Sony c37a power supply issues

[Woot]

Hey All,

Just wondering if someone could please help a n00b [me] to troubleshoot a bad Sony c37a power supply.

So I was lucky enough to acquire a pair of Sony c37a's, which I was told were in completely working and original condition with only one of the tubes being replaced at some time.

When I received the mics, one of the power supplies would turn on (power lamp on) but was putting out a very very low, distorted signal (using the same mic that sounds great through the other supply).

The supplies are the solid state CP-3B variant.

I have read some other threads here on these mics and supplies but I am still a little stuck. I am also new to the concept of a floating heater voltage so many of the readings don't make much sense to me.

The first thing to note on the power transformers is that the B+ secondary has a used 240V tap (pin 7) and also an unused tap at 220v (pin 6). On the power supply that is not working it looks like the 240V winding is open (whereas I am reading 2k resistance across pins 5 and 7 on the good one).

As a remedy, someone has just shifted the diode down to the 220v pin 6. The rest of the circuit seems to be original and identical to the working power supply.

Needless to say, the voltage readings that I'm getting on the B+ for the broken supply are less than those from the working supply (but only slightly).

The reading across both supplies H+ is ~17.5 VDC (again this I don't understand because I thought the maximum heater voltage should be around 6.3V).

Is this going to be the cause of my problems? Or would you expect the mic to still work, just with a slightly lower output?

Any help or advice greatly appreciated!

Regards,

Matt

 

[gridcurrent]

The supplies are the solid state CP-3B variant.
I am also new to the concept of a floating heater voltage so many of the readings don't make much sense to me.
The reading across both supplies H+ is ~17.5 VDC (again this I don't understand because I thought the maximum heater voltage should be around 6.3V).

1.  measure the heater voltage across pins 1 and 5.
2.  the heater circuit is elevated via the 50k and 100k resistors.
3.  B+ voltage at pin 4 is typically +270 VDC. 
4.  if you feel ambitious and build new supplies, a 5:1 ratio output transformer performs well.

 

[Woot]

Hi Grdcurrent,

Thanks for the reply.

Heater voltage across 1 and 5 (no mic plugged in) good supply= 17.5VDC; bad supply = 17.3 VDC
B+ voltage across 2 and 4 good supply is 309VDC; bad supply is 260VDC

Thanks for your time.

Cheers,

Matt 

 

[radardoug]

You need to connect the mike to the supply to verify voltages. Measure voltages at the microphone by removing the lower bell.
Dont do this with power on!
Heater voltage should be 6 - 6.5 volts across the filament. Dont try and measure to ground. The reason the filament is elevated is that the mike operates a cathode follower circuit, and the cathode is at about  half the plate voltage. The filament to cathode voltage rating for the tube is limited, and so the filament needs to be near the same voltage as the cathode.
Using the 220 volt tap will give a bit higher plate voltage, and also filament voltage.

 

[Woot]

Hi Doug,

Thanks for the reply!

I was actually mistaken, the pin 7 secondary is 240V, and the pin 6 secondary tap is 200V (see attached).

The measurements are as below. Same mic, after 10 mins warm up on each supply. A is "good"; B is "bad".

                A                          B
H      6.65 VDC          6.65 VDC
EP    248.4 VDC      211.7 VDC
EK  156.6 VDC        131.8 VDC

Cheers,

Matt

 

[mjrippe]

Voltages are not TOO far off, perhaps the issue is elsewhere?  Check the 6uf cap and both the low cut and output impedance switches (exercise/clean them).

 

[radardoug]

Have you tried swapping tubes? Based on these voltages the voltage is not a problem. Otherwise its capsule.

 

[Woot]

Thanks for both of your replys.

mj - I measured the 6uF caps in circuit on both supplies and they both read 6uF.

Interestingly I can get a reading of ~120nF across the smaller cap in this section on the good supply (in circuit) but no reading on  this cap in the bad supply.

Would this cap going bad cause this problem?

Doug - I was using the same mic for both the supplies in the test... Are you saying that the mic may not like operating at the lower voltages for some reason?

Cheers,

Matt

 

[Gus]

If I understand the first post you tried the two microphones on both supplies and the problem stays with the power supply part.
Sometimes with a problem like this checking all  the parts is a good way to start

Something to note is parts of the microphone circuit are in the supply box.
The voltages posted don't look like they would cause the distortion problem if they were taken with the microphone attached to the power supply
Can you check the resistance of the 220V B+ winding on both transformers to make sure the 220VAC wind is good in the bad one
If the 220VAC to 240VAC part of the winding is open you need to think about how that could have happened.
I would check every resistor and cap in the power supply.  Resistors for value and caps for value and leakage

Check every thing from the 6uf to the output.
Check the switch check, dirty contacts can cause issues.  Operate the switches many times if they do not measure good with a meter. I don't like to use contact cleaner unless I have to
Check the resistors for value and the caps for value and leakage
Check the output transformer and inductor if you have the tools for this

 

[Woot]

Thanks for the reply Gus.

If the advice is that the lower voltages on the bad supply are not likely to be contributing to the issue, then it has to be  problem in the "mic" part of the power supply circuit, the microphone itself being tested as fine because it works well on the other supply...

Additionally if I'm reading the schematic right, the choke shouldn't be contributing to the circuit in "M" mode, and if it was a problem with the choke, then the problem would only be present with the choke in circuit....

I have tested the windings of the output trafo's in both supplies and they test identically.

So that leaves the few resistors and the couple of caps...

I'm going to change out the 0.01uF cap as this is the only component that is not measuring the same as the other supply....

And give the switches a good working....

Let's see how we go...!

Cheers