Telefunken M60 pair

[mikeyB]

Hi All,
Been asked by a client to repair a pair of Telefu*k - en M60s.
Very erratic behaviour - intermittent ticking etc.
Absolutely no info on web and no other problem mics that I can find????
Build quality is good, capsule look outstanding! The only thing I thought wrong was very little solder used on joints.
Had a go at soldering half a dozen joints or so, reassembled and bingo!! now works. Proceed to secure with 3 tiny retainer screws, try again, BAH faulty again.
There is about 10 resistors, a polysty cap from the capsule and a few elec caps. Transformer output. The only active device is a dual fet LSK389. Is it likely the intermittency of the pcb is spiking the FET to death? The mic has a faint distorted output when subject to a high spl but nothing otherwise.
To me that sounds like biasing is way off (probably if the biasing is intermittent).
I'll try and trace out the cct, resolder all the joints and try again.
(I am using antistatic precautions while testing)
It is difficult to bench test as the outer body holds the capsule in place.

The mics were sent to Telefunken for repair just out of warranty, worked for approx 2 month then faulty again, sent back again but they insisted they were working fine!!! and returned to my client, again, not working!!

Any advice greatly appreciated.

 

[k brown]

Intermittent clicks can sometimes be sign of trapped moisture in capsules. This can make the noise infuriatingly intermittent. Did Tele replace them?

 

[Gus]

If you remove the capsules and with the PCB inside the body do you still get ticks?

The first post tells us a few things
It most likely does not have a DC to DC converter having only one active device reported in the first post so no switching noise from a DC to DC.
there was a thread sometime ago. I wonder if the M60 circuit is close to the same
https://groupdiy.com/threads/tf11.76082/page-3Myself and others started to trace a TF11 from pictures on the web

Check the voltages to ground for the
Drain
"top" of the drain resistor, Is the drain resistor a 15k?
Source

 

[mikeyB]

Cheers for the replies. Tried with Omni capsules which never get used. All capsules look very clean and free of moisture. If it is loud clicks caused by bad solder joints, can this damage the capsule. I'd imagine the fet getting hammered first??

 

[Delta Sigma]

I had a Beesneez Lulu with a similar problem. If I removed the set screws, it seemed to work properly. There was a poor connection between XLR and PCB. Pretty sure I just removed all solder and then resoldered everything.

I'm guessing the 48V was being shorted or opened.

 

[Gus]

Found this with a search, some pictures of inside.

http://recordinghacks.com/microphones/Telefunken-USA/M60-FEThttps://www.bonedo.de/artikel/einzelansicht/telefunken-elektroakustik-m60-fet-test.html

 

[mikeyB]

If you remove the capsules and with the PCB inside the body do you still get ticks?

The first post tells us a few things
It most likely does not have a DC to DC converter having only one active device reported in the first post so no switching noise from a DC to DC.
there was a thread sometime ago. I wonder if the M60 circuit is close to the same
https://groupdiy.com/threads/tf11.76082/page-3Myself and others started to trace a TF11 from pictures on the web

Check the voltages to ground for the
Drain
"top" of the drain resistor, Is the drain resistor a 15k?
Source

Happy (belated) New Year, had to have an enforced break but back on it now!!
Hi Gus, yes Drain feed res is indeed 15K.

I went through the stages of:
Given the mics. One working (for reference), one dead.
Bit of soldering, both working (for a while!)
Then repaired one stopped working, then the "good" one stopped working!!??!!??
Couldn't fathom it out, so traced out the circuit (suspecting the FETs failing).[See attached]

Enforced break over New Year, now back on it.

Both mics working - Yippee!! - hold on.........they have both failed again!!
What could it be????

They started out healthy output at power up, then got noisy after a while, then went really low distorted output if switching +48V off and on!!

Left overnight and tried again today, same thing!!!

Then I spotted something!! C3 is a 10n ceramic cap (blue body tells me it could be a Z5U type but not definite). Then I remembered! My first job as a design tech was to change out a shed load of Z5U decouplers on circuit boards, they were failing short cct!
About 5 years ago, my scope only scanned on right half of screen, dicky timebase but couldn't find the fault.
On further switching on the scope, I had a whiff of smoke! Opened it up, found the charred resistor and the only possible way it could have smoked is that the connected cap went short cct. Sure enough it was the cap shorted which then pointed me to another cap in the timebase cct. Both caps (and nuked res) replaced and hey presto! scope is now all working correctly!!

