Audio amplifier repair - bad capacitors

Hello,

When repairing audio amplifiers, would an ESR meter or a capacitance meter be more useful in identifying bad capacitors in circuit?

Would either of these be any good:

'High Precision L/C Inductance Inductor Capacitance Multimeter Meter LC200A Tool | eBay'

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'Capacitor/Capacitance ESR IN-Circuit Inductance Resistance Meter LC Meter dc 9v | eBay' (Error)

Thanks.

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M.Joshi
Reply to
M.Joshi
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I've been selling the Bob Parker ESR meters (now BLUE) kit since 1999 and they has been a great boon to technicians for fixing monitors, switching power supplies, and other devices with electrolytic capacitors. Great for caps larger than around 2ufd.

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Unfortunately smaller value caps (

Reply to
John Robertson

I dont think esr can prevent audio amplifiers from working it prevents power supplies (especially switching) yes there are lot of switching power supplies that may fail due to bad esr capacitor but in an audio amplifier there is quite never a switching power supply

but an esr meter is very useful to the technician but if you do only audio repair, you won't use it very often.

see below my web site for free esr schematics

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Kripton 
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Reply to
Kripton

+1.

The Blue ESR meter saves a lot of time. Often the biggest delay is waiting on the spare parts.

tm

Reply to
tm

Don't work on many amps, do you?

Class D are all switching units. The power supply as well as the amp.

Reply to
tm

I think the point is that generally audio amplifiers these days don't have that many electrolytic capacitors. The circuitry is generally direct coupled, even at the output. That's a big difference from early transistor amplifiers that suddenly used a slew of electrolytics.

And most amplifiers still use linear power supplies, so not only are the filter capacitors operating at a relatively low frequency, but they are few in number. If you've got hum from the amplifier, chances are the filter capacitor is bad. IN a switching supply, the supply may never start working if an electrolytic is bad.

Like I said earlier, one reason for ESR meters is the rise in the number of electrolytics. If you can't easily tell which is causing the problem, it makes sense to have a meter that will check the electrolytics in circuit. Especially if the circuit is still relatively "foreign" like a switching supply or other digital circuitry. An audio amplifier is generally still a familiar thing, check where the audio disappears, check for voltages, etc. There are a lot ore telltales than in a switching supply where mutliple things interact and still many people haven't had much experience with them.

In antique radio circles, it's all so much easier. 2 or 3 electrolytics, might as well just replace them, if they aren't bad now, they will go bad. ANd yes, if it's old enough, just replace the other capacitors, since old enough means odd types of capacitors that can go bad. The electrolytics are easy, not many of them. The others people know from experience, so if you've got the radio open, might as well do them all.

Even odler transistor radios tend to get th shotgun treatment. There are more electrolytics than in tubes, but still not that many and likely froma period when they would be failing by now. But again since the number are few, it;'s just as easy to replace them as to spend time trying to figure out what's wrong.

In modern equipment, not generally audio amplifiers, the circuit isn't obvious (and often no schematic), and so many electrolytics that it starts being cheaper to find the bad ones than just shotgun them all.

It really depends. For someone just doing amplifiers, an ESR meter might not be that useful, they have other means of evaluating things. If they did other things, then an ESR meter becomes valuable, and then they can use it on amplifiers, too.

Michael

Reply to
Michael Black

I would definitely want one with ESR-measuring capabilities. From what I've read (and in my own experience), electrolytic caps which start to fail will usually show a significant increase in ESR well before their capacitance changes significantly.

That one looks similar to the AADE meter

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and might be one of the "knock-offs" that this page refers to.

I do like the AADE meter - it seems to work very well for what it's intended to do. It does capacitance measurements over a wide range (pF up to 1.5 uF).

However, it is not intended or advertised as being useful for in-circuit measurements, it doesn't have ESR-measuring capability, and it isn't designed to measure the capacitance of all but the smallest electrolytic caps.

The one I see with this description looks more like what you'd want... its capacitance measurement range covers typical electrolytics (but not smaller film/ceramic caps), and it does multi-frequency measurements which will give you both the capacitance and the ESR.

The two types of meters you've described here are really complementary sorts of instruments... they both measure capacitors, but their ranges of measurements barely overlap. They're both useful but do different sorts of things, and you wouldn'd do wrong by having one of each type.

The third type of measurement you might want to do for looking at failed caps in an audio amp, would be a leakage test. Inter-stage coupling caps can become leaky, especially if there's no bias voltage across them most of the time (or if they're reverse biased) and this isn't something which will show up in a capacitance or ESR test. Testing for leaky 'lytics is probably best done out-of-circuit, though.

