What should be done as part of an "Inspection & Service" for an amplifier?
EDIT: I want to learn the process I professional tech would take in servicing an amp. My intention is not to make more money selling something with misleading claims. *I just want to learn what a professional service would entail*
EDIT 2: I have no intention of making a guarantee, express or implied, about the quality of inspections or service! Whenever I sell something, I like to go over it and see if anything is obviously broken or could be repaired, and I note that I've done that. I want to learn the process a proper tech would take so I can be more thorough, for my own experience and practice. When I sell something I would state what I have done, and the state the amp is in based on my testing. No warrantee or guarantee implied!
I have Hafler DH-500 that I want to sell. Before I do so I want to do give it a proper look-over. I've already given the amp a basic function test. Seems to work. My next step would be to set the bias and DC offset and see if they hold their values. I don't have a distortion analyzer to confirm the specified THD+N, although I could test some of the other values.
EDIT: I want to learn the process I professional tech would take in servicing an amp. My intention is not to make more money selling something with misleading claims. *I just want to learn what a professional service would entail*
EDIT 2: I have no intention of making a guarantee, express or implied, about the quality of inspections or service! Whenever I sell something, I like to go over it and see if anything is obviously broken or could be repaired, and I note that I've done that. I want to learn the process a proper tech would take so I can be more thorough, for my own experience and practice. When I sell something I would state what I have done, and the state the amp is in based on my testing. No warrantee or guarantee implied!
I have Hafler DH-500 that I want to sell. Before I do so I want to do give it a proper look-over. I've already given the amp a basic function test. Seems to work. My next step would be to set the bias and DC offset and see if they hold their values. I don't have a distortion analyzer to confirm the specified THD+N, although I could test some of the other values.
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Its important to check the correct fuses are fitted and the earth connection is solid, basically all the safety issues come first.
If possible test with a dummy load and 'scope, make sure its stable upto full power, and handles full power for short periods without issue. And a brief check of clipping recovery is reassuring (amps that are borderline stable can be triggered into oscillation this way - you want to know this before expensive speakers are connected). If you can inject a dc signal and test protection kicks in that's good too.
Then worry about THD! Its more important the amp isn't going to fry the user or the speakers.
If possible test with a dummy load and 'scope, make sure its stable upto full power, and handles full power for short periods without issue. And a brief check of clipping recovery is reassuring (amps that are borderline stable can be triggered into oscillation this way - you want to know this before expensive speakers are connected). If you can inject a dc signal and test protection kicks in that's good too.
Then worry about THD! Its more important the amp isn't going to fry the user or the speakers.
Oh no, that's the way to blow the output devices - or the power supply! Its amplified DC that might damage speakers that you want to emulate, thus provoking the protection.
Just a small DC voltage after the input blocking cap might do it, and you might have to bypass any DC-blocking cap on the feedback network too. Simply a 3 Hz input signal might be enough with a bit of volume - 3Hz should look like DC to the protection circuit I'd hope.
Just a small DC voltage after the input blocking cap might do it, and you might have to bypass any DC-blocking cap on the feedback network too. Simply a 3 Hz input signal might be enough with a bit of volume - 3Hz should look like DC to the protection circuit I'd hope.
Could I adjust the DC offset pots out of spec until they trip the circuit?
This amp is rated to 255wpc, and I've read reports that it doesn't clip until >400wpc. That's a hecken big dummy load! I can definitely build one, but I wanted to make sure that I'm not going off on the wrong track before I put together half a kV of resistors.
This amp is rated to 255wpc, and I've read reports that it doesn't clip until >400wpc. That's a hecken big dummy load! I can definitely build one, but I wanted to make sure that I'm not going off on the wrong track before I put together half a kV of resistors.
Well if you want confidence that its in proper working order I don't see how you can get away with just running it at low power. Perhaps use 32 ohm load for clipping test or something like that. A set of four 8 ohm resistors can be series-parallel for 8 ohms or all in series for 32 ohms. Or used in pairs for 4 or 16 ohms. Always handy to have a bunch of them on some heatsinking...
BTW there are waveforms with a very high crest factor specifically for testing clipping without needing full power, such as a sinc pulse.
BTW there are waveforms with a very high crest factor specifically for testing clipping without needing full power, such as a sinc pulse.
With most commodities it is prudent to be fully qualified in order to make a claim that something is of sound condition, and command a higher selling price. It’s an old amp, many things can go wrong with those, much of it unseen, like corroding components regularly creating bad connections to the circuit boards, that I know of.
I would be more inclined to sell it as-is, quickly, and show that the dc offsets and bias are adjusted, and to not make any further claims.
I would be more inclined to sell it as-is, quickly, and show that the dc offsets and bias are adjusted, and to not make any further claims.
