Intermittent amp channel

[pypes]

Hi all, I have a Carver TFM-15CB amplifier and the left channel goes out intermittently. I bought it used and it worked fine for a week or so and now it tends to crackle in and out. When it goes out, if I tap the side of the case it comes back in but will eventually cut out again.

I don't know much about amps, and don't have a multimeter to check things with though if I need to I can borrow one. My question is whether or not someone experienced this problem and can point me to something specific to check. Is it just a lose connection that I should be able to find relatively easily and resolder or is it likely to be something requiring more experience to fix?

Thanks in advance,
Leon

[PassingInterest]

Re: Intermittent amp channel

Since tapping on the side makes the problematic channel come back on again, that does seem to indicate a loose connection.
The connection may be a cracked solder joint.
If so, it can be resoldered, but it is best if this is done by an experienced solder-jockey.
Isolating the problem in the first place can be done by removing the chassis cover and carefully tapping on different areas of the board or on components with a pencil eraser while listening until the problem is narrowed to a specific connection.
But again, it's best to leave this operation to qualified service personnel, because of the very real possibility of electrocution.

[Æ]

Re: Intermittent amp channel

Hi all, I have a Carver TFM-15CB amplifier and the left channel goes out intermittently. I bought it used and it worked fine for a week or so and now it tends to crackle in and out. When it goes out, if I tap the side of the case it comes back in but will eventually cut out again.

I don't know much about amps, and don't have a multimeter to check things with though if I need to I can borrow one. My question is whether or not someone experienced this problem and can point me to something specific to check. Is it just a lose connection that I should be able to find relatively easily and resolder or is it likely to be something requiring more experience to fix?

Thanks in advance,
Leon

Any number of different things could be happening. With older pieces of equipment the volume control, various selector switches and all the RCA jacks eventually become oxidized or worn. So, I'd concentrate on those first. If you do find a loose connection, that is the easiest thing to fix.

Regardless of what is causing the fault, you will have to determine it by a process of elimination. Not likely you'll need a meter at this point.

[Æ]

Re: Intermittent amp channel

Since tapping on the side makes the problematic channel come back on again, that does seem to indicate a loose connection.
The connection may be a cracked solder joint.
If so, it can be resoldered, but it is best if this is done by an experienced solder-jockey.
Isolating the problem in the first place can be done by removing the chassis cover and carefully tapping on different areas of the board or on components with a pencil eraser while listening until the problem is narrowed to a specific connection.
But again, it's best to leave this operation to qualified service personnel, because of the very real possibility of electrocution.

Before taking the cover off, I'd wiggle all the input connections (RCA jacks) the volume/balance knobs and all the front panel switches. It could even be a speaker output connection problem too.
The real possibility of electrocution is minimal. You can touch something "hot" and you'll only end up pulling your hand away quickly. When I'm working on equipment it's on a bench/table and I'm wearing shoes, so there is no real chance for excessive current to flow through me.

[AJ]

Re: Intermittent amp channel

The real possibility of electrocution is minimal. You can touch something "hot" and you'll only end up pulling your hand away quickly. When I'm working on equipment it's on a bench/table and I'm wearing shoes, so there is no real chance for excessive current to flow through me.

This isn't good advice in my opinion. Whether the possibility is minimal or not, it's not something I fool with. KNOW what you are touching/doing at all times when troubleshooting.

As for the problem, it could be any number of things, but most likely a loose connection somewhere. Unplug the amp and take the cover off. Look over all the connections at the input/output terminals. If this amp has ribbon cables or other types of connectors, re-seat them.

Look for cracked or cold solder joints. If you find joints that look suspicious completely remove the old solder then resolder the connection with a clean tip. Do not just reflow the old solder; make sure your soldering iron tip is clean.

[Æ]

Re: Intermittent amp channel

Well, I usually know what I'm doing. But for those who don't, maybe they just shouldn't take the cover off!
You may care to notice that I said "Before taking the cover off."

Typically the only high voltage present is at the input of the power supply, keep your fingers away from the power supply and you'll be fine.

