Peavey special 212 transtube problem

[streamz1]

Hello, I lost gain on my amp. I turn up post gain and pre gain pots and get loud noise. The more I turn them up the more noise. Tube emulation still works. It will come thru the noise. Any suggestions?

[streamz1]

I need to add that I have tried several guitars and cables , still the same problem.

[oc disorder]

Hi streamz1 and welcome to the forum.

As the pre gain and post gain level controls increase the noise when turned up
the offending "noise maker" must be before these controls.
If the "noise maker" was after the pre and post gain pots (eg power amp ,effects loop) the controls wouldn't make any difference to the noise.
From the diagram the lead pre gain (VR5) and the Ultra Pre gain (VR 10) take the signal
from the relay contacts K1B which is basically the switch that chooses between clean
and lead.
The only active parts that could be faulty before that point would be Q1 & Q2 which are
2SC1740 low noise ( supposed to be ! ) transistors (NPN 40v 100ma).
However these also supply the clean channel so the noise would have to exist in that
channel too although as there is not a dramatic gain boost after them in clean mode
it's possible the noise is not very noticable but still present.
As a starting point I would change those transistors and while there check the 3 diodes
near that point CR7 ,8 & 9 which is easily done with the diode test function in a multimeter .If the diode is working properly you should get a reading indicating a voltage drop of .5 to .7 of a volt in one direction and hardly anything (open circuit) in the other.
Need to check the meter manual for what sorta results to expect from the diode test function.
Can you hear similar noise but much quieter in the clean channel if you turn it up ?

[streamz1]

Peavey special 212 transtube problem

oc disorder

Yes, noise is present on clean channel also starts at about 5. Not as noisy.

Thanks for the help!

[streamz1]

OC,
I did a search for transistor 2SC1740 low noise NPN 40v 100ma. Can't seem to find any except on ebay. Listed as universal transistor. Would you have any suggestions on where I could order quality parts.

Thanks,
Streamz1

[Enzo]

Where did you look? First place I went was Mouser and they sell them for 40 cents. In stock at present.

You may note there are two part numbers of them in stock. the ones ending on R have slightly higher gain than the ones ending in Q, but both will act similar.

A very reasonable sub would be a 2SC1815, got them in your drawer?

[streamz1]

Nope, all I got are 2N5087, I will order.

Thanks
streamz1

[Enzo]

Got any 2N5088? 2N5087 is PNP.

Asian types, ones with 2SC numbers have the lead arrangement ECB left to right across the front. American types like the 2N5088 have an EBC pinout. Obviously you can't just stick the 2N in place of the 2SC. But if you stick the left leg of the 2N5088 in the left hole, the emitter will be right, then rotate the transistor sideways. If you are careful, you can then stick the center base leg in the end hole, and the right end collector leg in the center hole, and it ought to work fine. Follow me?


As a professional shop, I stock a large selection of both American and Asian transistor types. But if it made the difference between finishing a repair and having something sitting around for that one little part, I'd make that substitution in a heartbeat.

[streamz1]

Enzo,
Your awesome, you just taught me a very important lesson, myself being a amatuer things are making more sense. This is one of my personal amps and I did order the 2SC1740 transistors so I can wait a few days. Yes I do follow you, you are very good at directions. Would have substituted if I had not ordered. Do you recommend lifting one leg when checking diodes?

[Enzo]

Depends on where the diode is in what circuit and what I want to test for. I wanted someone to lift diodes in some other repair around here, but that was to disable the circuit they were in.

If a diode measures shorted, then I unsolder one end and retest, just to see if the diode itself is shorted or if something around the diode is making it seems so. On the other hand, if I am looking for shorted things and the diode doesn;t measure shorted, I may leave it alone. There is nothing in any circuit that can make a shorted diode look unshorted.

Maybe my diode test on my meter tells me the drop voltage is very low on some diode or there is reverse leakage. First I look at the schematic to see if there are parallel paths. A diode with a 33 ohm resistor in parallel will test low. And a diode in reverse across a transistor will have the transistor juntions in parallel. SO you might read the diode one way, and the reverse test is actually readig the transistor.

SO it is a matter of what you are looking at and what you want to know. Mostly i test them in circuit, and if they seem OK< they probably are.

[streamz1]

Well found the time to take amp apart and have problems..I replaced Q1 .. Q2 2SC1740 with 2SC1815YTE2FT . Now no power.Transformer gets warm, nothing on the knobs, no noise, no nothing. Transistors I replaced were very close to eack other. flat part of transistor facing each other. I assume the ECB is the same since both start with 2SC ... Not sure what I did wrong. any guess's

Thanks,
streamz

[streamz1]

I thought for awhile and it came to me, I forgot to connect main power plug connectors. Took it apart again. Well all is good. Noise way down. I want to thank everyone involved. You guys are awesome! I do have some knowledge with Guitar electronics, so drop me a line. Thanks, Again..Streamz1

[streamz1]

Well I played thru peavey for awhile and discoverd another noise problem if not related. New transistors helped, but this loud swooshing and screaming noise is coming from gain bottom button. As soon as I push it in volume goes way up and lots of noise. I can turn the pre-gain knob next to the button all the way down to minimize noise. Any thought's?

Thanks,
Streamz1

[streamz1]

Update: It is GAIN SWITCH (13)
Boosts the Gain of the Lead and Ultra channels, respectively. Depress to the “in” position to activate. Press it in and loud noise starts

[oc disorder]

well this is a little puzzling.
See my fuzzy attached picture.
The lead and ultra gain channels look much the same.
The inputs of both come from the same point.
The push gain button (13- S4B) on the ultra channel brings in Q17 & Q18
wheras on the lead channel its push gain (8- S2B) brings in Q9 & Q10.
As the noise is only on the ultra channel I would assume the noisy component has to be
in that channel.
I would have thought it was most likely Q17 and Q18 if it wasn't for this comment.

I can turn the pre-gain knob next to the button all the way down to minimize noise. Any thought's?

This made me think that the noise has to be before the gain control as you can turn it down.
But.... seeing the lead channel doesn't suffer from the same fate I can only assume
that the first two transistors Q13 and 14 are somehow unstable and turning down the gain control helps tame them.

With something like this I would probably get a bunch of transistors from peavey
and substitute - the cost involved being negligible compared to the time spent tracing
the fault.

Have you checked the board again incase you got a solder splash somewhere or
maybee a not so good ground in that area ?