TOA 900 series ii Amplifier P-912MK2

[Abdl78611]

Thanks, Bro For sharing this information

[X006480jr]

Re: TOA 900 series ii Amplifier P-912MK2

Update: The secondary problem (faint volume level) was due to a bad integrated circuit M204 in the "'remote master control circuit" of the preamp (a dual OpAmp: NJM4558DD). I detected it using freeze spray. Bought, installed, works, done. The only remaining problem is that I had already purchased a used / identical amp from eBay because we needed it for our church PA system ... so now I have a spare unit that I don't need! Guess I'm headed back to eBay or Craig's list or something.

[X006480jr]

Re: TOA 900 series ii Amplifier P-912MK2

I wanted to thank you guys for your insights, and perhaps return the favor by relaying my recent experience.

My TOA A-912MK2 amplifier died after a power surge. It was stuck in “protect mode”. Based on Post #2, I found that the idle voltage was +6 Volts instead of Zero (thanks “redwire”!), so the protection circuit was working as designed. Post #6 provided a schematic showing the expected voltages at various transistor leads (thanks “Bterrier”!). So, I worked backwards from the output transistors in the Power Amp circuit. Transistor Q101 was shorted. I replaced it (and its mate, Q102), and I validated the adjacent resistors, which were OK.

That fixed the Protect Mode problem! Yay! But it revealed a second problem – the output volume is extremely low. I haven't started diagnosing this new problem yet, but the pre-amp is the primary suspect because the power amp seems to work fine … this unit lets you feed a line-level signal directly into the power amp (bypassing the preamp), whereupon the sound comes out of the speaker just fine. I'll post progress as the saga continues.

Some side notes:

This is a new hobby for me (recently retired, Yay!). I've fixed 5 amplifiers so far, but I need better understanding and experience to streamline my diagnostic procedure. Basically, I measure each component in-circuit in sequence (R101, R102, etc. then C101, 102, etc.), listing the results on a piece of paper. Some are inevitably suspicious, so I de-solder and re-measure them, recording the new results. Naturally, I try to narrow my focus to a given circuit first, such as the protection circuit, or power amp circuit, or the pre-amp circuit, if I can.

I purchased an LCR meter, which supposedly isolates components so you can test them in-circuit, but I still found some components that measured way-off spec, only to measure fine once removed from circuit. So, I think the LCR meter speeds things up, but it's not perfect.

I was able to test most of the transistors in-circuit, de-soldering only a few that tested suspicious. Perhaps I have some dyslexia, because my first round of testing found all the transistors OK, only to realize that I had confused the pinout on two of them (ECB vs. BCE). Yes, one of those was the defective one! Doh!

I had difficulty finding replacement transistors. I purchased transistors with comparable electrical specs, only to find they had different pinouts. Don't laugh, but I installed them anyway by bending the leads around each other into the proper holes (using a piece of wire insulation as a protective sleeve). Viola! It looks ugly, but it fixed the Protect Mode problem. If I get the secondary problem fixed, I'll go back and shop for the correct transistors (I've resumed the hunt, but it's been exasperating!).

I was careful to avoid electrocution. That might be second nature to you professionals … and I've been fixing electrical appliances devices all my life … but probing dozens of tiny electrical connections in such tight quarters was a bit intimidating. I built a “dim bulb tester”. I wore heavy(ish) rubber gloves. I removed all metal jewelry. I purchased multimeter test leads with longer probes (and covered most of the long metal probe with heat-shrink tubing, except the tip, to minimize the risk of contacting something inadvertently). I don't have a Variac, nor an isolation transformer, but I wore sneakers with rubber soles. I used just one hand at a time, and I avoided touching anything with my feet or second hand that might constitute ground. So far, I'm alive to tell about it.

I wanted to give kudos to the manufacturer, TOA. They responded quickly to my request for a Service Manual. They offered advice that the Power PCB is often the culprit behind Protect Mode situations. And they still sell that PCB. This wasn't my problem, but it's good to know that they support their products long term, unlike some manufacturers. They said this amp has been in production for 30 years! The one downside is that their schematics don't indicate voltages like the one in post #6 does.

I hope my experience helps someone else with their amplifier repair.

[Peterand]

Re: TOA 900 series ii Amplifier P-912MK2

Thanks so much for the info folks!

I would have figured it out but this Forum is awesome!
I was in the Electronic Repair business back in the mid-late 80's. No capacitor testers available, no Internet, no computers.
This is way better!

We have a ChurchBell program that rings throughout the Town and the amp quit with the Protect Light on.

Tested the electrolytic capacitors. C120, C117 and C116 were bad. Changed them out and started working.

[amusetronics]

Re: TOA 900 series ii Amplifier P-912MK2

I fixed mine, check all the caps in the relay circuit. C114 and c116 were bad, check c118 ci20 because mine were gone from heat.

[RichyRix]

Re: TOA 900 series ii Amplifier P-912MK2

Thank you RJ. Actually I used to sell Toa installed hundreds of them. Have a call in to see how much replacement pcb is, as I misread this thread i need the scheme for a-912mk2. I'll see if they'll send that to me.
Thank you for replies

[R_J]

Re: TOA 900 series ii Amplifier P-912MK2

Good luck with that, Some don't like to share, or give a thank you even.
See if this one is close

[RichyRix]

Re: TOA 900 series ii Amplifier P-912MK2

Hi, yes I saw that but poster said he received the manual from Toa thought he may be able to send it to me. The scheme posted is almost impossible to work with.😊

[R_J]

Re: TOA 900 series ii Amplifier P-912MK2

Check post #6, I think thats the best that can be found as far as a schematic.

