Tayda Rangemaster build bias trouble

[scotheath]

what are the voltages on the other transistor pins?

cheers

With BC109 biased as low as can go at 8.75v. E=.31, B=.87, C=8.75v
                  biased as high as will go at 8.52v. E=0 , B=0, C=9.52v.
Again I'm not really sure what any of that tells me except data sheet says

what those voltages say it that there is not enough current entering the Base of the transistor. i would question the value of the  trim pot, those voltage would make absolute perfect sense if the trim pot was 50k instead of 100k, you might want to check that value. you could put a 56k in series with the existing trimmer, by bodging the board, or you could get yourself a new 100k trimmer.

cheers

Its absolutely a 50k trim pot from Taydas parts list,  UGH!!
I'm assuming by bodging you mean adding a 56k resister from base of transistor to what leg of trimmer pot. I just might have a 100k trimmer but would hate to screw up board pulling the 50 k trimmer

[PRR]

Either mis-built or an extremely unlucky combination of tolerances.

The suggested values "do" work, Ge or Si, for a wide range of typical small transistors.

Try changing R1 470k to 330k (or tack 1Meg in parallel with your 470k). Does it get better?

[idy]

genuis yes, leave the trimmer, pull one pin, put a 50 or Ks between the board and  the pin. But need to figure out which pin...

[scotheath]

Either mis-built or an extremely unlucky combination of tolerances.

The suggested values "do" work, Ge or Si, for a wide range of typical small transistors.

Try changing R1 470k to 330k (or tack 1Meg in parallel with your 470k). Does it get better?

I tacked a 1m resister in parallel with R1 (470k) yes got better, tacked on another 1m and then another and now can bias between 9.53 and 6.14v

[PRR]

Input impedance is getting very low. An alternative is to go back to the stock 470k resistor and make the BIAS pot 250k or 500k.

But these extreme hacks suggest a mis-build or an ill transistor.

[scotheath]

Input impedance is getting very low. An alternative is to go back to the stock 470k resistor and make the BIAS pot 250k or 500k.

But these extreme hacks suggest a mis-build or an ill transistor.

Thanks for the help.
I tested both resistors and all four caps before soldering into board. I did not test the 50k trim pot or the 10k log pot. I've tried 3 other BC109, 4 diffirent BC108 and then tried 4 different 2N3904 transistors, Not sure how it could be a mis build, but youre right, something is wrong.

[idy]

And remember, the doc says 50k trimmer but the schematic says 100k! and increasing trimmer that or adding a 50k series R will have the same biasing effect as your parallel Rs...but without lowering input impedance!

OR you could try a 100k trimmer or you could add a 50k series R to the trimmer...Its something you might try....

like a 100k trimmer or a series R. You don't seem to like that idea...

[scotheath]

And remember, the doc says 50k trimmer but the schematic says 100k! and increasing trimmer that or adding a 50k series R will have the same biasing effect as your parallel Rs...but without lowering input impedance!

OR you could try a 100k trimmer or you could add a 50k series R to the trimmer...Its something you might try....

like a 100k trimmer or a series R. You don't seem to like that idea...

Just didn't want to burn the board, been there!! I got the 50k trimmer out without issue. installed sockets and put in a 100k trimmer, Much better!! Tomorrow after work I'm gonna try a 250k I have in guitar parts bin, (with the 100k still can't get to zero, not sure if it matters but gonna give it a try). That should work the same ? trimmer pots are linear?
Thanks to all for the help. Now onto the Tonebender  gonna have to cross reference parts list to schematic for discrepancies. Maybe get a range of trimmer pots on next order. Thanks again

[iainpunk]

And remember, the doc says 50k trimmer but the schematic says 100k! and increasing trimmer that or adding a 50k series R will have the same biasing effect as your parallel Rs...but without lowering input impedance!

OR you could try a 100k trimmer or you could add a 50k series R to the trimmer...Its something you might try....

like a 100k trimmer or a series R. You don't seem to like that idea...

Just didn't want to burn the board, been there!! I got the 50k trimmer out without issue. installed sockets and put in a 100k trimmer, Much better!! Tomorrow after work I'm gonna try a 250k I have in guitar parts bin, (with the 100k still can't get to zero, not sure if it matters but gonna give it a try). That should work the same ? trimmer pots are linear?
Thanks to all for the help. Now onto the Tonebender  gonna have to cross reference parts list to schematic for discrepancies. Maybe get a range of trimmer pots on next order. Thanks again

i think having an accessible bias pot on a tone bender is a nice feature, to take you from broken distortion to smooth boost.
i have never come across a log trimmer, so i assume trimmers are linear.
a higher trimmer resistance means better input impedance. the exact trimmer resistance also works together with the input cap to determine the High Pass frequency, so changing the cap might be in order if you change to un-original values.

i also think that the 50k trim pot in the parts list is wrong, because the maths suggest that your problem is the exact outcome to expect with a 50k pot there.

cheers

[idy]

Hallelujah