Badcaps.net Forum
Go Back   Badcaps Forums > Troubleshooting Hardware & Devices and Electronics Theory > Troubleshooting Audio Equipment
Register FAQ Calendar Search Today's Posts Mark Forums Read

 
Thread Tools Display Modes
Old 04-15-2018, 05:28 PM   #1
Bterrier
Member
 
Join Date: Apr 2018
City & State: CT
My Country: USA
I'm a: Knowledge Seeker
Posts: 57
Default TOA 900 series ii Amplifier P-912MK2

Hello everyone,

I have a TOA P-912MK2 70v amplifier. The protect light is on and the amplifier will not function.

Can someone assist me in troubleshooting this issue? I have only done a basic visual inspection to ensure the issue isnt obvious(checked fuses on board)
Bterrier is offline   Reply With Quote
Old 04-16-2018, 11:31 AM   #2
redwire
Badcaps Veteran
 
Join Date: Dec 2010
City & State: Alberta
My Country: Canada
Posts: 984
Default Re: TOA 900 series ii Amplifier P-912MK2

Typically, shorted power amplifier transistors trip the DC protection circuit.
Normally at idle you have 0V DC at the loudspeaker terminals, unless transistors have failed.

I could not find a service manual or schematic for the "P" series, but did for the "W" series (wall mount) and "A" series, like A-912MK2 which may be similar. It looks like the same family 906 or 912 or 924 just different power output.

I would start testing transistors in the power amplifier section, and look for burnt parts. The output transistors, drivers, pre-drivers, emitter resistors I would test.

I see TOA forgot protection diodes to the massive 70V line output transformer, so it's not a tough design despite the reliability claims.
redwire is offline   Reply With Quote
Old 04-16-2018, 05:57 PM   #3
Bterrier
Member
 
Join Date: Apr 2018
City & State: CT
My Country: USA
I'm a: Knowledge Seeker
Posts: 57
Default Re: TOA 900 series ii Amplifier P-912MK2

Quote:
Originally Posted by redwire View Post
Typically, shorted power amplifier transistors trip the DC protection circuit.
Normally at idle you have 0V DC at the loudspeaker terminals, unless transistors have failed.

I could not find a service manual or schematic for the "P" series, but did for the "W" series (wall mount) and "A" series, like A-912MK2 which may be similar. It looks like the same family 906 or 912 or 924 just different power output.

I would start testing transistors in the power amplifier section, and look for burnt parts. The output transistors, drivers, pre-drivers, emitter resistors I would test.

I see TOA forgot protection diodes to the massive 70V line output transformer, so it's not a tough design despite the reliability claims.

Thanks, I was able to obtain the service manual from TOA tech support today. I will start by checking the components you mentioned.

I am new to this type of electronics work, so although I have the service manual, any advice or input is appreciated.
Bterrier is offline   Reply With Quote
Old 04-21-2018, 09:26 PM   #4
Bterrier
Member
 
Join Date: Apr 2018
City & State: CT
My Country: USA
I'm a: Knowledge Seeker
Posts: 57
Default Re: TOA 900 series ii Amplifier P-912MK2

I tested the transistors which appear to be good. Also tested many of the resistors and there are a few which are questionable to me. R146 and R145 are rated for 680 ohm. I am getting a reading on both at 1.9k.

For the diodes, D104 gives me a reading of 0.57 and when reversed I get a reading of 1.229. D109 gives me a reading of 0.62 and 0.65. There are about 5 diodes in total that I am getting questionable readings from, but again I am new to this so maybe I am not testing properly? I have my DVM set to diode mode when testing.

Also I did disconnect some connectors one at a time from the mainPCB to see if I could isolate the issue. When I disconnect CN106 from the PCB, the protect light turns off. This is the connector that connects the condenser to the PCB.

At CN 106, I get a voltage reading of 32.5 across pins 1&5.
At CN601 I get a voltage reading of around 32.5v on pins 1&2 and the same reading on 2&3

Also, there is a small area on the mainPCB which appears to be slightly blackened in the area of Q105 and Q106, although I did test those transistors and they did appear okay.
Bterrier is offline   Reply With Quote
Old 04-22-2018, 11:33 AM   #5
redwire
Badcaps Veteran
 
Join Date: Dec 2010
City & State: Alberta
My Country: Canada
Posts: 984
Default Re: TOA 900 series ii Amplifier P-912MK2

If you can post the schematic you obtained, that would help.

