- Welcome to North Reading Engineering HiFi Forum.
An Open Forum for Audio Enthusiasts and Professionals
Recent posts
#91
Vintage McIntosh Amplifier Repairs, Restorations and Modifications / Re: MA6100 clean up
Last post by nre - Mar 27, 2023, 05:50 AMThere are five boards located in board edge connectors, each is held in place with a L-bracket that pulls the board against the chassis, remove the screws and wiggle-lift the boards off. A squirt of electronic grade alcohol where the board is inserted in the connector makes this effort much easier. The finger contacts are Pb-Sn plated.
The two in the bottom of the photo are the preamplifier boards, the two above those are the power output boards and the one at the top is the phase inverter/voltage regulator board.
The two in the bottom of the photo are the preamplifier boards, the two above those are the power output boards and the one at the top is the phase inverter/voltage regulator board.
#92
Vintage McIntosh Amplifier Repairs, Restorations and Modifications / MA6100 clean up
Last post by nre - Mar 27, 2023, 05:43 AMWith the power supply board replaced in my own MA6100, and the thing still on the bench, a cleanup seemed to be in order. The preamp and output stage chassis screen cages are easily removable, there's two sheet metal screws holding each in place. Remove those and the cages will fall free from the chassis.
This unit has been in continuous used since 1976, the year I purchased it new from Tech HiFi, Cambridge, MA! I've only been in it once, to replace the large filter cans and a few of the electrolytics. I also brought it to a McIntosh clinic, it was opened up there as well.
This unit has been in continuous used since 1976, the year I purchased it new from Tech HiFi, Cambridge, MA! I've only been in it once, to replace the large filter cans and a few of the electrolytics. I also brought it to a McIntosh clinic, it was opened up there as well.
#93
Vintage McIntosh Amplifier Repairs, Restorations and Modifications / Re: MA6100 Power Supply Board
Last post by nre - Mar 26, 2023, 05:17 AMHere is the R1.1 version of the power supply board.
To make the board easier to install, the thru holes for the transformer leads are larger. There's a footprint for each capacitor across the main power supply rectifiers and the RC filter at the output. Both will accommodate either a ceramic disk or any PCM5 film type such as a WIMA AC Pulse polyethylene for the rectifiers or bypass type for the filter.
Board top:
Bottom:
Installed:
To make the board easier to install, the thru holes for the transformer leads are larger. There's a footprint for each capacitor across the main power supply rectifiers and the RC filter at the output. Both will accommodate either a ceramic disk or any PCM5 film type such as a WIMA AC Pulse polyethylene for the rectifiers or bypass type for the filter.
Board top:
Bottom:
Installed:
#94
Vintage McIntosh Amplifier Repairs, Restorations and Modifications / Re: MA6100 Power Supply Board
Last post by nre - Mar 19, 2023, 09:07 AMThe demonstrator worked flawlessly.
I have another customer interested in a power supply board for his MA6100 so I've revised the R1.0 to 1.1 configuration. The R1.1 will incorporate solder pads and silkscreen footprints for AC pulse film capacitors in the output stage bridge rectifier circuit and general purpose polyester film in the output stage RC circuit. The thu holes for the transformer leads are larger in diameter to facilitate easier looming of the leads. The holes will also accommodate passing thru of #6 forks which could be soldered to the ends of each lead and allow screw disconnects of the lead to the Keystone terminals. The traces are duplicated on the bottom of the board resulting in 2X the total copper shown in the figure below.
I would prefer to use wire wound, non-inductive power resistors for R313/314 but cannot find them. I did manage to find a supplier for NIB Sprague KOOLOHM, non-inductive resistors and ordered a box of 1500 Ohm, 5W axials for this project. The 1500 Ohm resistor (R314) operates at its rated power and gets quite hot. The orginal McIntosh parts are simple wire wound, power resistors potted in ceramic boats.
In this design iteration, I've provided a location to use a PCM 5 film cap across each of the mains diodes. Either a ceramic disk Z5U or Polyethylene film such as the WIMA FKP2 would be suitable.
Top copper:
I have another customer interested in a power supply board for his MA6100 so I've revised the R1.0 to 1.1 configuration. The R1.1 will incorporate solder pads and silkscreen footprints for AC pulse film capacitors in the output stage bridge rectifier circuit and general purpose polyester film in the output stage RC circuit. The thu holes for the transformer leads are larger in diameter to facilitate easier looming of the leads. The holes will also accommodate passing thru of #6 forks which could be soldered to the ends of each lead and allow screw disconnects of the lead to the Keystone terminals. The traces are duplicated on the bottom of the board resulting in 2X the total copper shown in the figure below.
I would prefer to use wire wound, non-inductive power resistors for R313/314 but cannot find them. I did manage to find a supplier for NIB Sprague KOOLOHM, non-inductive resistors and ordered a box of 1500 Ohm, 5W axials for this project. The 1500 Ohm resistor (R314) operates at its rated power and gets quite hot. The orginal McIntosh parts are simple wire wound, power resistors potted in ceramic boats.
In this design iteration, I've provided a location to use a PCM 5 film cap across each of the mains diodes. Either a ceramic disk Z5U or Polyethylene film such as the WIMA FKP2 would be suitable.
Top copper:
#95
Vintage McIntosh Amplifier Repairs, Restorations and Modifications / Re: MA6100 Power Supply Board
Last post by nre - Mar 11, 2023, 02:39 PMReady for bottom chassis cover.
#96
Vintage McIntosh Amplifier Repairs, Restorations and Modifications / Re: MA6100 Power Supply Board
Last post by nre - Mar 11, 2023, 12:32 PMR1.0 demonstrator/prototype Complete!
#97
Vintage McIntosh Amplifier Repairs, Restorations and Modifications / Re: MA6100 Power Supply Board
Last post by nre - Mar 11, 2023, 05:44 AMR1.0 demonstrator board fits, always a good thing!
#98
Vintage McIntosh Amplifier Repairs, Restorations and Modifications / Re: MA6100 Power Supply Board
Last post by nre - Mar 04, 2023, 11:20 AMHere's a photo of thermal distress that the factory board will experience (pulled from the internet). The step down resistors are causing the board to bake, the leads are also involved in the distress.
#99
Vacuum Tube Amplifier Repairs, Restorations and Builds / Re: Juicy Music Peach II Pream...
Last post by nre - Feb 26, 2023, 11:09 AMThe unit came in with two broken RCA inputs which I removed.
The RCA jack is NLA.
There are many configurations similar but I've yet to find a similar solder pad pattern. One simple fix is to hang a female RCA "dongle" from each which will provide connectivity. The alternate is to drill the chassis for panel jacks which I haven't done. I may tape them both with Kapton and leave them. Should the owner wish to add jacks the diffiult part, removing the OEM units, is done.
The RCA jack is NLA.
There are many configurations similar but I've yet to find a similar solder pad pattern. One simple fix is to hang a female RCA "dongle" from each which will provide connectivity. The alternate is to drill the chassis for panel jacks which I haven't done. I may tape them both with Kapton and leave them. Should the owner wish to add jacks the diffiult part, removing the OEM units, is done.
#100
Vacuum Tube Amplifier Repairs, Restorations and Builds / Re: Juicy Music Peach II Pream...
Last post by nre - Feb 26, 2023, 06:48 AMThis assembly will reach about 170F at equilibrium, measured at the top of one of the fasteners holding the device down. The OEM nylon spacers would creep under these temperatures resulting is the output of the device to the PC board faulting. The aluminum ones will be better able to handle the temperature. A 6.3-6.5VDC potential should be measured at the test points.
