Help! Stuck In Protect Mode

[Gary D Olson]

I have a top-of-the-line mid-to-late 1980's Class-A 2-channel Denon DRA-1025R receiver. I'm its original owner and can attest this is an absolutely terrific piece of kit, very "70's like" as opposed to computer controlled. I haven't been able to use it for several years because it goes into protection mode within 1-2 seconds, however, it sometimes lasts a whopping 30 seconds before I hear the dreaded 2nd click, and sometimes the first click never happens.

I cracked it open this morning and looked very closely for cold solder joints. Non visible and everything seems solidly connected to the boards. After giving the controls & switches a good cleaning, I powered it on and it lasted a few minutes. Before it clicked into pm, I connected my multimeter to the speaker terminals and determined DC offset was near perfect on both channels.

Any help pointing me toward the most likely cause/s would really be appreciated.

Gary

 

[ryuuoh]

If you can, access the input pins of the protection relay when it is in protection.
That is quite a beast of a receiver.

 

[Gary D Olson]

If you can, access the input pins of the protection relay when it is in protection.
That is quite a beast of a receiver.

IIRC, those pins will be inaccessible due to another board positioned below the board on which the relay sits. By chance, have these relays proven unreliable?

 

[elnaldo]

you need to determine if you have DC at the output when it's protected. If not, probably the circuit driving the relay is failing. If you have the right voltage at the relay coil, and the relay is not engaging, the relay is bad. Sometimes a capacitor or a transistor not doing its job at the protection circuit will pull down the relay coil voltage.

If you can't access the relay itself, locate with the schematic the components connected to it, and measure there. Usually, a resistor in series with the coil, or a diode in parallel with it can be used to measure the relay voltage. To measure the DC offset, locate the power amp output (perhaps "emitter resistors", or some IC output...) and measure DC there.

 

[Gary D Olson]

you need to determine if you have DC at the output when it's protected. If not, probably the circuit driving the relay is failing. If you have the right voltage at the relay coil, and the relay is not engaging, the relay is bad. Sometimes a capacitor or a transistor not doing its job at the protection circuit will pull down the relay coil voltage.

If you can't access the relay itself, locate with the schematic the components connected to it, and measure there. Usually, a resistor in series with the coil, or a diode in parallel with it can be used to measure the relay voltage. To measure the DC offset, locate the power amp output (perhaps "emitter resistors", or some IC output...) and measure DC there.

Any recommendations as to where to get a service manual for this? I see where they're available but I don't know which sellers to trust. Thanks for input.

 

[Goldie99]

Hi - if you just need a schematic, you can use www.hifiengine.com (you have to join, but it's free, reliable, and with no malware that I've ever seen). The service manual is there, with schematic - the manual's in German, but for the schematic it makes little difference.

 

[LexDM3]

Before you go any further, check to see if that Denon has the famous current sensing resistor issue. I picked up a DRA-685 that was stuck in protection and that was the problem. Here is the information that helped me fix mine.

Apparently Denon didn't use resistors with high enough power ratings and they open up. Check that before you go into the relay circuit. It's a common problem with many of that series from Denon but I'm not sure if the 1025 uses the same current sensing method.