?Microphonic problems on inputs of glensound preamps?

Hi,
I thought I'd post this in a new topic after the initial discussions on the Glen Sound Services MX6/2/222 desk

Having done some further testing it really seems like there's problems and they're really puzzling....

Basically (and all 6 pres seem to behave like this) - when the pre is set to microphone input and without a mic even being plugged in (!) you can tap on the casing of the mixer and this is transfered into the circuit - I don't know what to call this other than using the word 'microphonic'..

Also the sound is really thin and nasty (not dark as strangeandbouncy described the preamps as sounding..)

So there's definitely a problem.

I don't have a service manual (anyone happen to have one? I can't find very much info at all on the net). Of course I wonder if the power supply is working correctly.. But without reference manuals its kind of tricky to figure out how things should be.

Most of my work is pretty lofi so this is a very new area for me - there's two parts on the preamp board that are particularly puzzling:
1) is that an input transformer -> the big round thing just above the two output jacks?
2) the two things next to the jack sockets - one either side of the big mic/line push switch - are these some sort of noise choke? On one of the preamps one of these parts is affixed to the panel..

Here's an image:


So - from my limited knowledge of such things, the areas I wonder about are:
1) Power supply - testing with oscilloscope seems as though the power lines are stable - but they're potentially higher than expected - something like 35v and 50v (presuming for the phantom power)
2) There's a 741 opamp in each preamp - I doubt this'd be the problem in this case, but may be noisy and I figured to change them sometime
3) could the electrolitic caps have dried out? (or does that only happen on power filtering caps?)


Any help would be really appreciated - the fact that all 6 preamps seem to be the same is puzzling..

Mighty thanks,
TomBugs
Bristol, UK

Everything is microphonic to some extent, although tubes are famously so. Tap various components to see where the offender(s) is (are). If nothing on an isolated board seems to be doing it then perhaps the problem is in your edge connectors or other aspects of the busing.

All 'lytics dry out whether due to internal heat or external---it's just that the power supply ones tend to do so sooner as they often have substantial ripple current and dissipation. Also, 'lytics have a tendency to unform a little just sitting on the shelf, although modern parts are far less susceptible. But these look like pretty old parts.

"Thin and nasty" sounds like loss of low frequencies, and could well be some out-of-tolerance-low coupling/d.c. blocking capacitors. But since you say all are the same one would wonder if there isn't something elsewhere in the box doing this.

I don't know with certainty what all of those unknown parts are, although the one on the left of the switch looks like a multi-turn ferrite choke, and the one to the right like a polystyrene capacitor, but tracing the circuit a bit ought to give some good clues. Someone else in here will probably recognize them. That probably is a cup-core transformer or something similar.

741's are of course ancient and pretty terrible for audio, although I haven't had the experience of them aging and getting noisy per se. But then this whole piece is something of an antique and I suspect not a particularly treasured one at that.

[quote author="bcarso"]Everything is microphonic to some extent, although tubes are famously so. Tap various components to see where the offender(s) is (are). If nothing on an isolated board seems to be doing it then perhaps the problem is in your edge connectors or other aspects of the busing.
[/quote]

Take a pencil and tap on individual components until you find the offender and replace it. Check the power supply as well. I know it's ghetto, but it works...

I hope I've helped...
:guinness:

Thanks a bunch both of you..!..

[quote author="bcarso"]Everything is microphonic to some extent, although tubes are famously so. Tap various components to see where the offender(s) is (are). If nothing on an isolated board seems to be doing it then perhaps the problem is in your edge connectors or other aspects of the busing.
[/quote]

Tapping is the way then.!.

I was very surprised to be measuring the taps at something like 4v PtoP - that's quite a lot for anything...

And the fact that all 6 pres seem to be the same would point towards problems elsewhere -- I'll look more at the busing and try some more around the power supply.



Hey and thanks for the info about caps and other bits - that's all very interesting and useful to know.

I'll report back I guess.
Cheers!

It's likely to be the input transformer that is acting as the transducer converting mechanical displacement impulses to voltage output. However, try it again with a 600 Ohm resistor across the inputs and see if it still does it. Some of those old BBC circuits were designed for a 600 Ohm environment (not bridging), and that means terminating everything and allowing for the 6dB loss each time.

When describing the sound as "thin and nasty", are you referring to the case of microphone in to line out? If so, what type of microphone? Again, this may be a case of matching the input impedance. Does it sound the same with line in?

For the opamps, you can do a lot better than 741s these days. It would need a glance at a schematic (either official or reverse-engineered from the PCB) to recommend alternative types. Crucial to this is whether the opamp gain is switched between mic and line settings.

Hey thanks a lot for the input transformer pointer -- I did read a little and wonder whether BBC standards could be strange and cause problems - I'll def. try out the 600r across the inputs. Hmm, transformers are not something I know very much about...

Well, the mic input testing I just did quickly with an sm57 that happened to be to hand. And, yes, it was mic input to the direct out of the preamp - I can't remember what my findings with line inputs was - should test again.

Ah, reverse engineering - tracing the schematics - YES that's a great idea.

Thanks so much.

Does the code: MSC 5060 PMT / 7717 on the transformer mean anything to anyone?

Change ALL of those blue electro caps (can I see a tant there too? change it!), clean the pots and switches and then check all the solder joints of the switches, pots and jacks and resolder if they look even a little sus.

Then start trouble shooting.

When youre tapping, is the gain up or down? It should be down and it might be an idea to short the mic input while youre tapping.

If theyre still microphonic after you do these things, let us know.



M@

Cheers Matt - yes, replacing all those components is certainly a good idea (and, yes, there is a tant cap too..)

Findings of today::

Tapping - certainly does seem as though the input transformer is the culprit - tapping other parts gives some noise but the transformer gives LOADS. Although, yes, I have been testing with the gain quite high, but I figure my initial investigations are still valid (?!). I really should do some reading about input transformers - I know very little about them and wouldn't know where to start in finding a replacement..

And the line input DOES seem to be sounding ok.

Right, I'm going to try to figure a schematic from it....


And I guess that asap I should upgrade all the components - socket the IC for example.

Thanks again for all the help.

Tapping anything on the board will couple mechanically to the input transformer.

You say that all the channels behave the same. That pretty much rules out individual component faults, so I would not recommend that you spend the time replacing all the resistors and capacitors unless you have evidence of a failure. As a separate matter, it would be worth upgrading the 741 opamp, as I mentioned previously.

Microphone transformers when run with their inputs open circuit can be quite senstitive mechanically, and I think you are just looking at a design effect which will disappear when you connect a low impedance microphone (e.g. dynamic). Again, I don't think there is a fault. I have a 4-channel DI box that has transformer outputs and it sings like crazy if I wind up the mixer gain on a channel that happens to have no input connected.

Cleaning switch contacts is a good idea, and the pots too if they show evidence of noisy operation.