Studio Projects T3 circuit

[LevinGuitar]

I once found (by a chance) a broken wire inside the plastic cover of the transformer leads in a SP TB1, so just to exclude that case, measure resistance between your transformer leads:

Between Red and Yellow (should have around 1.35K) and between Blue and Green (should have around 40R).

Also you can measure resistance between pins 5 & 6 of your microphone 7pin connector: if you still have that 40R, the audio output path works.

After this, look for capsule connections and capsule membrane shape itself. Also check out all component and tube socket soldering

 

[Newton50]

Digital multimeter was used for measuring voltages. T3 power supply was turned on, cable connects the mic with the power supply.

 

[Khron]

Digital multimeter was used for measuring voltages. T3 power supply was turned on, cable connects the mic with the power supply.

So what sort of AC voltage were you expecting to see on the output transformer's secondary? Especially with (i'm assuming) a very slow-responding handheld multimeter?

 

[Newton50]

Good question. Wasn't sure what voltages to expect.

 

[Newton50]

I once found (by a chance) a broken wire inside the plastic cover of the transformer leads in a SP TB1, so just to exclude that case, measure resistance between your transformer leads:

Between Red and Yellow (should have around 1.35K) and between Blue and Green (should have around 40R).

Also you can measure resistance between pins 5 & 6 of your microphone 7pin connector: if you still have that 40R, the audio output path works.

After this, look for capsule connections and capsule membrane shape itself. Also check out all component and tube socket soldering

I appreciate the troubleshooting tips!

All of the resistance values mentioned, I measured and they're close to your specs.

Also I measured around 40R between the 2 XLR pins on the power supply box, so there is continuity in the audio path from the mic to the output.

I checked out the capsule and connections are solid and there didn't appear to be damage to the membrane shape.

Soldering looks solid and nothing looked loose after applying a little pressure on wire solder joints. I may just get my solder iron and carefully re-flow all solder joints.

This is a novice question, but should I see the tube "glow" at all when the mic is powered up? I see no "glow" when it's powered on.

Thanks again!

 

[LevinGuitar]

Of course it should, a little. Check out voltage of the pin 4 of the tube socket, it's where the long yellow wire goes. Should be close to 6.3v.
Also check for continuity from pin 5 and 9 of the tube to ground, it should be a short.

 

[Migs 31]

I appreciate the troubleshooting tips!

All of the resistance values mentioned, I measured and they're close to your specs.

Also I measured around 40R between the 2 XLR pins on the power supply box, so there is continuity in the audio path from the mic to the output.

I checked out the capsule and connections are solid and there didn't appear to be damage to the membrane shape.

Soldering looks solid and nothing looked loose after applying a little pressure on wire solder joints. I may just get my solder iron and carefully re-flow all solder joints.

This is a novice question, but should I see the tube "glow" at all when the mic is powered up? I see no "glow" when it's powered on.

Thanks again!

“This is a novice question, but should I see the tube "glow" at all when the mic is powered up? I see no "glow" when it's powered on.”

Answer : “Let your Soul glowwwww...”
Yes, the heaters should glow a faint orange. Check it in the dark. In daylight it can be hard to see sometimes. Check you have 6.3 V at the heaters.

M

 

[Newton50]

Of course it should, a little. Check out voltage of the pin 4 of the tube socket, it's where the long yellow wire goes. Should be close to 6.3v.
Also check for continuity from pin 5 and 9 of the tube to ground, it should be a short.

Nope, the 6.3v is not there, it measures 0v. Tube socket pin 4 measures a short the mic case/ground, but only when the T3 cable is hooked up to the power supply.

The yellow wire is connected to pin 2 on the T3 mic connector. And the short is occurring on the power supply, mic pin 2 to gnd.
The cable and mic alone aren't shorted pin 2 to gnd. Now I'm getting somewhere.

The yellow wire is connected to the power supply PCB, so I'm going to take it up for inspection.

Anyone have schematics for the power supply? Thanks so much!

 

[Newton50]

Update. Measuring a short across Electrolytic cap C10 (16V, 10,000uF) that's connected to the yellow wire.

Also, R8 measures shorted. The first two color bands are Red, Black, (2, 0) but it's hard to determine color of the 3rd band.
Does anyone know the value of R8?

Both parts measure shorted after being removed from the circuit board. Haven't measured a short across any of the other components.

Thanks again.

 

[Newton50]

Update. Measuring a short across Electrolytic cap C10 (16V, 10,000uF) that's connected to the yellow wire.

