mrz80
I guess it's in my blood
So while I'm waiting for the last batch of caps for the 1700 power supply overhaul http://audiokarma.org/forums/index.php?threads/mac-1700-goes-under-the-knife.699390/
I decided to pull the 4100 out of the family room and go ahead and attend to some of the things that I was going to deal with after I finished the 1700 - intermittent FM stereo (this cropped up once before - pull the IF/MPX module and clean the card edge and chassis connector) , scratchy controls, and some or all of the indicator LEDs just not indicating anything. The wife was starting to complain, even going so far as to threaten to buy a new receiver if things didn't improve, so let's dig in...
Got the unit apart, and did the usual suspects - clean the pots with F5, and all the switches and PC board cable connectors on with D5, blew out all the dust and cat hair, got some smudges off the glass, that sort of thing. BTW, an old toothbrush and some dish soap worked great for getting all the finger schmutz out of the serrations on the knobs.
After a good deal of studying of the schematic I thought I had a handle on what might have been going on, but a couple hours of chasing voltages around led me to the conclusion that something was very wrong with the input switching; control voltages that were supposed to be turning LEDs on and off and muting unselected inputs on the preamp board were just not there; NONE of the LEDs were lit and ALL of the inputs were active! (That led me to notice that, according to the schematic and contrary to the sales lit, the inputs are, in fact, physically switched on one deck of the input switch, which explains why I don't hear all the inputs at the same time!)
The high (well, low) point of the evening came when I figured out how to pull the indicator board from behind the front panel without disassembling the panel. Unfortunately, I managed to unhook the spring on the tuning-cap to knob string. ARGH! I found a couple questionable solder joints on the indicator board, but even after fixing them and putting the board back in (and getting that infernal dial cord sorted out!), still no joy. ARGH again! Then, in the middle of another round of "poke everything with a voltmeter and marvel at the unchanging control voltages no matter how the input switch was set", lo and behold everything Just Starts Working Again. All the indicator lights followed the input switch, the appropriate dial pointer LEDs lit for AM and FM, and the AFL lock and Stereo lights lit up on FM! WHAT THE HEY?!?!! Turned the 4100 off and on several times, even banged it against the workbench a time or two, and it all still worked. SO weird. And as an added bonus, I found a couple of 1847 bulbs in a drawer in the garage, so I got the meters lit up again too. No telling what of the combination of poking and prodding, diassembly, cleaning, and reassembly actually got it all going again, but I'll take it any way I can get it. Game set and MATCH!
I put it all back together and stuffed it back in the stereo cabinet, and reveled in the return of rock steady, noise and intermittent cutout free FM stereo. With a twinlead dipole thumbtacked to the wall I was getting stations all over Florida. VERY nice. Then, about 5 minutes into my stereophonic bliss, there was a loud *crack*, the right channel output meter spiked, and with a muted *click* I lost all output. AAAAAAAAA! What was THAT?!?!!? *sigh* I knew it was all too good to be true...
With the receiver opened up on the workbench once more, the Mark One Schnozola was brought into play, and by homing in on the burnt smell I quickly localized the issue to the right channel driver board area. Ah, THERE was the problem: a grey wire (ground, by convention, in this receiver) had been resting on the connector pin feeding 48V to the driver board; looks like it just chose last night to wear through the last of the insulation and arc over, shorting the +48V rail to ground. Nothing but that one wire seemed damaged, so I just wrapped the bare spot in electrical tape, rerouted the wire away from the driver board connectors, and powered it up again. Phew! +48V, -48V, +18V, all measured fine. Back into the stereo cabinet it went, with a HUGE sigh of relief as the receiver lit up and still sounded fine. Let's hear it for protection circuits!
I decided to pull the 4100 out of the family room and go ahead and attend to some of the things that I was going to deal with after I finished the 1700 - intermittent FM stereo (this cropped up once before - pull the IF/MPX module and clean the card edge and chassis connector) , scratchy controls, and some or all of the indicator LEDs just not indicating anything. The wife was starting to complain, even going so far as to threaten to buy a new receiver if things didn't improve, so let's dig in...
Got the unit apart, and did the usual suspects - clean the pots with F5, and all the switches and PC board cable connectors on with D5, blew out all the dust and cat hair, got some smudges off the glass, that sort of thing. BTW, an old toothbrush and some dish soap worked great for getting all the finger schmutz out of the serrations on the knobs.
After a good deal of studying of the schematic I thought I had a handle on what might have been going on, but a couple hours of chasing voltages around led me to the conclusion that something was very wrong with the input switching; control voltages that were supposed to be turning LEDs on and off and muting unselected inputs on the preamp board were just not there; NONE of the LEDs were lit and ALL of the inputs were active! (That led me to notice that, according to the schematic and contrary to the sales lit, the inputs are, in fact, physically switched on one deck of the input switch, which explains why I don't hear all the inputs at the same time!)
The high (well, low) point of the evening came when I figured out how to pull the indicator board from behind the front panel without disassembling the panel. Unfortunately, I managed to unhook the spring on the tuning-cap to knob string. ARGH! I found a couple questionable solder joints on the indicator board, but even after fixing them and putting the board back in (and getting that infernal dial cord sorted out!), still no joy. ARGH again! Then, in the middle of another round of "poke everything with a voltmeter and marvel at the unchanging control voltages no matter how the input switch was set", lo and behold everything Just Starts Working Again. All the indicator lights followed the input switch, the appropriate dial pointer LEDs lit for AM and FM, and the AFL lock and Stereo lights lit up on FM! WHAT THE HEY?!?!! Turned the 4100 off and on several times, even banged it against the workbench a time or two, and it all still worked. SO weird. And as an added bonus, I found a couple of 1847 bulbs in a drawer in the garage, so I got the meters lit up again too. No telling what of the combination of poking and prodding, diassembly, cleaning, and reassembly actually got it all going again, but I'll take it any way I can get it. Game set and MATCH!
I put it all back together and stuffed it back in the stereo cabinet, and reveled in the return of rock steady, noise and intermittent cutout free FM stereo. With a twinlead dipole thumbtacked to the wall I was getting stations all over Florida. VERY nice. Then, about 5 minutes into my stereophonic bliss, there was a loud *crack*, the right channel output meter spiked, and with a muted *click* I lost all output. AAAAAAAAA! What was THAT?!?!!? *sigh* I knew it was all too good to be true...
With the receiver opened up on the workbench once more, the Mark One Schnozola was brought into play, and by homing in on the burnt smell I quickly localized the issue to the right channel driver board area. Ah, THERE was the problem: a grey wire (ground, by convention, in this receiver) had been resting on the connector pin feeding 48V to the driver board; looks like it just chose last night to wear through the last of the insulation and arc over, shorting the +48V rail to ground. Nothing but that one wire seemed damaged, so I just wrapped the bare spot in electrical tape, rerouted the wire away from the driver board connectors, and powered it up again. Phew! +48V, -48V, +18V, all measured fine. Back into the stereo cabinet it went, with a HUGE sigh of relief as the receiver lit up and still sounded fine. Let's hear it for protection circuits!
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First off, the indicator LED for FM, and the FM-side LED on the pointer, don't light up any more, but all the others still do. Annoying but minor. Probably still some dirt on a switch contact or somesuch. The big issue is that, after a few minutes (the time varies) of glorious stereophonic sound, I get a noise like a burst of static from the right speaker, and the speaker relay trips out. Power the unit down for a good long while and everything's back to normal. Lather, rinse, repeat. I'm going to put it up on the bench with a dummy load and start poking with a voltmeter again, but where to start? Bad relay? Flaky thermal cutout? Power supply SNAFU?
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