Most probably indeed the relays.
Schrack has good relays if you plan a search.
Hugo
Schrack has good relays if you plan a search.
Hugo
It seems pretty conclusive. Link the contacts out to prove it but turn the amp on without speakers connected and only connect them after the amp is on. Make sure you link the right points as you don't want to either short a channel or cross link the channels.
Schrack is a relay brand? Also do you think i should order new ones from mouser as well? Do people create fakes of those too?? 😅😅
Actually i plan to remove them, connect them on my bench power supply to turn them on and measure resistances.
After that i will try to see if they can be opened and cleaned up. I will probably end up buying new ones but i have to verify what i just saw on the oscilloscope.
Will be back with pictures 😁 Thanx!!
I would not advise cleaning them because most of the time
the contacts are burned or corroded anyway.
Hugo
the contacts are burned or corroded anyway.
Hugo
Ok then. I will take them out, measure them, disassemble them to have a look at the contacts and once i verify the fault i will replace them all together. Thanx
Replaced the relays with the same specs about 10€ each (brand:sony).
Everything is working fine now and i'm pleased with the results. All 4 channels have the same output now.
The previous relays had numerous problems. One was measuring 1kohm resistance on one of the two contacts and when I opened it up i saw that the relay connections where not touching enough. I had to push it manually to close. Same story woth the other contacts with the exception that they where measuring 1-2ohms instead of close to zero.
Here are some pictures from the old relays (black) and the new ones (blue):
Everything is working fine now and i'm pleased with the results. All 4 channels have the same output now.
The previous relays had numerous problems. One was measuring 1kohm resistance on one of the two contacts and when I opened it up i saw that the relay connections where not touching enough. I had to push it manually to close. Same story woth the other contacts with the exception that they where measuring 1-2ohms instead of close to zero.
Here are some pictures from the old relays (black) and the new ones (blue):
Excellent news, thanks for the update 👍
Dodgy speaker relays with high and intermittent contact resistance are a surprisingly common issue.
Dodgy speaker relays with high and intermittent contact resistance are a surprisingly common issue.
Honestly i would have never thought of it since i'm not experienced. But thank god i thought of tracing the signal up to relays, after excluding the possibility of un-even gain between the channels, otherwise i would still scratch my head and loose sleep on what i did wrong 🤣.
Anyways thank you for your precious help!
It seems a standard enough relay, Omron and Panasonic also make relays.
The brand in the picture is a new one for me.
A tip for those who came in late: most relays have a standard footprint in the same rating, so they can be swapped out with those of a different brand....
A search turned up Fujitsu as well, but no longer made by them.
This is a 24V DC coil relay, with two sets of N/O contacts.
One more thing, if I have said something already posted, mods, please delete, I have not read the entire thread.
The brand in the picture is a new one for me.
A tip for those who came in late: most relays have a standard footprint in the same rating, so they can be swapped out with those of a different brand....
A search turned up Fujitsu as well, but no longer made by them.
This is a 24V DC coil relay, with two sets of N/O contacts.
One more thing, if I have said something already posted, mods, please delete, I have not read the entire thread.
Good to read that you will finally be able to listen to a well serviced amp 🙂
Hugo
UPDATE AFTER SEVERAL MONTHS OF USE AND FURTHER RESEARCH
To anyone attempting to repair a unit like this i have the following remarks so that you don't go through what i just did:
1. After repair NEVER test this amp without the back panel attached and all the screws attached. This amp uses the back panel as ground and if not connected the amp will continuously blow the outputs. That was the main reason mine was failing again and again. I was testing it without the amp board fully mounted to the chassis!
2. Be carefull of fake transistors. One of the main reasons i had to repair this amp repeatedly was fake transistors failing after a while.
3. Always check the solder connections in the pre-driver transistors. They tend to break because those get pretty hot and they have no cooling block on them.
4. Always check the relays. Even though they are sealed type they still loose conductivity over the years leading to uneven stereo image or downright no output at all.
That's it! To anyone reading this in the future i hope this helps!
To anyone attempting to repair a unit like this i have the following remarks so that you don't go through what i just did:
1. After repair NEVER test this amp without the back panel attached and all the screws attached. This amp uses the back panel as ground and if not connected the amp will continuously blow the outputs. That was the main reason mine was failing again and again. I was testing it without the amp board fully mounted to the chassis!
2. Be carefull of fake transistors. One of the main reasons i had to repair this amp repeatedly was fake transistors failing after a while.
3. Always check the solder connections in the pre-driver transistors. They tend to break because those get pretty hot and they have no cooling block on them.
4. Always check the relays. Even though they are sealed type they still loose conductivity over the years leading to uneven stereo image or downright no output at all.
That's it! To anyone reading this in the future i hope this helps!
Alex, nice to read you and thanks for this write-up.
However, not attaching the back panel seems strange in relation with blowing outputs.
If that back panel would serve to make ground connections between parts, that would be very bad engineering practice.
All other points are very helpful, thanks.
Hugo
However, not attaching the back panel seems strange in relation with blowing outputs.
If that back panel would serve to make ground connections between parts, that would be very bad engineering practice.
All other points are very helpful, thanks.
Hugo
I was not expecting that to be the case either. The amp board does mount to the bottom of the chassis as well. BUT there are a few screws that mount the output binding posts to the BACK chassis and those threads have a small metal part that connects the back plate to the amp board through those screws. The input board has those metal parts as well which are threaded and a screw goes through them.
The same thing applies to the pm7200 model as well.
A channel on youtube uploaded a video stating the same things i did in this post here is the link for future reference:
The same thing applies to the pm7200 model as well.
A channel on youtube uploaded a video stating the same things i did in this post here is the link for future reference:
Weird. I repaired quite some amps (not this one IIRC) and never encountered something like that.
But it's good you found out, output transistors can be expensive, as you learned. 🙂
Hugo
But it's good you found out, output transistors can be expensive, as you learned. 🙂
Hugo
You could put an extra chassis ground, with lugs, like those in cars, as an extra precaution, in case you forget and attempt service later.
At least that will make you pause and think just why the wire is there.
Use 2.5 to 6 square mm, whatever is handy. Make sure clean metal contact is made.
I tend to use MOVs across relays, to reduce contact burning, YMMV.
At least that will make you pause and think just why the wire is there.
Use 2.5 to 6 square mm, whatever is handy. Make sure clean metal contact is made.
I tend to use MOVs across relays, to reduce contact burning, YMMV.