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MC2105 Rebuild

Motnick said:
I also noticed that the positive 35v rail drops to 0v almost immediately when the unit is powered off, while the -35v rail takes a minute or more to reach 0v. Is this the expected behavior, or should they both fall slowly to 0v together?

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Nice main caps! a dark blue. As for voltage bleed-off, the original caps in John's 2100 took ~2 mins to drop to under 5 volts. One can was dropping about 1/3 faster. Looks like there are a pair of 2-watt bleed resistors in plain view. Wonder how well matched they were/are?

-Greg
 
Interesting to know that the caps in your unit take about 2 minutes to fall under 5 volts.

In my unit the -35 volt rail takes a little more than one minute to bleed down to -5 volts, while the +35 volt rail only takes about 5 seconds to fall to zero volts.

I measured both bleeder resistors and they are both still very close to 1000ohms. I tried swapping the two filter caps, but have the same result. I also removed the +/- 35 volt from the input board to see if something on that board was dragging the +35 to zero, but again had the same result.

I’m not sure if this is an issue or not, but it is concerning that my unit does not behave the same as your 2100.
 
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Motnick said:
Interesting to know that the caps in your unit take about 2 minutes to fall under 5 volts. In my unit the -35 volt rail takes a little more than one minute to bleed down to -5 volts, while the +35 volt rail only takes about 5 seconds to fall to zero volts. I measured both bleeder resistors and they are both still very close to 1000ohms. I tried swapping the two filter caps, but have the same result. I also removed the +/- 35 volt from the input board to see if something on that board was dragging the +35 to zero, but again had the same result. I’m not sure if this is an issue or not, but it is concerning that my unit does not behave the same as your 2100.
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At least you have some info on the issue now—via process of elimination. You could pull the PO boards and swap them, or examine pins 6 and 14 `cold´ for more clues. Ideally both sides would discharge roughly at the same rate. If one side takes 2 min to bleed off, and the other takes 1.5 mins, I'd say that's no worry.
 
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Did you have the driver boards installed when you tested the voltage bleed off?

My capacitors bleed down together slowly to 0 volts if I have both driver cards out. If I install even one driver board then the +35 volt falls fast to zero. This has always been the case with my unit. Even before I did any updates to the driver boards. I had forgotten this, but found it in my notes from back when I installed the new main capacitors.
 
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Motnick said:
Did you have the driver boards installed when you tested the voltage bleed off?

My capacitors bleed down together slowly to 0 volts if I have both driver cards out. If I install even one driver board then the +35 volt falls fast to zero. This has always been the case with my unit. Even before I did any updates to the driver boards. I had forgotten this, but found it in my notes from back when I installed the new main capacitors.
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Yes IIRC, it was just powered down, I was about to go in there and poke around looking for RED and GREEN voltage wires to the Outputs to ID them. I posted that (powering up with driver cards removes) but then deleted it a few mins later. Apparently it's ok to power up with PO boards removed and the outputs energized.

Next time I have the 2100, or over the guys house to replace Input PCB caps, will check the bleed-down times again.
 
Gregory said:
Next time I have the 2100, or over the guys house to replace Input PCB caps, will check the bleed-down times again.
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That would be great!

My amp does play on for awhile after it is switched off which I think is the expected behavior since these amps do not utilize a speaker relay. I’d have to measure to see how long, but I’d guess around 10 seconds or so at least.
 
Motnick said:
That would be great!

My amp does play on for awhile after it is switched off which I think is the expected behavior since these amps do not utilize a speaker relay. I’d have to measure to see how long, but I’d guess around 10 seconds or so at least.
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Interesting on the play on. My 2100 powers off and no play on.

My Mc1900 receiver does that though. It also powers up slower. Had it checked out and service by tech couple of years ago. That unit plays great and looks good too.
 
I see the voltage ratings are certainly high enough. The 2100 schematic, and I'm assuming the 2105 is similar, has 200V, 200V, 150V, 150V cap sections from transformer end to `supply rail´ end. OTOH new caps today are physically smaller than years ago.

I asked Hayseed NOT to put a sticker on my two multi-caps. I worked with him on the 2505 cap specs, and said he could later make the piece available to other Mc users. He sent TWO empty test cans to check for height and clearance to the main caps, etc. It's really good to work with others constructively.

2505_test_cans.jpg
Test cans.

Same idea with JB's 2100, I'm designing an arrangement of discrete axial caps and `solder lug terminal strips´ to mount on the `barrier wall´ under the chassis.

