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Old 11-29-2022, 09:58 AM   #1
waspie
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Default Aiwa GX-770 Amplifier Protection Circuit

I am by no means an expert on this type of troubleshooting but I can follow direction real well.

I'm no stranger to using a meter and soldering iron so I'm looking for some help in fixing an old amplifier/system I got. This is part of a system "V-1200" by Aiwa where there are several components. My first problem seems to be the amplifier.

It seems to be in protection. When first turned on I hear the inital relay coming on but never the second relay which is connected to the protection IC. I don't see any obvious bad components on the board. If I ground the control side of the protection relay nothing blows up... the relay clicks on and I hear a loud hum from the speaker but this may be due to the pre-amplifier. When the pre-amp is plugged in the 125hz bar of the spectrum analyzer on the amplifier lights up as if its getting a fairly loud signal at about that frequency. I have not yet attempted to jumper the protection relay without the pre-amp as they seem to need each other to work. I may try to jumper them together without the audio out portion connected but I need to find some jumpers to do so and I'm at work currently.

So, I'm asking for help in figuring out why the protection circuit isn't enabling the relay.

The repair manual is available online here:
https://archive.org/details/manual_B...e/n11/mode/2up
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Old 11-29-2022, 10:06 AM   #2
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Default Re: Aiwa GX-770 Amplifier Protection Circuit

Some snips from the manual to highlight what i'm referring to
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Old 11-29-2022, 06:15 PM   #3
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Default Re: Aiwa GX-770 Amplifier Protection Circuit

Jumpering the relay with no source connected still produced a significant hum = bad caps, right?

Pulled the 2x 56v 5600uf and 2x 56v 6800uf caps and put it back together as little as I could. powered up and the protection relay is closing now.

3 of the 4 caps I can set upside down and they sit flat but 1 of them definitely has a slightly pushed up top. I guess i'll try replacing these 4 caps first
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Old 11-29-2022, 08:12 PM   #4
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Default Re: Aiwa GX-770 Amplifier Protection Circuit

Removing the main filter caps and powering up the amp is just plain stupid, you may have damaged the amp beyond repair
The HUM is likely caused by shorted STK output ic's not bad filter caps.
Check the voltage on IC5 and IC6 pins 4,7 It should be near 0 volts, I suspect it is not and likely closer to +or- 50v

You should be able to check the resistance of the internal transistors, check between pins 9and 8 also between 2 and 3

Last edited by R_J; 11-29-2022 at 08:25 PM..
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Old 11-30-2022, 08:06 PM   #5
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Default Re: Aiwa GX-770 Amplifier Protection Circuit

Quote:
Originally Posted by R_J View Post
Removing the main filter caps and powering up the amp is just plain stupid, you may have damaged the amp beyond repair
The HUM is likely caused by shorted STK output ic's not bad filter caps.
Check the voltage on IC5 and IC6 pins 4,7 It should be near 0 volts, I suspect it is not and likely closer to +or- 50v

You should be able to check the resistance of the internal transistors, check between pins 9and 8 also between 2 and 3
no need to be rude, i didn't know.

Measuring resistance between 2 and 3 / 8 and 9 on both ICs shows near infinite.

I guess I was expecting the typical burnt component smell if these were bad and it's not there
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Last edited by waspie; 11-30-2022 at 08:26 PM..
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Old 11-30-2022, 08:28 PM   #6
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Default Re: Aiwa GX-770 Amplifier Protection Circuit

Pins 4 & 7 are the output to the speaker, and should be less than 1 volt, pin 2 is the positive 54v supply, pin 9 is the negative 54v supply.
The inputs pins 1 & 10 voltage needs to be checked as well, The circuit is DC coupled so a faulty component anywhere can cause the output not to be balanced causing the amp to go into protection. (DC voltage can kill a speaker)
You can power on the amp to check voltages, but you must have the power supply filters in place as raw unfiltered dc is not good for the components.
Set your meter on DC and connect it between ground and the output. you can check for the voltage on L1 and then L2. These voltages need to be less than 1vdc

Last edited by R_J; 11-30-2022 at 08:34 PM..
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Old 11-30-2022, 08:44 PM   #7
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Default Re: Aiwa GX-770 Amplifier Protection Circuit

Voltage difference between STK pin 1-10 should be around 4VBE for ~2.8V and anything much higher means the IC is blown. Pin 10 around +1.4V and pin 1 around -1.4V at least.
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Old 11-30-2022, 09:14 PM   #8
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Default Re: Aiwa GX-770 Amplifier Protection Circuit

Got to agree with RJ and redwire. For me, I would have started at the STK power IC’s and see which one is having DC on the output pin, making the protection relay trip.
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Old 12-01-2022, 08:11 AM   #9
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Default Re: Aiwa GX-770 Amplifier Protection Circuit

Quote:
Originally Posted by CapLeaker View Post
Got to agree with RJ and redwire. For me, I would have started at the STK power IC’s and see which one is having DC on the output pin, making the protection relay trip.
If it's worth anything, the relay was never engaging even from start. Not sure that that actually changes anything


Thanks guys, I'll put the caps back on and test.
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Old 12-01-2022, 04:28 PM   #10
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Default Re: Aiwa GX-770 Amplifier Protection Circuit

usually if the relay doesn't click, the amp is in protection mode because of DC in one of the channels. Now that can be at the final stage or even down at the pre amp stage. It also could be something simple, like cold solder joints on these STK bricks.
It's all a learning curve and I am sure the next time you know how to go about it.
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Old 12-05-2022, 06:07 AM   #11
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Default Re: Aiwa GX-770 Amplifier Protection Circuit

Quote:
Originally Posted by CapLeaker View Post
usually if the relay doesn't click, the amp is in protection mode because of DC in one of the channels. Now that can be at the final stage or even down at the pre amp stage. It also could be something simple, like cold solder joints on these STK bricks.
It's all a learning curve and I am sure the next time you know how to go about it.
definitely!