If a resistor (in circuit) reads higher than rated value, reverse your meter's leads and see if the reading changes. Sometimes DC voltage stored on a capacitor nearby upsets the multimeter reading. Otherwise, the resistors are damaged I would say.

If the output transistors shorted, the fusible emitter resistors 0.22R would be open circuit. Some resistors used here are fusible types and look fine but can go open circuit. They have a square symbol next to them on the schematic.

I would keep checking parts. Or you can take more voltage readings with the amp powered on, carefully at each transistor and write them down. This is another way to troubleshoot.
redwire is offline   Reply With Quote
Old 04-23-2018, 07:24 AM   #6
Bterrier
Member
 
Join Date: Apr 2018
City & State: CT
My Country: USA
I'm a: Knowledge Seeker
Posts: 57
Default Re: TOA 900 series ii Amplifier P-912MK2

Thanks, I will double check those resistors and reverse the leads to see if the readings change.

See attached schematics and troubleshooting guide.


p912mk2 schematic.jpg

p912mk2 schematic2.jpg

p912mk2 schem3.jpg

p912mk2 troubleshoot.jpg
Bterrier is offline   Reply With Quote
Old 04-23-2018, 10:43 AM   #7
R_J
Badcaps Veteran
 
Join Date: Jun 2012
City & State: ☻
My Country: Canada
I'm a: Professional Tech
Posts: 2,701
Default Re: TOA 900 series ii Amplifier P-912MK2

R145 & 146 are 2k 2watt, so they are ok.
What is the voltage that is measured at (L101) or where the two .22 ohm 5 watt resistors connect together. Note the polarity of the voltage if it is not zero. You can NOT look at a transistor and say it looks ok, you need to at least check them with a meter, even if the transistor is leaky or if a resistor is (ie higher than its value) is enough to unballance the amp and trip the protection circuit

Last edited by R_J; 04-23-2018 at 10:49 AM..
R_J is offline   Reply With Quote
Old 04-23-2018, 12:02 PM   #8
redwire
Badcaps Veteran
 
Join Date: Dec 2010
City & State: Alberta
My Country: Canada
Posts: 984
Default Re: TOA 900 series ii Amplifier P-912MK2

There are many different approaches to fixing audio power amplifiers.
You can start at the power supply, the amp input, the output, the middle etc. or use your eyes to see damaged parts or, use educated guesses. The point is to be a detective and have fun.


Note this amplifier uses a servo circuit op-amps M101 which work to keep the (idling) output at 0VDC. I would also check if its power +/-15V from zeners D109, D110 is OK. Sometimes the amp is fine, but it's the support circuitry that is misbehaving.
redwire is offline   Reply With Quote
Old 04-23-2018, 10:09 PM   #9
Bterrier
Member
 
Join Date: Apr 2018
City & State: CT
My Country: USA
I'm a: Knowledge Seeker
Posts: 57
Default Re: TOA 900 series ii Amplifier P-912MK2

Quote:
Originally Posted by R_J View Post
R145 & 146 are 2k 2watt, so they are ok.
What is the voltage that is measured at (L101) or where the two .22 ohm 5 watt resistors connect together. Note the polarity of the voltage if it is not zero. You can NOT look at a transistor and say it looks ok, you need to at least check them with a meter, even if the transistor is leaky or if a resistor is (ie higher than its value) is enough to unballance the amp and trip the protection circuit
Yes I see now that R145 and R146 are 2k on the schematic. I was looking on the parts list which shows them as 680 ohm.

I did measure the voltage at L101 and is 0V.

For the transistors, i did use a meter to test. I set the meter to diode mode and tested across B&C, then reversed the leads. I don't remember the voltage reading but I got 0L one way and a reading when reversed. Is this the correct way to test transistors?

