Gefell mic repair..

[Svart]

Ok so a while back we had a crazy storm around the city and lightning struck around the studio.

Needless to say I had some lightning damage to various devices but what really puts a hurt on this is that one of my Gefell m930s took a hit as well and now has a low output.

I hadn't used it since the lightning and didn't have time to troubleshoot it but over the weekend I took a look.

I'll have to describe this circuit for you as it's not very conventional.

The phantom comes in and is split, one direction goes through diodes to the photovoltaic system that generates the HV bias, the other direction goes through resistors and to the amp board. the amp board is a tiny ceramic subtrate daughterboard that is packed with sot-23 parts.. ALL OF WHICH HAVE THEIR SMD CODES SCRAPED OFF!!

uggh talk about adding insult to injury.

Ok so I pulled the amp board off the main PCB. the HV bias now comes back on. It seems that the amp board is pulling the summed phantom power down to something like 3 volts instead of around 30v(compared to a working m930).

I sat down with a diode tester and compared every part to a working amp board from another m930.

I found one part that seems to be faulty. the diode testing shows it's a NPN. Ohming the good part shows numbers in the high Megohms, the bad part shows mid kiloohms.. obviously bad.

I was able to put the part under a microscope and through a miracle was able to make out the first number, 2 and what looks to be the bottom of a G and a W.

2GW=bc850

so far so good.

the main board has some sot-23 parts for the HV section and those parts are not scraped, a 4GW is present which is a BC860.

So I assume that it's safe to guess that most parts on this board are BC8XX.

I suppose I will get a BC850 and swap the part out and see what happens.

Worst case I send this back to Gefell and pay through the nose for repair.

any opinions?

 

[pucho812]

sounds like you doing a good job fault finding....

 

[Svart]

well it's a lot of guessing right now. I emailed Gefell this morning but they haven't gotten back to me yet about purchasing a new amp board. I doubt they will just sell me the little board, I'll probably have to send the thing in for service unless the BC850C is the culprit.

I'm going to have to place an order for bc850C.. 1.25$ for 10 parts, 10$ in shipping and handling.. :roll:

 

[[silent:arts]]

[quote author="Svart"]I emailed Gefell this morning but they haven't gotten back to me yet ...[/quote]
oh, Gefell and Internet - this can take a few days.
but good work finding faults

 

[pucho812]

when my buddy's PSU went up, A trasistor had gone out from a power spike so it doesn't suprise me that that a transistor could be at fault here.

 

[gyraf]

Try contacting Gefell by phone - they are really nice people to deal with. And get access to a FAX-machine - they'll most often be able to fax you the schematics in less than a day.

And their repair service is decently-priced.. ..nice company indeed..

Jakob E.

 

[Svart]

ok for those following this, I replaced the part in question with a BC850, as planned, and the mic is now back to working perfectly.

I had already tracked overheads with mc012s in the meantime, they sounded good, but not as good as the gefells but that is another story.

I put the mic up next to the other of the pair and did an A/B comparison.

I could detect no difference in level, level handling, coloration or noise.

As of now I am fully convinced that the part was indeed a BC850.

That's all kids, it's fixed and I used them for vocals/guitars the other day with no problems.



:thumb:

 

[[silent:arts]]

:thumb:

 

[blacier]

I have screen dents on my Gefell m930, that I want to remove. Does anyone have tips on how to take it apart? Thanks!

 

[Whoops]

I have screen dents on my Gefell m930, that I want to remove. Does anyone have tips on how to take it apart? Thanks!

Try to contact Gefell,
I'm sure it's not that hard.

or post in Klaus Heyne's Mic Lab  at prosoundweb forums, a lot of useful knowledge and info on mics there:

http://repforums.prosoundweb.com/index.php?board=27.0

 

[jonasmeurer]

Hello! I have a m930 with the same issue, i know its been a few years but do you remember witch one of the transistors you replaced? looked at it under a microscope but couldn't make out any writing on them, and sadly also don't have another m930 to compare to.