Help needed, Primare A10 crackling and popping.

[Brillegeit]

Hi guys!

I was recently given a Pirmare A10 Integrated Amp with some problems, now I want to try to fix it.
The preamp in it works, but the poweramp is faulty.

Here are the symptoms:
I turn on the amp, with speakers connected, the volume is at 10 and there is some sound in both speakers. If I turn the the volume up, or down, it crackles and pops, and then one of the channels gets really loud and distorted, and the other channels has no output. This continues until I fear for my speakers health, and turn off the amp, about 10 seconds later..

As mentioned, the preamp-part works, I can connect it to my poweramp and it works just fine, volumecontrol and input-swicthing.

The output-stage consists of 2pairs of Toshiba 2SC5200/2SA1943 bipolars. Could these be faulty? Or does this sound like a cold solder, bad connection, output relay, or leaky capacitor?


Any input/help will be greatly appreciated!

- Bjørn

[Spiny]

If turning the volume causes noises, replace or try switch cleaner on the volume pot. then clean all other mechanical contacts and re-flow all solder joints, a comon fault on older gear is dry joints and pots that do not work.

[Brillegeit]

The volume pot. seems ok, since it controls the volume of the preamp just fine. The voulme pot. is connected to an IC, on a pcb in the front of the amp, and then via flat-cable connected to the preamp-pcb and then to poweramp-pcb. Since the preamp has full functionality, I figure the fault is somewhere on the poweramp-pcb?
I`ll try to pull the poweramp-pcb out of the chassis and do a complete check with a magnifying glass, hopefully I`ll find a dry joint, as you say! :-)

Thank you for your response.

[Brillegeit]

Two pins of an IC in the DC-servo was shortcircuited due to a bad solder. A big bead of solder on one of the pins had a slight connection to the next pin on the IC. I broke the shortcircuit, and the amp now works flawlessly!

[VictoriaGuy]

Good for you!
DIY!

[Brillegeit]

As the title says, I didn`t fix it afterall. Few weeks after I thought I`d fixed it, it started acting up again!

Since last time I have changed the volume attenuator-chip (LM1972), dc-servo-opamp, and protection opamp.

I have manage to diagnose it a bit further though; the problem is totally random, the amp can work for several weeks, and then suddenly starts acting up again. After I changed the chips mentioned earlier it has been working for many weeks, and then, yesterday it started again.

The problem seems to be only in the right channel, sometimes it goes mute other times it gets 10times louder than the left channel. And this only happens when changing the volume. The volume is indicated in an lcd-display from 0-79, and it is totally random at which setting the problem occurs, and if I change the volume back down or further up it goes back to normal. It happened a few times yesterday, but worked fine when I turned it off, when I turned it on this morning it wouldn`t come out of protection, had to power-cycle it a few times before it worked. It has also c***ked up a few times today, but is currently working, volume changing works fine.

I`m thinking maybe the problem is located in the right poweramp-section? Maybe output-devices? Any ideas on what could be causing this is much welcome!

I`ll post the schematic for the right poweramp later. (on different pc)

Bjørn

[Brillegeit]

Here are the schematics for the right poweramp. And also an overview of the main pcb.

I noticed earlier that the output devices in schem. are 2SC4278 and 2SA1633, but in my amp they are 2SC5200 and 2SA1943. It doesn`t look they have been changed though. I guess the Toshibas are better though.. :-)
There is another thing I find odd, Q14, 2SC3420 connected to the pcb via cables, is mounted on top of Q51, 2SC5200, effectively using it as an heatsink! Now that can`t be a good idea, can it?

Bjørn

[technical guy]

From technical guy (paul),
Had a primare a10 in for repair with a report of "no audio".
closer investigation i found 'speaker protect relay not being switched ,due to one of the regulater i.c,s had failed. The minus 35volts was low at approx 15.5volts--(reg8),a type :ka337 3 terminal 1.5 amp ,adjustable regulator.
Not sure if the crackling/popping noises are directly related to one of the regulator devices failing,found that there are : plus 5 volts,plus 6.5 volts,minus 6.5 volts,and plus and minus 35volts---derived from lm317t/lm337t type regulators.
I also have known the metal can type smd electrolytics to physically leak electrolyte--BE VERY CAREFUL WHEN DESOLDERING SMD COMPONENTS,AS THE PCB AND NEARBY COMPONENTSCAN BE EASILY DAMAGED.
From close inspection of the pcb,there is around power amplifier/protection a few metal can smd type capacitors--47uf 50v and 3.3uf 50v by protect cct,and 1uf 50v and a 3.3uf 50v by d.c./servo i.c.'s next to driver transistors.
hope this may help.

[whizgeek]

Q14 is the bias current temperature compensation, so has to sense o/p devices.
replace R121 trimpot anyway, but measure voltage across R4/R5 first to check bias current.

also meter DC volts across the output when fault occurs: may help determine if DC or ACsignal faults