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Posted to sci.electronics.basics,rec.audio.tech
royalmp2001
 
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Default How to test how good my PSU electrolytics are in an old amp?

Hi,
I got hold of a 20yr old dead Adcom hifi power amp recently together
with the service manual.
The fuses on each +ve line have blown, the fuses on the -ve lines were
ok.

I have not tried the amp out - I have taken out these supply line fuses
(between the psu and the main electronics), therefore isolating the
power supply to the two amp boards, and checked the psu with a
voltmeter and oscilloscope.

The psu seems fine.... + and - 49.4V out, and all lines giving out
about 10mV ripple. One rail had 20mV ripple.

As this is quite an old amp, and I understand the electrolytics go
first (after pots, swtiches and connectors), what tests can I do on the
caps to see what kind of life I can get out of them. I can replace
them, but if it is not necessary I don't want to do it.
Thanks, guys.

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Posted to sci.electronics.basics,rec.audio.tech
mc
 
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Default How to test how good my PSU electrolytics are in an old amp?

Measure effective serial resistance with an ESR meter. But if they are
giving no symptoms, there's probably no reason to test them. You could try
paralleling another good capacitor with each of them temporarily to see if
there's a real drop in the ripple.

Age alone is not what kills them; it's a kind of electrolytic deterioration
that can be prevented by using them regularly. In fact, they may be better
after a month of use than they are now.

"royalmp2001" wrote in message
ups.com...
Hi,
I got hold of a 20yr old dead Adcom hifi power amp recently together
with the service manual.
The fuses on each +ve line have blown, the fuses on the -ve lines were
ok.

I have not tried the amp out - I have taken out these supply line fuses
(between the psu and the main electronics), therefore isolating the
power supply to the two amp boards, and checked the psu with a
voltmeter and oscilloscope.

The psu seems fine.... + and - 49.4V out, and all lines giving out
about 10mV ripple. One rail had 20mV ripple.

As this is quite an old amp, and I understand the electrolytics go
first (after pots, swtiches and connectors), what tests can I do on the
caps to see what kind of life I can get out of them. I can replace
them, but if it is not necessary I don't want to do it.
Thanks, guys.



  #3   Report Post  
Posted to sci.electronics.basics,rec.audio.tech
Roy L. Fuchs
 
Posts: n/a
Default How to test how good my PSU electrolytics are in an old amp?

On Sat, 6 May 2006 17:51:04 -0400, "mc"
Gave us:

Age alone is not what kills them; it's a kind of electrolytic deterioration
that can be prevented by using them regularly. In fact, they may be better
after a month of use than they are now.


They "dry out". Age is the main factor for determining how close to
"dried out" they might be.

The only exceptions are hermetically sealed military versions of EL
caps.

Most all commercial EL caps are vented and eventually will "dry out".

The easiest way to tell is to compare it with another of the same
form factor, by weight. No instruments required other than an
accurate scale.
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Chris
 
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Default How to test how good my PSU electrolytics are in an old amp?


royalmp2001 wrote:
Hi,
I got hold of a 20yr old dead Adcom hifi power amp recently together
with the service manual.
The fuses on each +ve line have blown, the fuses on the -ve lines were
ok.

I have not tried the amp out - I have taken out these supply line fuses
(between the psu and the main electronics), therefore isolating the
power supply to the two amp boards, and checked the psu with a
voltmeter and oscilloscope.

The psu seems fine.... + and - 49.4V out, and all lines giving out
about 10mV ripple. One rail had 20mV ripple.

As this is quite an old amp, and I understand the electrolytics go
first (after pots, swtiches and connectors), what tests can I do on the
caps to see what kind of life I can get out of them. I can replace
them, but if it is not necessary I don't want to do it.
Thanks, guys.


Hi, Royal. The first check should be in the power supply itself. You
seem to have already begun this. Try no load, then add some power
resistors to approximate the load of the amplifier. Look at the loaded
ripple, and see if it's anywhere near what it should be. For a 1 amp
load, you should get 1 volt of ripple for an 8300uF cap. If your cap
is 8300uF and you have 2 amps, you should have 2V of ripple. Double
the load, double the ripple. If your capacitance is doubled, your
ripple should halve. Work it out on paper beforehand, and compare your
calcs with your observations.