You take a bigger risk getting into your car and driving on public roadways, but I bet it doesn't stop you from driving!

This isn't good advice in my opinion. Whether the possibility is minimal or not, it's not something I fool with. KNOW what you are touching/doing at all times when troubleshooting.

As for the problem, it could be any number of things, but most likely a loose connection somewhere. Unplug the amp and take the cover off. Look over all the connections at the input/output terminals. If this amp has ribbon cables or other types of connectors, re-seat them.

Look for cracked or cold solder joints. If you find joints that look suspicious completely remove the old solder then resolder the connection with a clean tip. Do not just reflow the old solder; make sure your soldering iron tip is clean.

[rone]

Re: Intermittent amp channel

As for the problem, it could be any number of things, but most likely a loose connection somewhere.

I have an amp with a wonky speaker protection relay that goes in and out, if turning up the volume sharply brings the sound back, it could be a relay of this type.

[PassingInterest]

Re: Intermittent amp channel

Before taking the cover off, I'd wiggle all the input connections (RCA jacks) the volume/balance knobs and all the front panel switches. It could even be a speaker output connection problem too.
The real possibility of electrocution is minimal. You can touch something "hot" and you'll only end up pulling your hand away quickly. When I'm working on equipment it's on a bench/table and I'm wearing shoes, so there is no real chance for excessive current to flow through me.

Good advice on checking external connections first and volume/balance knobs.
I'm perfectly comfortable working on this kind of equipment also, but then I am an electronics technician.
I figured if my warning scared him off, it was for the best and if it didn't, then he would be all the more careful working inside the chassis.
And, you're right--the main danger is near the power supply.

AJ offered some sound advice, as well.

Rone--I'm so glad you said that! I didn't even think about a possible relay problem. Relays are often used to delay connecting the output to the speaker until the equipment has had a chance to stabilize after power-up.
This avoids power-on/off "pops" heard through the speakers.
Relays can become weak or even get dirty/oxidized contacts.
Gold plated contacts shouldn't need to be cleaned of oxidation, but you are right to suggest a possible relay problem.

Let us know how it goes.

[lhwidget]

Re: Intermittent amp channel

I have an amp with a wonky speaker protection relay that goes in and out, if turning up the volume sharply brings the sound back, it could be a relay of this type.

I've also seen similar results with connector corrosion and failing line level patch cords. Just zing the volume (very briefly) and the failing channel would come back for a while.

The only advice I'd give concerning safety around a power amp is the ability for the power supply caps to store (sometimes a LOT) of energy. Some amps will have resistors to drain the power supply caps, some won't. Usually, the amplifier will drain the caps, but if the amp is muted immediately (a few milliseconds) after turn off the caps may hold their charge. My point is that it's possible for the caps to remain charged for quite a while after an amp is unplugged. Checking the rail voltage and/or discharging the cap with a 500 Ω 10W, or a 1000Ω 5W resistor is the only way you'll know what to expect.

They will be fully charged when working on an operating amp. Accidentally shorting charged power supply caps to chassis ground or the 0V rail can vaporize 1/4" off the end of a screw driver and could cause a capacitor to explode. Either way, the shrapnel's gonna hurt.

[AJ]

Re: Intermittent amp channel

Did a quick google search and found that there is atleast one relay in the center of the main board. If it's in the speaker protection circuit and not for soft-start, power switching or something else, it could cause your problem if one set of contacts was partly oxidized.

[pypes]

Re: Intermittent amp channel

Thanks for all the advice! I did wiggle the speaker connections / RCA inputs before posting... and I moved the gains up and down as well.

So after I posted I took the cover off as the channel didn't want to come back in. After I did that, I gave a sharp rap on the side a little further back and the channel came back on and stayed on. Quite weird... not sure exactly why that happened but I listened to it for another 45 minutes to an hour without it cutting out again.

