Germanium transistors in early Sennheiser MKH microphones

[Gerard]

I have some old Sennheiser MKH 110 mics which are noisy, and one that is dead. The latter I loaned to someone who plugged it into a P48 supply and blew the transistors; the MKH 110 uses an 8 V power supply.

According to the circuit diagrams I can find, the MKH 110-1 used an AT 125 in the oscillator and an AC 125 transistor in the output stage. I can find AC 125 transistors at a price, indeed I still had a couple of AC 126 transistors from my first interest in electronics around 1970, when I was only 12 or 13 years old.

What is the difference between the AC 125 and AC 126? Looking in my ancient copy of Towers, I can't see any significant difference. Would AC 126 be a suitable replacement for AC 125 in an RF-biassed microphone?

I can find nothing online or in Towers about the AT 125. Does anyone know what it is and what might be a suitable alternative?

 

[Whoops]

What is the difference between the AC 125 and AC 126?

The same except AC126 has a bit higher HFE

Would AC 126 be a suitable replacement for AC 125 in an RF-biassed microphone?

Yes, it would be the same or marginally better

I can find nothing online or in Towers about the AT 125. Does anyone know what it is and what might be a suitable alternative?

I think AT125 doesn't exist, never seen any germanium transistor that starts with "AT"

I think the transistor is an AF125, as the schematic has lower resolution the F looks like a T, but if you look closely you can see the "F" there
So it should be an AF125, which makes much more sense.

 

[Gerard]

Thank you very much. Just spotted your reply.

I have some AC126 transistors, so OK there. Someone else has also suggested AF125 for the oscillator just based on the circuit topography and oscillator frequency; I wonder if I can still find AF125 transistors.

 

[Whoops]

I wonder if I can still find AF125 transistors.

I'm sure you can find them.

I can also check in my stock the next time I go to the workshop, I have a big stock of Germanium transistors.
If I forget please remind me by PM

 

[gyraf]

Look at the F at the 200pF - the transistor is certainly marked AT125

Could it be a internal name for a selected AF125?

/Jakob E.

 

[analogguru]

AF125 have a smaller case (like BC107) and 4pins .... so a picture could be helpful to unravel the mystery

 

[Gerard]

Well, I opened up a couple of MKH 110s and did not find a single AC 125 or AT/AF 125. Instead I found what might be a BC 5608 or BC 6608 (partially covered in red paint, so probably 'selected') and a something-something 5608 in one mic, and a BC 107 and a BC 214K in the other one. Does that make any sense to anyone? These are both MKH 110 mics, not MKH 110/1 or MKH 110/2.

 

[Khron]

BC560B, perhaps?

 

[Gerard]

Yes, I think you are correct; now I look again, the last character does look like a B. I wonder what the red paint signifies? Maybe selected for 8 MHz? Could a BC560B operate at that frequency? fT seems to be typically 250 MHz, so 8-10 MHz should not be a problem for it.

 

[Tubetec]

Bear in mind , these mics are old ,they may have been modified along the way with whatever transistors came to hand , might be part of the reason they sound poorly .
Chances are these mics went through several revisions as silicon technology advanced down through the years , Its hard to find people with the expertise that are even willing to service these MKH's
As you might have seen Ger , I've scored a pair of MKH mic bodies plus capsules minus interference tubes , they look like 415's 416's , I was thinking of a xlr barrel jack with a mini 12v cell inside rather than the bother of trying to use phantom or dedicated mains psu .

 

[Tubetec]

Ger ,
I said I'd post a link to the technical docs section just in case you hadnt found it .
https://groupdiy.com/threads/sennheiser.45159/
Seems to be details of several revisions of the MKH .

 

[Gerard]

Ger ,
I said I'd post a link to the technical docs section just in case you hadnt found it .
https://groupdiy.com/threads/sennheiser.45159/
Seems to be details of several revisions of the MKH .

Hi; Just seen this. Thank you.

 

[Gerard]

Well, through the kindness of John Willett, I now have the service manual extract for the MKH 110; the component list and circuit diagram. Theis confirms that the transistors are BC 560B. http://sound-link.co.uk/docs/MKH 110 Service Manual.pdf. This should greatly aid repairs.

 

[jokeramik]

The differences between germanium transistors and silicon transistors are very significant.
Best regards!