Used some of the Easter holidays to test some different older circuits, and among them the power amplifier section of the McIntosh MC2120 and MC2200 (same circuit used in both but different no. of output devices).
What puzzles me is the bias adjustment. The Service Manuals just states you to measure the total power consumption of the amp during bias adjustment, but I measured the Voltages Base to Gnd of the output devices.
In the manuals are stated that with no input the Voltages on the Bases should be +0.4 and -0.4V respectively.......
I get app. -0.38V on negative side, but each time I try to adjust bias up so the positive side measures app. +0.4V the amp suddenly trips into Class A with app. +9 and-9V on the Bases ?
The funny part of this is if I keep the possitive Base Voltage below app. +0.3V and the negative at -0.38V it works perfekt using a 3R3 dummy load. Full output without a glitch of crossover distortion on the scope, symmetrical clipping and just a little bit of overshooting when tested with square waves.
The circuit has been checked again and again several times a day for many days without I could find any errors in my circuit (it is after all a quite simple circuit).
Any experienced McIntosh people here that has an explaination ??
Thanks in advance 😉
What puzzles me is the bias adjustment. The Service Manuals just states you to measure the total power consumption of the amp during bias adjustment, but I measured the Voltages Base to Gnd of the output devices.
In the manuals are stated that with no input the Voltages on the Bases should be +0.4 and -0.4V respectively.......
I get app. -0.38V on negative side, but each time I try to adjust bias up so the positive side measures app. +0.4V the amp suddenly trips into Class A with app. +9 and-9V on the Bases ?
The funny part of this is if I keep the possitive Base Voltage below app. +0.3V and the negative at -0.38V it works perfekt using a 3R3 dummy load. Full output without a glitch of crossover distortion on the scope, symmetrical clipping and just a little bit of overshooting when tested with square waves.
The circuit has been checked again and again several times a day for many days without I could find any errors in my circuit (it is after all a quite simple circuit).
Any experienced McIntosh people here that has an explaination ??
Thanks in advance 😉
I usually apply a 1 volt sine wave and monitor the output for crossover distortion with a 4 ohm speaker connected.
I raise the bias very slowly until crossover distortion just goes.
This has always given good results.
I raise the bias very slowly until crossover distortion just goes.
This has always given good results.
And it sure does 😉 And it is very stable....
Just wonder why it trips into Class A so brute. No middle way ??
Jan,
I repaired an MC2200 last year and tried using the "total power consumption" method and couldn't get it to work that way. I finally ended up adjusting the bias to just eliminate of the crossover spike while monitoring the output of my HP 334 distortion analyzer. The adjustment was very touchy and easily went too high. This was after a total rebuild of both heatsink assys. using parts obtained from McIntosh. My "golden ear" friend was happy with the results so that's the way it is to this day.
Craig
I repaired an MC2200 last year and tried using the "total power consumption" method and couldn't get it to work that way. I finally ended up adjusting the bias to just eliminate of the crossover spike while monitoring the output of my HP 334 distortion analyzer. The adjustment was very touchy and easily went too high. This was after a total rebuild of both heatsink assys. using parts obtained from McIntosh. My "golden ear" friend was happy with the results so that's the way it is to this day.
Craig
Nigel;
Thanks I'm aware of that 😉
But this just happens with a 1/1000 turn of the 100R multiturn pot....
Normally you have a little slope when adjusting bias.
Thanks I'm aware of that 😉
But this just happens with a 1/1000 turn of the 100R multiturn pot....
Normally you have a little slope when adjusting bias.
Thanks Craig 😉
Nice to know this is common reaction. Then I don't have to search more for errors in my circuit 🙂
Looking forward to listen to it within a week or so. Last time I heard a MC2120 was 12 years ago 🙂
Nice to know this is common reaction. Then I don't have to search more for errors in my circuit 🙂
Looking forward to listen to it within a week or so. Last time I heard a MC2120 was 12 years ago 🙂
Recaping my MC2200
Hey Guys,
A little bite of info for a freshmen here: does a recapping of the MC2200 requires a bias adjustment?
