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drjb 3rd January 2012 02:59 PM

MR 67 Coupling Caps
Hi, I'm trying to figure out which caps (I assume output coupling caps) most influence the sound of my MR 67. A schematic is available here:

Am I right to think that C53/C54 and C56/C57 will influence the quality of the sound, the most?

I thought I'd like to upgrade these caps to PIO or VCaps, depending on size and cost.

Any and all info much appreciated!



DF96 3rd January 2012 03:35 PM

The LF point is set at the input to the audio circuit (as it should be) by C79/80 together with the 1M resistors. This means that the electrolytics in the audio output stage will not have much signal voltage across them, so cannot generate much distortion. There may be some advantage in a small increase in the value of C54/57 to, say, 22uF as this will avoid an LF rise. Apart from that I would leave it as it is. Sound quality of an FM tuner is set mainly by RF and stereo decoder circuitry, not the tiny bit of audio stuff at the end.

drjb 3rd January 2012 03:42 PM

DF96, sounds like this is a plan I should abandon. I changed the old, tired, stock output tubes, which transformed the sound; was hoping for the same with the output caps.


DF96 3rd January 2012 03:56 PM

You could check the caps for value and leakage, as they may have deteriorated. If OK, leave alone. If not, replace.

It's a pity the stereo decoder sits right on the page boundary as it makes it hard to follow the circuit!

drjb 3rd January 2012 04:27 PM

Overall, it's in great condition, no issues with it, at all (now that I've replaced the output tubes and one other).

It's not the best manual, but at least it's available.

Jebem 3rd January 2012 08:20 PM

Nice Tuner from Mcintosh.
As I see it, old equipment tends to have more than one component with altered values, like capacitors and resistors.
So, it takes a lot of effort and work to put the equipment back in good shape.

For the resistors, use a good multimeter and check them, and replace all the ones having a drift bigger than 15%, in particular for the high values in the 100Kohm and above region.

For the electrolytic capacitors, I would replace all of them, not just the two 1uF output caps that You mention.
There is a reason for this: these type of caps have a limited life, especially when operating in warm environments like tube systems. Most likely, old electrolytic caps will shows leaks or even worse, they just dry and loose capacitance.

At least consider replacing the power supply caps and all power supply bypass caps with new electrolytic ones of good quality.

For the output capacitors, just use new good quality electrolytic caps, or if You fancy expensive better caps, use MKP type as a minimum - try WIMA caps, they are good and not really expensive ones.
As DF96 states, it is not worth to use VCaps on this design.

And let us not forget a basic rule, if we remove one old resistor or capacitor from the circuit, the best procedure is to replace it with a new one. Just in case.

drjb 3rd January 2012 09:48 PM

Thank you, Jebem!

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