PC2002M - one channel dropping out at low volumes
- Thread starter DWJames
- Start date
DWJames
Active Member
Yeah, just checked the service manual and some of the plugs are the same, others are reversed. Looks like a good way to get confused and let the magic smoke out!
Yeah, if I crank it, it pops back to life. There's also another issue on that channel where it intermittently gains about 15db and sounds overdriven and harsh. Cranking the volume seems to solve that too. I'll pull the DB and PDB boards when I get a minute and reflow the joints.
Yeah, if I crank it, it pops back to life. There's also another issue on that channel where it intermittently gains about 15db and sounds overdriven and harsh. Cranking the volume seems to solve that too. I'll pull the DB and PDB boards when I get a minute and reflow the joints.
fredcohiba
Super Member
I had the same issue on my CR-2040. Deoxit to the volume control and 100 twists back and forth - issue went away. Give the pots another shot and really work them.
DWJames
Active Member
Thanks. That's what I did already, but I can easily give them another go.
I can certainly see how it could be the attenuators, but I'm confused at the fact I can twist the attenuator whilst the fault is present and it doesn't clear the issue.
Is it enough to let the deoxit seep into the joins between the sections of the attenuator body or do I need to somehow prize them apart a little? It's hard to tell how much goes in and how much drips around the sides....
I wonder if it's easy to swap the cables/plugs for the attenuators to their opposite channels. That'd very quickly prove if that was the issue.
thanks,
James
A couple of things to double check while you have the amp partially disassembled and have the DeOxit out:
On the back panel there are a number of slide switches: Your may vary slightly from the pic depending on country of use, binding post types, fuses.
Occasionally these slide switches get corrosion or are incompletely moved to the correct position, or they have never been moved! Once in a while, they present with signal issues.
The top center switch changes from stereo to mono operation. The switch above the XLR jacks lifts the ground on the XLR inputs. The switch above the 1/4 inch phone input jacks selects either the XLR balanced or 1/4 inch unbalanced. If you can get to them (front or back), while powered down, spray some Deoxit into any access hole (or into the front of the switch while it is pointing up (amp sitting on faceplace), let sit for a couple of minutes, then using a small flat blade screwdriver, exercise each switch left and right about 10 times each. Clean up any residue, and then set the switch firmly to the setting that you need.
Which input are you using, the XLR or 1/4 inch (the 1/4 inch are wired in parallel for daisy chaining to other amps)? Are you using any type of RCA-1/4 inch adapter? Each input (XLR or 1/4 inch) should also be cleaned with DeOxit in the plug and Jack and inserted/pulled at least 10 times, with a final cleaning and Deoxit the last time, wipe off any excess residue.
While this does not directly address the issue of why increased power gets the channel going, it may relate to why one channel goes low and sounds distorted, at least from a contact standpoint. Your other problem sounds like either a power supply stability problem somewhere at the differential or predriver stage. The replacement relay should have solved the question of bad relay contacts.
If the unit cuts out, does the relay click back open, or have you checked DC offset at the relay input points (which should show offset values no matter if the relay has engaged or not)?
On the back panel there are a number of slide switches: Your may vary slightly from the pic depending on country of use, binding post types, fuses.
Occasionally these slide switches get corrosion or are incompletely moved to the correct position, or they have never been moved! Once in a while, they present with signal issues.
The top center switch changes from stereo to mono operation. The switch above the XLR jacks lifts the ground on the XLR inputs. The switch above the 1/4 inch phone input jacks selects either the XLR balanced or 1/4 inch unbalanced. If you can get to them (front or back), while powered down, spray some Deoxit into any access hole (or into the front of the switch while it is pointing up (amp sitting on faceplace), let sit for a couple of minutes, then using a small flat blade screwdriver, exercise each switch left and right about 10 times each. Clean up any residue, and then set the switch firmly to the setting that you need.
Which input are you using, the XLR or 1/4 inch (the 1/4 inch are wired in parallel for daisy chaining to other amps)? Are you using any type of RCA-1/4 inch adapter? Each input (XLR or 1/4 inch) should also be cleaned with DeOxit in the plug and Jack and inserted/pulled at least 10 times, with a final cleaning and Deoxit the last time, wipe off any excess residue.
While this does not directly address the issue of why increased power gets the channel going, it may relate to why one channel goes low and sounds distorted, at least from a contact standpoint. Your other problem sounds like either a power supply stability problem somewhere at the differential or predriver stage. The replacement relay should have solved the question of bad relay contacts.
If the unit cuts out, does the relay click back open, or have you checked DC offset at the relay input points (which should show offset values no matter if the relay has engaged or not)?
DWJames
Active Member
Thanks.
I definitely cleaned and wiggled al the back switches when I got the amp a year ago, but I'll give them another quick go for good measure.
