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Author Topic: Peavey Session 400 1976 troubleshooting  (Read 911 times)

Roundtone

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Peavey Session 400 1976 troubleshooting
« on: July 09, 2019, 02:38:02 PM »
Greetings kind people. I am in the middle of a repair on a Peavey Session 400 from the 70’s. So far I have replaced a resistor in the Reverb send/rec section and re-attached two components where one leg had lifted off the PCB. I am getting output now but I have an issue in the power supply rails to the output transistors. I am getting a solid +50 vdc on one rail and -50 vdc on the other. I can signal trace an input signal and 50vdc  off of 5 of the 8 sand box resistors in connection with the output transistors. The other 3 have 104vdc and a buzz. I pulled one of the output transistors (5 seem to be working fine) and tested with diode checking and it appears to be ok.I also have no signal at the send of the reverb circuit section, but the return and tank are ok. I’m not that experienced with SS power section diagnosis, so I’m hoping someone here might give me some advice on where and how to start best. Thanks for reading/thinking etc - (I have a schematic from Peavey BTW)

Jazz P Bass

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Re: Peavey Session 400 1976 troubleshooting
« Reply #1 on: July 10, 2019, 12:59:47 AM »
Please post the schematic.

Those 'sandbox resistors' are most probably the output ballast resistors.
The speaker output is taken off of them.
Therefore there should be zero dc voltage on them.

Roundtone

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Re: Peavey Session 400 1976 troubleshooting
« Reply #2 on: July 10, 2019, 10:57:00 AM »
Schematic

Enzo

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Re: Peavey Session 400 1976 troubleshooting
« Reply #3 on: July 10, 2019, 12:52:31 PM »
If you have 50v rails, it is about imposible to get 100v anywhere in there.  Make sure your meter is connected to ground, usually chassis, and make sure the board ground is in fact connected to chassis.

Roundtone

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Re: Peavey Session 400 1976 troubleshooting
« Reply #4 on: July 10, 2019, 01:05:12 PM »
I will double check this evening and post voltage readings. Thank you

Roundtone

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Re: Peavey Session 400 1976 troubleshooting
« Reply #5 on: July 11, 2019, 12:36:03 AM »
Voltages attached

Roundtone

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Re: Peavey Session 400 1976 troubleshooting
« Reply #6 on: July 11, 2019, 12:46:23 AM »
I was measuring to the voltage rail as well as looking at mV as V, so you will see a much different situation. (I am assuming that the power amp is working correctly maybe not at peak performance, but output is strong and without hum). All voltages were taken between chassis ground and leg on my chart. The Reverb circuit is still not functioning. Is the voltage  variance on the ballast resistors something of concern?

phatt

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Re: Peavey Session 400 1976 troubleshooting
« Reply #7 on: July 11, 2019, 08:34:46 AM »
I may not be reading it right but the best I can make out is that it all seems ok.

Easiest way to grasp this is maybe I try and explain the basic concept.
It's a split supply and the output sits at Zero VDC.
It's never perfect so always a little DC offset. So 160mV of DC on the output (Pos or Neg) is normal, as long as it's close to Zero.

So the Emitters of the top 3 Transistors are going to be very close to Zero VDC.(which is what i think I'm reading) While the Emitters of the bottom 3 Transistors are going to be within a bees dick of the Neg rail hence you noted -52Volts on those resistors.

The basic plan of these circuits is to keep the base's of the two input transistors which form a LTP (long tail pair) as well as the output as close to Zero as possible, Feedback (that 100k) helps to keep this in check.

The diode string (2 in that dotted box and the one below) bias the output Tr's. The 2 Tr's beside the bias are just for safety,, they limit the output current if the output is shorted out.

The Tr down the bottom left is the voltage gain and then sent to the output TR's which turn that into current to drive the low Z speaker.
Hope that helps.

without a full schematic No way to tell what might be wrong with Reverb.
Phil.

Roundtone

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Re: Peavey Session 400 1976 troubleshooting
« Reply #8 on: July 11, 2019, 10:07:31 AM »
Thanks Phil - I appreciate the explanation very much. I need/want to comprehend the theory of operation of everything I work on. I have much to learn.

The Reverb circuit (preamp) schematic is posted earlier in this thread. I will post voltages again tonight for that part of the circuit (at the correct magnitude to ground) if you or anyone else wants to take a look. I have successfully tested the wires/jacks/tank and all are ok. Here is a close up - the +/- 50 Vdc input voltage is good.

Roundtone

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Re: Peavey Session 400 1976 troubleshooting
« Reply #9 on: July 11, 2019, 11:27:53 PM »
Here are some voltages and a layout shot of the reverb output section to the tank input. The dual molex connector on the left edge of the photo is the send to the tank. The one on the right is the return from the tank output.
« Last Edit: July 11, 2019, 11:30:49 PM by Roundtone »

phatt

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Re: Peavey Session 400 1976 troubleshooting
« Reply #10 on: July 12, 2019, 08:29:02 AM »
Thanks Phil - I appreciate the explanation very much. I need/want to comprehend the theory of operation of everything I work on. I have much to learn.
You are welcome. :tu:
This guy has a ton of well written pages and I highly recommend it;

Main page;
http://sound.whsites.net/index2.html

Click on Articles,, Projects,, and Site Map to find a ton of in depth info :dbtu: :dbtu: :dbtu:
I've found it easier to use the site map to find stuff of interest.

The Reverb circuit (preamp) schematic is posted earlier in this thread.
Durh :-[ silly me yep I should pay more attention,,  :crazy2:

I can't see the driver plug clearly but the return plug has 3 wires.
This makes me think there might be a ground missing for the drive.
Darn hard to explain this with words but always remember you have to check the ground path of circuits as well as the hot wires.
If driver output is grounded via the return plug it may not be connecting back through the tank. look at the schematic might help.
Phil.
« Last Edit: July 12, 2019, 08:32:02 AM by phatt »

Roundtone

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Re: Peavey Session 400 1976 troubleshooting
« Reply #11 on: July 12, 2019, 01:35:00 PM »
In review of my comment I realized that the connector shown on the right is actually the preamp output to the power amp (the tank return is out of the photo). There are only two wires from the reverb circuit send to the tank. Also there are only two wires from the tank out to the preamp reverb recovery. They all share common ground. What range of voltage should I expect to see on the signal send to the reverb tank? Guessing... my reading is low and the tank is absorbing all the signal. This might point to a faulty drive transistor?

Jazz P Bass

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Re: Peavey Session 400 1976 troubleshooting
« Reply #12 on: July 13, 2019, 10:14:27 AM »
The first reverb circuit transistor, on the left, (3904) has weird dc voltages.
The collector and the emitter are the same voltage.
It may be shorted.

This circuit is an amplifier.
It should be able to drive a speaker.
That said, the -7Vdc on the tank coil is not a good sign.
I would expect zero volts dc.

Roundtone

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Re: Peavey Session 400 1976 troubleshooting
« Reply #13 on: July 24, 2019, 11:22:53 AM »
Wanted to follow up on this repair - customer needed the amp back so I stopped working on it. I did replace the transistor with no change. I also pulled both of the larger transistors and they seemed to test ok. Baffled would be my state. Hopefully I can understand the dynamics going on here one day when the amp returns. Thanks for the help!

 

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