. I'll check the diodes to confirm

 

[petehall347]

if all that's wired to transformer is the bridge and bulb is lighting up it has to be bridge or transformer ..
transformer on its own wont light any lamp as far as i am aware ...
as conrad stated i would have also normally checked the bridge diodes with meter first but it was going well this way .

 

[bhuskins]

One of the 1N3492R (black lettering ones) is bad for sure...shorted.

So, what's the minimum specs on a modern full wave rectifier replacement I should use? Also I'd assume I'll still need to use the capacitor between the AC fed leads? .1uf 150V. Any suggestions for it as well?

 

[bhuskins]

I'd suspect a 35A 400V rectifier is ample...is there anything special about the .1uf 150V capacitor that goes between the 2 AC feeds?

Also, looks like the green leads go to the negative side (-35v) and the red leads go to the positive side (+35v)...I'll add o ring crimps to the + and - outputs to make the connections easy at the big caps screw terminals. Not sure yet on the best way to mount the rectifier. Any suggestions?

 

[bhuskins]

Replaced the bridge rectifier with a new modern full wave rectifier and finished the recap as well as mounting all the new outputs...sounds perfect and runs nice and cool.

Thanks to all that helped!

 

[damacman]

These are fabulous sounding amps. Enjoy!