[RichyRix]

Re: TOA 900 series ii Amplifier P-912MK2

Hi, any chance you can forward service manual to me. In same boat protect on.
Let me know.
Thanks

[Bterrier]

Re: TOA 900 series ii Amplifier P-912MK2

I've also a protect light issue on my a-912mk2. Did you ever resolve your issue?

I've also a protect light issue on my a-912mk2. Did you ever resolve your issue?

Just seeing this message. I haven't had much time lately to work on this project, hoping to resolve this issue soon. I did replace transistors Q114-117 as well as both emitter resistors. I think the protect light may be off now, but I am having an issue with the fuse blowing, so it is hard to say for sure.

[Tim Polzin]

Re: TOA 900 series ii Amplifier P-912MK2

I've also a protect light issue on my a-912mk2. Did you ever resolve your issue?

[R_J]

Re: TOA 900 series ii Amplifier P-912MK2

How was Q111 faulty? was it shorted? if yes which pins were shorted, If it was shorted E-C check R117, R129 and R130. If it was shorted C-B, check Q108, R114, R115
If Q111 was shorted it would definitely cause Q117 and Q116 to be turned on HARD. and maybe even shorting them again. It would definitely cause the fuse to blow!

[R_J]

Re: TOA 900 series ii Amplifier P-912MK2

The bridge rectifiers are likely ok, all they are is 4 seperate diodes in one package. the fuse is blowing due to over current by the amp outputs being biased on hard.
MJE15035 (PNP)
2SA1837 will also work but it is a to220 all plastic (metal tab is not exposed)

[Bterrier]

Re: TOA 900 series ii Amplifier P-912MK2

I have found Q111 to be faulty. Part number 2sa1011e

I believe this may be a discontinued item, so I am not finding a data sheet to reference for matching a replacement transistor. Can anyone point me to a comparable replacement?

[Bterrier]

Re: TOA 900 series ii Amplifier P-912MK2

ok thanks for the advice re: the transistors. I will use a variac for applying power and monitor voltage at L101.

Going off the service manual for blowing fuse F101. it wants me to test bridges d112 and d601. Can you tell me how to go about testing these?

D601 part number - d15xb60

D112 part number +s2vb

[R_J]

Re: TOA 900 series ii Amplifier P-912MK2

You say you are holding off replacing q103-q113 Are they bad? did you remove them? Remember I can't tell what you have done at your end. All those transistors are DC coupled (direct coupled) if even one is leaky it will apply the wrong voltage to the next one. if the base to emitter voltage is about 0 -.3 volts roughly the transistor is not really turned on, but once that e - b voltage is .6 or higher that transistor is turned fully on. in other words there is the collector and emitter are connected together. so if the collector is at 45 volts and it is conducting to ground via a .22 ohm resistor. BOOM Thats roughly 205 amps through the resistor!
Thats why a variac is used, by increasing the ac slowly you monitor the voltage at L101, and see if it stays around 0 or if it is going to the + or - side there is still a problem.
At least place a light bulb in series with the ac line to limit the current .
BTW I hate typing
as for the transistors, look up each data sheet and compare them, Vceo, Ic and Pc (collector power) are important, also pay attention to the lead configuration, they could be different (e-c-b or e-b-c)
I would just use originals and not start subing
The 2sc949 is close to the 2sc2240, but collector current is only 50ma vs the 100ma for the 2sc2240, so it would likely work for testing with no input, once the amp is turned up the transistor would likely fail taking out the rest of the transistors etc..

[Bterrier]

Re: TOA 900 series ii Amplifier P-912MK2

I did replace the larger tranistors q114-117. I actually only did 2 at first, as well as the dual emitter resistor. Powered on and blew the other emiiter resistor, so then I replaced the other 2 transistors

Still blowing the fuse, but for the brief half second the unit is powered om, I believe the protect light is now off. Im a bit confused because when I initially started this process, the unit would stay powered on without blowing the fuse.

I have been holding off on replacing q103-q113 because I realizd when I was replacing q114-117 that I would get a totally different reading once I removed the transistor from circuit. So I am not sure they are actually bad.

I have an old onkyo which I have been slowly removing parts from to use for repairing other equipment. I found some a949 tranistors and k332 tranistors. Would the a949 be an okay replacement for q103-108? (for the ones that are PNP) and would k332 work as a replacement for q109-113?

What else could be causing this fuse to blow? Im not great at reading the schematics to attempt to diagnose an issue. I understand the symbols within a schematic, but identifying potential issues is beyond my current ability.

[R_J]

Re: TOA 900 series ii Amplifier P-912MK2

If you are not in a hurry, get them from China/ebay https://www.ebay.com/itm/5pairs-2SD6...-/261049128012
another option is the NTE subs, 2sb647 = NTE383

[Bterrier]

Re: TOA 900 series ii Amplifier P-912MK2

R121 and R122 are basically in parrallel, one end is connected together and the other ends are connected via R123 and R124 so R121 or R122 will read one half or 500 ohms (0.493k)
R127 and 128 will each read 1K because R126 is open.

Thanks for the clarification.

After learning how to properly test a transistor with your assistance, I went back to the area of the board that looks overheatd and retested all of the small transistors in the area. About half of them tested bad.

I have the part numbers but cannot locate them because I believe they have been discontinued. How do I go about matching up replacements?