SoundTech 6150 Powred Mixer

[km6xz]

OK, we are getting somewhere. It is unlikely to have two Zeners fail in a similar manner in different circuits so the common element in all these level anomalies is the opamp IC101. Is it possible to pull it out? I would suggest putting a socket in its place so you can re-install it or a replacement easily after a few more tests with the IC out of the circuit.

[cbass]

Ic101 is not socketed all i have on hand is 16 pin sockets.Kinda of a wierd thing is that I could not get Pins 4 and 8 to short to ground untill after I turned the power on and then back off again.After it sits powered off for a while the readings rise they are reading around 12mohms now.

[cbass]

I cut the 16pin socket I had in half and installed it on the board put in an IC I pulled from something else.Voltages appear normal now.No DC on speaker outs.Thank you very much I would have never got this far with your guys help.I learned a lot about troubleshooting Poweramps.Thank you.

[km6xz]

That is a strange reading. Pull the IC and turn it on again to see if both the positive and negative 15volts are present and stable at pins 4 and 8. If they are, replace the IC, and if not, you have only a few components to consider, the caps and Zeners

[cbass]

Yes I replaced the IC101 and now have 15volts on pin 4 and 8.The power amp seems to be working but now the only way I can get sound is to plug into the power amp in.The mains volume doesnt do anything.None of the channels work

[Jazz P Bass]

You will want to check that the connector to the preamp is intact.
The preamp IC voltages should also be checked.
Monitor the Main Out jack to verify whether or not you have a preamp signal.
If yes, check that the EQ In & EQ Out jacks are working.(or jumper tip to ring)
They both have a normally closed contact to allow signal passage when nothing is inserted.

[cbass]

I can't tell you guys how much i Appreciate your help.Iwas really hoping that the this thing would work after fixing the PA.Then I would Have an excuse to have a beer at 2 in the afternoon.I'll make sure connection is good and check ic voltages

[km6xz]

The mixer power supply is separate from the power amp, check the plug and cable that connects the low voltage power supply and the mixer. With all the work being done in the power amp section the plug that connects the mixer supply might have gotten disconnected. If it is OK, see if you have a functioning low voltage supply at the connector.

Congratulations on getting the power amp working....go have the beer anyway, the mixer will be easier to resolve.

[cbass]

2 of the pins on the connector Measured +40VDC Is this odd?I'm not getting voltage on the rails going to the mixer channels.Should I be looking for around 15volts?

[km6xz]

Unfortunately I do not have the full schematic, the channel section is missing but the master and effects is there.
There are a couple pins that should have about 40volts on them, coming from the power amp supply. The mixer supply has two regulators and its own bridge rectifier. Check the two fuses in that second supply, and see if there is about 20-24volts on the collectors of Q117/118. Those are the two regulator pass transistors. Their bases will be held to about 15v by the Zeners. The emitters of the pass transistors will be 15v

[cbass]

It appears that Voltage is getting through the connector to the preamp board but i cant find any voltages after that.

[km6xz]

With such a dramatic drop out of several power lines....stopping dead in its tracks after entering the mixer, I would keep looking for connectors or a worst case scenario of a cracked pc board. It would defy logic for two separate rails to die at the same time, without pulling much current. A short somewhere would have taken out the fuses.
What is the history of this, has it been used by for for a while and suddenly stopped working or was it a new item to you and it never worked...yet...for you? I suspected something like the latter when you wrote that someone might have put Q104 in backwards.. I usually end up with things when people or shops give up on them. I like the chase but when a bad tech has swapped a lot of parts it always makes it harder than if they just gave it to me with the original problem.

[cbass]

Its my bandmates it was supposed to be in fine working order.As soon as it was plugged in it blew one of my mains.Last time I do that with out checking first.

[cbass]

Two wires on the conector.One reads around +12VDC other around -12VDC unplugged.When i plug them into the connector on the master board the same wires read barely over 0 volts.

[km6xz]

The rails are 15volts which you should be seeing at the connector so the 12 volts indicates it is not a direct connection from the main connector running to the power supply. Do you notice any difference measuring at the 15 volt regulators when plugging in the master section?
When the connection that reads 12 volts is disconnected what is the resistance to ground from either 12 volt wire. It might be a by-pass cap or an IC is pulling the supply down, it could be anywhere on the board. With some test instruments it is not hard to find them but I assume you do not have a milliohm meter or a constant current regulated bench power supply.
It is 2 am here in St Petersburg so I am signing off for tonight, the GF is calling me to bed.....Good luck, I'll check the progress tomorrow...


I just noticed on the partial schematic that each section of the master and effects has low ohm resistors isolating the section. Finding them and seeing which has 0 volts on the circuit side of the resistors is the guilty party, the others will have some level above 0.