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Thread: PV Session 400

  1. #1
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    PV Session 400

    Howdy Gang,

    Long time lurker, first time poster. I have an issue I need help with.

    Amp is a 1974 Peavey Session 400 (the original, not 400 Limited from the 90's)

    The amp has a distorted output. I have checked and have no DC on the speaker out. Preamp signal fine.
    I have good rail voltage coming from transformer to board. I have followed +/- 55v to try and find the problem.

    -55 is good all the way to the outputs. Somewhere I am loosing +55 before it gets to output transistors. No voltage on 1/2 of the 5 watt resistors. The last place I have +55v is on the read wire leaving the board going to the first output transistor.
    I diode checked output transistors per Enzos instructions on several other threads. The look good to me.

    I suspect the - 55 side is the only side producing output.

    If anyone has any ideas, I would appreciate it.

    Thanks.
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  2. #2
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    Quote Originally Posted by justin25 View Post
    -55 is good all the way to the outputs. Somewhere I am loosing +55 before it gets to output transistors. No voltage on 1/2 of the 5 watt resistors. The last place I have +55v is on the read wire leaving the board going to the first output transistor.
    Welcome to the place.

    If the +55 voltage wasn't there, you would have dc on the output. I think you are looking for the +55 voltage at the wrong place, it will be on the cases of the output transistors and not at the ballast resistors.

    Did you diode check the other transistors and the diodes on the board as well?

  3. #3
    Old Timer J M Fahey's Avatar
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    Agree and add: +50/55V would be in the cases of the top 3 power transistors and not on the top 3 ballast resistors.
    The reverse is true for the bottom 3 transistors and ballast resistors..
    52 Bill likes this.
    Juan Manuel Fahey

  4. #4
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    Thank you both very much. After looking at schematic, I see where I was measuring voltage in the wrong place.
    Let me do this:
    Re-install output transistors
    check for +55 on the cases of the top 3 transistors.

    I did not check the other transistors on the board. Diodes checked out ok in circuit. (i did not remove one leg)

    I will report back what I find after measuring +55 in the right place.

    Thanks again.
    Justin

  5. #5
    Supporting Member Jazz P Bass's Avatar
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    Check the voltage on the base of the output transistors.
    They should barely be on (ie: .600 volts)
    Also, when you are on the + - 55 volt rails, flip the meter to read volts ac.
    That will give you an indication of the health of the main capacitors. (ripple wise)

  6. #6
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    Update.
    Installed output transistors after I tested them.
    Amp immediately blew fuse. looking at board layout I posted, the 3 right TO-3's were a little bit above room temperature. I did put the insulating washers between transistors and back of amp.


    About the time I think I understand these things something like this happens. I am sure I have made a mistake as the amp would power up, the output was plenty loud, just distorted.
    It may be best at this point to forget what I think I know and just take step by step instructions from you guys. Deal?

    One more thing of interest, p/s caps and all coupling caps were replaced by Peavey about 8 months ago. Amp has been working fine 3-5 nights a week.

    Thank you all.

    Justin

  7. #7
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    Update #2:
    I found the issue I caused. One of the insulators under the output transistor was torn, causing a short.

    I am back to square 1.

    The preamp signal is great
    The power amp output is distorted. I do not know why.
    I diode checked all 6 TO-3 outputs. They tested ok. I also replaced the one I blew by shorting it out along with it's 10w resistor which went open when the TO-3 frosted.

    I have checked all diodes on board in circuit.
    I have checked output transistors.
    I have both +/- 50v.

    Any ideas where to look next?

    Thanks again.
    Justin

  8. #8
    Supporting Member Jazz P Bass's Avatar
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    Post #5?
    "Check the voltage on the base of the output transistors.
    They should barely be on (ie: .600 volts)
    Also, when you are on the + - 55 volt rails, flip the meter to read volts ac.
    That will give you an indication of the health of the main capacitors. (ripple wise) "
    You have to look at the amp for what it is.
    What is it?
    It's a Class B output section.
    Meaning?
    Meaning the output transistors should barely be on.
    Working back, the drivers should also barely be on.
    Start taking some basic voltage measurements now that the amp is holding steady.

  9. #9
    Senior Member Enzo's Avatar
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    Agree^^^

    The outputs should have just around a half volt B to E. Note the six power transistors, four are output, but two are wired as drivers. I'd look for that half a volt across the 1 ohm resistors. Those drivers also want about a halof a volt from B to E on them. But remember that the E for the driver is already at half a volt above the output xstr E because it is connected to the base of that xstr. (xstr = transistor. Easier to write) The base of the driver has its own 22 ohm resistor down to output E, so just measure across the 22 ohmers, and expect to find about a volt (two half-volts).

    The 430 and 431 before those power xstrs are your predrivers. Note the negative side one is "upside down" because this amp is quasi-complementary. Moving further left are small xstrs 3642 and 3638. Those are limiters. Next to each isw a diode. Lift those two diodes to remove the limiters from the circuit. The amp will run without them for now, and if that clears the distortion, then the limiters are bad, or one is anyway.

    Left of those is a dual diode in the dashed line box and another diode. Those are in series between bases of the predrivers. The dual diode will be clamped to the heat sink (rear panel) near the output xstrs. Measure voltage drop across that dual diode. I expect it to be about a volt, maybe 0.8v to 1.1v or something. If it is shorted, the amp will work, but increased crossover distortion. If it were open, you'd blow fuses. And the diode with the resistor in parallel, is it shorted?

