MA5100 buzzing in the left channel

[FireGuruMN]

I have embarked on a quest to get all of my old audio equipment (Heath-kit, Carver, Phase Linear, Sansui, Marantz, Luxman since name dropping seems to be popular here ) in working order. I will then in a big rack with a patch bay to select which device I will listen to on a given day. But I have never had any McIntosh so I picked up a MA5100 which seemed like the appropriate vintage to fill in the 1960's. The display circuit on the amp was blown out because they had the wrong bulbs. I fixed that first. Ya GOTTA have a working display on a Mac. The amplifier put out relatively good sound but the bass was mushy. So I ordered up a set of Sprague Atoms and soldered them in the preamp and driver boards and what ever that circuit by the balance knob is (sorry for the display of ignorance). I am relatively sure one even had a broken lead on the right driver board. After doing the last cap on the preamp board I powered it up. Now I have a buzzing in the left channel. Like a wire shield fell off or something. I can't see anything obvious. I did have a problem with popping in the left channel but that went away when I took the front metal piece off. I think it was a pinched wire. Or maybe it was a short that I lengthened and that created the problem. The buzz goes away when you hold the phase switch in the exact middle which essentially disconnects the left preamp channel from the driver board. Interestingly the buzz also goes away when you turn the treble all the way up or down. The buzz is not affected by the input selector or really anything on the front panel. It gets louder and softer with the volume control. Where should I continue my investigation?

Interestingly has anyone ever noticed that the polarity on the C20 capacitor is backwards on the circuit diagram of the service manual? I checked the voltage on the circuit board and installed it to match. That keeps the beautiful symmetry of that boards design. Looking at pictures on like it seems like half are one way and half are the other. Maybe it is not enough voltage to cause smoke.

Just to declare everything: When trimming the leads off one capacitor the trimmed piece went flying away and I have never found it. Could it be lurking around in the amp causing this problem? I doubt it. It is probably lost on my workbench somewhere.

I don't know if it is normal or not but almost any wire on the preamp that I get close to will cause a buzzing in both channels.

Any encouragement and/or support will be appreciated.

 

[c_dk]

Welcome to AK.

I could not hazard a guess what has happened to your unit without seeing it in person.

 

[FireGuruMN]

I once saw a sign at a steam thresher's reunion: "A steam engine is the most human of all man's inventions". It was on the side of a 3 story steam generator with a 30 foot flywheel and more pistons and rods than I could count. I think this amplifier might have it beat though. I pulled the whole box apart again to see if I could spot anything visible before I started into testing components. When it was apart I polished the shafts of all the knobs and miraculously the buzz went away. Did it disappear because of the polishing or some short that I lengthened during the disassembly? It does make a bit on sense that turning the treble knob removed the buzz if the bottom of the treble shaft was less oxidized. Either way I reassembled the box and checked for the buzz every screw I put in. It never reappeared. Now about those scorch marks by all of the wire wound resistors inside the amp. Is that normal. See pictures. I see it in some pictures of some amps but not others. The amp sounds much better with the new caps. No mushy bass. still a little buzz in the air but that might be the transformer. I have to bring this into the big room and hook it up to some real speakers. oh yea, my deoxit is in the mail. I will need that on my next project: Heathkit AA-100.

 

[FireGuruMN]

Nope, I hooked it up to some "real" speakers and the left channel is not as "sweet" as the right channel. Something is burned out somewhere.

 

[mnmmt]

Have you checked the wire wound resistors or the other resistors to see how far off they are? I found about half the resistors in my mc250 to be out of spec, which definitely throws the sound off. Also cleaning the switches and pots can clear up distortion.

 

[FireGuruMN]

Can I check them when they are installed on the circuit board or do they need to be pulled out (on one side) to prevent other components affecting the resistance reading?

 

[mnmmt]

When i did mine, i checked them in circuit first. If they aren't right next to a cap in the circuit, i felt like i got accurate readings. The ones that seemed off i then pulled a lead to check. Also, figure out which ones are bias resistors and make sure they are spot on. I replaced the bias and wire wound resistors just to be sure.

