M 70 Protection Lamp

B

bikerdon

Active Member
I picked up some yamaha gear from a sale, and this was included. When I got it nothing came on except the protection light. I changed out the 6 caps on the protection board, and now the display lights up, but the protection light is still on.

I can hear a very faint buzzing sound, if anyone has anything obvious to look for I would appreciate it. I have this, the tuner and the preamp....this is the only thing that doesnt work..

TIA
 
For the buzzing look at C607 on AC input control board NA07771 (its in the rear behind the power transformer on one of those circuit boards. It may be split open and look burnt.

search yamaha recall on X-power supply models--this includes the M-70--C607 needs to be replaced with a new X rated suppression cap.
.22 uf at ,at least 250vac.

For the protect -look over the Power C board NA07760 or NA07761.(Up front between the front panel on the powertransformer) look it over for swelled up or overheated electrolytic caps (shrunken plastic sleeves.)
 
Last edited:
but the protection light is still on.
Click to expand...

First you need to check for any DC offset -prior to the protection relay..

I think the service manual provides test points for this measurement.
 
I recplaced all 6 electrolytic caps on the NA07761, they are brand new Nichicon.
I looked at that X cap on the Contorl board NA07772 and it "appears" to be good...

I pulled up the service manual to test the DC Offset, and this is what I found.

L channel TPO TPE 57 mv
R channel TPO TPE 56 mv

Power Supply Votage TP HB 69.1
 
L channel TPO TPE 57 mv
R channel TPO TPE 56 mv
Click to expand...
Not enough DC to trigger protect circuit. Need to check the protect circuit supply voltages..
 
Unless I am reading something wrong, (highly likely) the service manual says I should be reading 50 mv on those pins +-5 mv.....

How do I check the protection circuit voltages?
DC
 
Just a note....the buzzing sound I hear reminds of the sound i heard when a relay was not getting enough voltage to close on a Pioneer unit I had, though this is much fainter...
 
Check fuses F701 & 702 with your meter.
There should be around 11.5-12vdc at the emitter and collector of TR702.

Have you replaced caps ...
C701 4700µf/16vdc
C702 10µf/16vdc
C703 1µf/50vdc
 
The fuses are good.

C701 UHE1E472MHD 4700uf 25v
C702 UKT1C100MDD 10uf 16v
C703 UKT1H010MDD 1uf 50v

As I was testing, the relay clicked and the protection light went off.
Now it is up and running, all my solder joints look good, but may one was not making good contact?
 
As I was testing, the relay clicked and the protection light went off.
Now it is up and running, all my solder joints look good, but may one was not making good contact?
Click to expand...
Cold solder connection on that board somewhere.
 
I went back over the solder connections and some others that I didnt replace, now the unit is powered up and running nicely....

thanks for all your help, I really appreciate this resource!
Don
 
Are you getting 11.5 - 12 vdc on TR702 emitter and/or collector?
 
I have to open up the unit and test it. I know where that transistor is, but I am not exactly surre how to test it for voltage.

It takes a minute or so for the protection light to come off when I first turn it on, after it has been on for a while I can shut it off and turn it back on it it only takes about 5 seconds....there must still be energy stored in the caps after it has been on....

I tried to look up how to test transistors for voltage on the internet, but I couldnt find any articles....

Thanks again,
DC
 
bikerdon said:
I have to open up the unit and test it. I know where that transistor is, but I am not exactly surre how to test it for voltage.

It takes a minute or so for the protection light to come off when I first turn it on, after it has been on for a while I can shut it off and turn it back on it it only takes about 5 seconds....there must still be energy stored in the caps after it has been on....

I tried to look up how to test transistors for voltage on the internet, but I couldnt find any articles....

Thanks again,
DC
Click to expand...

http://www.audiokarma.org/forums/showpost.php?p=390997&postcount=1
 
All voltage measurments are at "chassis ground" reference.

Ground the negative meter lead to "chassis" ground. With the positive meter lead - probe the transistor leads.Be extremely careful not to short out two transistor leads or you will cause further damage.


Note: Suggest acquiring a pair of "mini grabber" test leads for your meter.
http://www.amazon.com/Banana-Minigr...924677&sr=8-5&keywords=minigrabber+test+leads
 
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have had flaky transistors do this sort of thing .sometimes nudging them a little will make them work again for a while .or gently heating them .
 
I tested it as you described, I am not getting anywere near 12 vdc
I am testing
.68 on the left lead when looking at the PCB diagram
.003 middle
.000 right...
 
OK.My bad! .On the M-70 you can't use chassis ground. Use TPE test point E for the ground.
Around that transistor TR701 you should be reading
C---14vdc
B---12vdc
E---11.4vdc
 
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