petehall347
the brandy coffee man
its the unused contacts that's throwing me as nothing can get welded to them if truly unused .
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Why Does My Marantz 250 Amp Keep Eating the Protection Relay?
- Thread starter Davidmac5
- Start date
Go ahead and install the new relay but then put an oscilloscope on the output and make sure it isn't oscillating. I think the amp may start to oscillate after a few hours when the transistors warm up. Then it passes lots of high energy high frequency through the relay contacts which may be burning them, although it should damage your tweeters as well.
Coolhandjjl
Embiggened Pompatus
I cut mine down a bit
You guys mentioned this and I agree with:
The offending contact (arm) isn't fully closing. It's hovering in space arcing to the N.O. terminal. Getting hotter than **** but enough air gap that isn't not melting anything yet. Then when (what?) fails and that spring pushes that hotter than the sun arm contact onto the N.C. terminal, it melts.
Whew, that's a stretch but this is a mystery.
I do like the idea of the unused terminal being grounded against the case but the terminal leg would show that I don't see it.
Carry on.
The offending contact (arm) isn't fully closing. It's hovering in space arcing to the N.O. terminal. Getting hotter than **** but enough air gap that isn't not melting anything yet. Then when (what?) fails and that spring pushes that hotter than the sun arm contact onto the N.C. terminal, it melts.
Whew, that's a stretch but this is a mystery.
I do like the idea of the unused terminal being grounded against the case but the terminal leg would show that I don't see it.
Carry on.
petehall347
the brandy coffee man
it does look like its been hot here .
I replaced the relay with a rewired MY2 and the amp came out of protection and the idling current and DC offset were right where I left them before the melt down. I checked the voltage at the relay and it was 24.93 VDC. The amp played perfectly for about 30 minutes and then the right channel started making a popping sound ( I still had it on the DBT and it never lit up ) I shut it down and noticed that the relay was warm to the touch. Now when I turn on the amp it does come out of protection but the popping of the right channel starts immediately. I guess my meltdown of the relay is being caused by an issue with the right channel after it gets warmed up but what could it be? Every critical part on that amp board has been replaced.
Popping if it’s intermittent could be a bad solder joint or damaged track on one of the semiconductors? Are the outputs securely into the pin holders? I do notice the relay on my 250 gets warm too but haven’t had any issues.
I would like to offer the following having completed my Marantz 250 update. OLD PARTS may look soldered by have tons of oxidation on the leads. I clean them off the best I can before populating a pc board, but I always trim them down to a minimum after soldering and reflow over the cut end to make certain I have a good electrical connection. As mentioned before, it could be something intermittent due to heat after warmup and dimensional change causing a highly resistive joint.
I thought of a novel idea as to see if there is DC present at the output. You can short the inputs and operate the amplifier without signal. You say that the bias is good, but more importantly the DC offset is set low. If you connect two LEDs, one in each direction on the offending channel with a 1.5k Ohm resistor in series with each LED, you would perhaps see one (or both) flicker on if there was DC present at the output after that 1/2 hour or so. This may be the noise you hear in the speakers. Depending on which LED lights up, it would let you know if it is the positive or negative rail being the offender. It's a start.
I thought of a novel idea as to see if there is DC present at the output. You can short the inputs and operate the amplifier without signal. You say that the bias is good, but more importantly the DC offset is set low. If you connect two LEDs, one in each direction on the offending channel with a 1.5k Ohm resistor in series with each LED, you would perhaps see one (or both) flicker on if there was DC present at the output after that 1/2 hour or so. This may be the noise you hear in the speakers. Depending on which LED lights up, it would let you know if it is the positive or negative rail being the offender. It's a start.
GRonald
Active Member
The Meter board is also connected to the output pins of the relay; I suggest disconnecting the meter board connections and testing to see if that is causing the problem, since you have burnt relay contacts, but your speakers have suffered no damage.
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Also another thought make sure meter board solder points aren’t touching the vu meter housing it’s very close there
After some poking around I have zeroed in on the relay/ps board as the cause of the popping sound. I barely touched C303 on the relay board and the speaker popped and the relay went into protection, I touched it again and the relay started clicking on and off repeatedly and the meter lamps were blinking on and off until I pulled the plug. I'm assuming some of the components on the relay board were damaged when the relay shorted out. I'm going to go ahead and rebuild the board but I'm having a problem finding replacements for Q302 SS47 Q301 and Q303 SPS439. What current equivalent transistors would be good replacements for SS47 and SPS439?
I suggest recapping it first, then see if it's now reliable. That may just fix it. Look for bad solder joints, especially on any components that normally do power like big resistors.
Saw this response a couple of days ago. Makes sense to me! If only the OP had a scope for his regular troubleshooting queries.....
The root of the mystery to me is how that NC terminal melting? That's such a common relay, we had hundreds in operation for decades at work and quite often I'd seen the plastic cover black with soot and\or melted. But fused contacts in those specific relays? I think I've seen that twice in my whole working career (building automation).
And of course "the hook" to my interest is this NC terminal isn't connected to anything.
Ss47 - mje182g
Sps439 - bc546 maybe mpsa42 also
Well I replaced the capacitors on the relay board and it's acting normal again, but I still have issues with the right channel amplifier board. After putting it all back together I fired it up and it came out of protection and played beautifully for about 30 minutes and then went back into protection and I now have 51.2 VDC on the output pin 19 when I had zero before and 4.1mV idling current when I had 13.2 before, no damage was done to the relay or anything else. This has been consistently the same problem with this amp, right channel plays for a while then goes out. As I stated I have totally rebuilt both boards, the only thing I'm wondering about is the 39 Ohm 1 watt resistors I replaced, I used metal film resistors instead of carbon film resistors, is that a mistake? I know I'm close to figuring this out but something on that right channel board is eluding me.
Using metal film resistors would not cause this. Possibly a bad solder joint or a cracked trace. Are there any parts you didn't not replace? If you allow it to cool down does it work again? If so you can buy some freeze spray, run it until it fails then spray a small amount of freeze spray on each component while monitoring the DC voltage on the output until it drops. The last component you spray is the culprit, or nearby traces.