Carver M1.5 Crackle Noise

[undefinedza]

Hello all,

I've recently acquired a Carver M1.5 and upon listening have found that there is a faintly audible crackle sound.

Diagnostic indicators:

1. It's definitely worse when the interconnects are unplugged.
2. It actually seems to get better when the amplifier has been on for a while.
3. The crackling becomes louder with an increase in volume, but not in direct proportion.
4. The crackling isn't constant... It comes and goes quite frequently.

Pretty weird. DC offset on both channels is below 15mV.

Any ideas?

Cheers,
Chris

 

[Tajzmaj]

Hi.
I have some experience with PM 1.5 which is basically the same amp. My advice would be recap it and properly adjust it again. It is an old amp. But of course good one. Funky unit. If you need I have service manual for PM1.5. It can be used for some guidance. Email me if you need it.
And it is also important amp is properly grounded.
That buzzzz or putputput...or something like that is generated by ''dimmer'' psu , that ''magnetic field coil''.
Cheers, Taj

 

[undefinedza]

Update:

The crackle definitely comes from both channels - but is worse in the left channel.

I found two burned resistors on the right channels board (you can see the same resistors on the left hand board), picture here: http://imgur.com/XZFtVvP

Otherwise the amplifier looks pretty good under the hood. It's a bit dusty, but none of the other components look burnt and none of the capacitors are bulging.

The crackle is definitely worse when the interconnects are not plugged in - might this be an indicator that the power caps are fine? I'd like to replace them in time, but I'd rather not replace them now unless I know they're the problem.

Cheers,
Chris

 

[indianajo]

Small crackles probably won't be the power supply caps. Shorts in those would be huge crackles.
I've found small crackles in tantalum coupling caps, that I bought new with the crackle in them. Before the internet, you bought at the store and had to be "happy" with what was available. Having the right numbers on the outside was as far as the store was prepared to guarentee their parts.
The best way to isolate would be with an oscilloscope. Or, you can listen with a sound probe (amp with a DC blocking capacitor on the input and a zener across the input after the cap to reduce huge pops when the DC level changes in the input. If going though the stages, the crackle is only after a certain stage but not before, the problem is somewhere around that stage. Be careful, this amp has dangerous voltages in it, use only one hand at a time and clip the negative of the meter or probe to frame. Wear safety glasses, exploding parts can damage your eyes.
Good luck. If you use google the right way, the carver schematic diagram has been uploaded on this forum before.

 

[undefinedza]

Thanks indianajo.

Could this not be a grounding issue?

 

[indianajo]

Crackles can be bad solder joints, whether on the ground or the hot side. They can be oxidized pressure connections between brass/tin parts, as the ground lugs to case. Also any push in connector that is not gold/palladium/rhodium plated or soldered. (Almost every connector in commercial amps). The scope or sound probe may focus your eyes on just where. Or you can look at every solder joint or connection and hope you spot it. Can also be bad connections within a transistor, resistor, diode, or capacitor. Some guy with a Peavey bass amp on I&A thread just paid 50E in Portugal for an "expert" to find a crackle, which turned out to be the leg corroded off a zener diode at the solder joint. Poorly washed flux can do that in 30 years. Knowing which part or joint is the cause takes analysis.

 

[Tajzmaj]

Hi. Those 2 resistors are R42, R43. They are part of ''+ rail commutator''. Once more I'm offering service manual. Amp boards are the same as in PM1.5. If you are not sure how to repair it, please get some experienced technician. This amp is more complicated because of it's triple rail design and ''magnetic field'' power supply. I doubt there is some exotic oxidation present. It's been probably shorted at full power and/or there is some dry small cap. I agree that crackling is not coming from smoothing cans. My guess is that something happened to that commutator circuit and it might be behaving quite randomly now. You will need scope, generator and chunky dummy load. Be aware there is very high voltage present. I don't know which version of PM1.5 is but even at ''L'' version there is some +-70V and +-124 at ''normal version. Please be careful.
Cheers

 

[undefinedza]

Hello gentlemen.

Thank you for your responses!

My electronics experience is severely limited, and I don't have access to a scope.

I think that I may take this amplifier to a professional.

Tajzmaj - thank you for the offer, but I actually already have the service manual for the M1.5.

I checked the DC rail voltage at the 130V filter caps and it's currently at 110V rather than 124V. Could this be the problem? I doubt so.

Chris

 

[Tajzmaj]

I don't think it's the problem. Of course you can try to adjust properly. It will probably change the '' rhythm '' of noise if it's related to psu ''pumping''. But still you have those burned resistors. I would strongly recommend to take it to some tech. I have several PM1.5 and it is definitely worth to recap it (completely-even all those small caps), maybe you can leave those 2 big filter cans. If you still have installed 2 smaller dark blue double caps there are substitute boards available on ebay. Than everything should be readjusted. After all this it will show how good it is. Very powerful and quite decent sounding. Maybe little on the ''hard'' side but still ok. And of course almost unlimited power for home use.
Cheers
50 80 V Capacitors to Repair Carver M 1 5 M 1 5T PM 1 5 | eBay

 

[indianajo]

Good technician training is to purchase a 24 to 48 v power transformer, some filter caps and rectifiers, a junk car speaker, a heat sink (or old aluminum scrap) and an LM3886 amp IC, and build a sound probe. Check the chip amp thread for schematics etc, or download the LM3886 datasheet from datasheetcatalog.com. The amp plus a blocking cap (.47 uf @ 600v) on the input, and a back to back zener diode (2.7 v) from input + to ground, makes a sound probe. Then you can look for where the popping sound starts in the schematic. The Carver is a very difficult amp to learn the basics on. the triple power supply is tricky, and the voltages could be lethal. I learned solid state amp repair on a dynakit St120, a very simple 14 transistor amp with one power supply. In those days I had no schematic diagram or service manual or help, I had to draw my diagram and guess at a lot of things that had been burned away.

 

[Conrad Hoffman]

I wonder what ever happened with that? I'm working on one right now with exactly the same burned parts. The high voltage commutator transistor failed, plus some other stuff. Definitely not for the beginner.