Using a small portable solid state amplifier to trace an audio circuit???

Question

I have a small portable solid state amplifier that can run off batteries or a wall wart. (12V).

I want to trace the circuits in a solid state Hammond Organ. (To listen for audio signals).

So, this amp is ready to go. Has a speaker and a wall wart connected.

I could also use some powered computer speakers too.....

My question is this:

To trace this organ (or any other audio signal), do all I need to do is isolate the input of this amp, using a capacitor? Then connecting a test lead and probe to the cap? (I assume shielded wire should be used from amp to the probe).

What size cap?

Since I work on a lot of tube stuff....

Could this also be used to trace tube amplifiers, assuming I use a cap that is rated at 600V or more?

I know, the main thing is to make sure that the high voltage DC (B+) dont get into the amplifier and ruin it.....

I was looking on ebay for "Signal Tracers", and could not believe the high prices. An old Micronta (Radio Shack), tester that tests both AF and RF, sold AS-IS for $50 and did not even include the test leads. I know the RF test lead is more complicated and has a circuit inside the probe. No, I wont pay that much..... I'll wait till I find one that is priced reasonably....

I dont really need to test RF at the moment anyhow.

John Robertson · Accepted Answer

Something in the order of 0.1ufd will work fine. Yes. Look up the old Heathkit Audio/RF Signal Tracer or the far more  recent Dick Smith Signal Tracer.  I have both and they work just fine. The DS version is the size of a  smallish, fat, remote control. I need to scan those and host them someday. Hence the isolation capacitor. The RF test is a trivial change - you simply need a diode. See the  Heathkit schematics. John :-#)# --  (Please post followups or tech inquiries to the USENET newsgroup) John's  Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9 (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)                                "Old pinballers never die, they just flip out."

mike · Answer

I'd use back-to-back diodes from the amplifier input to ground and a big series resistor and series cap in the probe.

I don't know anything about the equipment you're testing, but I think it would be risky (possibly fatal) to assume that there's a ground point in the system that isn't somehow connected to the AC line.

tabbypurr · Answer

You also need a pot on the input, or large swings will produce severe distortion. I'd add a fixed resistor too, who knows what the amp input can handle.

if it's not low impedance and you care about hum

size it to pass 20Hz

A spike of 600v into an unknown solid state amp is likely to kill it very dead. Your input cap won't prevent that. Add a resistor & diodes to protect it.

You can always use your audio to test the rf envelope. Just needs diode & capacitor.

NT

tabbypurr · Answer

Yes, some tube kit is live chassis. NT

oldschool · Answer

What do you mean by back to back? Woundnt a diode short the audio signal to ground? Maybe I'm not picturing this right....

I dont work on any of them AC-DC sets (live chassis) without an isolation transformer. In fact I dont usually evne get that stuff with live chassis. It's not worth salvaging.... That was a poor design right from the start.

Using a small portable solid state amplifier to trace an audio circuit???

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