Aion Rangemaster Biasing

[cnspedalbuilder]

Hello, I'm now coming back to my next problem circuit. This one is downright embarrassing--a Rangemaster, on an Aion PCB. Should be dead simple. It's a Japanese NOS PNP transistor from Small Bear specifically tested to be suitable in a Rangemaster, and the PCB has a charge pump to take standard center negative power supplies. I tested the transistor before install and it tested fine (don't have the values written down but it was in the Rangemaster zone). Schematic is on the Aion BOM but I've also pasted a link here:

I cannot get it to bias properly. I am supposed to get voltages at the test points somewhere from -6.8 to 7-7.1V. Currently, closest I can get is -7.77 volts at the test point, with the trim pot at the opposite end, it goes to -7.88V.

Here is what I have tried so far: I initially used the recommended resistor values suggested by small bear (Rb=47k, Re=4.3k) but that didn't work well. So I tried the values suggested in the Aion BOM, but still no luck. The trimpot that I put in didn't quite go up to 5k so I tried a 1.5k value in R3 to try to get more resistance. Aion had said that I might need to try 47k in R2, but that didn't work either. So TLDR, I have the stock Aion build, but with a 1.5k resistor in R3.

I'll post my PCB shortly below. Any ideas? I feel like this should be the easiest build in the world! 

[cnspedalbuilder]

Here are pictures. Although the transistor orientation looks a little weird when you look at the legs, I verified the C B and E on a component tester and lined them up with the C B and E pads on the PCB. So the orientation should be right.

[nickbungus]

Hi.  What does it sound like?  If it sounds like out should or if it sounds good to you then does it matter?

[antonis]

I cannot get it to bias properly. I am supposed to get voltages at the test points somewhere from -6.8 to 7-7.1V. Currently, closest I can get is -7.77 volts at the test point, with the trim pot at the opposite end, it goes to -7.88V.

Than shouldn't happen..
Try to replace R3 and BIAS trimpot with a resistor of an arbitrary value of 3k3 - 3k9 to see what you get..

[mozz]

Put the stock value on the emitter and bias with the base 68k resistor.

[duck_arse]

you haven't given us the base and emitter voltage readings yet.

[cnspedalbuilder]

Thanks everyone:
Base Voltage: ~ -136mV
Emitter Voltage: -86mV

Antonis and Mozz, I can try those steps, but is it unusual that it's not able to bias with the values in the build doc?

[tl-ww-ext]

Had this happen to me with a RM build on a BYOC board.  Low tech solution: I installed the battery clip & used a battery with ~8v.  Biased pretty close & sounds great.  Just a thought,

T

[cnspedalbuilder]

Thanks, ideally I'd like to put this on my pedalboard, so I'm hoping to avoid a battery.

Also, I forgot to mention this earlier but in my initial testing the circuit sounded terrible and actually cut the volume a little. That was before several iterations, but given that the bias is still not right, I'm pretty sure it won't sound good.

[duck_arse]

Thanks everyone:
Base Voltage: ~ -136mV
Emitter Voltage: -86mV

Antonis and Mozz, I can try those steps, but is it unusual that it's not able to bias with the values in the build doc?

if you sling a 470k resistor to supply, and a 6k8 resistor to ground, the midpoint will sit at about 0V13. if, however, you were wanting to fit a 470k and a 68k, your base voltage junction would sit at about 1V14 instead. check very carefully your colour bands.

[cnspedalbuilder]

Duck, I pulled the R1 and checked color codes on R1 and R2 and they check out to 470k and 68k. Continuity also checks out. One odd thing is that the resistances measure differently in the circuit than they do when the components are out.

I'm puzzled. It looks like the power supply is working properly, right? I wonder if the problem has to do with the transistor legs. I hooked them up according to my component tester, but the base leg is on the opposite side from what is indicated on the PCB.

Alternatively, is it just the case that you get these weird voltages sometimes depending on the transistor? Should I just socket a pair of resistor pads and try out different values to see what works? If so, which one is best to try?

Thanks everyone.

[cnspedalbuilder]

added info: re-verified the transistor and CB&E are in right spots according to component tester.
HfE=45
Uf=54mV

[Slowpoke101]

Some component testers have difficulties with germanium transistors and do not show the correct results. It is not uncommon for them to transpose the collector and emitter connections.
What is the type number (any markings ) of your transistor and can you see any markings that indicate the manufacturer ?

[duck_arse]

what's that you say? a japanese transistor from small bear? in that case, don't mess about. see the above pic, yours WILL match. in the pic you can see a little tab - this called the emitter tab. often missing from TO-1 cases, but replaced with a collector dot - a dot of paint up the side where collector resides. and the base will be at the point of the triangle. this rule holds well.

the plastic pack BC series transistors also follow this pattern [with suffix denoted exceptions], either triangle lead layout, or the flat of the body to the hypotenuse of the triangle.

[cnspedalbuilder]

What?? A component tester can get confused between C, B, & E?!?!? Interesting!

OK the transistor is labeled 2SB77. Duck Arse, it does not have a tab or a dot. This appears to be the datasheet:
http://www.datasheet39.com/PDF/824089/2SB77-datasheet.html

It says it has a "collector mark" but I'm not sure what to look for. There is an A (or isocelese triangle, not sure which) inside a circle on one side.

I'm not 100% sure but it appears that the C and E are switched relative to what I see on the datasheet.

[cnspedalbuilder]

addendum: On closer inspection, I think the orientation is correct.

[Kipper4]

it does not have a tab or a dot. This appears to be the datasheet

It says it has a "collector mark" but I'm not sure what to look for

The mark is on the underside of the rolled rim under the metal case as is seen in ducks picture.
Does it not have that?

[Slowpoke101]

One more silly question; What is the actual voltage being produced on pin 5 of the TC1044 ?

What voltage does your meter measure it at ?

[mozz]

Go back to stock values 68k and 3.9k. Set trimmer to zero ohms or jumper it.  Pick the base resistor, 68k, raise value to 75k or 82k, 100k, see what that does, lower value, see what that does.

Here is a Japanese transistor viewed from the bottom.

[Kipper4]

I see now it's a different package.
No painted dots or indents in the can?
Hmmmmm
What does the seller say?