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General testing procedure for amps

ocdecio

ocdecio

The proof of the restoring is in the listening.
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After many restorations I am still uncertain as to the proper sequence to ensure an amplifier is in good working condition and ready for performance testing. Last week I had a 2238 that on the surface was working but 5 minutes into testing it, smoke came from an emitter resistor and a driver went with it. It was a faulty but not blown output (it tested OK for the most part - I don't have a curve tracer that probably would have caught an issue).

My current procedure is - DBT increasing the lamp wattage from 40 to 100, 150, 200 and 300W. Measure temperature of pre-drivers and drivers with moderate load (just normal music playing on speakers). Track bias on both sides. If both sides agree, move up the wattage until plugged on wall. This last step is where I wanted to ask - what tells you a receiver/amp is "good to go" and be fully tested on the wall without fear of a major blow up?
 
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ocdecio said:
After many restorations I am still uncertain as to the proper sequence to ensure an amplifier is in good working condition and ready for performance testing. Last week I had a 2238 that on the surface was working but 5 minutes into testing it, smoke came from an emitter resistor and a driver went with it. It was a faulty but not blown output (it tested OK for the most part - I don't have a curve tracer that probably would have caught an issue).

My current procedure is - DBT increasing the lamp wattage from 40 to 100, 150, 200 and 300W. Measure temperature of pre-drivers and drivers with moderate load (just normal music playing on speakers). Track bias on both sides. If both sides agree, move up the wattage until plugged on wall. This last step is where I wanted to ask - what tells you a receiver/amp is "good to go" and be fully tested on the wall without fear of a major blow up?
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I don't see anything wrong in your methods. If anything they may qualify as overly cautious.

The failure you describe would be almost impossible to catch IMO, as I imagine all the components were fine but a semi was getting ready to let go under normal operation.

Unfortunately semiconductors aren't like tires... you can't look at them and think "yeah, this one's got almost no tread left." :)
 
I guess I have a simplistic view of when to go from DBT to the wall. If the bulb dims in the first few seconds, there clearly isn't a short. I check bias and offset to make sure they are not way out of line, and if not, I plug into the wall to adjust bias and offset to spec.
 
Ray Gianelli said:
Unfortunately semiconductors aren't like tires... you can't look at them and think "yeah, this one's got almost no tread left." :)
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Quite true. For now I am doing like an hour light listening on the wall after all DBT passes, monitoring both bias and DC. If they stay quiet I am taking as a "proof of a negative" essentially - no visible problems, load testing can start.

I just wante to see if anyone had a safety step that I didn't think about. Thanks!
 
ocdecio said:
Quite true. For now I am doing like an hour light listening on the wall after all DBT passes, monitoring both bias and DC. If they stay quiet I am taking as a "proof of a negative" essentially - no visible problems, load testing can start.

I just wante to see if anyone had a safety step that I didn't think about. Thanks!
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My acid test is to run them for several days at low volume, with bursts at high volume. Then power cycle them several times a day. If they make it through that then I'm more comfortable shipping them.
 
Ray Gianelli said:
My acid test is to run them for several days at low volume, with bursts at high volume. Then power cycle them several times a day. If they make it through that then I'm more comfortable shipping them.
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That's exactly the type of procedure I was looking for, thank you. I always get anxious deciding when to ship and this looks like a good one.
 
ocdecio said:
After many restorations I am still uncertain as to the proper sequence to ensure an amplifier is in good working condition and ready for performance testing. Last week I had a 2238 that on the surface was working but 5 minutes into testing it, smoke came from an emitter resistor and a driver went with it. It was a faulty but not blown output (it tested OK for the most part - I don't have a curve tracer that probably would have caught an issue).

My current procedure is - DBT increasing the lamp wattage from 40 to 100, 150, 200 and 300W. Measure temperature of pre-drivers and drivers with moderate load (just normal music playing on speakers). Track bias on both sides. If both sides agree, move up the wattage until plugged on wall. This last step is where I wanted to ask - what tells you a receiver/amp is "good to go" and be fully tested on the wall without fear of a major blow up?
Click to expand...
Post repair I always use them for a week, at least, more if I am shipping. Cuts down on 95% of returns, especially on restored units. I say 95% because the other 5% they come back for a switch cleaning because the owner let it sit for 2+ months before hooking it up.
 
goldswimmerb said:
Post repair I always use them for a week, at least, more if I am shipping. Cuts down on 95% of returns, especially on restored units. I say 95% because the other 5% they come back for a switch cleaning because the owner let it sit for 2+ months before hooking it up.
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That's a great strategy - were you able to catch an issue while using them that would have popped up with the customer? In your experience what is the issue that occurs the most?
 
