Novice question - How test amp repair


2020-02-04 4:58 am
I'm still practicing my soldering, but soon will attempt to replace a spkr relay on my late-80s receiver.

Though the repair seems straight ahead, I want to "test" the repaired unit before hooking it up to my prized speakers.

What is the simplest/cheapest way to test?

Buy (2) cheap, bare, 8 ohm drivers?

Thanks in advance for any input.
2 tests.
One, no load, shorted input (use RCA cable & an alligator clip lead), No DC on output. Use a DVM. No more than 200 mv.
Two, sounds good. Use a cheap speaker from the charity resale shop or a car speaker from the junkyard. 4 to 8 ohms, or 3 to 7 ohms with the DVM on ohms scale. For better protection, you can put minus to minus electrolytic capacitors in series with the speaker. Old ones that are suspicious are fine for this. 1000 uf will get you some bass, 10000 uf would get you as much bass as you can stand. Sounds good? great.
Test 3, full power watts test. Use an analog VOM with 20 vac or 50 vac scale, or a scope. (I saw an analog VOM at the discount store last week for $25, those are fine for this). I use FM radio as signal source, pop/rock/hiphop stations are very compressed and run near full power all the time. With speaker rated higher wattage than the amp will put out, or an 8 ohm or 4 ohm resistor of enough wattage, turn volume up until you hear clipping. (has a certain sireny sound). With scope can look for flat tops on the sine waves. Back off volume a little. Then measure AC voltage on the speaker. W=(V^2)/Z where z is speaker impedance (an 8 ohm speaker has ~ 6 ohms resistance, 8 ohms impedance). Watts = rated watts, your repair is successful and your electrolytic caps aren't dried up. Note scope trace, average Vac=.7 Vpp (peak to peak). You can cook at full power for an hour if your wish, to test your solder joints against heat and test the fan & heat sink capability.
One 100 W amp I re-e-capped because it went silent at age 28, and was measuring 2 watts when I got there 2 days later: it wouldn't go over 25 watts with just the rail caps replaced. I had to replace all electrolytics until about the 14th or 15th cap made it go up to 100 W. Weird little 75 uf 50 VNP (non-polar) cap, very hard to find in stock anywhere. The manufacturer sells entire amps as a replacement part, not the cap. About $2800 for a new 100 W amp. Proud of themselves. They do have good reliability, noise rejection, ability to put out full wattage for hours.
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2020-02-04 4:58 am
I appreciate your lengthy response. Unfortunately, your explanation is far from understandable from a novice's viewpoint.

Could you possibly expand on this explanation.
Let's start with this line:

"One, no load, shorted input (use RCA cable & an alligator clip lead), No DC on output. Use a DVM. No more than 200 mv."

I take it "no load" means not to hook anything up to the speaker terminals. What does "shorted input"mean? What am I shorting and why?

"No DC on output" - are you saying, ideally there will be no DC on outputs? Again, are you talking about the speaker terminals?

Lastly, are you saying that I could have as much as 200mv DC on the speaker terminals?

Let me reiterate that I appreciate the courtesy of your response.
One shorts the input of the amp to prevent noise pickup. The air is full of radio frequency (RF), especially as now everybody carries a cell phone. If your amp has 1/4 phone plug jack instead of RCA jack, you need a 1/4 phone plug cable, although usually alligator clips will not stay on a 1/4" diameter piece of metal. In that case having a new 1/4 phone plug that you can take the cover off and access the solder terminals to short it is useful. Alligator clip leads with rubber boots are hard to find, since you can kill yourself with them on voltages over 80. I think my buddy bought his at harbor freight (everything else they sell is trash, IMHO). I bought mine at Radio Shack, now bankrupt. The 600 v rated alligator clip lead that Fluke sells is so safe it is perfectly useless for testing anything but enormous 480 v electrical panels.
No load means nothing on the speaker terminals but the probes of the DVM on DC volts scale.
Correctly repaired amps will produce 0 v DC on speaker at idle. 200 mv is acceptable. More than that the speaker will start heating up.
For the 2nd music test, I use a 1/8" stereo earphone plug to two male RCA plugs adapter, to put music out of a cheap battery radio into the amp. I think they call 1/8" stereo plugs now 3 mm or something. A sine wave (signal) generator will also work, but is obviously more expensive. Don't use a cell phone, if the amp puts DC out the input jack, you've blown a $600 cell phone. Use a $10 radio, walkman, or something disposable. Check radio with earphone before test to be sure it is putting sound out the earphone jack (battery isn't dead). Turn down to modest volume to avoid clipping; earphone drive is up to 7 vac and consumer amp input full volume is 1.6 vac or 2.0 vac.
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2020-02-04 4:58 am

I'm sure this is a classic example of two persons at opposite ends of the learning curve, in which the more knowledgeable person (you in this example) inevitably make certain assumptions concerning what the other knows/understands.

I have read and re-read your first paragraph, but still do not understand what I am shorting and how I am shorting it.

I assume you are talking about the rca inputs (CD, AUX, PHONO)
If so, what am I doing to them exactly? Am I connecting L&R channel together?
Where am I clipping the alligator clip? Where am I clipping the other end?

Thanks again for taking the time.
Sorry, it has been 61 years since I began electronics repair.
You use the selector switch to activate one of the 3 input - say CD.
Then you plug a RCA male to male cable in that socket, the CD one. For testing both sides at once, you can put cables in both left & right. I find these RCA cables at the charity resale shop, old DVD player and such triple cable sets. Blue yellow green or yellow white red doesn't hurt the electrons flowing through it. Check continuity pin to pin & ring to ring with the ohms scale of DVM, sometimes for $1 you get a bad cable. From farnell or RS, quality is assured.
Then you take the alligator clip cable, and short the ring of the plug, to the pin of the plug. For 2 sides at once, you use 2 alligator clip cables.
If you have shorted both inputs, with power on, each speaker output should be less than 200 mvdc. If not, that channel still has a problem.
If problem, you search internet for schematic of that amp, and search diyaudio for repair threads about that amp.
Relay replacement usually only causes a dead channel output, due to miswiring, wrong pinout, or bad solder joint.
BTW, I like teaching, but can't spend time in public. I'm part native Am, catch up to 10 respiratory diseases a year when I work with children. This is fun, I type 250 wpm since I'm a amateur pianist.
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