Upon taking off the cover I noticed that the power supply is right near where giving a tap on the chassis would bring the channel back on... I should note for you that rapping on the amp at other spots (front, other side, top) did NOT consistently have an effect while tapping near the power supply always brought it back on.

The cover is still off.. but I just got back from the pub and figure I'll wait to go digging around in there. I should have sometime tomorrow after school to take a look at it... first thing will be turning it on and seeing if it cuts out again... tough to trouble shoot wiring problems while it's not a problem! lol

[dthomas]

Re: Intermittent amp channel

Thanks for all the advice! I did wiggle the speaker connections / RCA inputs before posting... and I moved the gains up and down as well.

So after I posted I took the cover off as the channel didn't want to come back in. After I did that, I gave a sharp rap on the side a little further back and the channel came back on and stayed on. Quite weird... not sure exactly why that happened but I listened to it for another 45 minutes to an hour without it cutting out again.

Upon taking off the cover I noticed that the power supply is right near where giving a tap on the chassis would bring the channel back on... I should note for you that rapping on the amp at other spots (front, other side, top) did NOT consistently have an effect while tapping near the power supply always brought it back on.

The cover is still off.. but I just got back from the pub and figure I'll wait to go digging around in there. I should have sometime tomorrow after school to take a look at it... first thing will be turning it on and seeing if it cuts out again... tough to trouble shoot wiring problems while it's not a problem! lol

Probably not the power supply as this would be common to both channels. I would guess as has been pointed out you have a bad solder joint in the suspect channels circuit. If you are comfortable with the cover off and know how to not get yourself electrocuted. The pencil eraser tip is good and anything that is insulated or made of plastic that can used to tap around will help you narrow down the area where it is occurring. A hard rap impacts everything in the chassis, you want to find the area where a gentle nudge will create the issue. Once you have located the region that seems to be affected resolder all the joints in this region if the affected solder joint is not obvious. Then recheck to see if the problem has vanished.

If you have a schematic and the problem is isolated to a single channel you can use the schematic to isolate the circuit in that region and narrow down your search extensively. Also if you know of a reputable technician a repair like this is simple and should not cost much.

[jeff_free69]

Re: Intermittent amp channel

I have an old NAD 2200 power amp with the same problem. A little googling turned up that this is a common problem with the relay contacts.

Before shipping it out for repair ($175 + shipping), I decided to try cleaning them myself. if done very carefully, worst case i ruin the relays. I removed one (had to desolder) and worked it over with Craig Deoxit. Once I realized there's a flip top, I cleaned the other while still on the board.

So far so good , no smoke or sparks

[Æ]

Re: Intermittent amp channel

I have an old NAD 2200 power amp with the same problem. A little googling turned up that this is a common problem with the relay contacts.

Before shipping it out for repair ($175 + shipping), I decided to try cleaning them myself. if done very carefully, worst case i ruin the relays. I removed one (had to desolder) and worked it over with Craig Deoxit. Once I realized there's a flip top, I cleaned the other while still on the board.

So far so good, no smoke or sparks

They actually make a burnishing tool for relay contacts (I have one).
Chemicals or spray cleaners will remove dirt, but will not remove burned or pitted contacts. You need to slide something in between the contacts to polish the surfaces. Something like paper or a business card is good, but those may be too gentle. If that doesn't do it then you need to step up to something like 2000 grade wet or dry sandpaper.
I had some micron grade lapping sheets that worked wonders on hard surfaces and precious metals.

[pypes]

Re: Intermittent amp channel

Alright so I've let this go for a few days because I don't get much time to sit down and listen for extended periods and I wanted to test the relay guess. As it turns out, everytime the channel goes out if I crank the volume it immediately comes back in. It's done so every time and I've tried it at varying times over these past couple days to make sure.

The other thing I've noticed is that if I'm listening to it loudly it never goes out in the first place - it only seems to drop the channel at middle to low volume levels - does this make sense with our hypothesis that it's the relay?

If so: how do I go about finding / cleaning the relay? I don't know what one looks like or where it would be located - I'm a n00b when it comes to circuitry.

Thanks again for all the help!