![]() If the 3080 pin 5 measures OK, there may be something else messed up with the bias circuit for the 3080. Ordinarily I'd expect the 100K in the emitter circuit of Q5 to limit current to the 3080, but I can see some situations that would feed too much current in there. If it doesn't measure like a single base-emitter when powered, the 3080 can't work. Internal to the chip there is effectively a transistor base-emitter between pin 5 and pin 4. If that's zero or more than about 0.7V, the 3080 is dead. Measure the voltage between pin 4 (-15V) and pin 5. If some situation fed current into the control or Q5, it could kill the 3080. This circuit seems to be relying on using PNP Q5 to limit that current and modulate it with a variable amount of voltage fed into the depth pot on Q5's base. I also have a ton of germanium PNPs, would they work?ĬA3080s are fragile and fussy about the current fed into their Iabc pin (modulation pin, pin 5). Is Q5 (KTA1266GR) shot? What may I replace it with ? Would a general purpose PNP like a 2N5087 work (different pinout though). Q5 collector / CA3080 input pin5 = 14,7V non oscillating Q5 base = 30mV non oscillating (same as diode D8 output) 650, which is fine I guess? - I know very little about solid state stuff) (note: these last two measurments led me to think D8 was shot but I switched off the amp and used my meter in continuity / diode testermode and read. Here's what I got, starting at the Speed pot. ![]() Is it normal for a transistor amp?Ģ/ I then took my measurements with a random circuit ground as a reference (a big solder spot next to the Reverb pot ground). I then realised the circuit ground was not chassis connected. I've taken voltage readings along the LFO path.ġ/ First, I took my measurmeents with my multimeter black wire connected to the chassis (as I always do with tube amps). Thanks very much for your help, I appreciate it. Please let me know if you need voltage measurements and at which points, and if anything in this circuit is more likely to fail.Īny help is greatly appreciated, thanks so much in advance! the problem would then lie around the Depth pot.īut What do I know really? This is just a guess. ![]() That leads me to think the oscillator part of the circuit does work (the IC3 NJM062D). One thing I can tell you as a hint: when the amp is on and I touch the Speed pot solder joints or traces next to it with my fingers, I can hear a ticking sound through the speaker, that goes slower / faster as the Speed pot is adjusted. ![]() I would like to repair it and not just swap components mindlessly hoping for the best. Rest of the amp is fine but probably distorts quicker than it should (not sure about that though). Adjusting the Depth or Speed pots produces zero effect. Schematic is below (trem ckt is at the bottom. I just got this Vox Pathfinder 15R and the tremolo does not work (tried a footswitch too - nothing). ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |