What to know to begin fixing amplifiers [video] | Hacker News Reader

What to know to begin fixing amplifiers [video]

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62 pointsbrudgers34 comments

The volume pot went in my 1992 Kenwood amp (18th birthday present). It crackled quite horrendously as they tend to do as they age.

I took that amp back and forth to uni each semester carefully boxing it up each time, took it to Germany, back to the UK, and it's still with me in the US, albeit currently in its box.

Huge sentimental value.

A couple of years ago I found the (impressive) service manual for it that had the part number for the potentiometer. Unfortunately the part is no longer available, it has a special loudness tap on it. I replaced it with a pot without the tap, I never used loudness anyway. Works great. It's a brilliant amplifier.

The service manual explains how to re-calibrate the amplifier current biases. So I checked it and it was still in spec.

https://www.audioservicemanuals.com/k/kenwood/kenwood-ka/161...

I also have a same era CD player that still works, I replaced it's laser maybe 10 years ago. Both purchased from Sevenoaks Hifi in Kent, by sending off a check in the mail.

you have sentimental attachment to a...uh...amplifier?

I can see this happening. My lounge doesn't look the same since my favourite (big) speakers got evicted in favour of a minimalist / soundbar thing that I hate.

I just got very used to seeing (and hearing) those big black boxes and they have been in every house we lived in for over 30 years.

If you're used to seeing (and hearing) something then having affection for those objects is quite common I would have thought.

Music equipment is among the most sentimental of all human possessions. In fact virtually all the musicians I’ve ever met believe in “MOJO”.

Is MOJO an acronym in this case? I consider myself an amateur musician but I'm not familiar with this term.

sentimental value refers to the memories associated with an object, not the object itself.

I have had it for 30 years, I have listened to hundreds of CD with it (that I also still own). Thousands of hours. I know what it sounds like. It was £210 - I paid for a good chunk of it myself from my first job at Tesco in the smallest city (w/ cathedral) in England, pushing trollies. And! I fixed its crusty old volume pot for a couple of bucks instead of throwing it in the trash.

That's how sentimentality works... for me anyway.

I've been able to quiet noisy volume potentiometers--where the problem is dirt inside the control--by squirting contact cleaner into the pot and rotating the knob. I have the advantage of having a (now almost empty) aerosol can of no-longer-legal CFC contact cleaner (I bought it many years ago and save it for the good stuff), but I'm guessing what whatever the good stuff is these days for cleaning circuit boards is likely to work just as well.

As someone who lives way down south, and incidentally spent a brief but memorable bit of time helping monitor the ozone...

Maybe try some of the new stuff, and if it works OK, take the old stuff to haz waste next time there's a good opportunity? I understand that the can of CFCs is a "drop in the ocean", but the mindset change is more significant.

Fair enough. In my defense, I've had the same can for almost 40 years, use it judiciously, and guesstimate that it's kept at least a couple of thousand US$ worth of electronics in service that would otherwise have needed expensive repairs or, worse, been disposed of and replaced by new stuff, with all of the environmental consequences those entail. But I will explore alternatives, and I see that there's a new version of the product (Chemtronics Electro-Wash) that doesn't have CFCs. Thanks for the nudge!

If you find something new that works as well then please let us know!

All good! I think we can all appreciate the proverbial giants who've lifted us to where we are, and simultaneously acknowledge that they left some nasty shits here and there.

Good reminder to clean the pots in my amp too - I've got an old Leak Stereo 70 that occasionally makes some odd noises or drops one of the channels, usually it gets sorted with a firm smack.

Silicone contact cleaner works also very well for old pots.

that could certainly have helped though it was a bit beyond crackling, a channel would drop out completely.

Maybe if I had tried that several years earlier the lubrication may well have extended the pots life.

I have an old NAD integrated amp from the same era - it keeps trucking along as it ever has. It's been rather sad to see their reputation for reasonably-priced, reliable kit take a nosedive in the last decade or so.

I was quite jealous of NAD equipment around that time. Never owned any though.

