Transistor question for Enzo

[stokes]

As I've said before,I know nothing about solid state circuits,I have avoided them like the plague in the past.So here I go,moving into the 1960's.I'm trouble shooting a solid state Echoplex,a Sireko to be precise.I did a voltage check and I found one of the TIS98's is reading 2.7 vdc on the emitter,that,according to the schem should show .6 vdc.The base and collector are closer to spec,base has 1.9,should be 1.2 and the collector shows 10.4 and should be 11.8 as per the schem.Does this sound like the problem is with the transistor itself,or should I check what appears on the schem to be the "cathode resistor/bypass cap"? I will have to pull the board to get to the resistor/cap,so I am asking before I start pulling boards.Of course I welcome comments from anybody,I addressed this to Enzo since he had encouraged me in the past when I made the "I dont know squat about solid state" comment,to see the similarities between tube and ss circuits,and was helpful when I had a question concerning the only other transistor amp I ever worked on.

[olddawg]

Can you post the schematic? What does this transistor do? Is it signal? Is it an oscillator? Is it switching or regulation? In general, any bipolar silicon transistor will be biased on when you have a .6-.7v voltage drop across the emmiter-base junction. You might want to check that. BTW, I find a lot of bad small electrolytic caps in old SS stuff line level more than bad transistors.

[Enzo]

Obviously the tube/transistor parallels are limited. But here they can be instructive. Think of that transistor now as a triode. The "cathode" (emitter) voltage is high and the "plate" voltage is dragged down. (collector) The transistor appears to be conducting too hard. Either it is leaky, or it is being turned on too hard at its base.

Of course it could just be an out of tolerance circuit.

SWap the transistor for another of the same type. Does the unit use more than one? Swap them. Or the transistor has its base overbiased. check all the resistors associated with that base. And check for a leaky coupling cap coming into that grid. Just as you would a red plating power tube.

Can you link a schematic? I don;t have the energy to look for it here at the moment - maybe tomorrow... If the transistor is part of an oscillator, there may be a nearby trimmer

[Enzo]

And old dawg is right about the caps. And I would say in my own experience, that the small lytics used as couplers are the worst offenders. 1uf 4.7uf, 2.2uf, etc.

[jrfrond]

To me, the caps are the primary offender, especially in solid-state gear. I find most often that they will use small 16V coupling/decoupling caps in circuits that are using a 15V supply (Hey Enzo, ever work on old Korg synths?). If you superimpose signal voltage swing on these puppies, the voltage rating is exceeded on a consistent basis, compromising the lifespan of the caps. We can now get these same size caps in a 25V or even 35V rating, which is MUCH better and more reliable. Bad transistor? Unlikely. But a leaky cap will feed DC to parts of a SS circuit where it doesn't belong, and because the operating voltage is so low, even a few mV can wreak havoc.

[stokes]

Heres a link to the schem http://schematicheaven.com/effects/sireko_se1.pdf I am talking about Q4.The 2.2k resistor from emitter to base measures okay at 2.2k.The .01 cap I am refering to is the one connecting the base to the 22k resistor on the wiper of the 500k sustain pot,that pot is not working at all,it is just above Q4 next to the circled number 3 .Someone added another .01 in series with this one,I am going to remove one of them.It is a bit of a PITA trying to trace things in this unit,I will have to rig some kind of cradle to hoild it so the components are accessible while I do so.Thanks for the replies so far,guys.