BPM CR 10 resistor shorted/ replacement

Help Support GroupDIY:

Studiogearlover

Well-known member
Joined
Mar 14, 2017
Messages
138
hey Guys, hope all is well with all of you today

I have a BPM CR 10 from 1998. It is / was really a sweet mic but it is not working sadly.  Dismantled it, no loose wires. No burning sign of any components. Electrolytic reads spot on, all seems to be alright besides the R1 resistor (please see attached)

It should be 154K but measures 5.8Ohms on my DMM. Set to test continuity and it seems that this resistor is off, there is continuity.

I do not have the exact value but I do have 150K resistor for the exact value. Will I be safe with this considering that I am replacing also the opposite R2 also with 150K?

Will this less 4k difference do anything or any damage to other components?


Just checked the diodes in circuit... D1 and D2 next to the resistors conducting DC both directions and also D3 next to the screw left side  in the middle on the pic, this is a slightly bigger diode also measuring voltages in both directions...were these failed because of the faulty resistor R1?

Please help me our if you can :)

Thanks dudes!

 

Attachments

  • PC020219.JPG
    PC020219.JPG
    429.8 KB · Views: 19

rogs

Well-known member
Joined
Jun 23, 2006
Messages
317
Location
UK
It's extremely rare for resistors to go short ...Have you removed the resistor from the PCB and confirmed that it is not something fitted across the resistor that you are measuring?

A schematic would be useful.....If you could post that, it could help a lot....
 

Khron

Well-known member
Joined
Apr 8, 2010
Messages
1,881
Location
Finland
I'm "smelling" a Schoeps circuit, and that 154K may well be between the base and collector of one of the output PNP's.

It might be worth measuring it in both directions, and see if you get the same "5.8 ohms".

When you say those diodes "conduct in both directions", how did you determine that?
 

Studiogearlover

Well-known member
Joined
Mar 14, 2017
Messages
138
rogs said:
It's extremely rare for resistors to go short ...Have you removed the resistor from the PCB and confirmed that it is not something fitted across the resistor that you are measuring?

A schematic would be useful.....If you could post that, it could help a lot....

Hey thanks for your help.

you are totally right.. removed the resistor and out of the circuit testes fine.

I am thinking of the diodes then... both small diodes next to R1 and R2 testing for continuity and with diode testing mode both direction display voltages..these diodes are so small that I barely can make it out whats written on it...maybe B3?

Sadly I do not have a schematic for this..all I know that these early BPMs are the simplified design of the U87.

I have not tested for voltages as I donno where to start  :-[

 

Studiogearlover

Well-known member
Joined
Mar 14, 2017
Messages
138
Khron said:
I'm "smelling" a Schoeps circuit, and that 154K may well be between the base and collector of one of the output PNP's.

It might be worth measuring it in both directions, and see if you get the same "5.8 ohms".

When you say those diodes "conduct in both directions", how did you determine that?

Hey thanks for the reply and help ! :)

I did remove both B3 diodes.. DMM set to continuity. Both directions there are continuity. Then set the DMM for diode testing mode. Both diode in both direction measures 0,004V .... not sure if this is alright tho...
 

Khron

Well-known member
Joined
Apr 8, 2010
Messages
1,881
Location
Finland
If those 154k resistors measure in the single-digit ohms in circuit, and especially since the diodes / zeners that were across the output PNP's are shorted, there's a good chance the PNPs themselves are toast as well.

There's no U87-ing in here, though. See attached schematic for reference.

Diode-test mode for diodes is exactly what that was meant to do. With the black probe on the cathode (band end) and the red on the anode, on a healthy diode, you should be seeing about 0.6V, and "OL" / out-of-range the other way around. Same goes for the zener diode in the middle of the board.

 

Attachments

  • Schoeps.jpg
    Schoeps.jpg
    234.1 KB · Views: 7

Studiogearlover

Well-known member
Joined
Mar 14, 2017
Messages
138
Khron said:
If those 154k resistors measure in the single-digit ohms in circuit, and especially since the diodes / zeners that were across the output PNP's are shorted, there's a good chance the PNPs themselves are toast as well.

There's no U87-ing in here, though. See attached schematic for reference.

Diode-test mode for diodes is exactly what that was meant to do. With the black probe on the cathode (band end) and the red on the anode, on a healthy diode, you should be seeing about 0.6V, and "OL" / out-of-range the other way around. Same goes for the zener diode in the middle of the board.

Hey thanks

yes that is exactly what I was doing using the DMM.. both diodes out of circuit in both directions measuring 0.004V in both directions on both diode.