 

[FireGuruMN]

After playing the amp for an hour or so I noticed the heat sink in the cage on the top towards the FRONT of the amp was warmer than the heat sink towards the BACK of the amp. I believe the back heat sink is driving the RIGHT channel. So something is making the left channel work too hard and ending up with bad results.

 

[FireGuruMN]

I'll check on the values. I have seen this question asked before and it seems to be unique to just some of the MA5100's: "What are the potentiometers on the driver boards for?" I have never seen an answer. They show as fixed resistors on my schematic. I have never seen any measurement point or desired value to use to adjust them. But that is probably not something that you would know.

 

[mnmmt]

No idea, but in other amps i have owned, trim pots are used to set the bias current and to adjust dc offset at the speaker taps.

 

[mnmmt]

I would suspect high bias caused by some off upstream components.

 

[mnmmt]

Does the schematic show the tolerance of the fixed resistors? The bias resistors on my amp are 1% tolerance, so that may give you some idea as to what the trim pots are for.

 

[FireGuruMN]

I'll have to do some digging to determine which are the bias resistors on the circuits. All of the wire wound resistors are spook close to the values printed on them. It is a bit of effort to determine the values for the other resistors. I will either have to remember color codes or compare the circuit layout to the schematics. The resolution of the schematics makes I have makes it hard to read the values.

Also, I had my transistors/heat sinks for left and right turned around. According to the manual the left channel is towards the back and the right channel is towards the front. That means my right channel was getting warmer. That would say to me that it was getting a signal and working to put it out. The left side was not getting a full signal and so it didn't have anything to push out.

I found only 0.02 volts of DC offset on the speaker terminals.

Note 2 in the service manual says that lower serial number have pots instead of resistors for R221 and R222. Still does not say what they do.

 

[FireGuruMN]

I checked all of the resistors on both driver boards. The values are pretty much identical on the entire board. The potentiometers were somewhat different. On the working right/board it was around 975 ohms. On the left/bad board it was around 600k. I adjusted the pot on the bad board to match the good board. I did not note any change. Apparently it is a closely guarded secret where we are actually supposed to measure the bias voltage on these amplifiers. I guess on the later model there was no adjustment so they chose not to provide any measurement either. But I did measure several voltages across the boards. On the top of the 0.33 ohm wire wound resistor R237 and R238 I had 22mv on the good board and only 7mv on the bad board. This is with the amplifier warmed up and no inputs on the back side. I have located a good scan of the schematic (attached) and now I can see that there are some voltage values on the transistors. I think I will go take some readings and see how they match. Maybe the voltages are not such a secret after all. Looks like any failed transistor on that board would have a significant affect on the output results. And it still bothers me that the 3.6k wire wound resistor R231/232 is getting hot enough to burn the board and darken the metal plate. But that was apparently happening long before I took possession.

 

[FireGuruMN]

I measured all of the voltages shown on the schematic. All of the voltages on the large transistors were shockingly (pun intended) close to their intended voltages. The voltages on the driver boards were somewhat off from the voltages indicated on the schematic but they were pretty much the same between both driver boards. The voltage I was concerned about earlier is basically a 0.0 volt reference. So something must be putting a trickle of voltage on it compared to the chassis ground I am using. Given that all these voltages are some close to the factory specs I am leaning back towards the cursed treble knob. My deoxit arrives soon and I will apply a proper dose to see if that brightens up my day. All the bits for my Heathkit AA-100 rebuild arrived today. Maybe I will go back to tubes for a while. Or aren't I allowed to say Heathkit in this forum?

 

[c_dk]

There is a schematic in the Akdatabase or you can buy one from Mac parts.

Those resistors get hot by design, a design from 50 years ago.

 

[mnmmt]

The pots and switches would be my next suspect too. I have had issues with the tape monitor switch with other brands. Use f5 on the pots and d5 on switches. Guitar center carries both types if you are in the metro and want to pick it up.