C-Audio SR-707 Protect Issue

simonp1100 wrote:

Yes you could swap the boards over to see if it fixes the protection issue.

The only thing to set up on each board (if you swap them over) is to set up the bias voltage for each side, this is done as follows:--

Using a DC multimeter connected it between the gates of the 2SJ50 & 2SK135 output MOSFET's for channel A, power amplifier up & adjust the bias current preset (VR1 IQ TRIM) on the driver board for 350mv, leave on for 10-15 minutes and re-adjust for 350mv.

Do this again for channel B.

hello

I wonder are the points to probe for bias voltage highlighted in green below?

I cant see "2SJ50 & 2SK135" as you suggest. My mosfets are called k135 and J50. There are 16 each side (8 x 2) A total of 32 attached to the chassis/heatsink, unlike the pdf picture below ( maybe the picture below is the sr606?)

thanks again hopefully almost there. LOL

simonp1100 wrote:

Measure between the N drive and P drive on the circuit.

Hi after swapping the channel boards over it is now ' out ' of protect mode Tongue

with a nice firm click sound.

I owe you a drink at least. : )

i tried to adjust the VR1's to get 350mv. but the least I can get with these variables is 363mv both sides.

i have an old speaker attached (an awia, small cheap hi fi speaker) and strong line level sound source, and the amp

does work.But it is strange, I thought this amp, being so powerful, would only need a 1/4 turn on the volume to make these small speakers loud/distort, but I have to go almost the whole turn on the front panel volume control. Confused

i shall run an experiment tomorrow , input pink noise 0db and measure the speaker output with a multimeter and see what happens.

Thank you very much for your help , it was so cool when those output relays clicked and the yellow led went out Wink

Edited by qwerp - 11 March 2019 at 12:45am

studio45 wrote:

Glad yours is working too qwerp :)

I also can't get the bias set right on ch B. I am reading 396mV between the drives, which seems too high. Although it does pass audio, sounds fine and doesn't seem to be running too hot.

Adjusting VR1 does not affect the reading, in either direction. I have already checked out the BF422 transistor, 1.5k and 2.7k resistors and they all measure as good.

I even added 1k in series with VR1 in an attempt to gain more downward adjustment range, but I still just get that 396mV figure, no matter where VR1 is set.

Just tried substituting an MPSA42 for the BF422, in case my transistor tester is lying to me, but no change, still settles at 396mV.

Any ideas?

hi are you measuring between the orange and pink wire (crocodile clip to the actual wires) on the rear of the mosfet circuit board? with it set to dc on multimeter? Thats how i read on a sr707.

good idea adding a resistor. i was considering changing out the vr1 to a different value pot to allow me to get 350mv. But is it really that necessary to hit 350mv exactly?

Did you manage to change the fan?

I am baffled why my amp seems not so powerful. i do not have a high watt speaker to test, but i thought it would distort this cheap naf old speaker I have here. maybe i do not fully understand the science? Maybe with a powerful high watt speaker this amp will show its true power ...or maybe someone changed a resistor somewhere near the opamp to reduce the level so to prevent someone blowing up a set of speakers at its last installation ( gain reduction of some type) ?

( to clarify....the diagram above is wrong the green circles were just a guess. (the points to probe are the pink and orange wire sold points on the rear of the mosfet pcb)

Edited by qwerp - 11 March 2019 at 7:59pm

studio45 wrote:

Glad yours is working too qwerp :)

I also can't get the bias set right on ch B. I am reading 396mV between the drives, which seems too high. Although it does pass audio, sounds fine and doesn't seem to be running too hot.

Adjusting VR1 does not affect the reading, in either direction. I have already checked out the BF422 transistor, 1.5k and 2.7k resistors and they all measure as good.

I even added 1k in series with VR1 in an attempt to gain more downward adjustment range, but I still just get that 396mV figure, no matter where VR1 is set.

Just tried substituting an MPSA42 for the BF422, in case my transistor tester is lying to me, but no change, still settles at 396mV.

Any ideas?

The bias current should go up and down as VR1 is turned, if it's not then check the preset pot, this maybe the reason it will not adjust.

DO NOT ADJUST VR2 this is the common mode rejection pot for the input stage and is set at C-Audio and only needs setting up if some components on the input stage have been changed / replaced. Dead

Edited by simonp1100 - 11 March 2019 at 10:34pm

simonp1100 wrote:

The bias current should go up and down as VR1 is turned, if it's not then check the preset pot, this maybe the reason it will not adjust.

DO NOT ADJUST VR2 this is the common mode rejection pot for the input stage and is set at C-Audio and only needs setting up if some components on the input stage have been changed / replaced. Dead

Changed out VR1 too....I can measure the resistance changing when I turn it. I've also checked the two driver transistors just after the bias stage, and they measure fine.

On the subject of leaky transistors. My tester sometimes shows a leakage result (ICEO) of 1uA, which then normally disappears if I repeat the test.

Is 1uA considered a lot of leakage, would this result indicate I should replace the device? I note that on datasheets, leakage is normally quoted in nA so 1uA would be 100x more...

Edited by studio45 - 12 March 2019 at 11:00am

Studio45 - Repairs & Building Commotion Soundsystem -Mobile PA

Don't suppose anyone on this thread has the schematics for the SR404, SR606/707 and the XR3801? Embarrassed

Edit: That they'd be willing to share?

Edited by RoadRunnersDust - 12 March 2019 at 9:50pm

www.guildfordcablecompany.co.uk