Also, R8 measures shorted. The first two color bands are Red, Black, (2, 0) but it's hard to determine color of the 3rd band.
Does anyone know the value of R8?

Both parts measure shorted after being removed from the circuit board. Haven't measured a short across any of the other components.

Thanks again.

Also, do you have supplier recommendations for ordering part replacements?

 

[Khron]

Tube socket pin 4 measures a short the mic case/ground, but only when the T3 cable is hooked up to the power supply.

What about pins 5/9 to ground?

Measuring a short across Electrolytic cap C10 (16V, 10,000uF) that's connected to the yellow wire.

Both parts measure shorted after being removed from the circuit board.

Capacitors usually have (at least) two terminals, so C10 is connected between the yellow wire and....?

10.000uF will take quiiiiiiiiiite a while to charge up from the (perhaps less than) 1mA that your meter can provide, on the resistance measurement range - it WILL appear as a short-circuit until that happens. You'll want to keep your meter probes connected to the capacitor terminals for a good couple dozen seconds and let the cap charge to its final voltage, before believing the meter reading you see. "It's how you use it..."

Regarding that possibly-2-ohm resistor... "Measure shorted" is really vague. What sort of resolution does your meter have on the lowest resistor range? 0.1 ohm, or lower? The "beeper" function on your meter is a very vague "go / no-go" thing, and it'll beep when the resistance measured is anything under 10-100 ohms - check your meter's specs.

Are C10 and R8 in the power supply?

 

[LevinGuitar]

Well, the big electrolytic can fail, and would leack to ground the heaters voltage. With that cap removed do you have 6.3v on the tube?
Of course you need a cap there but just for the test you can take a look for voltage presence without it.

 

[mjmajilton]

Update. Measuring a short across Electrolytic cap C10 (16V, 10,000uF) that's connected to the yellow wire.

Also, R8 measures shorted. The first two color bands are Red, Black, (2, 0) but it's hard to determine color of the 3rd band.
Does anyone know the value of R8?

Both parts measure shorted after being removed from the circuit board. Haven't measured a short across any of the other components.

Thanks again.

R8 is 20 ohms. Not sure how many watts.

 

[Newton50]

Thanks, I will do some further investigation.

 

[Newton50]

What about pins 5/9 to ground?




Capacitors usually have (at least) two terminals, so C10 is connected between the yellow wire and....?

10.000uF will take quiiiiiiiiiite a while to charge up from the (perhaps less than) 1mA that your meter can provide, on the resistance measurement range - it WILL appear as a short-circuit until that happens. You'll want to keep your meter probes connected to the capacitor terminals for a good couple dozen seconds and let the cap charge to its final voltage, before believing the meter reading you see. "It's how you use it..."

Regarding that possibly-2-ohm resistor... "Measure shorted" is really vague. What sort of resolution does your meter have on the lowest resistor range? 0.1 ohm, or lower? The "beeper" function on your meter is a very vague "go / no-go" thing, and it'll beep when the resistance measured is anything under 10-100 ohms - check your meter's specs.

Are C10 and R8 in the power supply?

Great questions. So the negative side of C10 has been pulled out of the circuit. That pad is shorted to gnd.

I borrowed a BK Precision LRC 4-wire meter today.

R8 resistance measures 1.97 ohms on "slow" and C10 has 4.15mF or 4150uF capacitance. swapping lead polarity made little difference in measurements. C10 only 1.8 ohms across it after 15 seconds.

 

[Newton50]

Well, the big electrolytic can fail, and would leack to ground the heaters voltage. With that cap removed do you have 6.3v on the tube?
Of course you need a cap there but just for the test you can take a look for voltage presence without it.

Will test this out...

 

[Newton50]

Good news, there's about 6.6v measured on the yellow wire with C10 pulled out of circuit. Off to find a suitable replacement.
I'll check with Mouser for 16V, 10,000uF. Hopefully they will sell to an end user, or consumer.

I'm also concerned about that R8 reading only 2 ohms, so I need to look for a 20 ohm replacement.

Thanks to all for the troubleshooting tips!

 

[LevinGuitar]

You can also buy it (or similar) at your local electronic repair store, saving shipping costs.
My R8 also measures 2.1R btw.

 

[Newton50]

Great, your R8 has the same color bands as mine.
Caps ordered from Mouser. I called around and couldn't find anything locally.

 

[Newton50]

Okay, a new cap in C10 was installed. The 6.3v is present and the mic is now working fine! Your help in pinpointing the problem is appreciated!