While you're at full load, run it for a few minutes, and feel the body
of the cap. It shouldn't be getting more than slightly warm. If any
of them are getting toasty, leakage current is probably well on its way
to cooking the cap.

One good way to check older large electrolytic power supply caps if
you've got time is to disconnect one end, and then charge them up to
rated voltage with a fairly good sized series resistor. For instance,
if your caps are rated for 63WV, try charging them up to 60V through a
10K 1/2W resistor. If the cap can't make it up to anywhere near
voltage, or the voltage starts ramping up, and then either stabilizes
or goes down on its own, you're SOL -- the cap is bad. Once the cap
voltage stabilizes at close to your applied voltage, read the voltage
across your 10K resistor with your DVM. If it reads more than a couple
of volts (a couple of tenths of mA of leakage), that indicates bad.

You're saying that the PSU seems to work fine when the amplifier is
disconnected, and you'd like some information on troubleshooting the
PSU caps. I'm not sure I follow you. You might want to apply yourself
to the amp itself, which seems to be where the problem is.

It sounds like the power supply lines on the amp board are shorted out
(likely a cap on the amp board?). Now you can get a start on tracking
down short circuits if you've got a floating 13.8V regulated bench
power supply, a DVM with a 200mV range, and some power resistors in
your junkbox. If you set up something like this (view in fixed font or
M$ Notepad):

|
| 100 ohm 3 watt
| + ___
| o-----|___|-o---------------o
| | ,
| | ,´
| | ´
| .-. |
| 13.8V 1 ohm | | /+\
| | | (DVM)
| '-' \-/
| | |
| | \.
| - | \
| o-----------o---------------o
(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

you'll be able to make up a crude short finder, which shouldn't damage
even semiconductors in the circuit. Use jumpers to clip the 2 lines
parallel to the 1 ohm resistor on the probes of your DMM, as close as
you can get to the tips of the probes. Now place your probes across
the shorted power lines on the amp board, and move them around to try
to get the minimum mV reading on the DMM. This will work down to a
couple of milliohms, which might get you pretty close to where the
short is, or at least narrow it down by quite a bit. The response is
obviously non-linear, but it becomes most sensitive (as well as useful)
at less than ten milliohms. (Note: you're not reading milliohms here,
you're reading the voltage across the probes in parallel with a 1 ohm
resistor. However, a reading of 0.1mV greater than the minimum reading
you can get with the probes touching each other will correspond to
roughly 1.4 milliohms extra resistance.)

If you've got a more expensive DMM with a 20mV range, you can click
down to less than two tenths of a milliohm per count at the low end,
which should walk you right to the short.

The advantage of this little trick is that you'll never apply more than
140mV or so to the board, so there's essentially no chance of smoking
anything, even a JFET input.

If you narrow it down to one area of the board, and you think a cap
might be suspect, use a solder sucker to open one leg of the cap, and
see if the short is still there.

There are easier ways to do this, and some rather expensive instruments
exist which can do some cap testing in-circuit (Sencore), but for
hobbyist repair work this should be sufficient.

Happy hunting
Chris

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royalmp2001
 
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Default How to test how good my PSU electrolytics are in an old amp?

Thanks Chris for such an excellent detailed and knowledgeable reply.
You sound as if you are in the repair/service business....



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Roy L. Fuchs
 
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Default How to test how good my PSU electrolytics are in an old amp?

On 6 May 2006 15:41:21 -0700, "royalmp2001"
Gave us:

Thanks Chris for such an excellent detailed and knowledgeable reply.
You sound as if you are in the repair/service business....


The best thing to do if you want another twenty plus years out of
the equipment is to gather all the values up, and purchase all new 105
C units, and then replace them all. At that point, you have restored
the circuit to original specs, and have increased the life of the unit
ten fold over what it could be expected to last in its current state.

I have a 25 year old laser disc player that most assuredly has EL cap
problems. Other than that, it will be fine when I finally get around
to servicing it.

