Xr600c mixer amp, advice.

[Tassieviking]

I would not give up yet, the hard part is likely over and now its just tidying up left.
First check to make sure you have +-15V going to the pre-amp board, I don't know what the in/out switch does, but flick that up and down to make sure that's not the problem.
If you have a pre-amp pedal plug that into the return jack to test the amplifier, you can also use the send of another amp and plug it into the return jack on the mixer.
It might just be a few more lessons in fault finding you need, and this mixer wants to be your teacher =)
Make sure the mute button is not on if it has one.

Never throw something like this mixer out, the transformer alone might be worth hundreds of dollars, and you might need one for a project one day.
If the power amp is now working you have 3/4 of a great solid state amp already made, pre-amps are easy to make, I have made several different ones from scratch.

[Tassieviking]

I have had a look at the schematic and the only LED I see is on the power amp section, is this the status light you mean ?
I think its in the power switch on the front panel, the main power switch is on the back panel and the switch marked power might be like a mute switch ? I'm not sure on that one.
I think you should start from the beginning and check the following:
DC on the speaker out wires
+- 52V supply
+- 15V supply
+-16V supply
+-15V on U1, the TL074 IC on the power amp board, pins 4 and 11 (It might be easier to measure across C45 and C46 for this)
If all these voltages are ok then plug a guitar or pre-amp pedal into the socket marked PWR.AMP and see if you get any noise out of the speaker.
A guitar into the PWR.AMP socket will not give you a lot of noise but you should be able to hear it when you strum the strings.
Give the cabinet a light thump and see if the reverb springs make a noise, naturally turn the volume and reverb pot up for this.
Let us know what happens.

[Oldmactech]

Thanks for keeping me going.

There is 4.1mv DC at the speaker terminal.
I get nothing through the guitar cable plugged into the power.amp input.

Novice question. The 52v test, AC or DC? Measure to ground from each pin or across the pins from the red wire?

Thanks.

[Tassieviking]

Like I asked you in post #16, measure across the capacitors, C26, C30 .for the 52VDC, and you can measure DC, but the AC measurement can also explain things.
If you have too much AC across those caps then there is still something wrong in that part of the circuit, the less AC across those caps means that the capacitors (C35, C36) are working good.

Explanation of bridge rectifier with diodes:
https://www.elprocus.com/half-wave-a...acitor-filter/

https://www.electronics-tutorials.ws/diode/diode_6.html

[Oldmactech]

Here goes:

Across c30 72.6vdc fluctuates from 7.5vac but settles on .1
Across c26 59.9vdc fluctuates from 11.5vac but sellers on .1.

thanks for the help.

[Oldmactech]

Also remeasured c41-c44
c41 .88vdc
c42 .02vdc
c43 15.56 (might be -15.56 but I don’t know he polarity of this little cap)
c44 16.19 (same as above)

j

[Tassieviking]

Can you please check that you installed the diodes correctly.
The voltage across C30 and C26 should be very close to each other, the schematic indicates 52V but it depends on the ac voltage coming into the diodes from the re/red-yellow/red wires.
Check the fuses for the +-16v circuit, C41 and C42 should have around 16V dc across them.
Make sure you have installed all the plugs correctly.
There is still something wrong with the power supply somewhere.

You had the correct voltage across C41 and C42 before, around 15.7V across each one.

[Oldmactech]

Retook some readings.
c26 59.7vdc
c30 67.9vdc
c41 .5vdc
c42 -.5vdc

c45 15.3vdc
c46 -16.1vdc

i checked a few of the 5w resistors and r64 appears to be open.

I re-examined the diode placements and they are all correctly oriented but I’ll check again. They were tested for continuity when installed but I can check again. I installed to additional diodes on the riser board that contains the output transistors but they are oriented correctly as well.

Fuses good.

[Oldmactech]

Btw , I’m have the amp connected to a dim bulb limiter, would that affect the voltage readings?

[Helmholtz]

Btw , I’m have the amp connected to a dim bulb limiter, would that affect the voltage readings?

Of course. The bulb takes away voltage from the amp, so all secondary voltages are lower than normal.

[Oldmactech]

Well….took the limiter out of the circuit and it immediately blew the 8 amp power fuse. Have to find a source for some. Something shorted for sure.

[Tassieviking]

When you get a fuse we will have to find out why the voltages are so astray.
First reconnect the light bulb limiter.
Disconnect the 2 cables running to the power transistor boards.
Disconnect the cable running to the pre-amp boards
Power up and start measurements.
Measure the AC voltage coming in to the fuses (this supplies the 16V circuit that goes to the pre-amp I think)
Measure the DC voltage across C37 and C38, (this is the voltage going to the 15V DC regulators (the diode to the ground pin boosts the voltage to 16V)
Measure the voltage across C41 and C42, this is the voltage coming out of the 15V regulators (should be close to 16V DC)
Measure the voltage at the plug that goes to the pre-amp that I have marked on the drawing (on the far left)
Measure the voltage across C45 and C46, this supplies the +-15V to the amplifier itself. ( U1, U2 etc.)
And check the voltage across C29/C30 and also C25/C26. This should be around 52V DC each.

By checking the voltages with the pre-amp and the power transistor PCB's unplugged we are trying to see if the power supplies are good.

[g1]

When you took it off the limiter and the fuse blew, that was with no load connected? And the bulb had been dim before you removed it? What wattage bulb?

[Oldmactech]

Weird. I couldn’t read the fuse rating but faintly looks like 8amp.
Picked up 8amp 250v fuses. Replaced the fuse. Re-installed limiter in the circuit. (75w bulb)
immediately on startup bulb went 100% Brite and stayed that way, even a bit of noise- not connected to load or the preamp section.

I have not touched it since the first fuse blew.

I disconnected each power connection from the board one at a time and switched it on. Still 100% brite until the AC connection off the switch.

I have changed the c35 and c36 and the diodes cr-16 through cr19 so I must have mixed up polarity somewhere (but I have triple checked).

I’ll take a look at tassievikings schematic tomorrow, must be something I’ve either overlooked or something.

thanks.

[Tassieviking]

The schematic says 8amps on 110volts for the main fuse, but if you live in a country with 220-240volts mains you should most likely use a 4 amp fuse for the mail fuse.
The 2 fuses on the PCB should be 1amp no matter where you live.
If you have the board out, maybe you can take some photos of the board and post them here, sometimes a fault is obvious to someone else even though you don't see it.
Photos of the front and back would be good, sometimes the service manual stays the same even when the manufacturer changes the PCB outlay or circuit over time.
What happens if you unplug the two AC 3 pin plugs from the transformer to the PCB and turn it on ? Measure the ac voltage on the transformer wires while they are disconnected.