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Dead MR78 on my workbench

PE9ZZ

PE9ZZ

Event Horizon
Subscriber
Just joined the ranks of McIntosh owners by receiving an MR78 tuner off Feebay. High on my want list for a long time. Got it from a German seller for €575 inklusive Versand. It was sold as defective. I opened it up and found the transformer still wired for 120 V. I quickly changed it to 240 V to prevent accidents and fired it up. No sound as promised but the LO works and all incandescent bulbs too, including the dial pointer. Work has already been done to it but right now I have no idea what's wrong with it. Did I get a good deal? We'll see. Will post updates as work progresses.
 
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Well, it's been resurrected. But before that I got a weird fault. I plugged it in and my AC current meter and overload indicator went nuts. I immediately yanked the power cord and found one of the power supply diodes (D504) had shorted:
IMGP2383_crop.jpg

I quickly replaced all three diodes with 1N4007s (and the mains fuse from 2 A to 0.3 A) and all was well:
IMGP2384_crop.jpg

I then soon discovered the tuner was demodulating signals but not decoding them. The dual opamp IC301 had some flakey voltages (pin#1 was at positive rail) so I kludged an NE5532 adapter board to replace it. Annoyingly the mystery NS dual opamp has its inputs exchanged so I had to cross wires:
IMGP2390_crop.jpg
Dropping the board into the socket for IC301 did not immediately work due to lead C's on the minus inputs. The NE5532 does not like those and oscillates. Removing the C's produced a nice MPX signal to the decoder diodes (not visible in this pic):
IMGP2392_crop.jpg

Right now I am not impressed by the stereo performance (separation) but I've just started.
 
I had that same damn op amp go bad in my MR78.

It’s a very complex tuner to align and rest assured by now the crystal filter center frequencies have drifted a bit. And the discriminator is a complex delay line so it’s not adjustable, as I recall. I spent a lot of quality time with the sig gen, spectrum analyzer, scope, and distortion analyzer dialing mine in.
 
w3jn said:
It’s a very complex tuner to align and rest assured by now the crystal filter center frequencies have drifted a bit
Click to expand...
The super narrow filter seems OK but the narrow filter loses the signal that goes into it. Because the super narrow filter also goes through it it disappears as well. Right now I dunno whether it's borked or just misaligned. I poke around with my scope at 10.7 MHz because my spectrum analyzer is too much of a load. Maybe I'll have to make a FET probe or something to reliably make measurements. Will also help with my scope. Even the capacitance of a 1:10 probe is enough to upset the measurements around T203/4.
 
Yeah, put a 1k or so resistor in series with the spec a lead so you don’t load down the filter. Or use a scope with the sig gen cranked accordingly.
 
I happen to have an active probe and I can even connect it to my spectrum analyzer. It clearly shows that the normal filter works fine but both narrow filters have a huge insertion loss. I don't have a proper 2.5 mm HEX tool to align (only a 2.0 mm one and I can't find them anywhere) but just poking an Allen key in the core shows that it's already at max. So I'm at a loss what's going on here. Will make some screenshots/provide numbers to illustrate.
 
A propos loss: the normal filter has this output:
normal_IC201_in.jpg

The narrow filter has 19 dB more insertion loss:
narrow_IC201_in.jpg

This is the input of IC201 but the second filter pair behaves the same way. Surely the narrow filter should have a similar loss to the normal one? The switching voltages seem OK but I will check tomorrow to make sure I'm not overlooking something.

BTW- I remembered I have an active Tektronix probe that I can hook up to my SA.
 
You'd think so. It's just a multistage LC filter. Did you try tuning +/- 10.7 to see if it peaks off-frequency?

luckily the narrow filter isn't a crystal filter - just switched/cascaded L/C circuits. Might want to make sure the switching diodes are OK. Looks like a mix of germanium and silicon, which is strange.
 
w3jn said:
You'd think so. It's just a multistage LC filter. Did you try tuning +/- 10.7 to see if it peaks off-frequency?
Click to expand...
Yes, I did. The response centers nicely around 10.7 in all three modes but it is rather bumpy in normal mode. The diode switching looks OK. I also noticed the curious mix of silicon and germanium. With my Hi-Z probe I can also look at the tops of the LC circuits and these look fine as well. I will have to do some more digging (and locate a 2.5 mm hex tool that can adjust dual slug transformers).
 
Correction: the bumpiness is from the frontend. I did some more testing and I found that the normal filter also has a rather high insertion loss. With -50 dBm at the input the IF signal into the IF strip is -28 dBm. The normal filter puts out -49 dBm (21 dB loss):
IMGP2437_normal_out.jpg

and the narrow filter -67 dBm (39 dB loss):
IMGP2438_narrow_out.jpg

The plots were made with my SA on MAX HOLD and tuning the MR78. While both filters center well they both are asymmetric suggesting they're in need of alignment. I don't have the .100" HEX alignment tool but I've ordered it. Until then no updates...
 
I was lucky I had the designer of my 78 restore it and update it 10 + years ago. He's a consultant now for Audio Classics in Vestal NY. It came back sounding better than ever. Te MR 80 was looked at and fixed up by the Audio Classic techs and with a liitle help from the designer a few changes were made, too. Even though the 80 is known to have its problems I prefer using it as I listen to only two University channels. Guess we are all going to have to donate now.
 
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Update after all: managed to get some cyan/turquoise LEDs for the meters and they are awesome!
IMGP2435_crop.jpg

The Nichia LEDs illuminate the shiny RF box. The tuning meter has a black box behind it so it will need a reflective patch. Of course I used a lamp socket so the lamps can be re-inserted quickly (but why would you want that...).
 
PE9ZZ said:
Update after all: managed to get some cyan/turquoise LEDs for the meters and they are awesome!
View attachment 3565903

The Nichia LEDs illuminate the shiny RF box. The tuning meter has a black box behind it so it will need a reflective patch. Of course I used a lamp socket so the lamps can be re-inserted quickly (but why would you want that...).
Click to expand...
As for the reflective patch - aluminum tape used for ductwork in ac or for dryer vents would do well.
 
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