Hi all,
Got my hands on a lovely looking MAC 1700 that unfortunately has some internal issues. I'm going to do my best to describe them here and list the steps I have already taken in hopes of somebody being able to help me out with diagnosing and fixing the problem.
When I first got my hands on the receiver I plugged it into my DBT which lit up brightly, but the unit powered on. Running test audio through it revealed a horribly distorted right channel and a very weak left channel. Signal run straight from the Tape Out to a separate power amp sounded flawless. First move was to thoroughly flush all the switches and pots with DeOxit, but that yielded no improvement.
Next step was to investigate the power amp section. I tested both output transistors on each channel as well as the three smaller TO-3 shaped transistors mounted to the top of the chassis near the power amp heat sinks. They all tested fine.
Next I inspected the power amp circuit boards (marked 128-039 on the trace side) underneath the chassis for bad solder joints and damaged components. Nothing stood out visually but to be on the safe side I reworked the solder on both boards. As I was poking around for bad solder I noticed that vibrations (in this case taps on or around the left channel circuit board) would cause the left channel to come back in and the DBT to dim. The left channel is now passing a strong clear signal but will occasionally cut out again, DBT will light up etc... At tap with a screw driver will bring the signal back in.
At this point I was pretty sure the problems I was experiencing stemmed from the power amp circuit boards. To test that theory I swapped the left and right boards. Sure enough the left channel was now awfully distorted and the right channel displayed the cutting in/out problem.
Next I started testing transistors on the power amp boards. They tested fine with my DMM. I even went as far as to swap the transistors from board to board in hopes that the problems might shift sides. They did not.
Phew. So as it stands right now the problems I have described persist and I am wondering what to do next. I know I have done a lot so far, but in reality not much has changed in terms of solving (or even altering) the problems. Hopefully my tinkering won't shy the experts on this unit away from lending a hand.
As a side note, this receiver is packed full of old components which I am having trouble finding suitable replacements. The transistors mounted to the power amp circuit boards in particular are proving difficult to find even a hint of information on. Any thoughts on that little conundrum would be much appreciated.
Thanks for taking the time to read all this! Looking forward to any and all input.
Got my hands on a lovely looking MAC 1700 that unfortunately has some internal issues. I'm going to do my best to describe them here and list the steps I have already taken in hopes of somebody being able to help me out with diagnosing and fixing the problem.
When I first got my hands on the receiver I plugged it into my DBT which lit up brightly, but the unit powered on. Running test audio through it revealed a horribly distorted right channel and a very weak left channel. Signal run straight from the Tape Out to a separate power amp sounded flawless. First move was to thoroughly flush all the switches and pots with DeOxit, but that yielded no improvement.
Next step was to investigate the power amp section. I tested both output transistors on each channel as well as the three smaller TO-3 shaped transistors mounted to the top of the chassis near the power amp heat sinks. They all tested fine.
Next I inspected the power amp circuit boards (marked 128-039 on the trace side) underneath the chassis for bad solder joints and damaged components. Nothing stood out visually but to be on the safe side I reworked the solder on both boards. As I was poking around for bad solder I noticed that vibrations (in this case taps on or around the left channel circuit board) would cause the left channel to come back in and the DBT to dim. The left channel is now passing a strong clear signal but will occasionally cut out again, DBT will light up etc... At tap with a screw driver will bring the signal back in.
At this point I was pretty sure the problems I was experiencing stemmed from the power amp circuit boards. To test that theory I swapped the left and right boards. Sure enough the left channel was now awfully distorted and the right channel displayed the cutting in/out problem.
Next I started testing transistors on the power amp boards. They tested fine with my DMM. I even went as far as to swap the transistors from board to board in hopes that the problems might shift sides. They did not.
Phew. So as it stands right now the problems I have described persist and I am wondering what to do next. I know I have done a lot so far, but in reality not much has changed in terms of solving (or even altering) the problems. Hopefully my tinkering won't shy the experts on this unit away from lending a hand.
As a side note, this receiver is packed full of old components which I am having trouble finding suitable replacements. The transistors mounted to the power amp circuit boards in particular are proving difficult to find even a hint of information on. Any thoughts on that little conundrum would be much appreciated.
Thanks for taking the time to read all this! Looking forward to any and all input.
