Friday, June 3, 2016

McIntosh MC2500 Rehab

Mcintosh MC2500 Power Amplifier 

Today were showcasing a Black/Black plated McIntosh MC2500 Power Amplifier.
Albeit very large and heavy the MC2500 is a true work horse, capable of driving 1OHM loads at rated power via their innovative AutoFormers, and thermostatically controlled fan options adds to the versatility of the amp. Integrated with McIntosh’s renown Power Guard feature. Mcintosh’s Power Guard feature has evolved over the years and is still considered one of the best detection circuits out there, all in a fraction of a second the circuit analyzes the input/output waveforms for premature distortion( D+N), when D+N does occurring the proceeding input signal is dropped until a clean waveform is present and gain then compensated.  This particular rebuild, retained the original 33,000MFD capacitors after verifying at voltage levels ESR/ratings. 

Driver Assemblies PCBID: 046783
The MC2500 uses a dual symmetric NPN/PNP driver topography and Autoformers.  On these boards the original electrolytic were replaced with a audio grade Nichicon KG/KA and UVP Nichicon BP capacitors.  The tracking potentiometers were updated as well as the initial differential pair transistors with matched ZTX low signal transistors matched within 1% along with current monitor Q4.  Final stage drivers Q10/13 were updated with matched TO-220 cased 1220/2690 Bi-Polar transistors.  Remaining 4148 general diodes were updated with new 4148 type. Remaining film capacitors were replaced with high grade WIMA polypropylene type including the .47MFD electrolytic to a WIMA film. 

At this point the TO-3’s were removed and re-greased along with crimp thermostat/molex connector re-crimped and cleaned. At this time I like to pull the 33,000MFD assemblies and replaced the original .22/100 film with a MPK polypropylene .22/630 film capacitor.

Power Supply Assembly PCBID: 045743
The Power supply and Meter Control Assemblies both lie in the front of the chassis parallel to the main meter displays.  Both PCB’s are vertical molex oriented assemblies.  On the power supply assembly we replaced the electrolytic with all high temp (105C) long life Panasonic FR capacitors along with updating the two Y2 rated ceramic capacitors with Vishay X1Y2 type .01 capacitors.  Both regulating BJT’s Q3/Q2 which are TIP devices were updated to MJE15033/32 TO-220 transistors with new thermal compound applied. I want to note this assembly uses a MOTO MPSU57 transistor which has a fairly sketchy life, we replaced this with a MJE15030 type TO-220 BJT.

Meter Display Assembly PCBID: 045619

Located next to the power supply assembly the power guard detect and meter simply and driven via this assembly.  For this stage we used all high temp (105C) low impedance Nichicon PW capacitors along with UVP Nichicon Bipolar type. Updated the potentiometer network with Bourn precision devices and addressed Q1/2 which is a BC239 to a 5088 BJT device, we noted a leaking BC239 under load and notated a low gain (HFE-11) which resolves the intermittent power guard indication.  Both 1.5MFD films were updated to 1MFD Solen MPK polypropylene film capacitors.

The Input/RCA assembly was pulled and the electrolytic replaced with a high temp (105C) low impedance Nichicon PW, the RCA/Thermo/MONO switches were all deeply cleaned, many customer have noted issues with their thermostatic switches.    I highly suggest using LED's in this assembly, the original incandescents literally bake the lamp clips, we can tell from a prior shop had replaced with the lamps with mismatched voltage type that the clips are brittle and have broken away, LED's have almost no radiant heat to combat. 

Audio Notizen

1 comment:

  1. awesome unit. someday i'll have the bucks to get one.