412mn+246mL._AC_SX425_.jpg

That would be in keeping with builds of the era and clearly show what caps are in use.

-Greg
 
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I am working on a parts list for my input board. It looks like my board matches up closest to the input board in the later serial number mc2100 manual. However on my Input board C9 & C10 are standard axial capacitors. According to all of the 2100/2105 schematics that I have seen these two capacitors should be non polarized. Have these been replaced on my board? Should I use NP capacitors to replace these two?

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Motnick said:
I am working on a parts list for my input board. It looks like my board matches up closest to the input board in the later serial number mc2100 manual. However on my Input board C9 & C10 are polarized capacitors. According to all of the 2100/2105 schematics that I have seen these two capacitors should be bipolar. Have these been replaced on my board? Should I use bipolar to replace these two?
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I'd say use bipolar (polarized) caps for all six. According to the schematic C309 & C310 have -1V on them. Not all that high. I wonder what's likely to fail and hit them with DC?

However on the latest schematic there's no + or - marking for C309 & C310, or on the board diagram. The other four caps are so marked. Go by the existing cap polarities, + to the output pins?

-Greg
 
I had noticed early on that the headphone jack on this amp was broken. It took me awhile to figure it out, but I was able to find the correct replacement part on the Mouser website. The headphone jack on the MC2105 has a longer threaded portion than most of the jacks that I was finding. The length is important because it needs to come up flush with the glass face plate once installed, and the standard length jacks are a bit too short.

The replacement part that I chose is Switchcraft L112BX. It seems like an exact match to the original. So I guess I can check that one off of the to-do list.

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Motnick said:
I had noticed early on that the headphone jack on this amp was broken. It took me awhile to figure it out but I was able to find the correct replacement part on the Mouser website. The headphone jack on the MC2105 has a longer threaded portion than most of the jacks that I was finding. The length is important because it needs to come up flush with the glass face plate once installed, and the standard length jacks are a bit too short.

The replacement part that I chose is Switchcraft L112BX. It seems like an exact match to the original. So I guess I can check that one off of the to-do list.

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Very good!
 
The barrier strips on this amp were in pretty sad shape. One strip had broken at some point and someone soldered it to the lug. On the other barrier strip the Common screw lug was broken free inside of the barrier strip and would never tighten up. When you tried to snug it up the screw would just spin forever. Someone had replaced the screws with some Philips style, and the plastic strips between the terminals were all broken and chipped up.

I had an old Fisher receiver that is too far gone to repair and the other day I noticed that it used the same barrier strip terminals as the MC2105. It took me some time to do a nice job, but I was able to swap them out today.

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I was also able to upgrade my fuse holder cap thanks to that old fisher receiver. Mine worked but it had a crack.

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Motnick said:
The barrier strips on this amp were in pretty sad shape. One strip had broken at some point and someone soldered it to the lug. On the other barrier strip the Common screw lug was broken free inside of the barrier strip and would never tighten up. When you tried to snug it up the screw would just spin forever. Someone had replaced the screws with some Philips style, and the plastic strips between the terminals were all broken and chipped up.

I had an old Fisher receiver that is too far gone to repair and the other day I noticed that it used the same barrier strip terminals as the MC2105. It took me some time to do a nice job, but I was able to swap them out today.

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I bought a new fuse holder for the MR77.

Why are the screws painted red? You could easily replace ALL the screws on those strips, look at McMaster-Carr site.


-Greg
 
Gregory said:
I bought a new fuse holder for the MR77.

Why are the screws painted red? You could easily replace ALL the screws on those strips, look at McMaster-Carr site.


-Greg
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The original owner added red paint to the screw heads because his eye site was failing. Red for Right channel. I thought about removing it with some acetone while I had the screws out today, and I may still do that, but I decided to leave it for now as it’s part of this amplifiers history. Plus it’s on the back side and only I will ever see it.
 
Motnick said:
The original owner added red paint to the screw heads because his eye site was failing. Red for Right channel. I thought about removing it with some acetone while I had the screws out today, and I may still do that, but I decided to leave it for now as it’s part of this amplifiers history. Plus it’s on the back side and only I will ever see it.
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Ah, right channel of course, but the RIGHT speaker terminals are on the right, and likewise the left.

The barriers on the `terminal strip´ get broken when a slotted screw driver blade isn't relatively centered on the screw, and the torque will snap the barrier ends off. Typical of someone who can't see well.

That's the reason for replacing he screws with Phillips, they're self centering. But everyone knows that, right.


-Greg
 
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