I built my own ESR meter a few years ago, based on the Ludens design

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and it's been quite useful. Easily made using "junk box" parts, capable of doing in-circuit ESR measurements safely, and un-critical in its construction, it's a great weekend project.

Reply to
David Platt

There are still a significant number of older audio amplifiers that use ele ctrolytics in the coupling mode, so I guess an ESR meter is handy.

I have a Parker ESR meter and an older Sencore Z meter. Both have on occas ion reported normal ESR in an in circuit test that was false ie: removed ca p showed bad ESR out of circuit, and the circuit-sans cap, still showed goo d ESR with no cap installed). BTW, beware the resident troll that will pop up shortly and spew his vile garbage. Regulars are already ducking...

What I do now is mostly use my scope with the equipment up and running for both bypass and coupling caps. Anything that doesn't look right gets pulle d and checked on the Sencore for ESR, full voltage leakage, then recheck fo r ESR.

Reply to
John-Del

An ESR meter is a VERY useful tool for finding bad caps.

Most of these were found with an ESR meter over about a 3 month period: While some of these caps show obvious bulges, many of them have no visible signs of failure.

No. That's an L/C meter, not an ESR meter. You can have a bad capacitor, with a very high ESR and still show normal capacitance.

No experience with that one. I'm suspicious because it doesn't say anything about zeroing the probe cable resistance, which is necessary for in-circuit measurements.

Search for one of the Bob Parker ESR meter designs. I have one of the original Dick Smith variety. Works nicely. I suggest you read a little about how an ESR meter is used before buying.

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Jeff Liebermann     jeffl@cruzio.com 
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Reply to
Jeff Liebermann

I beg to differ slightly. Here's a lousy photo of the power amp and power supply section from a Behringer PMH518M mono amplifier that had a problem with bad capacitors: The two big caps (C6, C7), that are missing in the photo, were the primary culprits[1]. Their high ESR caused major voltage excursions to the regulator transistors, sufficient to blow up the devices and fry the traces. Other than some corrosive goo leaking from the rubber seal on the bottom of the caps, there was no visible evidence that the caps were not doing their job. I didn't need an ESR meter to fix such an obvious problem, but with less damage, it might have been helpful to verify that the caps were still usable.

I don't do much audio or video repair, but I am beginning to see Class E switched amplifiers, mostly in computah speakers, which more closesly resemble a switching power supply than a traditional linear power amplifier. I would expect capacitor ESR problems in those.

[1] Hint: The caps are 4700uf 50VDC running on a 40 VDC input bus to the regulators. Running caps at 80% of the maximum voltage, next to a heat sink full of very warm devices, is not a great design, even at 120Hz. I replaced them with 3300uf 63VDC caps.
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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Jeff Liebermann

"M.Joshi"

** An ESR meter is the only useful type.

The best one for all servicing work is the "Bob Parker" ESR and Low Ohms meter:

Just Google that name for heaps of hits.

... Phil

Reply to
Phil Allison

"M.Joshi"

** An ESR meter is the only useful type.

The best one for all servicing work is the "Bob Parker" ESR and Low Ohms meter:

Just Google that name for heaps of hits.

... Phil

Reply to
Phil Allison

"M.Joshi"

** An ESR meter is the only useful type.

The best one for all servicing work is the "Bob Parker" ESR and Low Ohms meter:

Just Google that name for heaps of hits.

... Phil

Reply to
Phil Allison

** Nonsense.

Electro caps get old and wear out no matter what gear they are in - heat is the main cause.

Audio amps get hot, tube ones very much so.

High ESR caps in an amplifier will cause hum, high and low frequency instability, loss of gain and even loss of signal entirely.

A good ESR meter allows you to find then before symptoms appear too.

... Phil

Reply to
Phil Allison

"Arfa Daily"

** A recent example of using my Bob Parker ESR meter was with a nicely made clone of a Fender "Princeton Reverb" combo amp. The amp was just a few years old and had all new components in it - PLUS a chassis made from Stainless Steel !!

However, it was riddled with faults like low frequency oscillation ( motor-boating), loud hum and high frequency instability when the reverb was turned up.

The ESR meter read off scale ( ie over 100ohms) on three sections of the main filter electro and 20 ohms on the fourth - it was one of these:

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New electros fixed all the faults.

When I opened the can of the old one, the aluminium strips linking the terminals to the insides were mostly eaten away with horrible corrosion.

My conclusion is that the electro was far from new - more likely 50 year old stock that someone has found and re-labelled.

BTW:

Old electros can often still work fine, but not if left in storage for periods like 50 years.

.... Phil

Reply to
Phil Allison

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