Do you know by any chance or someone here on the forum what is this diode B34 stands for? I can not see this tyoe anywhere..

So the resistors are fine. The diodes seems to be off... also could be the PNPs too?

 

Khron

Well-known member
Joined
Apr 8, 2010
Messages
1,881
Location
Finland
Studiogearlover said:
Do you know by any chance or someone here on the forum what is this diode B34 stands for? I can not see this tyoe anywhere..

So the resistors are fine. The diodes seems to be off... also could be the PNPs too?

Khron said:
If those 154k resistors measure in the single-digit ohms in circuit, and especially since the diodes / zeners that were across the output PNP's are shorted, there's a good chance the PNPs themselves are toast as well.

There's no U87-ing in here, though. See attached schematic for reference.
 

Studiogearlover

Well-known member
Joined
Mar 14, 2017
Messages
138

Thanks I understand that you were referring to the Schoeps design here but looking at the components in the  mic and the schematics are differnt.

the two PNPs are 2N5401 and the Schoeps schematic you posted referring to BC416C or equivalent.

The 2N5401  has much higher voltage and current values therefore I am not sure that the 6.2V zener diode and its value is sufficient.

Here is an article for reference to the "simplified" U87 design.

https://www.vintagemicworld.com/html/Mic%20of%20the%20month%20September%202014.html

 

Khron

Well-known member
Joined
Apr 8, 2010
Messages
1,881
Location
Finland
I seriously don't mean to sound condescending or anything even remotely similar to that, but... How should i say this...

Perhaps that... Not everything in electronics works as you might picture it right now :)

Schoeps used BC416's because back in the day, from what was available, that's what suited their design.
If we're talking collector-to-emitter breakdown voltage, "too much" is not a bad thing.

Being capable of "more" isn't necessarily a bad thing. If that were the case, then you "couldn't" be able to drive, say, a 300bhp-engined car at 30mph / 50kph, as you could a 50bhp-engined car. See where i'm going with this? :)

Just because an electronic component has certain capability-specs, doesn't mean it can "only" be used at the upper limit. If anything, it just means that it'll be under that much less strain (and risk of failure - in theory, anyway) than the lower-spec'd part.

The zeners in anti-parallel with them are there precisely to limit (ie. clamp down on) that potential voltage across the collector and emitter. So there's no "sufficient" that those to need to be.

Regarding these circuits - if it walks like a duck, swims like a duck and quacks like a duck, is the colour of the plumage really that crucial? :p

And yeah, marketing blabber is just that - marketing blabber ;) The only "simplified U87" thing i can take from that is the cardioid-only versus U87's multipattern. I see no description of (or claims made about) the circuitry used inside the mic, so...

 

Studiogearlover

Well-known member
Joined
Mar 14, 2017
Messages
138
Khron said:
I seriously don't mean to sound condescending or anything even remotely similar to that, but... How should i say this...

Perhaps that... Not everything in electronics works as you might picture it right now :)

Schoeps used BC416's because back in the day, from what was available, that's what suited their design.
If we're talking collector-to-emitter breakdown voltage, "too much" is not a bad thing.

Being capable of "more" isn't necessarily a bad thing. If that were the case, then you "couldn't" be able to drive, say, a 300bhp-engined car at 30mph / 50kph, as you could a 50bhp-engined car. See where i'm going with this? :)

Just because an electronic component has certain capability-specs, doesn't mean it can "only" be used at the upper limit. If anything, it just means that it'll be under that much less strain (and risk of failure - in theory, anyway) than the lower-spec'd part.

The zeners in anti-parallel with them are there precisely to limit (ie. clamp down on) that potential voltage across the collector and emitter. So there's no "sufficient" that those to need to be.

Regarding these circuits - if it walks like a duck, swims like a duck and quacks like a duck, is the colour of the plumage really that crucial? :p

And yeah, marketing blabber is just that - marketing blabber ;) The only "simplified U87" thing i can take from that is the cardioid-only versus U87's multipattern. I see no description of (or claims made about) the circuitry used inside the mic, so...


aaaaalrighty ! :) advice taken ... I understand.

So can I replace these zeners with any 6.2V zeners and the PNPs with this which is equivalent to BC416c ?

https://www.st.com/resource/en/datasheet/cd00003229.pdf
 

Khron

Well-known member
Joined
Apr 8, 2010
Messages
1,881
Location
Finland
That was indeed my point :) And yup, that's the path to take. Just be careful about the various pinouts of the transistors - they're not all the same.
 

Latest posts

Top