Troubleshooting OD850 hummingbird C1518 transistors

[press_record]

Hi i'm troubleshooting an OD850 hummingbird with 1x k30a input transistor and 4 C1518 transistors.
there are no schematics that i found, almost a big muff

This is the problem: Whenever there is a heavy strum the volume drops, but only with the gain high or on max. Much like an over the top, exagerated sag.
The gainknob is after the first transistor so the first transistor (k30a) could not be the culprit.

This is what i tried so far:
1. Although the supply voltage showed to be steady on the multimeter, I changed the elco since they are old and it would make sence if they were the culprit.
2. I changed the Ceramic caps so that the diodes wouldn't do anything funny while clipping. Measured them afterwards and sure enough they were bad also.
3. The problem persisted and i thought it might be the transistors:( so i put in some transistorfeet and tried other silicon NPN transistors Bc184, sn3904 and ksp2222a.
But all made the pedal have a much lower gain, less then unity even! I have no clue why, some even had Hfes of 600 and more??
I looked it up, but there's no direct replacement still available for the c1518

Please help!
Kind regards,
Zas

[Mark Hammer]

Um, those are 2SC1815 transistors, not 1518.  Just about any medium gain NPN will substitute.   But you should pay attention to the pinout.  The 1815 is ECB, while replacements like the 2N3904 or 2N5088 are EBC.

[press_record]

I took great care in putting them in the right way, i searched the pinout and made sure the pins were in correctly. still no luck.

[Mark Hammer]

Apart from the difference in transistors....is this your pedal?

[duck_arse]

damn your speedy fingers, M hammer!

[bluebunny]

damn your speedy fingers, M hammer!

"Can't touch this..."                                                                [ Coat on already... ]

[press_record]

Apart from the difference in transistors....is this your pedal?

Hi mark!

Yes that's about it! there's one more transistor, for the output stage, but the basic layout is very similar!

[R.G.]

It's clearly a bias problem; most likely not a transistor problem.

Very little of any use can be done without a schematic, so I suggest you start with the likely and trace it until you verify it is or isn't one of those and note any differences.

Then read and follow "Debugging: what to do when it doesn't work".

Anything else is like saying "Doc, my belly hurts. It's a kind of an achey pain. And I have this rash. What do you think it is?" by email.

[duck_arse]

your complete circuit may well be this, then:



or mentioned here:

http://www.kitrae.net/music/Images_Secret_Music_Page/Maxon%20D&S%20first%20edition_SUSTAIN%20T_no%20LED.jpg