Also, can you please advise on the proper way to check the 3 pin emitter resistors?
Bterrier is offline   Reply With Quote
Old 04-23-2018, 10:21 PM   #10
Bterrier
Member
 
Join Date: Apr 2018
City & State: CT
My Country: USA
I'm a: Knowledge Seeker
Posts: 57
Default Re: TOA 900 series ii Amplifier P-912MK2

Quote:
Originally Posted by redwire View Post

Note this amplifier uses a servo circuit op-amps M101 which work to keep the (idling) output at 0VDC. I would also check if its power +/-15V from zeners D109, D110 is OK. Sometimes the amp is fine, but it's the support circuitry that is misbehaving.
Thanks, I will check the voltage on D109 and D110 next
Bterrier is offline   Reply With Quote
Old 04-23-2018, 10:23 PM   #11
Bterrier
Member
 
Join Date: Apr 2018
City & State: CT
My Country: USA
I'm a: Knowledge Seeker
Posts: 57
Default Re: TOA 900 series ii Amplifier P-912MK2

I also failed to mention that there is a portion of the board which appears to have possibly overheated. Please see attached photo and let me know what you think.


PCB.jpg
Bterrier is offline   Reply With Quote
Old 04-23-2018, 10:40 PM   #12
R_J
Badcaps Veteran
 
Join Date: Jun 2012
City & State: ☻
My Country: Canada
I'm a: Professional Tech
Posts: 2,701
Default Re: TOA 900 series ii Amplifier P-912MK2

The 3 pin resistors are just two resistors with a common center pin (the single pin end), they should read .22 ohms (basically a short)
Since there is 0 volts on L101 which is the amp output. that tells me the amp is biased properly.
Have you checked the thermal cutout (listed as thermostat)
Check the voltages on M102 (protection ic) it also drives the relay. if the relay does'nt close you get no audio out. That circuit also drives the protection led
That pc board has been running hot for a long time to do that, it did not just turn dark all of a sudden. likely due to the large resistors in that area.
You may want to remove the pc board and check the solder connections, if the boards been running hot the solder can break down and cause poor connections, that might be whats wrong.

Last edited by R_J; 04-23-2018 at 11:00 PM..
R_J is offline   Reply With Quote
Old 04-24-2018, 12:20 AM   #13
redwire
Badcaps Veteran
 
Join Date: Dec 2010
City & State: Alberta
My Country: Canada
Posts: 984
Default Re: TOA 900 series ii Amplifier P-912MK2

That's a BBQ, Q105/Q106 appear to have been running very hot. These PA amps are on 24/7 too.

R111 is 270R on the schematic but pic looks like 27R installed on this board??? That could cause it.
redwire is offline   Reply With Quote
Old 04-24-2018, 10:26 PM   #14
Bterrier
Member
 
Join Date: Apr 2018
City & State: CT
My Country: USA
I'm a: Knowledge Seeker
Posts: 57
Default Re: TOA 900 series ii Amplifier P-912MK2

Quote:
Originally Posted by redwire View Post
That's a BBQ, Q105/Q106 appear to have been running very hot. These PA amps are on 24/7 too.

I agree that they appear to have been running very hot. Im wondering if somewhere within that area is the problem. I just cant seem to find it.


R111 is 270R on the schematic but pic looks like 27R installed on this board??? That could cause it.

I see what you are saying about R111. It shows 270 on the schematic, but the parts list shows 27R. I tested resistance on R111 so it is the 27R installed.

Looks like maybe a typo?
Bterrier is offline   Reply With Quote
Old 04-24-2018, 11:01 PM   #15
Thedoc298
New Member
 
Join Date: Apr 2018
City & State: Union City
My Country: United States
I'm a: Knowledge Seeker
Posts: 3
Default Re: TOA 900 series ii Amplifier P-912MK2

Quote:
Originally Posted by R_J View Post
The 3 pin resistors are just two resistors with a common center pin (the single pin end), they should read .22 ohms (basically a short)
Since there is 0 volts on L101 which is the amp output. that tells me the amp is biased properly.
Have you checked the thermal cutout (listed as thermostat)
Check the voltages on M102 (protection ic) it also drives the relay. if the relay does'nt close you get no audio out. That circuit also drives the protection led
That pc board has been running hot for a long time to do that, it did not just turn dark all of a sudden. likely due to the large resistors in that area.
You may want to remove the pc board and check the solder connections, if the boards been running hot the solder can break down and cause poor connections, that might be whats wrong.
I agree with the poor connection. I would flip that board over and fix all the crap solder joints and they will be their.
Thedoc298 is offline   Reply With Quote
Old 04-25-2018, 10:22 AM   #16
R_J
Badcaps Veteran
 