STKs should be here today and I'll have time to solder them in tomorrow.

I'll probably still be waiting as one of the larger of the two filter caps has a bulging top and replacements are on a slow boat.

I'm assuming powering it up with a bad cap is a bad idea.

As for the solder, it all looks pretty good. I haven't gone over it with a fine tooth comb or anything but its all nice and soft solder and there's no obvious cracks anywhere. The only 2 things remarkable is a previous repair to what looks like the circuit for powering it on and discolorations around where both power amps were, they seem get rather warm.
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Last edited by waspie; 12-05-2022 at 06:14 AM..
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Old 12-05-2022, 06:59 AM   #12
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Default Re: Aiwa GX-770 Amplifier Protection Circuit

took the covers off


*edit*

Looking at these with the caps off I don't see anything blown/burnt. All the wires are intact and all the components *look* fine.
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Last edited by waspie; 12-05-2022 at 07:57 AM..
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Old 12-05-2022, 08:08 PM   #13
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Default Re: Aiwa GX-770 Amplifier Protection Circuit

The discoloration on the PCB where the bricks are is very common. These bricks get hot and over the time the solder on the pins become cracked. The last brick amp that I repaired had exact the same symptom. Usually I hit the pins on the bricks with fresh solder first. That fixed the amp. Now if this doesn't fix it, you have to get the service manual and start tracing and finding the offending channel and go from there. Also make sure that there is nothing stuck in the headphone jack or a bad headphone jack. It will prevent audio coming out from the rear too. Look at the block diagram. You got the differential input amp, the driver amp and the final power amp. Check em out too. There are 2 detection circuits I see as well.
If you want something to read you can look at that goose chase on my Pioneer QX949. Given this one didn't have any bricks, but still was interesting how far back the problem was.
https://www.badcaps.net/forum/showthread.php?t=69650

Last edited by CapLeaker; 12-05-2022 at 08:30 PM..
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Old 12-05-2022, 08:56 PM   #14
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Default Re: Aiwa GX-770 Amplifier Protection Circuit

With STK's, I've found it's either broken bonding wires that a tap on the module with a screwdriver can flush out, or shorted transistors with sometimes a fused open bonding wire. You can test the internal transistors with a multimeter to confirm they are bad.
The old STK's look OK, I can't see any fried parts so I am suspicious of another problem.
It might have been run with a fault for a long time to cook and brown the phenolic PC board, but these are just cheap. NAD has the same problem with their amp boards cooking.

The trimpot SE12 looks badly cracked, I would check if it still works. The STK and output would have problems with DC offset.
There is a chance the input JFET's are either way off (DC offset trimpot) or they are damaged.
I would power it up using a variac, something like 1/2 or less AC mains and measure voltages. You could have a good STK but bad calibration (or dud dual JFET) on the JFET DC offset which would give you a DC offset at the amp's output, so the protection relay will not pull in.
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Old 12-06-2022, 06:02 AM   #15
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Default Re: Aiwa GX-770 Amplifier Protection Circuit

Great info, thanks fellas.

I did check the headphone jack previously and it looked good. After desoldering the STKs yesterday I briefly powered it and it was blowing the main fuse pretty quick.

I found one of the smaller of the two large sets of filter caps completely shorted. For the time being I'll be waiting on the new caps to come in.

I'll be checking those pots, thanks.
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Old 12-21-2022, 01:47 PM   #16
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Default Re: Aiwa GX-770 Amplifier Protection Circuit

Hey folks, caps finally came in.

To recap, at least 1 cap in both sets of the big caps were bad. one was completely shorted and the other couldn't hold voltage for any reasonable length of time.

The caps are soldered in. The STKs are currently removed.

Is it OK to put power to it with the STKs removed?
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Old 12-21-2022, 02:59 PM   #17
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Default Re: Aiwa GX-770 Amplifier Protection Circuit

Don't answer that last one, i didn't have the patience.

With 4 new caps in i powered it up and all the relays came on as i expected

Soldered the original STKs in, powered it up again and all the relays clicked.

I'll be putting it back together now but cannot test further until i have fuses again since i blew all of them from the bad caps.
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Old 12-22-2022, 06:36 PM   #18
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Default Re: Aiwa GX-770 Amplifier Protection Circuit

Success! the amplifier works! The original STKs seem fine (as shown in the pictures).

The pre-amp however does not. I'll start a new thread for that

I put it all together and am getting an intense hum around what I'm assuming as 120hz (the spectrum analyzer has 125 listed out).

If I unplug the pre-amp input to the amp the amp falls silent. Using the headphone output of a portable audio device and some test leads I input sound into the amplifier et voila! it works, both channels.

If i use a pair of headphones on the pre-amp and connect via the test leads and i can hear the hum. no matter what input (to the pre-amp) I use there's no sound besides the hum. anyway, i'll have a new thread for that asking for help.
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