There's some great 90's and early 2000's era stereo gear that's basically free or very cheap at goodwill and thrift stores these days. I feel like right now is the time to snag good CD playback gear before it goes into the upswing in price as collectors items. No one is going to be building or selling 200 disc changers or high quality portable CD players any time soon for example, but CD collectors will love to have those things in decades to come. CDs in general I think are prime for a comeback like vinyl and tape have seen. Right now you can find them and the gear to play them for dirt cheap--it won't stay that way forever.

Lots of great equipment at Goodwill and garage sales.

One challenge though with the older CD players is that the laser focusing mechanisms tend to go at some point. Happened to me on a few players. Some of these can be replaced but I imagine it's harder and harder to find the parts now.

I thought for sure it was going to be a link to Mr Carlsons lab: https://www.youtube.com/c/MrCarlsonsLab

I haven't fixed an amp (yet) but I have been able to fix a few electronic doodads and boards using EEVBlog.com forums. Great resource if you are into electronic repair or troubleshooting.

If you have a Marantz receiver/amplifier, this is also a good place to start:

https://irebuildmarantz.com/classic/html/restoration.html

As an ignorant person who occasionally repairs things, something I have often wanted is a handwavy guide to failure modes of components expressed in terms of their audible effect.

What I mean is something that would help me to take a problem like "the bass keeps popping out and back in again in the left channel" and make a logical decision about whether to look first at the output transistors, amplifier bias current, some capacitors somewhere, the volume pot, quality of contacts for the input or output connection, or something else entirely.

I'm not able to watch a video right now - does the "What Goes Wrong?" part of this go there? Is there a transcript or a source article?

I've done a lot of repair in my day. I don't have what you need, but at the same time, my experience has taught me that 90% of "electronic" problems are mechanical in nature, such as switches, connectors, potentiometers, and so forth. Once you get into the guts of the amplifier circuit, it gets hard because that circuit is a feedback loop, so the signal can't be easily traced from one end to the other.

In the evolution of my home audio equipment, the amplifiers are all Class-D IC's that are easier to replace at a board level.

Prophylactically replacing things that might be broken can lead to worse problems, especially since a lot of older consumer electronics had relatively delicate single sided circuit boards that were easily damaged by desoldering.

came to post this, been my experience to a t. always start in places where things move, get plugged in, switches, look for exposed/funky looking traces/boards/connections. almost always an easy win once you crack the thing out (which can be a struggle)

sometimes watching youtubers like bigclivedotcom cover failures of cheap electronics can show spectacular exceptions though. for some more realistic failures of common consumer hardware like laptops and cellphones (and general rework wizardry) Electronicsrepairschool is pretty hard to beat

https://www.youtube.com/c/Bigclive

https://www.youtube.com/c/Electronicsrepairschool

Bigclive is the best. Sometimes my mind uses his voice when I’m checking out a circuit board. “What to we have here? A silver metal tube, probably the clock crystal for this kitchen timer. Ah ha, it says 32.768 on the board, so it certainly is.”

At least 90% is mechanical. The list mentioned above plus corroded traces on circuit boards and broken external connectors (RCA jacks, etc.).

For older components, beyond switches/pots the only other things I've ever had to replace are electrolytic capacitors.

Well, if it's the bass alone and in the left channel, that strongly suggests a problem in the wiring of the left speaker. A continuity tester is your friend.

If you get a crackling noise that changes when you change the volume, the volume potentiometer either needs to be cleaned or replaced. Similarly if the volume knob has ranges where the sound drops out entirely.

If one channel doesn't work at all, you trace the signal path starting at the source.

If nothing works, look for a fuse (and look for something that might have caused a short).

> A continuity test is your friend.

Also a spray can of freeze (e.g. CRC Freeze Spray) is useful, and a jeweller's loupe (x20 magnification or so). If your phone takes good macro video, that can be really useful to look at several times (in the absence of a stereoscopic microscope).

Contacts are the first thing to suspect with any fault. Check inside the plugs on the cables.

