Mcintosh MA230 Bias Voltage

[Kingsin7]

I've begun to notice a slightly louder than usual hum coming from my Mac MA230 that I use daily in my office setup.

It has been about a year since I last reset the bias, so I decided to take it out of it's wood case today to make adjustments. After reviewing the manual, a voltage of 0.7v is needed for each of the four 7591 tubes.

I cannot measure an output of 0.7v when checking the voltages. As I turn each bias adjustment clockwise, the maximum I can reach is about 0.6v for a couple of the tubes, and the hum noise from the transformer increases significantly...

A measurement of 0.07 or 7mV is easily attained across all tubes and keeps the hum quieter than before I had made any adjustments, but the manual calls for that unattainable 0.7v. I do not recall how I set the bias last time, and I now question if I had done it correctly to begin with. The amp's audio output sounds great, but the hum from the transformer has been annoying me.

Is the 0.7v a typo in the manual or do I have an issue on my hands?
Thanks,
Steve

 

[dcgillespie]

Steve -- The manual indication of 0.7 vdc is the correct value. You would not want to use a setting of 7 mV as the output tubes would then operate with significant crossover distortion.

The fact that you can attain a setting as low as 7 mV, and cannot attain the recommended factory setting of 0.7 vdc, coupled with the hum you are hearing, strongly implies that one of the voltage doubler caps in the B+ power supply has failed. With reduced B+ (the power supply effectively only operating off of one half of the power applied to it), it would shift the tubes to a much colder operating position for the bias range provided by the bias controls, and also elevate hum, since the power supply would now be operating as a half wave design instead of as a full wave design as it's supposed to. I would check, or have your tech check those caps first thing (C42, C43 on the Mac schematic), which by now should be replaced anyway simply due to their age, and the work load they carry in the design. They are the hardest working passive components in the unit.

I hope this helps!

Dave

 

[Kingsin7]

I was praying for a typo...
Thanks for the very quick response Dave.

Here is a pic of the unit from when I first acquired it and went through it a year or so ago:

Could one of the 6 brown caps in the top center of the pic be the culprit you spoke of? Should I just replace all?

Sorry, I'm just not that great at reading a schematic, but especially schematics from Mac... Here's the MA230 schematic for reference:

 

[dcgillespie]

No, those are the coupling caps, and are almost certainly still very good. The caps I mention are in the round cans on top of the chassis, commonly called (appropriately) can caps. In your underside pick, they would be the two cans -- one above the other -- closest to the transformer on the left of of the unit. These are 150 uF caps rated at 250 vdc (being electrically very large, they are therefore physically very large. that's why they are mounted vertically in cans above the chassis, to conserve space under the chassis.). Note that one is mounted by way of an insulating bakelite wafer -- this because the case of the can operates at several hundred volts above ground. On the top side, it will have a (usually black) cardboard cover over it for the very same reason.

In any event, it is quite likely that one of these two can caps has failed. A simple voltmeter test will confirm if that is the case. As it appears that all of the power supply caps are original in this unit, it would be a good idea to change out all of them -- even if by some chance they are all still good. After 50 years or so, they can dry up internally and basically go "open", which is likely the scenario in your case, or develop a very high power factor, effectively becoming a short circuit, which if happened in this location, would surely blow the fuse. Either type of failure mode can happen, which it always invariably does given enough use and time. Some caps can go quite long before a failure happens, while some go much shorter. It all depends of how well the component was manufactured, how hard it works in the circuit, and the heat it has been subjected to over time.

Dave

 

[Kingsin7]

Thanks Dave
I considered replacing those caps when I first acquired the unit (traded a C24 and Ma6100 for it), but I was so pleased with the sound I just let it be.

I saw this kit with "Authenticaps" on Ebay with a higher capacitance, but I am always weary of Ebay seller gimmicks where they advertise parts as a kit for a specific unit (sellers do this with LEDs and such charging 5X what they are worth).

These seem very well suited for the task, but I was wondering if there are other (cheaper) caps on Mouser or Digikey that you or anyone else has had experience with?

 

[dcgillespie]

To my knowledge, Authenticaps have a good reputation. Like so many Mac products, the MA230 includes a thermistor device on the primary side of the power transformer to limit the inrush current when the unit is turned on. Increasing the capacitance can be a good thing, as long as it isn't taken to extremes. An increase to 200 uF or 225 uF would be fine in this position, and would help maximize LF performance. The 150 uF that originally Mac used was no doubt the biggest value they could get in the day that would physically fit in the allotted area. As long as there is no increase in physical size, then an increase in capacitance along the lines suggested above would be just fine.