After adjustments per the service manual, I usually do a load test right there and then. I shouldn't call it a load test, more of a where does it clip at test.
I will run a sine wave at 1\3 output for 5 minutes or so and then take a thermal image, more for fun than anything. I do look at the images to see if anything is "hot" that shouldn't be - but I'm saying this is a valid test at all. I can certainly use the thermal images at work to find building envelope issues, but ya mostly for fun on amps.
After that, I also then run a unit for days over a period of weeks, thermally cycle it multiple times and just listen for issues.
 
ocdecio said:
That's a great strategy - were you able to catch an issue while using them that would have popped up with the customer? In your experience what is the issue that occurs the most?
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I've caught issues on occasion, but it's few and far between and usually something I should have addressed in the first place. Usually things like a pop or other DC on a control or a switch that needs to be cleaned up.

It's mostly for peace of mind though, as it helps eliminate things if the customer has an issue.

With straight repairs it works out well because you'll know when you miss something, especially if you use it like it was yours. I've had many where it seemed the tech before me didn't bother to test all that much.
 
I find running a THD curve family can show up a lot of subtle problems. If it doesn't meet spec or the channels are a lot different, it has to be explained. This is a PITA unless you have some automated way of doing it. I also just sit and watch the thing, putting my finger here and there to look for unexpected temperature rises. I've found several amps over the years where the ratings of the outputs were just too close to the operating conditions. True on some Marantz and even on Crown. I had to choose devices on the curve tracer for those units. Something that doesn't help one bit is high line voltage and I seem to have that much of the time. Stuff in my lab has to survive continuous operation at about 128 VAC.
 
ConradH said:
I find running a THD curve family can show up a lot of subtle problems. If it doesn't meet spec or the channels are a lot different, it has to be explained. This is a PITA unless you have some automated way of doing it. I also just sit and watch the thing, putting my finger here and there to look for unexpected temperature rises. I've found several amps over the years where the ratings of the outputs were just too close to the operating conditions. True on some Marantz and even on Crown. I had to choose devices on the curve tracer for those units. Something that doesn't help one bit is high line voltage and I seem to have that much of the time. Stuff in my lab has to survive continuous operation at about 128 VAC.
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Great insight, thank you. I am working on a 2325 and I point a common temp gun (wouldn't dare use my finger lol) to the various places I think important to be balanced - pre-drivers seem to be an early indicator of issues for example, as well as some larger resistors. I don't yet have the luxury of a proper curve tracer and I am sure I could have prevented a couple of blow ups by as you say outputs close or below their ratings. Line voltage is an interesting point because if you look on the back it says - 120V, and it is very common here at home to have a wall voltage of 123-124V.
 
ConradH said:
I find running a THD curve family can show up a lot of subtle problems. If it doesn't meet spec or the channels are a lot different, it has to be explained. This is a PITA unless you have some automated way of doing it. I also just sit and watch the thing, putting my finger here and there to look for unexpected temperature rises. I've found several amps over the years where the ratings of the outputs were just too close to the operating conditions. True on some Marantz and even on Crown. I had to choose devices on the curve tracer for those units. Something that doesn't help one bit is high line voltage and I seem to have that much of the time. Stuff in my lab has to survive continuous operation at about 128 VAC.
Click to expand...

Any thoughts about buying and using something like a 15 or 20 amp Tripp Lite, etc. AVR to power up/protect your bench test equipment from that rather high mains voltage ?
I find it very comforting to do so for both of my benches, and my mains rarely rises above 126. v,& and it seems my test equipment runs cooler.
 
I'm a heartless brute and any test equipment that doesn't survive on my bench doesn't deserve to be there. :whip:
IR temperature readings are great, but you have to watch out for emissivity. A shiny TO-3 probably reads way too low. I often put a stripe of water-based black paint on things I want to measure accurately. It was expensive but I just had to have one of these- https://www.thermoworks.com/close-focus-ir/ It will read a 0.1" diameter circle.
 
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