Weird voltages on a twin face Ge side

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kurtis1

Member
I built this twin fuzz face from pedal pcb. The silicone side works fine but on the Ge side I can't get less than 8.2v on Q2.


The voltages are wierd all over the Ge side of the circuit. I've tried a different set of ac128 transistors and I get the same results. The voltages after R1 should be 10x what they are. What would cause this? Is C1 somehow dumping voltage to ground? Is that possible? I've attached my voltage readings, a gut shot and a diagram from electrosmash dictating the correct voltages.

Thanks


https://drive.google.com/file/d/11HeCZtQq7UNQSgBajusg15dBdL9xz1_A/view?usp=drivesdk


https://drive.google.com/file/d/11JzXTKd427Umf7C3R2YzHwNdkFP4bMte/view?usp=drivesdk


https://drive.google.com/file/d/11HwKKZ5r85hbMWDtrI2pKyV40K4kqqJr/view?usp=drivesdk
 
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music6000 said:
After everything said & done, I think you have counterfeit AC128 Transistors, where were they purchased from?
If they ae genuine, you have an issue in the soldering of components on the PNP side.
Click to expand...

These are the transistors I purchased, I also purchased their medium gain and their low gain sets, all give me this same issue... Is it possible that they audited and packaged counterfeit ones without knowing?

Ive also attached a picture of the readings from my cheap tester I tried them in.

Ive also tested every connection by bypassing the pcb/solder joints and using a jumper wire as I followed the connections through the schematic.

Ive also tested most of the components that are next to each other for shorts and everything looks good but I'll continue the audit.

20210421_085246.jpg

drive.google.com drive.google.com

20210421_085152.jpg

drive.google.com drive.google.com

Do the readings indicate counterfeit or do you not know until you actually try them in a pedal?

Thank you again for your help, this is my 6th build, I've always been able to figure any issues out. This twin fuzz face build was suppose to be the most simple build I've done. It's now proving to be the most difficult.
 
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I believe they are the real deal, Can you confirm that 1 - E , 2 - B , 3 - C matches the pinouts on the PCB
I can't make out the DOT on the Transistors ,Is it aligned with the Collector?

Twin Face AC128 PNP Transistors hFE.jpg
 
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These type of Motorola ? AC128 TO-5 have been the topic of discussion since around 2014, Your Tester is showing them as PNP & quite a few members swear by them as being a good tool!
The tab is pointing to the Emmitter so that also appears to be correct.

Can you give me what Voltages are at the 3 Red Circles hilighted? :

Twin Face Voltage Test.jpg
 
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music6000 said:
These type of Motorola ? AC128 TO-5 have been the topic of discussion since around 2014, Your Tester is showing them as PNP & quite a few members swear by them as being a good tool!
The tab is pointing to the Emmitter so that also appears to be correct.

Can you give me what Voltages are at the 3 Red Circles hilighted? :

View attachment 11280
Click to expand...

Screenshot_20210425-082523_Chrome.jpg

drive.google.com drive.google.com

All the voltages are negative volts (for got to indicate that)

The trimmer is still off the board but I used a jumper wire to connect the 2 top leads.

Input voltage from my DC brick is 9.8v

Edit :
I'm not sure I'd they're Motorola. They do have an "M" logo on the top though.
 
Last edited:
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kurtis1 said:

Screenshot_20210425-082523_Chrome.jpg

drive.google.com drive.google.com

All the voltages are negative volts (for got to indicate that)

The trimmer is still off the board but I used a jumper wire to connect the 2 top leads.

Input voltage from my DC brick is 9.8v

Edit :
I'm not sure I'd they're Motorola. They do have an "M" logo on the top though.
Click to expand...

kurtis1 said:

Screenshot_20210425-082523_Chrome.jpg

drive.google.com drive.google.com

All the voltages are negative volts (for got to indicate that)

The trimmer is still off the board but I used a jumper wire to connect the 2 top leads.

Input voltage from my DC brick is 9.8v

Edit :
I'm not sure I'd they're Motorola. They do have an "M" logo on the top though.
Click to expand...
The Voltages should all be around - 0.92 Volts with your Power Supply.
They are all connected by one trace!
Are you sure it wasn't 7.5k on your external Trimmer pot to achieve 4.5v on PNP 2 Collector.

Is your DMM a Good one ?
 
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music6000 said:
The Voltages should all be around - 0.92 Volts with your Power Supply.
They are all connected by one trace!
Are you sure it wasn't 7.5k on your external Trimmer pot to achieve 4.5v on PNP 2 Collector.

