TOA P-924MK2 on Protect

[JP-Stereo]

Hi, i have a TOA P-924MK2 with the Protect Led ON.

I've check the outputs transistors and nothing is shorted.
All the transistors are also passing the diode test.

I have the schematic of the old P-912A but it doen't have the Protect led on this model.

Anybody have a hint if there is another model that may have a similar Protect circuit ?

Thanks.

[x-pro]

What is the DC voltage at the amplifier output before the relay? The point where the L101 coil is.

[JP-Stereo]

Hi, I find something before testing for that.
DC must be there for shure.

After testing the diodes and the caps, i removed each transistor to test them out of circuit.

Pre drivers Q101 and Q103 were testing fine in diode mode but a leak test on an old Heatkit TR tester show major leak...
(The b-e and b-c diode test reading was a little lower on the leaky transistors)

I read somewere that since all the stages on this line are DC coupled, a leaky transistor may force all the following to get full on and pass DC..
By chance i have two of these amps, so i will replace these transistors for repair while ordering some.
Currently removing the tr from the second amp.

[x-pro]

You have taken the path of greatest resistance in troubleshooting.
The lack of a repair strategy leads to a lot of time.

[JP-Stereo]

I am always open to learn new strategy...
But i stay on known ground as much as possible, unless guided to try a new way.

Something has improved, no more Protect led after the 5 seconds delay and no smoke !

But the signal led stay off even if i feed a CD signal at the input while set at 0db.
No sound and no DC at the output, testing in 4 ohms with the output TX jumper removed.

Will look at the supply and try to follow the signal...

[x-pro]

No sound and no DC at the output,

where exactly, on L101 or CN102?

[JP-Stereo]

In fact it's 0.016vdc after L101.

I just scope R208 and R204 and i have no signal, in any input setting.
I removed D201-202 and C210-212 for testing = good, reinstall and now it's working.
Good signal and gain in the input and filter, even more i have output without DC.
I am reassembling it for duty testing.

[x-pro]

For the future. Always start any repair by measuring all voltages in the devices.

[JP-Stereo]

Thanks for the time saving hint, i will apply it.

Too good to be real...
At first everything looked fine, the Protect delay, the Signal led when volume goes up.
But the Normal led was flashing to the beat and the Peak was responding when pushing it...

After 5 minutes, the sound begin to fade and cut, comming back when the volume was moved.
and i lost the 3 first leds, Signal, Normal and Peak.
The Power led is ok and the Protect too at power up.

This amp is very similar to the first schematic, but one exeption is for the Signal, Normal and Peak leds that are not in the schematic.
I have another schematic that have more difference but at least a led circuit is there.

I figure, i will begin by checking the supply for this led board.

[x-pro]

After the signal fades again, immediately turn off the power and check the TH1 thermal fuse contacts and A1 relay contacts while they are "hot", located at the amplifier output, for zero resistance.

[JP-Stereo]

Hi, i choose to replace the 2 IC (M601 & M602 = 4558D) on the LED board (since they are coupled to the output line and i got them on hand.
The signal is the same before and after the relay but it did not fade again yet.
I didn't find any thermal fuse (TH1) on this board or on the heatsink.
I test it with a 4 ohm resistive dummy loads and with the integrated transformer, sound and signal led ok.
On a less than 3.75 ohm load i loose the signal Leds.
So the last problem seem to involve these two IC.

The initial problem was Q101 and Q103 leaking big time.
I also found leaking Q101 and Q103 on a second amp from the same venue.
Power surge or lightning probably.

On the second amp same model, Q304 (not on any schematic) was leaking too.
This is a kind of buffer between the main board and the Led board, on this model, CN302 have only 3 pin and the transistors are on the main board.

And Q113 A970 is leaking 1.5ua CtoE.
I checked with replacements and i have other transistor from different batches that leak 0.75ua and 1ua.
Since these are DC coupled what is the leaking tolerance limits ?
Is it really normal to get low leaking NEW transistors ?

Beside that i am only waiting for a 2SA1015 (Q304) feeding signal to the Led opamps.

[x-pro]

Q113 and Q112 are elements of the output stage protection circuit. They work when the current through the output transistors becomes high. This can happen with a very large signal and low load. It is necessary to try to drive the amplifier into such a mode. Opening at high current, they shunt the bases of the pre-output stage at Q110, Q111, thus limiting or disabling the output signal at the load.
In general, the amplifier will work without these transistors. Only at very high signal levels may a rattle appear.
Leaky transistors should not be used at all. If they do not behave badly now, their leakage will eventually increase to a critical situation.

[JP-Stereo]

Q112 is a A970 or 2SA970, both of my original (i have 2 of these amplifier) show a small leakage of 0.75ua and 1ua.
I verified a batch of new transistor i just received and it's the same, even a litthe worse.
Is it possible that at that level (ua) such a leak is not disruptive ?

One of the amp is working with this transistorcurrently.
Is someone have a 2SA970 that is not leaky at all ?

[JP-Stereo]

Just finished the second amp.
Same problem, Q101 and Q103, and Q102 was a substitute, so i changed all four including Q104.
Q112 was leaking CtoE over 1.5ua so i replaced it with one leaking 0.75ua.
Q304 was also leaking CtoE to the roof so replaced it.

After that, no Protect but no output or signal Led.
The signal was present at CN104-4.
There are 2 Zenner D104 and D105 protecting the input, both conducting both ways...
Once these ones replaced, good output.

Next i will test them at normal level for a day.
The problem seem to be from a lightning on the phone line wich feed these amp.
Only the inputs were affected.

[x-pro]

And Q113 A970 is leaking 1.5ua CtoE.
I checked with replacements and i have other transistor from different batches that leak 0.75ua and 1ua.
Since these are DC coupled what is the leaking tolerance limits ?
Is it really normal to get low leaking NEW transistors ?

To answer the question.
In most cases when repairing amplifiers, checking transistors for leakage is not done. It is enough to check the state of its transitions. Or when the amplifier does not behave in the best way when checking its modes and capabilities. Checking for leakage, both in terms of currents and other parameters, is done when installing new components of the pre-output and output stages. It is also done when debugging and building new Hi-Fi amplifiers. However, practically all elements are tested there.
In the device that has been in use, many things are already fine-tuned and balanced. Simply replacing a few transistors according to part number in different nodes will not cause a problem in most cases.
Of course, if there are a lot of same-type elements, it is more advantageous than one existing one. From the pile, it is better to choose the one with the lowest leakage current).
But sometimes it is possible to apply an analog.