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TOA amplifier repair 70v
TOA amplifier repair 70v
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Old 15th April 2018, 05:47 PM   #1
Bterrier is offline Bterrier  United States
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Default TOA amplifier repair 70v

Hello everyone,

New to this forum but I have an amp that I am trying to repair. It is a TOA 900 series ii P-912MK2 70v amplifier. The unit is in protect mode and will not operate. I pulled the cover to see if there is anything obviously wrong, which there is not. Fuses are good and capacitors seem to be intact.

Can you guys help me troubleshoot this issue?

Thanks in advance
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Old 15th April 2018, 09:58 PM   #2
AMV8 is offline AMV8  United Kingdom
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You may be able to download the service manual for your amplifier from the TOA website.
I do not have your amplifier but I do have other TOA amps such as the P300d and found detailed service and owner manuals for all of them on the TOA website in pdf format.

Don
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Old 15th April 2018, 11:02 PM   #3
Bterrier is offline Bterrier  United States
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Quote:
Originally Posted by AMV8 View Post
You may be able to download the service manual for your amplifier from the TOA website.
I do not have your amplifier but I do have other TOA amps such as the P300d and found detailed service and owner manuals for all of them on the TOA website in pdf format.

Don
Thanks for the advice. I did search the TOA website as suggested. I can only locate the operating manual, no luck in finding the service manual. If anyone knows where I can obtain the service manual, please let me know.

Or if anybody can be of assistance in troubleshooting. Thanks
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Old 16th April 2018, 01:37 PM   #4
indianajo is offline indianajo  United States
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One usually uses a DVM to find simpler problems in amps.
FIrst off, use a clip lead on the negative lead of the meter, to use only one hand testing. Current across your heart over 24 vdc can stop it. Jewelry on hands wrist or neck can cause class 3 burns at 1 vdc with enough current available. Take it off. Clip the negative lead of the dvm to the speaker return binder post.
Then measure the DC voltage at the input of the relay. You can look at pictures of relays on protection threads as here Niles SI-250 Fix or Re-Purpose? , or on a vendors website like farnell.com. If input voltage to contacts is over about 1000 mv there is a problem with the amp circuits.
If not, there likely is a problem in the DC detect circuit. Sometimes the e-caps dry up and cause undue sensitivity.
Other times just the contacts on the protect relay have oxidized and won't pass low voltage sound.
Have fun.
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Last edited by indianajo; 16th April 2018 at 01:52 PM.
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Old 17th April 2018, 12:28 AM   #5
Ian Finch is offline Ian Finch  Australia
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AFAIK, TOA provide the service manual with the product but there's a low-res schematic and bias setting procedure here: TOA idle current setting process??

There is another related thread with PDF files on that site too but as indianajo says, you need to start by measuring the amplifier's DC voltage at its actual output node (with no input signal or speakers connected) to see what is wrong and hopefully identify the most likely faults at the point where the protection circuit is triggered. Electrically speaking, that point will be just before the output relay contacts but it may be easier to locate by testing DC voltage (with respect to amplifier ground) at either end of the output coil, L101 or the low value resistor in parallel with it (can't read the part no. but it could be R130, 150 or even 180). After that point is a thermostat, relay and switching which routes the output to impedance matching circuits including the output transformer. In any event, you identify any power amplifier problem first.

For protection from further damage, always use a dim bulb tester aka lightbulb limiter in series with the mains supply to the DUT. It's simple and self explanatory protection for the amp but only use a old style, say 75W, incandescent globe here. Google will reveal dozens if not hundreds of different DIY assemblies to choose from but many are fitted with large globes to suit tube amplifiers. A small PA amp will draw much less idle current.
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Old 22nd April 2018, 03:23 AM   #6
Bterrier is offline Bterrier  United States
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So I am definitely a noob when it comes to this thoe of electronics work. I have done some testing, maybe not in the order I should be.

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 reading the, 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.

Can someone explain in layman’s terms what the actual output node is? (so I can chck dc voltage there). Are you speaking of the output terminals on the rear of the amp?

Also, there is a small are 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.
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Old 22nd April 2018, 11:43 AM   #7
indianajo is offline indianajo  United States
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All the tests you did are the least likely parts to be bad. At 10 years of age electrolytic capacitors inevitably fail because they are water filled bottles sealed with cheap rubber. If speaker wire becomes shorted, output transistors tend to fail from the heat, plus the emitter resistors. Sometimes driver transistors are inadequately heat-sinked to save money and fail. Output transistors are large, Drivers are medium sized. Plus if you removed and replaced those transistors & diodes to test them, you likely injected a lot of bad solder joints into your problem. Things go faster if you replace only one part at a time between overall system tests, to prove your work was good, not another problem.
The CN connector could be interesting, but if it connects a "condensor" (obsolete terminology) to the main board you have merely removed the power from the board. The main rail caps will have values between 3300 uf and 20000 uf. They are often connected to the main board by connectors, if not actually on the board. These rail caps are frequently bad, but you don't replace them first as as they are expensive and a badly burned amp that you are not going to fix, you don't want to spend that money first.
The speaker terminal will be after the protection relay, and will measure zero if the unit is in protection mode. Not interesting.
As in the picture listed above, the output relay will be near the emitter resistors, which are 5 watt or 10 watt boxes, usually white, near the output transistors on the heat sink. The values are usually between .51 ohms and .22 ohms. the junction of two of those resistors, one from an npn transistor and another from a pnp, or four if their are two pairs of output transistors, will go to the input of the protection relay. That is where you want to measure the voltage to speaker return. It should be less than 1 v.
The coil of the protection relay will measure some dc voltage, usually 24, when on, and 0 when off. Or like above link, some 24 v offset from a high power supply value. If the relay is not a 24 volt coil (will say on top usually) the coil voltage will match that of the marked value when on.
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Last edited by indianajo; 22nd April 2018 at 11:58 AM.
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Old 22nd April 2018, 12:33 PM   #8
simon7000 is offline simon7000  United States
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Usual failure mode is bad input transistors and their current source.
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Old 22nd April 2018, 05:29 PM   #9
Ian Finch is offline Ian Finch  Australia
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Quote:
Originally Posted by Bterrier View Post
.......Can someone explain in layman’s terms what the actual output node is? (so I can chck dc voltage there). Are you speaking of the output terminals on the rear of the amp?....
A node is a specific and identifiable point of interest in a circuit. The output node here is the common connection point for the 2 pairs of output transistor emitters, where they join together to sum their output currents and then connect it to protection circuits and output impedance matching devices as stated previously. In this PA type of amplifier, there is a lot of clutter associated with impedance matching to various speaker loads so the output node is way upstream from the output terminals and not shown by any marks other than the components referred to.

So, as said, you only need to locate coil L101, its parallel resistor or other parts at that electrical point on the circuit and just measure voltage between there and power supply ground. In a healthy amp, that would likely be <30 mV. In a blown amp, the voltage could be anything up to either power supply rail voltage of +/- 45V.
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Last edited by Ian Finch; 22nd April 2018 at 05:32 PM.
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