Thank you for the help
Hey Guys,
A little bite of info for a freshmen here: does a recapping of the MC2200 requires a bias adjustment?
Thank you for the help
No, though checking it tends to be a good idea in old amps.
Looking at the schematic, I'm not surprised bias adjustment is touchy. The Vbe multiplier controls base voltage of a degenerated push-pull common emitter amplifier which drives another push-pull common emitter job that drives the outputs. Smells like gain plus major nonlinearity.
Looking at the schematic, I'm not surprised bias adjustment is touchy. The Vbe multiplier controls base voltage of a degenerated push-pull common emitter amplifier which drives another push-pull common emitter job that drives the outputs. Smells like gain plus major nonlinearity.
Hi Sakharov,
Do you have a good soldering station (not iron), a good DVM and a decent oscilloscope? If you don't, then either buy these items, or do not touch the amplifier. Get familiar with making good solder connections and the art of desoldering before you touch any product. You really ought to have some proper dummy load resistors mounted on a suitable heat sink and a distortion analyzer. If you don't believe me, just read the service manual where it lists the required equipment you need to service this amp. There is a very good reason why these things are required.
To replace the aging capacitors in your amplifier requires that you know about the parts you will be installing. It's not about working off a parts list provided by another unknown person. Many times I have seen inappropriate capacitor types used, and / or installed improperly.
Question: What are the most expensive components (and can be delicate) in an amplifier?
The circuit board itself.
-Chris
Man, do I ever hate the term "recapping"!!! This simple sounding procedure can destroy a nice amplifier so easily, and there are more things to do than simply swap out capacitors.
Do you have a good soldering station (not iron), a good DVM and a decent oscilloscope? If you don't, then either buy these items, or do not touch the amplifier. Get familiar with making good solder connections and the art of desoldering before you touch any product. You really ought to have some proper dummy load resistors mounted on a suitable heat sink and a distortion analyzer. If you don't believe me, just read the service manual where it lists the required equipment you need to service this amp. There is a very good reason why these things are required.
To replace the aging capacitors in your amplifier requires that you know about the parts you will be installing. It's not about working off a parts list provided by another unknown person. Many times I have seen inappropriate capacitor types used, and / or installed improperly.
Question: What are the most expensive components (and can be delicate) in an amplifier?
The circuit board itself.
-Chris
Could be a bad bias pot, or even the plastic bias transistor (with its cable-tie to a driver).
Never mess with the frequency compensation caps, especially in the CFP feedback loop.
The other small electrolytics are common types from Digi-Key, Mouser, etc.
McIntosh MC2205 - Manual - Power Amplifier - HiFi Engine
The MC502, 2120/2125/2200/2205 use the same drive circuit, just more outputs and driver a lower impedance tap on the output autoformers.
Never mess with the frequency compensation caps, especially in the CFP feedback loop.
The other small electrolytics are common types from Digi-Key, Mouser, etc.
McIntosh MC2205 - Manual - Power Amplifier - HiFi Engine
The MC502, 2120/2125/2200/2205 use the same drive circuit, just more outputs and driver a lower impedance tap on the output autoformers.
There you have it....
Have made those quick and dirty replacements with multi thousand audio gear equipment and then having to make a walk of shame to the real specialist...
Yes good investments in secondhand Flukes and Tektronix stuff and a Erna solderingstation with different tips and zero to 400 Watts pot.
Now I've bought a McIntosh MC2205 which is IMO relatively easy to service because of their good service manuals, schematics and easy to remove printboards.
I have had a 16,000 USD pair (price in 1991) of big monaural Reference American amplifiers that did not have an official service manual nor any schematics other than inadequete drawings from some technicians that work on these beasts.
The Mac was "only" 1,200 USD in 1975.