Hello

I have a power amplifier C-Audio Sr707

The protect LED comes on when turning on it on. The fans come on too. but it does not amplify.

it all looks clean inside, the glass fuse inside is o.k.

I wonder does anybody know what the fault might be? Could it be a relay in this instance?

Thank you for any help you may offer..

The output devices are MOSFET'S (2SJ50 & 2SK135) NOT transistors. Check that all 3 relays on the soft start PCB board are o.k. (not burnt & are opening and closing) and also check on each driver PCB board the 27K 1 Watt resistors (R14 & R50) as they have a tendency to go open circuit due to heat and age.

The other thing you could check is to see if there is any DC offset voltage (above 1 volt) on each channel output (before the protection relays), if there is, this would give you an ideal which channel is faulty.

Good luck

Simon   

 Metal Oxide Semiconductor Field-Effect Transistors....

No, but there IS a difference between the two and i think that MOSFET would have been more of a correct & accurate statement rather than transistor.

Thank you very much

I tested the 27k 1w resistors called R24, R34, & R50 on my pcb,  they all tested o.k and looked fine.

fuse 1 is o.k on soft start board.

i can de solder and remove the soft start board, id be able to remove the relays as i have the de-soldering machine but there are no codes fotr replacements. I could test them, but am not 100% sure with 230v (I normally use 9v on breadboards)

it looks like the mains is rectified and that high voltage dc is used to trigger the relays. As you can tell my knowledge is limited so might be best if I change the relays and see if that remedies the fault.

i wonder do you know the part number?

thanks again : )

hi thanks for your reply.

Do you mean when the amp is on and the protect light is stuck on (fans run)

to connect a multimeter (set to dc) and probe the two points in the photo below?

thought id check to be sure as i am just a novice.

http://oi64.tinypic.com/xp30x4.jpg

thanks

I forgot to quote your email i am not sure if that makes a difference with notification. so posting again.

0.083v is what i see on my meter.

I hope that is good?

thanks for any further help

Xoc1 wrote: Check the amplifier for a DC offset before the output relay, The body of the output mosfets to the negative speaker binding post is a handy place to measure this. No point sweating over the relays if they are just doing their job and stopping your speaker from being damaged,

there is only 0.044v DC on the other side.

thanks again 

Here are some schematics of the protect circuit with its LED , just under the LED are two lines Labelled "CONT" and

"GND"(orange & pink) which connect to the output relays

2nd picture below is the output relay area and the soft start also.

Right click "view image" for a closer look.

Thanks for any help.

Hi,

The relays are T9AS1D12-24 & are 24 volt DC relays. Make sure these are working and switching correctly (contact wise).

I assume you have no high DC offset on each channel output, if each channel is around 10mv (no signal present), then i would suspect the protection circuit at fault (also check the thermal trips 3 & 4 on the circuit diagram, these should be normally closed) & then start measuring the diodes and resistors on the protection circuit.

If you have already removed the soft start PCB from the amplifier, then test each relay (power supply 24 Volts DC and a multi-meter)

Let us know what you find.

thank you for your reply

i think the soft start circuit would be easy to remove desolder slide out.

the protect circuit seems to be on the end of one of the channel circuit boards (A).

im sure i could remove it. but it doesn't look easy at all.

There are allot of wires in the way I cant see the two thermal components in series.

Are the two thermals and protect circuit behind the area highlighted below on chn A? I am not sure what the thermals  look like, although i could test them for continuity if i could find them. 

thanks for your help again.

it is rusty outside though.

but yes quite nice inside,

if the protect circuit and the thermals are behind that orange circle. i am a bit worried.

The protection circuit is on both boards, this is to allow easy change of boards (as they then would be identical) BUT only one board is being used / wired for the protection part.

The round bit of foam that is glued in place is only there for isolation between the output coil and the lid to stop it from shorting out against the case, just remove this and when amplifier has been repaired just glue back in the same position.

Also again check the the 27K 1 Watt resistors, i did have one in for repair where these were measured O.K. BUT the tracking on the underside was cracked and open circuit.

hi simonp1100

what do the thermals look like?  I may be able to probe behind with long probes to check.

thank you again

There are two thermal trips on each channel heatsink / metalwork. They are a 90 degree normally closed trip and a 50 degree normally open (for the fan and has a orange capacitor soldered across it). They are round black plastic disc's with terminals coming out 180 degrees apart. The ones you need to check is the ones without a capacitor fitted across it, they should be closed.

hi

I tested the thermal trips and the ones without the caps beep when I test for continuity.  the ones with caps do not beep but they have a resistance.

The 27k resistors are reading correctly. However,  on the pads behind them they are a bit golden/brown ( maybe flux?). So I tested from each leg of the resistors to the next pad ( to be sure the solder is connecting to the pad, and got a beep for continuity from each leg>pad >trace> next pad. so the resistors are still connected/ in the circuit.