For intermitttently disappearing and reappearing bass, though: probably the contacts are OK. Inspect (and if possible, re-make) solder joints in this part of the circuit - you may have a hidden dry joint somewhere. Then old electrolytic capacitors, especially tantalum ones, are sources of trouble.

Of course if the amp has been in a house that's been hit by lightning, or where an arc welder has been used regularly, then the possibilities widen.

One time I had an amplifier fail. I took it to a local repair shop that I found that had good reviews who charged me $150 to "look" at it. A month later, I called them to ask what the status was (because I hadn't heard anything from them) and they told me it couldn't be repaired. No other answer than that.

That was a great $150 lesson...

I had a similar experience once with a quadraphonic receiver from the 70s. When I went to pick it up the shop owner gave me the schematics and said he’d never seen anything that complex before. It was probably a dozen or more pages long. Even the AM/FM receiver dial pulley diagram was complex. I wanted to use it as a pet project but ultimately gave it away due to other priorities and lack of space.

It wasn’t completely broken though! Various features just didn’t work. The whole system was insane. Each speaker cabinet had (iirc) 15” subs in it. Nowadays we have Atmos height speakers that are marketed as something completely new but these speakers had vertically oriented treble horns even back then. Modern speakers certainly have frequency response, efficiency, and compactness going for them, but to this day I’ve never heard any consumer system (I used to sell them in college) that loud. I rarely turned the volume past three (out of ten). I was only brave enough to briefly turn it up to seven once. When I did lights I’d never seen work illuminated. I felt Led Zeppelin in my chest.

When I was doing repair, the shops referred to the charge for lifting your set onto their workbench as the "bench press." It was usually given as a minimum labor time charge, such as 1 hour. You were doing well if you could get most repairs done in less than an hour. This depended on having both excellent diagnostic skill, and the experience to know what component was most likely to fail.

I ended up in an Apple repair shop within a government agency that served school districts. Since we didn't need to make a profit, we skipped the bench press, and if your Apple needed 15 minutes of work (typical), we charged for 15 minutes, while also letting you watch.

The pinnacle was TV repair because TVs were expensive but cheaply made and relatively simplistic. There were not many different ways that they worked due to the evolution of cost cutting measures. There were also good published repair guides -- the Howard Sams "photofact" series.

Stereos were not all that much more complicated, but a contemporary repair shop would suffer from lack of sufficient business to sustain a working knowledge of failures and repairs, and often obsolete or irreplaceable components such as output transistors. Paying $150 to sustain the shop, and learn that your set is unrepairable, are not far-fetched outcomes.

Also (in the amplified musical instrument world that I now inhabit), there's a declining number of good techs. For one thing, those guys can make a lot more money servicing million dollar industrial or medical equipment.

I was more than fine with the $150... which is why I paid it. I've had motorcycle shops do similar things and that's fine too.

I think I was too subtle in my response though. The fact that they didn't call me after a month to tell me that it was broken beyond repair, feels kind of like they were never going to fix it.

I charge 300 quid to even look at a piece of equipment that's been "re-capped". Some people pay that, presumably because they realise that's the price of having the damage inflicted by the poke-and-hope brigade repaired.

My Quad 22 died in a flood. I mourn it, but in truth the wax potting had destroyed the wiring years ago. Rebuilds are "my grandfathers axe" and replace everything except your rusty Quad nameplate and the chassis. Re-making transformers is labour intense and KT-66 are very expensive.

I have a Sony discrete component bookshelf amp with slider volumes for L and R which I love, but it has never quite been the same since a cat we were house-sitting pee'd on it in the 80s. Some things don't mix with electricity. I had one power transistor replaced after that, but the volume L/R got out of whack. Now, it blows the safety fuse when I power it on, and I suspect it needs work, which may well head to "my grandfathers axe" in discrete componentry again.

I see this in my future.

InLaws have hoarded multiple supposed "audiophile" stereo systems from richer friends over the years.

I have zero knowledge or interest in the "audiophileness" of it.

But it'll be a fun project one day to see whether they work, fix if required and sell/give/dump/pull-apart-for-magnets/use-for-non-Newtonian-fluid-demos etc as required.

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