Sunday, 22 July 2018

Grundig GMU3 microphone mixer.

John got in touch ...

"Any room for a little 'un - this little 'un?
It's a Grundig GMU3 mic mixer from the late '50s. I was thinking "looks pretty, but maybe doesn't sound much good..." and it was possibly in line for e-Bay. Then I plugged it in. Plenty of crackle, gain not what it should be, but the sound quality coming though from just a bog standard dynamic mic had a real presence that made me think it has real possibilities and definitely not something I should overlook.
Might this be something you could take a look at?
Hopefully see you when I'm down soon."

Sadly, as John was coming down, I was heading off to Spain. He dropped it in with a mutual friend. It's got an odd selection of knobs (the mixer, not my friend).

Back panel has 3 mic inputs , and a "channel" input, which is at line level.

Incidentally, this unit was probably intended for high output crystal mics, but is quiet enough for use with an ordinary dynamic.

Output, however, is at mic level, to feed a Grundig tape recorder of similar vintage.

Removing the case reveals some nasty electrolytics...

Which are long past their best...

and a pair of lovely Mullard ECC83's (12AX7)...

The electrolytics are evicted , and modern replacements held in place with a dab of RTV.

... and there's a lot to do on the underside, a myriad of nasty waxy's.. all replaced. The 25uF cathode bypass caps all measure very leaky except one, which was in tolerance, and had reasonable ESR too, but all are replaced anyway.

The switches, pots and connectors are all cleaned up....

... and it all works very well!

Most of these units seem to have the later, EM84 type of indicator tube.

This has the type which has two "wings" that close up, and is the brightest I've seen for ages.

The 13A fuse in the plug replaced for a more sensible 3A....

The some more suitable knobs found.

And it's pronounced fit to travel back to Scotland to John!

Despite Grundig being a German brand, this was made in the UK!

The guilty parties...

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