It sounds like the thermal trips and resistors are o.k. so if you do have a very low DC level on each output (around 10mv on idle) then i would suspect the protection circuit has a fault.

You can swap boards (i.e. fit channel A driver board into channel B and visa versa) and i have done this before BUT then the amplifier bias current and DC offset would need to be set up on again on each board.  .

You could also check to see if you have the + and - 15 volts on each driver board (this is to supply the op amp on each driver board).

oh no, that sounds like a job for an experienced engineer.

i was hoping it was worth putting in new relays. but as the three on soft start seem to be working

( i think they allow power to the protection circuit)

and the 2 output relays will only work if the protection led goes out (i think from looking at the diagram)

so the problem is likely in the protection circuit area highlighted above ( orange circle)

typical, the trickiest place to get too.

The other channel board (B) seems to have a couple of parts missing in the protection area. so they are not identical.

this amp was working near 20 years ago. no one has been inside since.

it was only opened up a couple of weeks ago to see if it was a fuse.

I am almost tempted to take out channel A PCB and see if anything needs replacing in

the protection circuit as it is so tricky to get too at the moment.

Thank you very much  for your help much appreciated : )   

The only other thing you could try is to put a stereo signal into the amplifier and put the front volumes on low and then with an old scrap speaker (cheap car speaker etc), connect between the back ground speaker connection and the metal cans on one of the MOSFET's on channel A and see if sound comes out, if it does do the same for channel B and again if it does this would then give 100% that the problem is the protection circuit at fault. 

This above test can ONLY be done if you have 10mv DC or less on each output. 

thanks, I shall give it a go later on

I forgot the mention there are little variable pots ( Phillips screw driver type) on each channel board and they had oxidisation on them, like white crystals or fluff.  Im not sure if this made its way onto the contacts

Hi

yes there is amplification on both channels and sounds clean too

I was using an old cheap plastic speaker ( not sure of the ohms) wasn't all that loud.

So it is the protection circuit.   Oh my,  I shall have to lever it out and have a look.   : /  :  )

Yes - I was going to suggest this - but with a sig gen & oscilliscope! The best way to see if it is just a faulty protection circuit - or if there is more going on, eg high frequency instability/oscillation on the o/p. A simple multimeter check won´t pick that up.  

I suggested this as i assumed he does not have this type of equipment and would give a quick way of finding out if the protection circuitry is faulty or not and if sound is coming out whether it is distorting etc.

owch, i did the test with a speaker,  with 44mv DC present on one chn and 83mv DC on the other chn.  (These DC readings were taken before when with nothing connected in or out.) 

I do not have an oscilloscope unfortunately.

yes copy of post above >

Yes - I noted the 'scrap speaker' comment! I have a few of those around the place as well for 'real life' tests. Sometimes circuits will behave differently under load. 

if there is hf oscillation confusing the protection circuit

could i trick the protection circuit by disconnecting something ( the feedback?)

Glad we are getting there. 

You may need to take the board out and start measuring the components on the protection circuit. The boards come out easily as the are just on plastic PCB spacers (don't forget to push the small plastic release pin in before the board passes past the pillar).

Ok - at least you now know.

Another question: are the output devices getting very warm? That would be a sign of high frequency oscillation - fairly common with MOSFETs.

(Or bias too high.)

Thank you very much

I shall try and  fault find the protection circuit later on :  )  

i am wondering which components are you refering too? and would it be safe to touch with fingers lol : )

as i notice it takes a while for the caps to discharge. 

should i run it with the speaker connected for a few minutes?

I assume you are on about the 47K resistors that detect the DC voltage on each channel, the protection circuit only stops or controls the following:

1. DC on either channel.

2. Thermal / heat via the thermal trips.

3. Soft / start relays.

If the sound coming out sounds fine, i would still suspect the protection circuit. Check the components with reference to your circuit of the protection circuitry in an earlier post.

o.k I shall do , thank you !!!  : )

I mean the (metal cased) TO3 output MOSFETs - where you connected the speaker!

Under no-load, they could get warm, but no problem to touch. If they do get too hot to touch, then there is a problem! 

The caps should discharge themselves after a few minutes, but a few seconds with the speaker is fine. I usually use a 25W, 47R resistor to do the job.

Also the Zobel resistor if it's badly burnt is another way that the channel is oscillating (normally a 0.1uf capacitor and a 10R 2 Watt resistor in series connected between ground and the main output), but the old Hitachi FET's are very good.

You could measure the voltage between the gates of the MOSFET's (2SJ50 & 2SK135 on each channel) with no signal connected and let us know what this is as this will give the info for the bias current.

hi thanks

i shall test the temperature when the circuit baord is back in, as it is in pieces at the moment,

  : )

hello

I found two damaged resistors hidden away on the protect circuit,  they allow no current at all.

R10 and R7

I can not see which 'watt' rating they are. I wonder,  should I replace them with 1/2 watt metal film, same value?