Is your DMM a Good one ?
Click to expand...

Yeah, I noticed that on the wiring diagram that they're all connected. It looks like I'm getting the proper voltage to them with no trimmer pot and the trimmer pegs 1 and 3 shorted to each other... But the voltage at Q2 collector is way too high like this... If I install and dial back a 100k pot at the trimmer to +5v I don't get the -0.92v on those points you told me to test... They're super low, like -0.03v

I'm positive I was using a b100k pot... I tried a 25k pot at first and it couldn't get the Collector voltage down enough.

I'm almost positive that I read my meter right, it's a decent fluke 115 I believe. I also checked the trimmer I removed and it definitely had 10k accross it.
 
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Confirm that these Resistors have these matching Colours,
I don't know what else is causing your issue!

R1 - 33K = Orange, Orange, Black, Red, Brown

R2 -1K = Brown, Black, Black Brown, Brown

R3 - 470Rr = Yellow, Purple, Black Black, Brown

R4 - 100K = Brown, Black, Black, Orange, Brown
 
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music6000 said:
Confirm that these Resistors have these matching Colours,
I don't know what else is causing your issue!

R1 - 33K = Orange, Orange, Black, Red, Brown

R2 -1K = Brown, Black, Black Brown, Brown

R3 - 470Rr = Yellow, Purple, Black Black, Brown

R4 - 100K = Brown, Black, Black, Orange, Brown
Click to expand...

Yeah, just checked again. They're all the correct values. Here's some close photos.

20210425_200902.jpg

drive.google.com drive.google.com

20210425_201100.jpg

drive.google.com drive.google.com
 
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Okay, I pulled the Transistors out and traced board by following the wiring diagram. Everything has continuity to what it's supposed to. I checked every single solder joint against every single solder joint and have no shorts. The voltages are still all funky. It takes 22k at the trimmer to get the Collector down to - 5v after being on for 5 minutes it settles out to the trimmer needing about 72k to get - 5v at the Collector and holds at needing that.

Could my IC be bad? I've tried another one but I bought them in the same batch? They all put out the -9.5v

Would a safe, valuable test be to pull the IC, put +9v to the PNP circuit and try to see if NPN transistors work? Or will I burn something up here?

Man, I'm losing my mind here haha!
 
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music6000 said:
No, The IC converts it to Negative 9v which is what PNP Transistors need to operate.
That's why you can't daisy chain Germanium PNP Fuzz Pedals.
Click to expand...
I understand that, but I'm saying "convert" the PNP circuit to +9v so that I can then test NPN transistors in it. Or is there another reason why it wouldn't work? Like maybe the electrolytic caps won't work.
 
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Those two transistors are extremely leaky. 1st one has Iceo = 830μA and the 2nd one has Iceo = 1.2mA. I usually draw the line at 300μA. Personally, I would not install either one of those in a Fuzz Face. Can you measure the voltage across R4? Black meter lead on the end closest to the edge of the board, red meter lead on the end closest to the transistors.

You would have been better off buying Germanium transistors from Small Bear.
 
Last edited:
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It's all on one screen, you're seeing it in Kurtis1's pix.
HFE = DC gain, corrected for leakage
VBE = base-emitter voltage
IC = collector current for the HFE & VBE measurements
ICEO = collector current with the base lead open ("O" signifies open)
ICES = collector current with the base lead shorted to the emitter ("S" signifies shorted)

In a real circuit, the effective leakage will be somewhere between Iceo and Ices because some of the leakage current at the base is drained off by other parts of the circuit.
 
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Chuck D. Bones said:
Those two transistors are extremely leaky. 1st one has Iceo = 830μA and the 2nd one has Iceo = 2,200μA. I usually draw the line at 300μA. Personally, I would not install either one of those in a Fuzz Face. Can you measure the voltage across R4? Black meter lead on the end closest to the edge of the board, red meter lead on the end closest to the transistors.

You would have been better off buying Germanium transistors from Small Bear.
Click to expand...
Sorry for the late reply, it's my wife's birthday today so I've been occupied.

On R4 measured how you describe, I have -0.028v.

I gotta be honest and say that I never really understood what "leaky" is. All I know is that I've always heard it's bad.

I originally tried to order some pnp transistors from Small Bear but they've all been sold out. That's why I ordered from banzai music.

But if the transistors being bad is the issue then I'm incredibly relieved. I've been going insane here trying to figure this out! Thanks for your insight.
 
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