Join Date: Jun 2012
City & State: ☻
My Country: Canada
I'm a: Professional Tech
Posts: 2,701
Default Re: TOA 900 series ii Amplifier P-912MK2

I looked at a couple other schematics and R111 is shown as a 27 ohm
It may depend on the version 'A-912' vs 'W-912'
Here is another manual, schematic at the end
Attached Files
File Type: pdf toa_w-906a_w-912a_sm.pdf (3.33 MB, 20 views)
R_J is offline   Reply With Quote
Old 04-25-2018, 12:05 PM   #17
redwire
Badcaps Veteran
 
Join Date: Dec 2010
City & State: Alberta
My Country: Canada
Posts: 984
Default Re: TOA 900 series ii Amplifier P-912MK2

Let's go with R111 as 27R (sch typo is 270R) and look for trouble elsewhere.

R111 at 27R sorta matches the listed voltages for about 6.4mA flowing in each Q105, Q106, Q107, Q108. That's around 0.25W dissipation for them TO-92. Usually these voltage amplifier stages use TO-126 package parts and they run warm and need small heatsinks.

I'm mentioning this because it's hard to know if the hot spot is due to design at 24/7 or due to a failure, as Q107 and Q108 form a current-mirror so it's difficult to roast Q105, Q106 as the currents are constant.

If OP can measure some voltages on the transistors, or just keep testing parts. Q105/Q106 are probably dead.
redwire is offline   Reply With Quote
Old 04-25-2018, 09:32 PM   #18
Bterrier
Member
 
Join Date: Apr 2018
City & State: CT
My Country: USA
I'm a: Knowledge Seeker
Posts: 57
Default Re: TOA 900 series ii Amplifier P-912MK2

Measured voltage on Q105 and Q106. Base = 42.5v on both. Emitter = 43v on both. Collector= 1.1 and 1.19 I believe....was difficult to get probe in and ensure good contact
Bterrier is offline   Reply With Quote
Old 04-25-2018, 10:03 PM   #19
R_J
Badcaps Veteran
 
Join Date: Jun 2012
City & State: ☻
My Country: Canada
I'm a: Professional Tech
Posts: 2,701
Default Re: TOA 900 series ii Amplifier P-912MK2

If any of the voltages on the drivers were off, the amp offset would be off, and you said you measured 0 volts on L101, Just to double check, measure the voltage on the white .22ohm resistors, you can measure on the single pin end of the resistor
What voltages do you have on ic labled M102 ?
And check continuity between L101 and relay contact (where R139 connects to relay) That will check the thermal cutout

M102 drives the relay, the relay contacts are S1 that connect the amp output to the speakers. the output of that ic also drives the protection led. If its hard to get at the ic pins, find a component that is off the pin IE R142 should have +45 on one side and 7.3 on the other.

Last edited by R_J; 04-25-2018 at 10:33 PM..
R_J is offline   Reply With Quote
Old 04-25-2018, 11:01 PM   #20
Bterrier
Member
 
Join Date: Apr 2018
City & State: CT
My Country: USA
I'm a: Knowledge Seeker
Posts: 57
Default Re: TOA 900 series ii Amplifier P-912MK2

Quote:
Originally Posted by R_J View Post
If any of the voltages on the drivers were off, the amp offset would be off, and you said you measured 0 volts on L101, Just to double check, measure the voltage on the white .22ohm resistors, you can measure on the single pin end of the resistor
What voltages do you have on ic labled M102 ?
And check continuity between L101 and relay contact (where R139 connects to relay) That will check the thermal cutout

M102 drives the relay, the relay contacts are S1 that connect the amp output to the speakers. the output of that ic also drives the protection led. If its hard to get at the ic pins, find a component that is off the pin IE R142 should have +45 on one side and 7.3 on the other.

So I was attempting to measure voltage on the white resistors when I started to smell something burning and F101 (250v fuse) popped.

I do have continuity between L101 and r139.

I am now unable to measure voltage at M102 but I did measure it the other night. I believe it was 43v on one side and 144mv on the other side. I cant be 100% sure of thoses numbers though. I will have to check when I get a replacement fuse in.
Bterrier is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump



Badcaps.net Technical Forums 2003 - 2018
Powered by vBulletin ®
Copyright ©2000 - 2018, Jelsoft Enterprises Ltd.
All times are GMT -6. The time now is 10:10 AM.

Did you find this forum helpful?