Completed Stereomour—left channel crackles/pops/hisses/distorts [solved]

[funandfunny20]

So I checked the resistance of the fuse, and it jumps all over the place, eventually settling at zero.  I'm assuming it is not fried since I'm getting some reading, is that correct?

[Doc B.]

Make sure you have good contact of the fuse ends and the test leads. I often rub the probes against each other to clear any oils that might be on them, and then rub the probes against the ends of the fuse to make sure there is nothing interfereing with the metal to metal contact.

[funandfunny20]

Hey Doc,

I just did what you said and while meter does jump up slightly, it almost immediately settles at 0.  I'm going to make a run to radioshack shortly to get alligator leads and new fuse.

[Doc B.]

Clip leads are a good idea. It sounds like the fuse is probably OK, but a new one is cheap insurance.

If the new fuse doesn't bring things back to life the next step is to make sure the power switch is working. WITH THE AMP UNPLUGGED use your new clip leads to clip a test lead to each terminal on the power switch then measure the resistance when you throw the switch back and forth. In one direction it should be overlimit and in the other it should read close to zero ohms. If it reads overlimit on both settings it needs to be replaced.

[funandfunny20]

Okay, so the Radio Shack near me doesn't carry 1.5a GMA slow blow.  Only fast-blow.  I'm assuming that matters, right?


As far as the power switch goes, it does OL when switched, and it is at zero when flipped back.

I did some resistance checks, and A4, A1, C4, C1, and all the other points that are supposed to read 1.2k Ohms, only read .5k Ohms.

Does that help with anything?

Sorry for all the dumb questions!

[Paul Joppa]

That 1.2K is the fat resistors on each side of the driver socket. See page 66 of the manual for a picture. Are they the right values (1.6K and 4.7K)?

[funandfunny20]

Got new fuses, but I am still not getting any readings. Except, an odd reading is -reg, which is NOT 0—it reads .7 or something like that. 

Just to cover my bases, the larger holes of the tube sockets are closest to the hum pots, right?

[funandfunny20]

I also checked the resistors.  As far as I can tell, they are correct.  I have them exactly like they are in the pictures, including the direction of the words.

The most confusing part of all of this is that the amp worked (and sounded good) for almost two days…

[funandfunny20]

Any ideas?

[Paul Joppa]

If the 1.2K resistance (1600 parallel 4700) is really 500 ohms, then it is being shunted by something. The obvious candidate is the bypass caps on the PSU board. This could happen if they are in backwards. You can disconnect the wire that goes into the board terminals marked "K" and then measure the resistance again to test this theory.

[funandfunny20]

Removed both wires from the k terminals and I'm still getting the same results: A4, A1, C4, C1, and all the other points that are supposed to read 1.2k Ohms, only read .5k Ohms.


Any other suggestions?  I'm thinking about just rebuilding everything.  Is that a plan worth pursuing?

I'll attach some photos in a couple of minutes.

[funandfunny20]

Here are some photos.

[Paul Joppa]

Sorry to say this, but it's time to unsolder and detach one end of each of the 166K and 4.7K white resistors and measure each one by itself to determine which one(s) are actually not the right value.

It might be a good idea to check your meter with some other resistors to be sure the meter is OK and this is a real problem, not a flat battery or something! Try R1 and R2 on the C4S board, they can be measured without removing them.

[Doc B.]

Do you have your meter set on a megohm scale, perchance? Meters are notoriously inaccurate if set at high ranges to read low values.

[Paul Birkeland]

I am going to go ahead and concur that there is probably a meter issue here.

If A1/A4 and C1/C4 all measure around 60V DC, then you're not going to have 500 Ohms to ground from those pins.

It would be helpful to see a shot of your power supply board, as there may well be a cap in backwards.