HELP! Diode identification

[tabpat]

I've been working on this Project One Mark IIIB for weeks now (on and off). Long story short, the protection relay activates on the right channel - mid volume - with or without a load. I have isolated the preamp/amp, swapped amp boards and outputs (left to right), replaced all transistors, all of the zener diodes, checked all resistor values, recapped...literally EVERYTHING. The problem is always in the right channel. The voltage is the same on each amp, the Bias and DC are set to 30ma and hangs around 0mv +/- 2mv. That larger diode right under the relay is the only thing I haven't replaced. There are no markings...and since it's a Project One...there are no schematics. From this board, it goes to the speaker selector and out to the terminals. Any guesses as to a possible substitute?

 

[ivandezande]

If it's across the coil terminals, I'm sure it's just replaceable by a 1N4004. I feel like the relay drive transistor might want to be a little beefier as well.

 

[tabpat]

Thank you! I'll try that tonight - and I actually have some at home. The drive transistor has been replaced as well with a NOS transistor. If this doesn't work, I'm just going to have to bypass the relay and see if anything pops. This unit is really getting to me! I'm out of ideas...

 

[ivandezande]

NOS is a bold move and a small package for a driver...I'd put in a KSC2690AYS, it's recommended as a relay driver everywhere on AK and I've used it in many units, mostly Pioneer. Good luck, hope you get it working. If it has adjustment pots for the turn on voltage be sure to mess with those to see if it'll keep on, I've had issues with that before.

 

[Tom B]

Try and identify the leads going to the protect circuit from the Amp boards. It can just be the output lead, but other times it can be 2 leads coming from a transistor circuit on the driver board. These leads usually come into an array of small signal diodes (usually 2 for left and 2 for right. )If you can find it measure the voltage as you increase and kick in protection. Those diodes can go bad, and sometimes a small value cap in the protect circuit can be an issue.
I don't think that diode you are looking at is your issue, or the driver transistor. Those parts affect both channels not just the right as you are saying. But how do you know its the right causing it?
Disabling the protect relay circuit is never a good idea.

 

[tabpat]

Basically, I've run out of ideas. All of the diodes have been replaced. All of the transistors and caps as well. The problem stays on the right channel, even if I physically swap the amp boards and outputs. It has to be in the protection circuit. Even with the pre-amp decoupled, it does this. If something blows with the circuit bypassed, at least I can pinpoint what it is!

 

[Hyperion]

Disabling the protect relay circuit is never a good idea.

+1

If something blows with the circuit bypassed, at least I can pinpoint what it is!

Unfortunately if you do that, the 'something' might turn out to be your speakers.

So, this thing has 2 protection relays - one for the left channel and one for the right? correct?

 

[tabpat]

I test with 8 ohm Goodwill home theater junkers. I keep a pile of them for such an occasion. No, this has one relay that lifts both channels.

 

[tabpat]

Here is the entire board...

 

[Hyperion]

OK, so how do you know its the right channel that is causing the problem?

(sorry for what might seem like an interrogation - just trying to help )

 

[tabpat]

Any signal that I pump through it - either via the built-in preamp or using a music player feeding the main amp-in jacks - once the volume gets to a moderate volume on the right channel, the protection kicks in. Any input of any strength on the left input doesn't. So, let's say I chose the AM tuner running through the pre-amp section with balance control in the middle - once I go above the 10 o'clock volume position - circuit activates. If I turn the balance control all the way to the left, the circuit deactivates. The more signal I feed to the right though, the protection comes back on. The volume is equal and everything sounds great right up until that moment. Pulling the jumpers for the pre-out / main-in and using another source does not solve the issue. Once the right channel gets to a certain level, it cuts out. If I were to guess...at about 5 watts output, while the left goes flat out to 50 watts. Again, pulling and swapping the amp boards had no effect. The problem stayed with the (now left) amp acting as the right amp and vice-versa. It's always the right channel.

 

[tabpat]

I appreciate the "interrogation," BTW. I feel stupid for not being able to locate this problem. Everything seems to check. Soooo frustrating. It's tough without a theory of operation or schematic. I'm flying blind.

 

[Hyperion]

I think you should measure the DC on the right channel output (compare it with the left channel) when you deliberately induce the problem. Try to find a point before the relay contacts i.e. directly on the output to measure this. Try swinging the balance control (no input) and see if it changes the DC on the output.

I am presuming you have checked the power supply output voltages. (or as best you can without a schematic )

Might be worth checking the relay contacts too.

Good luck.

 

[tabpat]

Both amps are getting the same voltage. I've tried so many things...I'm sure I did measure the DC just before it clicks....but I will try it again tonight. I took this receiver on as a favor for a friend. I should have known better...

 

[elnaldo]

Protection circuits use to have a current sensor, some transistor with a resistor connected to the output which trips the protection if the current at that resistor goes beyond a threshold. I'd try to identify and search around that part. Perhaps a pnp instead of npn, perhaps a wrong pinout, some resistor out of specs, some replacement transistor with lower voltage rating...

 

[tabpat]

I would agree, but it started doing this with all stock components and it was never worked on. I have now replaced everything with no luck. I (once again) pulled all of the resistors and found nothing out of spec. So, maybe it's a voltage issue...but I have no idea what the correct voltages should be. The DC at idle has been re-verified and is sitting at 0mv +/- 2mv on both channels. Bias is 15ma across .5 ohm emitter resistors. It sounds very good. I'll test the resistors in the PS and see if anything is out. Other than that...I 'm about to give up.

 

[Oldsansui441]

Did you get a chance to try what Hyperion suggested and check DC on the right channel whilst inducing the problem?

 

[tabpat]

I did, but the readings were all over the place. They weren't much different than the left, however. I'm guessing that the meter was being influenced by the actual program material.

 

[elnaldo]

What if you invest some time to sketch a schematic of the output and protection circuit? At least the relay driver and the current sensor circuit.