I'm using the unbalanced connectors with some good custom made RCA to mono jack plugs, no converters.
I think what I might do is to try and swap the audio inputs from left to right just before they enter the PDA/PDB boards. I think the plugs are mirrored, but it's not normally that hard to swap pins over. That would tell me if I'm looking at a physical issue with the inputs and attenuators still.
when the channel cuts out, I don't think the relay changes. DC voltages and offsets measure well within the parameters according to the service manual, but I haven't measured them whilst a fault is occurring.
I definitely cleaned and wiggled al the back switches when I got the amp a year ago, but I'll give them another quick go for good measure.
I'm using the unbalanced connectors with some good custom made RCA to mono jack plugs, no converters.
I think what I might do is to try and swap the audio inputs from left to right just before they enter the PDA/PDB boards. I think the plugs are mirrored, but it's not normally that hard to swap pins over. That would tell me if I'm looking at a physical issue with the inputs and attenuators still.
when the channel cuts out, I don't think the relay changes. DC voltages and offsets measure well within the parameters according to the service manual, but I haven't measured them whilst a fault is occurring.
DWJames
Active Member
ok, some progress I think!
I reflowed all the joints on the PDB board, but that didn't seem to make any difference so I thought I'd devise some tests to try and work out what area of the amp was causing the issue.
As I mentioned before, I wanted to swap the audio inputs to the PDA/PDB boards over and see if the fault changed channels. I had a quick look at re-running the standard wiring, but it wasn't going to work without extra wire.
So I bought myself 2 of these for next to nothing on eBay:
They're PC Fan splitter cables and although they're hardly audiophile quality components, they do have almost the right plugs and are juuuuust the right length. I don't need them to be splitters. It's just a convenient way of buying the wires for cheap.
So I used them to run the A channel input to the B boards and vice versa
thusly
I didn't need to wait for long. The fault where the B channel got louder and distorted was now transferred to channel A. So it had to be something to do with the input sockets, balanced/unbalanced selector, attenuator or the mono/stereo switch for channel B. I re-cleaned all the sockets and switches with deoxit and had another go at the attenuators.
I wasn't satisfied that I had got the deoxit in properly previously, relying on it to seap between the joins in the attenuator casing. This time I undid each attenuator's screw bolts and took the back casing off
I was a bit worried that the various bits were going to spring out at me never to be found again, but I went slowly and carefully. Turns out I needn't have worried. Nice and easy and once the back case was off, there was perfect access to the contacts and wipers so I could give them a proper deoxit blast and be satisfied they were happy and clean.
All back together now and sounding on top form
No faults so far in 2 hours of listening. Hopefully that's got it.
Cheers again for the help guys
I reflowed all the joints on the PDB board, but that didn't seem to make any difference so I thought I'd devise some tests to try and work out what area of the amp was causing the issue.
As I mentioned before, I wanted to swap the audio inputs to the PDA/PDB boards over and see if the fault changed channels. I had a quick look at re-running the standard wiring, but it wasn't going to work without extra wire.
So I bought myself 2 of these for next to nothing on eBay:
They're PC Fan splitter cables and although they're hardly audiophile quality components, they do have almost the right plugs and are juuuuust the right length. I don't need them to be splitters. It's just a convenient way of buying the wires for cheap.
So I used them to run the A channel input to the B boards and vice versa
thusly
I didn't need to wait for long. The fault where the B channel got louder and distorted was now transferred to channel A. So it had to be something to do with the input sockets, balanced/unbalanced selector, attenuator or the mono/stereo switch for channel B. I re-cleaned all the sockets and switches with deoxit and had another go at the attenuators.
I wasn't satisfied that I had got the deoxit in properly previously, relying on it to seap between the joins in the attenuator casing. This time I undid each attenuator's screw bolts and took the back casing off
I was a bit worried that the various bits were going to spring out at me never to be found again, but I went slowly and carefully. Turns out I needn't have worried. Nice and easy and once the back case was off, there was perfect access to the contacts and wipers so I could give them a proper deoxit blast and be satisfied they were happy and clean.
All back together now and sounding on top form
No faults so far in 2 hours of listening. Hopefully that's got it.Cheers again for the help guys
fredcohiba
Super Member
You're a smart guy. I was hoping it was the attenuators, and I'm glad you were able to open up, fix and close again with no issues.
DWJames
Active Member
Just trying to use a little logic reasoning to make up for my lack of hifi repair experience and general crappy understanding of electronics
I really love how this amp's been designed with the service engineer in mind and having 2 of everything is really helpful for chasing down issues which only appear on 1 side
It all seems to be working very nicely so far. I should find out for sure later this week when I can get an extended listening session in. It's hard work, but somebody's got to do it!
