amplifier troubleshooting

That's the key!

The problem lies somewhere before the power amplification stage - probably with the pre-amp if there is one.

Try connecting the 1uF capacitor further back on the right channel circuitry and keep testing points further back until you cannot hear any sound - there may be a capacitor which has gone open circuit or possibly a dry joint.

My first checks would be on the decoupling caps connected to pins 3 (18), 5 (16), and 6 (15). It is quite likely that one of these is leaky or short circuit ( figures in brackets are corresponding pin numbers for other channel ). After this, check the caps in series with the input pins for being leaky. Failing any of these being the problem, then looking at the internals of the IC, and a typical application circuit, I can't see much else it can be bar the IC itself

Arfa

"Dave" bravely wrote to "All" (24 Feb 06 15:33:29) --- on the heady topic of "amplifier troubleshooting"

Da> From: "Dave" Da> Xref: core-easynews sci.electronics.repair:359019

Da> Awhile back I posted about an amp I am trying to repair. The amp is Da> composed of a +/- rectified and filtered power supply, an LM1036N Da> volume/tone/balance IC (two-channel), and a pair of LM1875T 20W TO-220 Da> amps which use bipolar power, no output filter caps. A bunch of Da> various resistors and caps and diodes. Very simple. I tested the Da> resistors and diodes in circuit, they look to be okay. I have no Da> reason to suspect the caps although I did replace one which appeared to Da> be open circuit. Da> The symptoms: No sound on the right channel. When I turn the volume Da> up quickly it makes a "thump" on the bad channel. So. I started at Da> the outputs and worked backwards. First off, I bridged the inputs on Da> the LM1875's with a 1uF cap. Voila! Sound. So, the speaker works, Da> the amp works.

Da> Another poster suggested that I use an analog meter to measure input Da> and output voltages of the LM1036N IC. So last night I soldered on Da> some test leads. I measured AC and DC voltages, and they varied a bit Da> from channel to channel, maybe 5V on one side and 4.2 on the other, no Da> catastrophic failure there. As an aside I kind of expected my analog Da> multimeter to look like a VU meter, moving up and down to the music, Da> but didn't see that at all, very steady. Then I noticed something: Da> when I measured the potential between the bad channel's INPUT to the Da> LM1036 and ground, I heard a scratchy sound come out of the bad Da> speaker. Hmm. As the meter is basically a big resistor in parallel Da> with the circuit, I figured I'd try a smaller resisiter. I changed the Da> scale of my meter from 30V to 10V. Now I get tinny sound. Changed to Da> 3V. Meter was off-scale but I now had pretty good sound. Didn't have Da> the balls to try shorting the two, I have been burnt (actually my Da> components have been burnt) by trying dumb-ass things like that using Da> the axiom "if a little is good, a lot must be REAL good".

Da> I have no formal training in electronics, but have done a lot of Da> reading on the good ol' internet and am keen to learn more. I am Da> completely perplexed as to what is happening. The only explanation I Da> can conjure up (and sorry if it sounds stupid but...) is maybe an Da> excess of DC current on my signal input which would screw up the bias Da> of the transisters in the mixer IC. There is a small electrolytic in Da> series with the mixer output and the amp input, plus a bunch of smaller Da> ceramics from the signal trace to ground so I don't think DC can be Da> getting to the amp itself... and if it were my little experiement with Da> the bridging cap, above, wouldn't have worked so well. Da> I have another LM1036 mixer IC, but don't want to replace something Da> that's not broken.

Da> Does anybody have any ideas? How can bridging my input signal to Da> ground with a resistor make sound come out? I've looked at this Da> circuit long enough that I can probably draw up a schematic if I can Da> find a few hours. Da> THanks

Da> Dave

If the cap between the preamp/mixer to the power amp is leaky then it might be biasing the amp's output to one voltage supply rail or the other. The smaller value resistor to ground simply allows the output to become biased closer to normal and sound can get through. Now that we know a little more a good guess is to change that coupling cap. The input resistor itself may be bad but that is easy to test on the ohm range.

Your other symptom about the thump when turning the volume pot quickly may be pointing to a leaky cap to the wiper of the pot. Check that.

A*s*i*m*o*v

... If there were no electricity, we'd all be ohmless.

But if that was the case then bridging the amps' inputs together with a cap wouldn't help, right? I mean the DC would still be getting to the bad channel amp's input to throw off the bias...

Am I missing something here ? I thought the OP said that he was starting to get some sound of sorts from the bad channel, when he put his meter on the input of the vol / tone control IC. He has already proven that both channels of the power amp work correctly, by bridging that IC's inputs. Thus, the problem must be in or around the LM1036 - see my post above. The volume control thump is nothing to do with DC or leaky caps at the control. This is a DC controlled electronic volume control IC. There is supposed to be DC on the wiper, as there will be on the tone and balance controls also, as they will all be fed from the internal zener reference on pin 17 of the IC.

Arfa

to

channels

is

on

Can I check the caps in-circuit or must I remove them? I've been checking them with a DMM at the highest-resistance setting. Electrolytics start at zero and climb to infinite resistance. The smaller the cap, the faster the rise in resistance. With film caps (tan, thin, round), though, I don't seem to go to infinite resistance... on EITHER channel, the good or the bad. So I conclude that my test method is inappropriate. Anyway, all of the resistors and caps between the mixer IC and the amps measure similarly from one channel to the other IN CIRCUIT.

Tan, thin and round sounds like typical disc ceramic caps. These are often used for decoupling purposes, and it is quite common for them to go leaky. You need to lift one end really, to measure them. They should read nominally open circuit both ways, as their value is usually small. As far as the electrolytics go, the sorts of readings that you are seeing, look ok. Ideally, they should be removed from circuit to check them properly, or tested with an ESR meter, but for the problem you have with the amp, it is unlikely that you have an ESR issue, or even one that requires the caps to be removed and tested more than you have already. If one of these caps was going to be the problem, it would more than likely be that it was short circuit, pulling the pin on the IC that it was connected to, to ground. Clearly, none of them are short circuit, and as you seem to get comparable readings on each one, to its corresponding one on the other channel, it's unlikely that you have any gone leaky. I think now, that there is little else that you can do, bar risk replacing the tone / balance / volume IC.

Arfa

nominally

Thanks, I was heading that way... I've got a new tone/balance/volume IC, it was

I think now, that there is little

Replaced the IC last night, problem solved. Thanks for your help.

Dave