    Left of the dual diode is a 50uf cap. ABove it are a couple 1k resistors, are they OK? Is the cap OK? I expect roughly 25v across the cap.

    Jazz asked you to measure the 55v rails with your meter on AC volts. Did you?

    This amp is almost 40 years old, every electrolytic cap in it is suspect. If the main filteres were weak, you'd probably hear hum. But the other caps, like the 50uf I mentioned can cause distortion if they fade away.

    Upper left corner, a diode from the +55 line feeds another 50uf cap acting as a decoupler for the differential xstr pair. Is that cap healthy? Lower left, see the 200uf/75v cap? Chgeck it, and also the cap just right of the differential xstrs. 25uf 25v. I'djust replace all those smaller caps. I am niot concerned over the small film or ceramic caps.
    J M Fahey likes this.
    Education is what you're left with after you have forgotten what you have learned.

  10. #10
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    Thank you both.
    I am sorry I overlooked the voltage readings on the transistors, Jazz.
    I will test the transistors, check ripple,Etc then post results.

    Enzo, I will then cary on with your tests. Thank you for providing so much theory with your post.

    I will report back tomorrow.

    Thanks again guys.

    Justin

  11. #11
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    Jazz,
    I have some numbers for you:
    I measured the "B" terminal of each output xstr.
    looking from inside the amp with the heat sink away from me, the left three measure about.1vdc when the amp is idling.
    The three on the right side measure -55.75vdc. This is where I may be confused and measuring the wrong terminal.
    I measured the "B" terminal of each output.

    AC ripple on the rails in 15mvac on the positive side and 14mvac on the negative side.
    Peavey service replaced main filter caps early this year.

    The driver transistors both measured.6 volts on the base terminal.

    Enzo,
    I am on to your tests now.

    Thanks

  12. #12
    Senior Member Enzo's Avatar
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    This amp is what we call quasi-complementary, meaning the plus side and the minus side are not mirror images. Both sides use NPN transistors. SO it is normal to get -55 on the bases of the negative side output transistors, and only a small voltage on the positive ones.

    SO for Jazz's tests, you need to measure the barely on voltage from B to E on each transistor, rather than from B to ground.
    Education is what you're left with after you have forgotten what you have learned.

  13. #13
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    AAAAHHHH!
    OK the light just came on. Thanks for the explanation Enzo.

    I started your tests and found C24 (the 50/50 cap you told me to check) open. The 22k resistor next to it is almost closed. It is showing like 2 ohms. The only cap I have on hand near that value is a 47/100 film cap, so I will get some coming.
    Should I worry about the resistor? The other 22k resistors measure dead on.

    I will retest the output xstrs the correct way and report my findings.

    Thanks again.

  14. #14
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    Quote Originally Posted by justin25 View Post
    The 22k resistor next to it is almost closed. It is showing like 2 ohms....
    Should I worry about the resistor? The other 22k resistors measure dead on.
    Is you amp different from the schematic that you posted? I don't see any 22K resistors in the power amp section.

    Often there are other components in the circuit that will cause a resistor to read lower than its' marked value. You may have to lift one lead of the resistor to get an accurate reading.

    Another way to measure the b-e voltage is to read both values to ground and then subtract to find the difference. If you have -55.6 vdc at the base and -56.3 vdc on the emitter, there is 0.7 volts from b-e.

  15. #15
    Senior Member Enzo's Avatar
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    47uf and 50uf are pretty much the same thing, 3uf difference is about 6%, right? Use the 47uf.

    I only have the schematic, so if by "next to it" you mean on the board, that doesn;t speak to me. I see a 22 OHM conected to that cap on the drawing, not 22k, it is the one serving the emitter of the 430C predriver and also the base of the 62188 driver. I can tell you that in the last 50 years, I can count all the shorted resistors I have found, and still have a lot of fingers left. Any chance you were measuring a 22 ohm resistor expecting it to be 22k? (22,000 ohms) I don't offhand see any 22k.

    If indeed that part measures 2 ohms, then that 62188 it connects to is probably shorted. That puts a shorted transistor and its 1 ohm emitter resistor in parallel with the 22 ohm, making it read and ohm or two.

    You ALWAYS worry about a resistor way off value. You can always unsolder and lift one end to measure it out of circuit. But resistors that are OK can have shorted transistors connected to them and they APPEAR shorted. You have to determine WHY a resistor measures funny. It is bad itself, or the circuit around it has a problem, ir it is simply the natural result of the circuit configuration.
    Education is what you're left with after you have forgotten what you have learned.

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    Thank you Bill.
    22 ohm, not 22K, sorry.
    I will lift one side when I replace the open cap.

    I appreciate the additional info on measuring B-E voltage. Between you guys, I am getting a better understanding. I will also test across the legs as Enzo suggested.

    Soon as I get a second, I will test the Xstrs correctly. It may be tomorrow evening before I get a chance.

    Thank you.

    Justin

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    Thanks Enzo.
    I figured the value was ok, I just need to know if the non polarized film cap I have on hand is ok to substitute the open electrolytic.

    The 22 ohm resistor that measured "closed" is the one that hooks to one leg of the open cap. I will pull one end and test it as well as give the driver another look.

    I will update soon as I make more progress.

    Thanks.

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    Would someone be so kind as to repost the schematic from the first post? Seems to be a dead link. If not, I will call Peavey on Monday and post it here.

  19. #19
    Supporting Member Jazz P Bass's Avatar
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    Here you go: Session 400_LTD.zip

  20. #20
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    Thank you!

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