Can anybody assure me this could be the case with the mics?? The cap isn't shorted powered off, but is it failing over time??
On many pieces of gear I've repaired from 80s/90s, I've seen the same problem with tantalums going short on power rails, (lexicon/eventide gear especially).
Has anybody seen these problems with shorted caps!

Gus: If the cct is similar to what you describe, what voltage should I be seeing at the bias points?

I've got some 10n X7Rs on order to replace the C3's.

Will keep you posted.

Any other pointers much appreciated!!

Edit! - Forgot to add, when the mic fails, on switching +48V OFF, the mic briefly works while the +48V fades away. Must be crossing correct bias point over the fade???

2nd Edit!! Just noticed cct doesn't show it, but XLR pin 1 is indeed connected to 0V (and not floating/No Connection)

 

[Khron]

A couple years back, a friend's Rode NT5 has gone a lot quieter than normal. Turned out the capsule bias voltage had dropped way below nominal. On a hunch, i replaced either the, or one of the two, 0603-sized ceramic caps filtering the output of the DC-DC converter. Bias voltage came back up to 50-60v or whatever, and the mic was again nice and loud.

I'd say you've got absolutely nothing to lose by replacing that 10n cap - even a film one would do, assuming it physically fits in there.

Also, for "dry" testing, you can solder in a ~50pF capacitor instead of the capsule, if it eases probing for voltages. Then, you could disconnect the end that goes to ground, and feed a very low level test signal, thus being able to make sure the circuit itself is fine.

 

[mikeyB]

Khron, great tip for "dry" testing, much appreciated.

 

[Gus]

Have measured the drain and source voltages?
I think the drain should be around 12.3VDC and the source about 7.8VDC
"top" of the 15k drain should be about 38VDC
if my TF11 trace from pictures and the sim is correct.

 

[mikeyB]

Hi Gus, thanks for the reply, when you ask about measured voltages, do i need to hookup a 50pf cap in place of the capsule as suggested by khron in the previous post, or just test without capsule?
Cheers in advance.

 

[Khron]

Those are DC voltages, so without the capsule should be fine.

 

[Gus]

mikeyB did you get a chance to measure the voltages?

The screenshot is a trace from pictures on the web(I had some help) it looks close to the trace that was posted before in this thread
Curious if the sim voltages are close to the real circuit.
The transformer in the screenshot is made up I would guess >=5:1

 

[Tim Campbell]

This may sound foolish but since the mic works fine without the screws in place but has problems when they are inserted did you make sure that the screws are not causing a mechanical short or causing the body to touch a trace?

 

[Tim Campbell]

This may sound foolish but since the mic works fine without the screws in place but has problems when they are inserted did you make sure that the screws are not causing a mechanical short or causing the body to touch a trace?

?

 

[Gus]

any updates

 

[mikeyB]

Not yet Gus, hopefully back on them this week

 

[mikeyB]

Have measured the drain and source voltages?
I think the drain should be around 12.3VDC and the source about 7.8VDC
"top" of the 15k drain should be about 38VDC
if my TF11 trace from pictures and the sim is correct.

 

[mikeyB]

Hi Gus, attached annotated PDF with the voltages and a few other points, Phantom from mixer only 42V. These are when the mic works, it's on soak, when it craps out, I will remeasure the voltages and post.

 

[mikeyB]

Durr!!! I don't have to wait for the good mic to crap out....I got the other broken one.
New schematic now showing suggested voltages Gus, working voltages and non working voltages.
So, changed the 10n cap C3 for another X7R - BINGO!! working again.....but only for about 10 mins then fault mode again. Would the voltage difference between the Fet's Drain and Source (0.04V) indicate the FET is going short cct? and if so, why does C3 die as a consequence? Will try yet another 10n cap, some 100V polyester caps on order.

Any thoughts, Gus/anyone???

Thanks in advance!!