I could replace the 27k resistors (brown ones on the channel A&B circuits )  while I am in here.

( would 1 watt be o.k again, or maybe go up to 1.5 watt metal film?)

i may replace the bf422 "TR1 " that is close to the failed resistors also.

here is a pic of the two that failed.

thanks : )

Just replace the lot with a crowbar circuit! Far more efficient!!  

Thanks

I am looking forward to the parts arriving. And shall post results,

I'm sure there are a few hundred people in the future interested reading this too, so

here is the full schematic  >

http://s000.tinyupload.com/download.php?file_id=83811718507945732789&t=8381171850794573278938941" rel="nofollow - http://s000.tinyupload.com/download.php?file_id=83811718507945732789&t=8381171850794573278938941

peace : )

hi

That is the only pdf I found.  C-audio email has no response.

I did notice the inventor in one forum somewhere, he was giving out support ( that was from a search result of about 8 years ago)

you say channel B is in protect, that is strange. surly a stuck output relay as only one side is not working?

i presume the protect light does not come on?  if it does come on then both channels should not work.

The protect should be for both channels. there is only one protect circuit (95% sure of that.)

BTW. im still waiting for my parts to arrive.

Hi,

Lee Basham is the guy i think you are on about, bu these amps are pretty easy to repair so any good electronics guy should be able to do them. The protection circuit is for BOTH channels, so if there is a fault on one side, it will shut down BOTH channels, so if you have one channel working and the other is not then it maybe a stuck / faulty relay or another issue (audio input circuitry).

I will check my paper schematics, can you answer the following:--

1. How many relays does the soft start PCB have ?.

2. Does the output terminals on the back of the amplifier go to the relays on the soft start PCB or each driver board ?.

3. Can you upload a picture of the driver board (around the protection section). 

In the mean time, can you look at the other board and take measurements on that one to see if any components are different in value etc ?.

Just waiting on a pair of new fans, as the old ones have lost all enthusiasm, barely even spin up. Then I'll have a look at the soft-start board as it sometimes makes a "skrrrt" noise on power up, as though a relay is arcing or bouncing. There's evidence of heat damage there, and a previous repair. Looks like a right PITA to get that board out though!

Hopefully I can put her back into service next week!

(well done with yours)

I replaced the broken resistors and a few other components in the same area.  turned it on and still in protect mode.

After, i checked the resistors again, they have not failed again. so it seems the parts are o.k.  i havent changed all the transistors though.

there is an R8 that is less than 1ohm .I cant see it on the schematic.

maybe a cap is not working well?  i haven’t a testing device for those.

. errg. 

hi there

my r8 is very near the protect circuit area

i am considering switching the channel  circuits over just to see if i can get the spare/other protect circuit working.

someone mentioned this could be done, but a bias would need to be altered or something along the lines? 

I can do the soldering but  worry about any imbalances that might need to be fixed if i switch them around.

seems like the only option at the moment.

thanks everyone

Can bias current and DC offset voltage be set with a multimeter? 

thanks for advice.

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.

Wow great, Thank you !!

I shall do so later on. 

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.

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

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 

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)

Studio45 on another subject, Have you altered vr2? Do you know if it is for dc offset or is it a gain trim?

thank you

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.

I have it connected to a 100w 8 ohm full range studio speaker,  using an unbalanced media player to unbalanced in. The output is quite loud ( loud enough I worry what my neighbours are thinking), it is full range. flat.  but not so loud i have to worry about hurting my speaker. i know  the speaker could be pushed more.

I can only imagine its been tuned to a lower volume i have noticed there is a sticker on the back saying LF amp 3.top layer so i presume this was from an array of speakers.. hence i feel there might be a pot to turn or a resistor to change to make it a standalone amp?

it is full range and sounds nice. just the level issue.

thanks for any help you can offer.

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...

Can you accurately measure AC voltage in vs voltage out? That will tell you if the amp still has all the gain it should. Then you could do a test into some sort of dummy load to see if it will sustain high power. For a quick test, three to five 3000 watt electric kettles in parallel will give you a load of about 4 ohms when hot. Or, a couple of old voice coils in a bucket of water works too :)

https://www.schematicsunlimited.com/c/c-audio

The final issue i had is pretty much solved

i simply increased the input level.

I have 3 settings on my soundcard 

using a 1khz sine wave  at 0.1db from the software altering the attenuation options (small relays on the card i think)

the multimeter readings i can have are:

1. .39v ,  loud output.

2. 1.12v  potentially very loud as one would expect  (no distortion)

3. 2.27v  would be very very loud if i turned it  up (with 2.27v sine wave input and the main volume dial just slightly up I can hear a very slight distortion, I presume this is from the tl071 opamp struggling with the input level)

so that's pretty cool! 

Thank you to all that helped. especially simonp1100 : )

No worries, glad to help.