MA 5100 restore blows fuse

I am working on a MA 5100 with the 2.5A slow blow fuse burning out. Serial number 29H36. Transformer labeled with the initials CSL on may 30th 1980.

Three of the four output transistors were leaking so I replaced them. However, when using a dim bulb tester there still seems to be an issue. With the dim bulb tester it shows I am getting approx 4 vDC at the speaker terminals with 6.75 vAC powering the transformer.

There is alot of black gunk/residue covering the wires leading out of the transformer(159-069), and the two channel power output boards(128-042) have some blackening near the resistor end of the boards. The 6 remaining transistors near the output boards are not leaking/test normally.

I have been working with two schematics that are far apart 22H and 30H, so that has been troublesome, but I am managing.

I am looking for advice on my next steps. I am leaning towards reworking the output boards or rebuilding the multicaps. Should I be pulling/testing diodes, resistors, and smaller caps first? Any direction from here would be greatly appreciated.

Bridge rectifier diodes would be a good place to look. I am sure there is also at least 1 if not 2 half wave also.

c_dk said:

Bridge rectifier diodes would be a good place to look. I am sure there is also at least 1 if not 2 half wave also.

Click to expand...

Damn, those are really soldered in there. I will check them out.

I have been microfocused on the bottom of the machine. Yesterday afternoon I flipped it over and removed the tray that holds the lamps in place and looked at the pre amp and filter circuit boards on the top and there are quite a few swollen mylars and a swollen spraque tube cap.

Rectifying diodes are good. I pulled a leg on all of them and they passed a small circuit with a 9v battery and a led. Checked ok with dmm as well. Now to determine the values of the color coded caps... :/

Are the multicaps or main filter caps drawing excessive current?

Ok, sorry for the absence. I just finished replacing all the mylar caps and the swollen sprague cylinder electrolytics. Dim bulb is still showing continuous without the power drop.

How would I check to see which caps are drawing excess current? Is there a simple reading I can take? I was researching and found many people solder a resistor in the circuit. Then measure the current across the resistor.

How would I measure excess current draw with limited power supply? Currently, I am at 9.75 volts AC.

I am looking at the multi section cans for a recap and one is listed as having two negatively rated voltage ratings. -100/-75V . I have never seen this before. Do they make caps rated for negative voltages? Is this a thing? I'm headed over to mouser to look at datasheets, but I thought I would post here first to see if it is just an issue of switching the positive and negative leads of a positively rated capacitor. Any help on this issue is greatly appreciated.

You have to put an ammeter in series with the cap.....or at the least lift the connection and see if the bulb dims considerably.

Thank you! That will save time.

I have also found a similar recap list from a 1700 on google. Apparently, the negative voltage rating has to do with the placement of the common ground. If I had taken a closer look at the cans I would have seen the markings reading can positive/can common.

Well more bad news. I pieced together new multicaps today. Currently it looks like a frankenamp. One multicap looked like it was drawing excess current as it was reading over limit. All together it still has an issue. I am done working for the day, but I am hoping the diodes that power two of the multicaps are leaky. After diodes I am going to look at the transistors on the boards then on to unhooking the transformer output cables and testing for voltages at full power. By Thors Hammer it will live again!!!

Found an open diode today. 070-031, which I have read can be replaced with a 1N4004. The diode itself is a 1N4820, it is noted as a signal rectifier for one of the multicaps. Hopefully, this will restore functionality to the power supply.

Replaced the diode and it is alive! However, within a minute one of my multicaps started to burst. It was the multisection that had negative voltage ratings. I hooked it up as described in the schematic, but when I took the connections off of the original multisection the input wires were reversed. As in, 306A d was connected to the delta, and the 306B delta was connected to the d. So I switched them, resulting in a blown cap. Did I make a mistake by switching those connections? Is there something I am missing about the negative rated multicap setup?

It lives

So I had to turn my brain on. The Mcintosh clearly reads negative voltage ratings on the mutlicap. Then after sitting down with a volt meter I realized that the cap is sucking up negative voltage and you cant place that negative voltage on a positive terminal of the polar cap. So I grabbed some new caps and soldered the positive leads together and connected the negative leads to the circuit and we are in business. It plays powerful music. There is a slight buzz on one channel but the power supply problem has been corrected.