Help with no relay click on an Akai AA1175 (including schematics)

[Zaos]

Hello everyone!

I'm pretty new around here, but I hear this is the place to ask if you need help repairing electronics.

I'm currently stuck on trying to figure out why TR16 isn't getting the 22V it needs to work. I've tested TR13-TR17 out of circuit thinking one of these was causing a strange problem.  I'm still fairly new to electronics repair, so there may be something that's just going completely over my head. I also tested R32, and it was within spec, and passing voltage.

Thanks!

[mzacharias]

The normal function of the protect circuit is to detect DC voltage at the output of each amp channel and act to prevent any DC of any magnitude from reaching the speakers.
Normally one should FIRST check the output transistors for shorts and if there are none, check for DC offset at either amp channel. Since you seem to have skipped these steps, and gone straight to the relay, I think it's fairly safe to say you got lucky and there is no catastrophic amp failure at this point.

Most of the voltage readings you need to take will be with respect to chassis ground. Normally you'll attach the ground lead of the digital multimeter to the chassis and set the DC voltage range to 50 volts to start with, since the raw voltage for the relay circuit is in that range, or use an autoranging multimeter. The coil voltage can be measured at diode D4. If the raw DC (48 volts or so) is shown at the cathode, and the same at the anode, then the relay coil is almost certainly OK but the relay is not being driven by it's associated circuitry.
At that point I would check for any DC at the amp output going to the relay. DC of 1-2 volts or more is enough to keep the relay from being turned on. There is also over-current sensing involving transistor TR12 and it's mate on the other channel.

If there is DC on one channel but not both, obviously that channel needs repair. If there is DC on both channels, this usually points to a power supply issue of some sort.

This ought to get you started. Report back what you find.

[Zaos]

Sorry! I forgot to add that I measured the DC offset of both channels and they were 25mv and 28mv respectively. This is a bit higher than normal right? But, not enough for protection to kick in?

I measured from the relay with the probe to ground to get the offset for both channels, as I think this is the right way to do it?

I also measured the relay coil voltage and both sides consistently showed around 47V or so.

Thanks a lot for the help! I'm still learning myself how to figure out and fix problems, and help from here is great!

[floobydust]

Can you (carefully) measure the voltages on TR16 and TR13.
C17 is the power-on delay so if it is leaky or shorted, that would not allow TR17 base voltage to come up and turn on the relay.
TR12, TR13 monitor over-current.
TR14, TR15 monitor DC offset at the amp's output.

Have you replaced the electrolytic capacitors, they must be getting old.

[Zaos]

The only electrolytic I did replace was C17, thinking it may help. I wanted to see if I could find what the source of the problem is before a recap, but a recap is obviously due

Thanks for helping!


TR16
E: -2mv
C: 47mv
B: .489V

TR13
E: 27.9V
C: .488V
B: 28.5V

[Zaos]

Just an update! I went back to the obvious, and checked the caps within the circuit. I replaced C18, C19, and C20 thinking that they were preventing the circuit from functioning properly. I'm fairly certain it was C18 that caused the fault now.

My heart dropped a little once I started it after replacing the caps as there really couldn't be anything else causing a problem. Then I heard the click!

The old amp now works! I'll attach a pic once I get it all back together, thanks for the help everyone!

[mzacharias]

Just an update! I went back to the obvious, and checked the caps within the circuit. I replaced C18, C19, and C20 thinking that they were preventing the circuit from functioning properly. I'm fairly certain it was C18 that caused the fault now.

My heart dropped a little once I started it after replacing the caps as there really couldn't be anything else causing a problem. Then I heard the click!

The old amp now works! I'll attach a pic once I get it all back together, thanks for the help everyone!

Terrific! I'm sorry I didn't spot this one on the schematic earlier. This cap is used to provide a sharp cut-off at power down. It lightly filters a DC voltage, which then disappears almost instantaneously when the power is turned off. This is to prevent pops etc at power down. If the cap is open circuit, the relay never comes on, as the circuit "thinks" the main power is off.

Great Job!

[Zaos]

As promised, a pic of the now saved receiver!
 
I actually didn't know that, thanks for elaborating what that cap did exactly. I'm slowly reading all of the awesome pdfs and books linked in the forums to get the hang of understanding everything.


What's awesome about this thing is it's a dual mono design, and 75W per channel.

[floobydust]

Ah my guess was shorted C18.
I notice a voltage regulator for the power amplifier front end and a unique VAS stage. This older gear from Japan always had something unique in the design.
Really worth repairing. If you have a loud party it can keep up, whereas modern stuff (Sony) overheats in a few minutes. I had to laugh at the tiny power transformer and heat sinks compared to something like this.