I think you should sell it "as is". IMHO an honest service technician is going to need hours to check it over properly and will normally cost you more than you can sell the serviced item for. I specialise in vintage rebuilds and upgrades for clients and honestly I do it for the satisfaction, not the money. I really cringe when I find a client sells one of the items I have worked on with a "professionally serviced" tag. Old gear is old gear and the owners fix it for love; not even God can make it bulletproof.
I think you’re misconstruing my intentions. I’m not intending to say anything was done that wasn’t done, imply that I’m a professional, or they it’s bulletproof. Perhaps I overgeneralized in my description in the first post. I’d also like to point out that no one is actually helping me learn anything (except for Mark). Of course I’m going to do a poor job if I don’t know what I’m doing, which is why I’m asking for help 
Bucks: Dude, not cool. Would you rather I not ask for help and just **** **** up? Every post I make helps me learn something, which helps me do something right.
Bucks: Dude, not cool. Would you rather I not ask for help and just **** **** up? Every post I make helps me learn something, which helps me do something right.
Hi SoaDMTGguy,
Okay, I am a professional audio tech and do perform checks on products for sale and just after purchase.
All you can say is that it works. There are a ton of things a tech checks unconsciously (a good one anyway). We also have the proper equipment and can confirm it meets spec. In the course of doing these checks, more serious issues often show up even though the amp appears to operate okay otherwise.
You can not honestly suggest the amplifier has been checked other than it works. Just like a normal person can certify a car.
Please don't try to add value when you aren't trained. It is cheaper to take it to a tech for test than to buy equipment, learn and do the same yourself.
-Chris
Okay, I am a professional audio tech and do perform checks on products for sale and just after purchase.
All you can say is that it works. There are a ton of things a tech checks unconsciously (a good one anyway). We also have the proper equipment and can confirm it meets spec. In the course of doing these checks, more serious issues often show up even though the amp appears to operate okay otherwise.
You can not honestly suggest the amplifier has been checked other than it works. Just like a normal person can certify a car.
Please don't try to add value when you aren't trained. It is cheaper to take it to a tech for test than to buy equipment, learn and do the same yourself.
-Chris
anatech: Like I’ve tried to clarify, I’m not seeking to add any value or make claims about the functionality or quality or reliability of the amp. I just want to know what someone like you would do.
If I understand what you’re saying, you would test it to make sure it meets its published specifications, and address or note any issues identified in that process. That makes sense to me. I’ve had equipment looked at by professionals before, and that sounds like what they did, too.
Thanks for explaining your process. That was very helpful.
If I understand what you’re saying, you would test it to make sure it meets its published specifications, and address or note any issues identified in that process. That makes sense to me. I’ve had equipment looked at by professionals before, and that sounds like what they did, too.
Thanks for explaining your process. That was very helpful.
Hi SoaDMTGguy,
At the very minimum, check the seals on the main filter capacitors. Confirm the bias currents are correct and the DC offsets are acceptable. I should warn you that many people blow amplifiers up checking bias currents because the probes slip.
You often cannot hear distortion when in fact there is a lot. That's why we not only measure it, but we look at the waveform on the scope. I also get a frequency spectrum display that shows the levels of hum and ripple as well as the degree and order of distortion products. To the trained eye, you can read a lot from those displays. I just caught an amplifier that has serious problems on a check that sounds okay on music and was passed by another shop.
Quite a lot of information is also gained on a visual check. I also check regulated voltages because you can have a total failure of a regulator and the equipment will still work for a while.
When checking anything, never use meter probes that are bare, and pointed ends will hold better. I also use leads with grabber clips. These are expensive leads costing over $50 for each set. Oscilloscope probes must never be X1. Good probes are always X10 or X100, switchable probes are disasters waiting to happen. Use epoxy glue to fix the switches into the X10 position. A cheap probe isn't something I trust, but I do use cheap ones for general probing as my good probes are $275 each. The cheap ones are glued to the X10 position and are discarded when any damage occurs to them. If I need a reading or important waveform, the good probes are used. You can't trust cheap probes for anything accurate or important.
There are many things we check unconsciously I can't even begin to list. But hopefully this basic list can help you out.
One final note. Cheap meters are often out of specification and those specs are extremely loose. A half decent meter will be made by Fluke or Keysight, maybe a couple others but the first two will hold their calibration and most will not. The AC frequency response will be over 50 KHz on any meter you can trust. Check that spec! Digital oscilloscopes can lie to you, and often do. The best instrument will be a decent analogue scope. Their are many good used ones available, a no-name product is a waste of money. After all, if you can't trust the readings, what good is the instrument in the first place?
-Chris
At the very minimum, check the seals on the main filter capacitors. Confirm the bias currents are correct and the DC offsets are acceptable. I should warn you that many people blow amplifiers up checking bias currents because the probes slip.
You often cannot hear distortion when in fact there is a lot. That's why we not only measure it, but we look at the waveform on the scope. I also get a frequency spectrum display that shows the levels of hum and ripple as well as the degree and order of distortion products. To the trained eye, you can read a lot from those displays. I just caught an amplifier that has serious problems on a check that sounds okay on music and was passed by another shop.
