Update 5-13-2020
After I performed all of the parts replacements described below the problem slowly started to re-occur. The symptom was that the right channel would intermittently develop a small DC offset and the gain would be reduced until a “pop” was heard then the problem would be cleared. This problem, which I had noticed on-and-off for nearly 15 years, really bugged me so I told myself: “I’m really going to fix this thing this time!”
I started with basic troubleshooting to isolate the problem, which I was determined to fix. So I began by switching the right (R) and left (L) channel power amplifier driver cards and, lo-and-behold, the problem stayed in the R channel. So, the problem was not in the driver cards!
Next, I unplugged the preamp-to-power amplifier separation links and the problem remained in the R channel. This told me the issue is not coming from the preamp and has to be something in the chassis or R power amplifier wiring. After probing around I noticed that while the DC offset was occurring the input signal ground reference to the R channel driver card was also showing a DC offset. This is supposed to be connected to ground but apparently wasn’t in the R channel. A check of the schematic shows the ground reference (pin 6) should be connected to the R channel power amplifier input jack on the rear panel (RCA phone jack) through a coax shield. This is the power amplifier’s signal input ground reference and it was disconnected and floating! Eureka – that could be a big problem! As the DC offset drifted higher it would trigger the amp’s protection circuitry resulting in an audible “pop” in the speakers – then the DC offset would return to zero and the cycle would repeat.
Rather than trying to dig out the coax from the wire bundle to find out where the connection was broken I just simply ran a small ground wire in parallel with the coax in the wiring harness from pin 6 on R driver card to the rear panel input jack. That fixed the problem once and for all.
I’m embarrassed that I didn’t notice this simple problem much earlier but I have to say that all of the component replacements have really made this amplifier sound better than ever. For such a high quality and classic piece of gear over 45 years old upgrading these components is well justified.
Update 7-21-2019
After extensive troubleshooting, it turned out that the intermittent noise problem and popping was caused by several problems. The major problem was a intermittent transistor in the power amp input differential pair. This caused the DC output offset voltage of the PA to drift intermittently triggering protection circuitry which would reset with an audible pop. Additionally there were dirty switches in the preamplifier that needed cleaning. The whole amplifier chassis was cleaned. I replaced the panel lamps with LEDs and replaced a number of electronic components with modern high quality types:
- All of the electrolytic capacitors
- All of the ceramic capacitors in the signal path with film capacitors
- All of the carbon comp resistors in the audio path with 1% precision metal film types
- The input differential amplifier transistors with pairs carefully matched for input offset voltage
With all of these changes, this amplifier performs flawlessly and sounds great! It definitely has that robust “McIntosh Sound”! It also meets its original published specifications. I have become very familiar with the design of the power amplifier and can see where this design has some novel features. Although the PA is an older “quasi-complimentary” Class AB design it works and sounds very nice. It appears to have been designed to have high linearity before feedback is applied. The amount of feedback is not excessive and the closed loop amplifier produces only minor amounts of predominately low order harmonic distortion. When operated near maximum 70 Watt per channel power output, clipping is very smooth and well defined without any “sticking” or instability. The massive power supply produces a lot of “punch” typical of McIntosh amplifiers.
Love this amp – It’s a keeper!
I had lamented the lack of good FM stations here in SoCal but have re-discovered KJAZ-FM, 88.1 MHz from Cal State – Long Beach. They play 24-7 jazz of all types and their audio quality is superb, especially on the McIntosh MR-78. They even have one of my favorite musicians, David Benoit, as the 8AM-Noon DJ Monday – Friday. You can find them online or through I Heart Radio (but this won’t sound as good as a MR-78) Sweet!
I have owned one of these great stereo amplifiers since the 1970s. This was one of the first solid state integrated amplifier units made by McIntosh Lab. It has been used very lightly over it’s life, including very little over the last 20 years, but now appears to have some intermittent noise problems. When this unit is discussed on internet forums the old electrolytic capacitors in the are frequently cited as a problem and the response is to replace them with modern parts. I have identified most of the electrolytics in the unit and have determined replacements that are intended for high quality audio use made by Nichicon in Japan.
Block Diagram
Power Amp Schematic
Power Amp PCB
The right power amplifier has a problem where intermittently the volume is reduced and the DC level measured at the output starts to rise. Eventually there is a loud “pop” and the output level and offset returns to normal. The pop is accelerated by cranking up the volume. This was verified by swapping the left and right power amp PCBs (noise followed the PCB). This could very well be a faulty capacitor.
Power Amp
Funny that the Right Power Amp is on the left side of the unit and the Left PA is on the right!
Preamp (Output section) Schematic
Preamp PCB
The left preamp output has a problem where there is sometimes a noisy sound after the amplifier power is turned off. This was verified by swapping the left and right preamp PCBs (noise followed the PCB). Since the power is turned off by turning the volume control all the way down this is coming from the last preamp stage which is after the volume control.
Regulator / Phase Inverter
Regulator / Phase Inverter PCB
Replacement Capacitors
That’s it for now. Caps are on order. We’ll see how it works.