Help teach a CS guy to test a tube amp!

[rayma]

I teach on the side (academic pricing) and am looking at this scope -
Analog Discovery 100MSPS USB Oscilloscope & Logic Analyzer - Digilent
It looks like the scope can handle 25V (doesn't mention RMS). I'm thinking getting a x1 and x5 probe?

It has differential inputs!
The built-in attenuator is either x0.2 (7Vpeak-to-peak) or x0.02 (50Vpeak-to-peak).
You'd need a x10 probe to measure more than 15Vrms.

Looks like you'll need the BNC adapter board to connect the probes.
Each probe can be set for x10 or x1 attenuation with a switch.

 

[jeffhigh]

At this point it may help to describe the first system to test. One of my wife's friends owns a record store and sells used equipment. Over the years we have picked up components. The current iteration of the system has some cracks and pops. I would like to work through the components and figure out which is the offender. For the most part, I have no desire to get into board diagnostics with this system. Just find offending parts.

I think I have gone as far as possible diagnosing with differing sources. I first gave up on the record player and switched to a digital input from iTunes through an airport express. It cleared up a bit, which helped me figure out how to dial in the record player. The system sounds a lot better now. However is still has a little noise unaccounted for. I know it is noise, because if I leave the system plugged in without a source, a bit of noise pops up.

QUOTE]

I'm inclined to think that putting an oscilloscope on the output is unlikely to aid you in diagnosing cracks, pops and the "offending parts"

 

[agabriel]

I put deoxit red/gold on order today.

How else would noise/pops be diagnosed?

 

[jeffhigh]

At this point, the FIRST thing to do would be to ensure you understand how to work safely inside an amplifier including using a multimeter to check power supply capacitors to ensure that they are fully discharged.
You are probably not , at this stage of your knowledge, going to be able to do too much more that cleaning valve pins and sockets.

 

[rayma]

I put deoxit red/gold on order today.

Also get some lintless wipes, like Kimwipes.

 

[rayma]

How else would noise/pops be diagnosed?

If the noises are loud, don't use your good speakers, use boom box speakers, etc. in case of damage.
It might be best to clean all contacts before troubleshooting, since some noises could go away after cleaning.
First isolate the noises to one channel, if possible, by swapping L for R the input cables to the amp.
If noises stay the same, they are in the amp, not the preamp or source.
Then swap tubes between channels, one type at a time, to isolate the noise to tubes (or not).
There could be bad solder joints, loose connections, bad resistors/capacitors.
The scope won't be much help for this troubleshooting.

 

[agabriel]

If the noises are loud, don't use your good speakers, use boom box speakers, etc. in case of damage.
It might be best to clean all contacts before troubleshooting, since some noises could go away after cleaning.
First isolate the noises to one channel, if possible, by swapping L for R the input cables to the amp.
If noises stay the same, they are in the amp, not the preamp or source.
Then swap tubes between channels, one type at a time, to isolate the noise to tubes (or not).
There could be bad solder joints, loose connections, bad resistors/capacitors.
The scope won't be much help for this troubleshooting.

Sounds reasonable. I have tools/parts on order. More "mentoring" may (will probably) be needed once it comes in.

 

[agabriel]

Hello,

Ok - my scope and associate parts are in. I also have a few minutes to play with it 🙂.

I'm starting with a pre-amp that has been a flaky lately (phono stopped working randomly) and it is currently not being used with the stereo. The preamp seemed like the safest place to start because it is the simplest setup.

I have two tests - one works as expected the not so much with the other.

In both cases a x1 probe is plugged into Channel #2. Case 1 has channel #1 on tape out and everything looks as expected. Case #2 has channel #1 on Output #1 and does not work (even with the volume turned up). Anyway, I'm using a few inputs that were all working last time, so not sure what is going on. What are your thoughts?

Anthony

 

[agabriel]

I may have just answered my own question.... I plugged it in to the stereo again and neither of the outputs are working, but the tape outputs are.