Quite a lot of information is also gained on a visual check. I also check regulated voltages because you can have a total failure of a regulator and the equipment will still work for a while.
When checking anything, never use meter probes that are bare, and pointed ends will hold better. I also use leads with grabber clips. These are expensive leads costing over $50 for each set. Oscilloscope probes must never be X1. Good probes are always X10 or X100, switchable probes are disasters waiting to happen. Use epoxy glue to fix the switches into the X10 position. A cheap probe isn't something I trust, but I do use cheap ones for general probing as my good probes are $275 each. The cheap ones are glued to the X10 position and are discarded when any damage occurs to them. If I need a reading or important waveform, the good probes are used. You can't trust cheap probes for anything accurate or important.
There are many things we check unconsciously I can't even begin to list. But hopefully this basic list can help you out.
One final note. Cheap meters are often out of specification and those specs are extremely loose. A half decent meter will be made by Fluke or Keysight, maybe a couple others but the first two will hold their calibration and most will not. The AC frequency response will be over 50 KHz on any meter you can trust. Check that spec! Digital oscilloscopes can lie to you, and often do. The best instrument will be a decent analogue scope. Their are many good used ones available, a no-name product is a waste of money. After all, if you can't trust the readings, what good is the instrument in the first place?
-Chris
Wow, that’s a lot of good info. Thanks! I have two Tektronix scopes, one digital and one analog. Is the idea the analog scope shows you directly what the wave looks like so you can see distortion and problems more easily?
My “good” meter is Klein. I’ve been thinking of getting a bench top scope, I guess I should accelerate that. I also need some form of distortion analysis. I have an HP 331A that doesn’t work. And I should finally get an external sound card for my laptop so I can run FFTs on it.
I’ve got grabbers for my meter. Cheap Chinese probes for my scopes. I’ll see about getting some better ones.
My “good” meter is Klein. I’ve been thinking of getting a bench top scope, I guess I should accelerate that. I also need some form of distortion analysis. I have an HP 331A that doesn’t work. And I should finally get an external sound card for my laptop so I can run FFTs on it.
I’ve got grabbers for my meter. Cheap Chinese probes for my scopes. I’ll see about getting some better ones.
So then take Marks Advice and go that way
Of course I’m going to do a poor job if I don’t know what I’m doing, which is why I’m asking for help
No, if the Customers comes back because you did a bad job, then you will know that you did a bad job.
But it gets worse if the customer don't come back and neither write to you or phone after he bought that amp from you..
Two possibilities:
1. He is totally satisfied and see no reason to inform you
2. He is totally Pi**ed because he thinks that you sold him some Junk.!
In this instance you will never know why! Remember that, and if he does not come back, then also this doesn't mean that you are a good tech. It just means that the AMP did not fail for the one or other reason. And the guy who bought it is satisfied with the way it is..
Or the Amp broke, but because it was an old amp 30years+ the guy who bought it don't care
BUT if you sell that AMP as SERVICED then you are RESPONSABLE for the AMP, because in that Case at least in EUROPE, you give out a WARRANTY for that Device that it has been Controlled;.
But that's far not enough, if you want to sell it as SERVICED then also you will need to list what exactly has been done..
And if the Amp then breaks on a Part which you have stated as serviced then you will be the one who pays the Repair.
At least in Europe it's like that. It's called after Sales Warranty. IF you do not set a time for that Warranty then, the max of time will be used.. *Switzerland* Can't tell you about America or Europe, as Swiss is not a member of these confederation.
Don't be picky with Bucks, he just meant it good. the same counts for me, because getting upset to respect someone else mind or thoughts, will not bring you further to become a good tech.
Sell that amp as it is..
Open Cover, clean out dust.
Hook it up to input source, and Loudspeakers, no need to test with a scope,
Listen to it, if you like the sound ,then close the cover and sell it.
Clean the Front Panel clean underneath, Open the bottom cover and CLEAN IT ALSO, clean the back, clean inside of the cover; and then sell the AMP AS IT IS AND AS IT HAS BEEN SEEN and Listened too. To do this proper you will spend more than TWO HOURS..
No one *Buyer* will use a scope at home to proof what the specs should be!
He will go home, to listen to the sound instead..
If you see dark resistors, or Caps which loosing dielectrics then exchange these with the same kind. if not then that's it.. if you see caps with Tops almost blown off or already bend, exchange, if you spot bad cable connections then fix them. This is more than enough.!
No need to check VDC at the output, because if there is more than 1 VOLT APPEARING, speaker protection would kick in...
A good tech has an EYE for such things, a bad tech is going to imply his own ideas to a device which he did not build. we call this modify to worse.
LEAVE IT AS IT IS, the best for you and you will stay free of hassles in your time as ongoing TECHGUY..
My two cents.