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C-Audio SR-707 Protect Issue
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simonp1100 View Drop Down
Young Croc
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Points: 1110 		Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 08 March 2019 at 3:46pm
Originally posted by qwerp qwerp wrote:


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).

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studio45 View Drop Down
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Points: 3864 		Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 08 March 2019 at 4:00pm
Protect light comes on, and channel B speaker relay does not click, as it gets no drive voltage. In my unit there seems to be a seperate protection circuit on each channel, and the Protect LED has two drive wires coming to it, one from each channel. I'm currently checking out all the silicon components. All power supplies are fine. Less than 0.05v DC measurable at the MOSFET bank outputs.

I've also found Lee Basham's posts but he seems to have been inactive for the last 8 years so I guess maybe he retired...I'd like to replace all the resistors he mentions in several posts, but without the right schematics or any board markings it is difficult to tell which ones they are!
Studio45 - Repairs & Building Commotion Soundsystem -Mobile PA
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simonp1100 View Drop Down
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Points: 1110 		Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 08 March 2019 at 4:20pm
The protection circuity is on both boards anyway (even in the later issues), this was done to allow either / or board to be used in assembly (unless the amplifier you got is a very early model). All of the SR amps i have repaired or updated (606-707) have had the protection circuitry on both boards and only one board would be connected to this part. 

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). LOL

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 ?.

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Edited by simonp1100 - 08 March 2019 at 4:34pm
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studio45 View Drop Down
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Points: 3864 		Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 09 March 2019 at 1:13pm
Didn't snap any pics unfortunately but got er workin again! Replaced the Zener for +18v, replaced a 24k/1 watt resistor that is probably "R1*" on the schematic, both were open circuit. That got it out of protect mode but still no sound from ch B, so replaced the TL071 et voila! she's a skookum choocher!

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!
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qwerp View Drop Down
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(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.  Dead
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simonp1100 View Drop Down
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R8 on my circuit is going to the opto sensor for the soft start circuit (10K 2.5 Watt) along with a 1K resistor mod & these i think are on the soft start PCB board.
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qwerp View Drop Down
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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
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simonp1100 View Drop Down
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Originally posted by qwerp qwerp wrote:


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


I was the one who suggested it but you need to re-adjust the bias current and DC offset voltage for BOTH boards, if you don't know how to do this i would just replace all the components for the protection circuit on the original faulty board.
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Points: 41 		Post Options Post Options   Thanks (0) Thanks(0)   Quote qwerp Quote  Post ReplyReply Direct Link To This Post Posted: 09 March 2019 at 5:27pm
Originally posted by simonp1100 simonp1100 wrote:

Originally posted by qwerp qwerp wrote:


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


I was the one who suggested it but you need to re-adjust the bias current and DC offset voltage for BOTH boards, if you don't know how to do this i would just replace all the components for the protection circuit on the original faulty board.


Hi thanks for your message again.

Could i switch the boards just to see if that fixes the protect issue?

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

thanks for advice.
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simonp1100 View Drop Down
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Points: 1110 		Post Options Post Options   Thanks (1) Thanks(1)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 10 March 2019 at 11:33am
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.

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Edited by simonp1100 - 10 March 2019 at 11:35am
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qwerp View Drop Down
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Wow great, Thank you !!

I shall do so later on.  Tongue
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Points: 4947 		Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 10 March 2019 at 5:33pm
Lee Basham works for Linea Research, not sure it would be fair to pester him about c-audio problems these days, perhaps he's happy to forget about them!
Kevin

North Staffordshire
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