Dave

 

[Kingsin7]

They offer 250uF drop-in replacements. I just placed my order. I'll report back in a couple weeks after I have them in.
Thanks for all the help Dave

 

[dcgillespie]

Those should be fine.

Good luck with it, and let us know!

Dave

 

[Kingsin7]

Dave
I got the Authenticaps in the mail and sat down at my bench to pop them in today. One of the voltage doubler caps is shot as you guessed. It's capacitance jumped around on the meter when tested. The other is within spec and I may put it up for sale to recoup a little of the $100 or so I spent on the new caps.


They are in place now but I noticed all the 7591 tubes are a little loose. Some pressure is needed to get them in/out, but once they are in the socket there is significant play when i wiggle them.

Is this an issue worth reconciling? Of the 4 inputs, only one seems to be a little chipped, but all tubes wiggle just the same when installed.
Please let me know what you think.

FYI the 7591 tubes are half RCA and half Mcintosh.
Thanks,
Steve

 

[dcgillespie]

Steve -- Glad you found the defective doubler cap. As for the tube sockets, with the unit (obviously) turned off, unplugged, and discharged, use a small jeweler's screwdriver to carefully tighten all the receptor terminals in each output tube socket. That should tighten up the tubes when installed.

Let us know if it biases correctly once your new caps are installed.

Dave

 

[Kingsin7]

So, I was able to get everything put back together and I ran the amp on my bench. All 4 output tubes were set to 0.7v and held that reading for about 15 minutes with no issue. The amp was hooked up to my test speakers and sounded good.

I moved the amp back to my office, hooked everything up and was only getting audio at about 1/4 normal volume over speakers and headphones...

I unplugged everything and took it back out of its case to have a looksee at the back. I then hooked it up again and the unit would not power on.

I put it back on the bench and opened it up to find this little metal disc connecting the fuse holder to one of the AC inputs had been disconnected.



I figured that when I plugged my turntable into one of those AC inputs, the metal prongs were pushed inward and caused the ancient solder joint on this disc to break, but after resoldering and powering on, I found that after about 10-20 seconds that little disc heats up and the solder loosens up, separates and the unit powers off...

What is it and what can I do? Is it some sort of 55 year old protection device?

 

[mnmmt]

That is a thermistor, also known as a current inrush limiter. It is there to help it power up slowly. Replace it with a modern cl-80.

 

[Kingsin7]

Thanks mnmmt
I ordered a new one.

Is it likely the thermistor failed as a result of the new caps being installed?

 

[clinic-audio]

Hum , just arrived a little bit late , many answers already arrive before mine !

hum noise is coming from the choke not your output transformer so don't worried just replace all multisection capacitors by new Authenticap and everything will go back to normal function !
For information , the fact that you can't reach the right bias voltage is coming from your output valves 7591A who are closer to the end. This is a good sign the day who you will ask yourself to know why .
Use brand new ones and you will adjust your both bias without any troubles
Replacing thermistor is a very good idea , yours is too old !

 

[Kingsin7]

The thermistor arrived and was installed today. I let the amp warm up for 20 minutes and adjusted the bias. All output tubes were able to be adjusted to and hold at .7v.

The amp is back in my setup and sounds great.
Thanks everyone for the help.
-Steve

 

[triode17]

I've begun to notice a slightly louder than usual hum coming from my Mac MA230 that I use daily in my office setup.

Did you try changing V5? Its cathode is sitting at 116v. This exceeds the Heater-Cathode voltage for a 12AU7, 100v max, steady. If the heaters are not floated on about 60Vdc, the heater to cath. insulation will form a conductor which will slowly connect them together, causing hum. This problem wasn't well known when this amp. was designed.

 

[Kingsin7]

Did you try changing V5? Its cathode is sitting at 116v. This exceeds the Heater-Cathode voltage for a 12AU7, 100v max, steady. If the heaters are not floated on about 60Vdc, the heater to cath. insulation will form a conductor which will slowly connect them together, causing hum. This problem wasn't well known when this amp. was designed.

Triode,
The hum is all but gone since I swapped caps and installed the new thermistor. The (very) slight hum now doesn't bother me.

 

[triode17]

Triode,
The hum is all but gone since I swapped caps and installed the new thermistor. The (very) slight hum now doesn't bother me.

Great ! All tubes amps. have some hum, I consider it, raw horsepower !

 

[mnmmt]

There is also a service bulletin out there for the transformer physically humming. The solution is to tighten the power transformer bolts or insert fiber shims between the end bells and laminations and then tighten the bolts. I can hear mine buzzing from about 3 inches away from the amp.

 

[buickguy62]

Just a comment, I continually am impressed with this forum, 1) the knowledge of the participants, and 2) the willingness to share and lead other members in this arena. Thanks from a member of the silent majority, it's a great source of information.