MC2100 restoration

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Starting a complete restoration of another MC2100.  If you're interested in purchasing the amp please contact me directly.  This project will include new paint, polished chassis, new McIntosh output power boards, new North Reading Preamp, new filtering caps, new hardware including machine screws, new output transistors, new multi-cap and other goodies.

The chassis is in excellent condition and the transformers are fine. 

Some photos of the teardown. 

The amp was purchased with many parts missing.  I have many of the parts on hand to restore the amplifier to near new condition!











   

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First thing to tackle is the small signal cables from the amplifier front-end potentiometers to the preamplifier board.  McIntosh factory leads are good quality, tinned, single conductor with metalized mylar sheild and tinned drain wire.  A pigtail was attached to the drain via a crimp which was then secured using shrink. 

I prepped the leads per the factory approach with the exception of soldering the pigtail to the drain rather than using a crimp fitting.



   

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Fuseholder: Bussman HKP 15A 250V

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Replace the crack prone, hard plastic factory chassis feet with rubber.  Includes integral washer installed.  Requires a longer sheet metal screw for installation into the chassis. 

https://www.mcmaster.com/9540K914/

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The McIntosh preamplifier board will be replaced with a North Reading Engineering preamp.

First consider the factory preamp board, the MC2100 preamplifier used low noise BC238 NPN amplifiers at Q301, 303 (top) and Q304, 302, each pair forming a differential input pair (one for each channel). These are paint coded orange-green by the factory, likely sorted by Hfe (DC current gain).



   

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Here's the static characteristic curves of the two devices shown in the first photograph.  The two form a differential pair for the input signal.  The plots are the collector current (Y-axis) as a function of collector-emitter voltage (X-axis) over a range of fixed base currents.  As is shown, there's differences between the factory pair, they can be better matched.

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The North Reading Engineering board uses a pair of BC550 small signal, low noise NPN BJTs to form the differential input pair.  There's a pair for each channel of the preamplifier board.  The static characteristic of a selected pair is shown and, as evidenced, are nearly identical.  These devices are from the same production lot which makes the job of sorting and finding similar pairs a bit easier.

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Here's the schematic for the North Reading preamplifier.  The pink highlighted areas show the primary differences between it and the factory preamp.



First (top) is a current mirror which loads the collectors of the differential paired NPN transistors.  A current mirror synthesized using individual transistors is a bit of a challenge and requires careful matching of the two PNPs used to fabricate the mirror.  On an integrated circuit, this is no issue, but here it's a compromise.  That said, after sorting thru 20 or so 2N5087s, all from the same production lot, I was able to find two pair that are quite close based on the static characteristic curves I measured.

To minimize thermal drift, mechanically uniting the two devices as shown below does improve things a wee bit.  The white stuff is thermal grease.  There're small metal clips for this sort of thing but the tie-wrap will do.



What the current mirror provides is a significant increase in differential gain which allows for some emitter degeneration at the input pair (not done in the factory unit either).  The currents are quite small in this design, on the order of about 200uA, which manages to keep thermal effects to a minimum.

The lower highlighted region is a current source which replaces the factory long-tailed-pair resistor arrangement which suffers from variations in tail current as the common mode signals change.  This effects the gain of the amplifier causing distortion.  The tail current is determined by one diode drop across the 2kOhm resistor (~215uA).

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Below is the factory MC2100 preamplifier board schematic which can be compared to above.

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Capacitor C1 in the North Reading preamplifier is straightforward to compensate for instability associated with feedback.  The effect of varying C1 from 10p to 100pF, in increments of 10p is shown in the simulation.  The response is simulated from 1k to 500kHz.  The preamplifier slew rate is lowered as C1 in increased, the price for stability.

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The filter caps for the new preamplifier board arrived this weekend which allowed me to complete the preamplifier board and begin testing it.



A few preliminary measurements to determine if the assembled board works and doesn't oscillate.  20kHz (top) and 10kHz square wave response, each channel.  Input is 180mVRMS.  Signals sourced from Standford Reasearch Ultralow Distortion generator set at 50Ohm output impedance.

 

20kHz (bottom) and 10kHz sinewave response.



200kHz sinewave.  Output reduces to about 1.43VRMS at 200kHz from 2.01VRMS at 20kHz or a change of about -2.96dBV

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Plot below is simulation results of open loop gain of each preamplifier.  A significant 8dB increase is realized in the North Reading design.  Experimentally, this is a bit difficult to measure (requires some fabricated circuits, low cost but time consuming).  Final %THD numbers of the preamplifier in the amp, which are straightforward to perform, support the simulation trends.  I'll simulate the closed loop gain and then shown the loop gain and explain why this is worth examining.

To simulate the open loop gain requires the use of a simulation tool called a Tian probe which is easy to execute in LTSPICE and I'll show that also.

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In the LTSPICE environment, it's relatively easy to execute a Tian probe analysis by finding the file in the LTC library (LoopGain2.asc) and pulling both the probe (highlighted in the box below), the simulation directive:

.step param prb list -1 1 ; set prb=0 to turn off probe

and the plotting expression:

-1/(1-1/(2*(I(Vi)@1*V(x)@2-V(x)@1*I(Vi)@2)+V(x)@1+I(Vi)@2))

from the file.

The probe is inserted on the inverting side of the differential pair after the feedback resisitor circuit.

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Prepping the bottom chassis cover for paint...

is done (my way) using abrasive loaded Scotch-brite, specifically the brown pad.  This is ideal for breaking the factory paint and prepping the surface for a fresh finish.  Emery paper can take the paint off completely which is not desired.  The best primer is the original paint.

Before any paint is applied, the chassis should have a matte appearance and be free of any visible scratches. It also must be impeccably clean and free of residual abrasive grit or paint soot.  A hard rub down with clean towel or clean tee shirt will eliminate any soot from the surface.  I do not use solvents to clean the surface, they're not needed.







Burrs should be filed down before paint too.  This will ensure the new feet sit firmly to the chassis.





       

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Two very light coats later.  I'll place the chassis in a small heating box set at about 100F for three hours to harden the paint and drive off any remaining solvent.