I have two ten dollar (thrift store) LD players, and one $30 Plays
both side, does it all model (also thrift store), so I am not in a
rush.
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Bob Urz
 
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Default How to test how good my PSU electrolytics are in an old amp?



royalmp2001 wrote:
Hi,
I got hold of a 20yr old dead Adcom hifi power amp recently together
with the service manual.
The fuses on each +ve line have blown, the fuses on the -ve lines were
ok.

I have not tried the amp out - I have taken out these supply line fuses
(between the psu and the main electronics), therefore isolating the
power supply to the two amp boards, and checked the psu with a
voltmeter and oscilloscope.

The psu seems fine.... + and - 49.4V out, and all lines giving out
about 10mV ripple. One rail had 20mV ripple.

As this is quite an old amp, and I understand the electrolytics go
first (after pots, swtiches and connectors), what tests can I do on the
caps to see what kind of life I can get out of them. I can replace
them, but if it is not necessary I don't want to do it.
Thanks, guys.


Why are you worried about the caps, when the first thing you should have
been worried about if the fuses were blown are the rectifier diodes and
amplifier output devices?

I would check ALL the output devices for shorts with a DVM. If they all
check out ok, then power the amp up. If you had a variac, you could
slowly spool it up. Check for output offset and any abnormal heating
of any device.

In my opinion, fix what is wrong and get the amp at least working before
you stray off into any modifications.


Bob


Bob

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jakdedert
 
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Default How to test how good my PSU electrolytics are in an old amp?

royalmp2001 wrote:
Hi,
I got hold of a 20yr old dead Adcom hifi power amp recently together
with the service manual.
The fuses on each +ve line have blown, the fuses on the -ve lines were
ok.

I have not tried the amp out - I have taken out these supply line fuses
(between the psu and the main electronics), therefore isolating the
power supply to the two amp boards, and checked the psu with a
voltmeter and oscilloscope.

The psu seems fine.... + and - 49.4V out, and all lines giving out
about 10mV ripple. One rail had 20mV ripple.

As this is quite an old amp, and I understand the electrolytics go
first (after pots, swtiches and connectors), what tests can I do on the
caps to see what kind of life I can get out of them. I can replace
them, but if it is not necessary I don't want to do it.
Thanks, guys.


Electrolytics are probably not your issue, although they do age. It
sounds like you have some sort of short on the B+ line. With the fuses
out of circuit, compare the resistance to ground of load side of the +/-
rails.

My guess is that you're gonna find a shorted output device....

jak

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jakdedert
 
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Default How to test how good my PSU electrolytics are in an old amp?

Bob Urz wrote:


royalmp2001 wrote:
Hi,
I got hold of a 20yr old dead Adcom hifi power amp recently together
with the service manual.
The fuses on each +ve line have blown, the fuses on the -ve lines were
ok.

I have not tried the amp out - I have taken out these supply line fuses
(between the psu and the main electronics), therefore isolating the
power supply to the two amp boards, and checked the psu with a
voltmeter and oscilloscope.

The psu seems fine.... + and - 49.4V out, and all lines giving out
about 10mV ripple. One rail had 20mV ripple.

As this is quite an old amp, and I understand the electrolytics go
first (after pots, swtiches and connectors), what tests can I do on the
caps to see what kind of life I can get out of them. I can replace
them, but if it is not necessary I don't want to do it.
Thanks, guys.


Why are you worried about the caps, when the first thing you should have
been worried about if the fuses were blown are the rectifier diodes and
amplifier output devices?

I would check ALL the output devices for shorts with a DVM. If they all
check out ok, then power the amp up. If you had a variac, you could
slowly spool it up. Check for output offset and any abnormal heating
of any device.

In my opinion, fix what is wrong and get the amp at least working before
you stray off into any modifications.


Bob

Oops, Bob...I gave him (some of) much the same advice before I read this
post....

jak


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royalmp2001
 
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Default How to test how good my PSU electrolytics are in an old amp?

Thanks Jak,
This design is a dual monoblock config...separate psu transformers, etc
per channel.
Resistance of +ve line to ground was about 68K on either channel, and
was 600K from -ve to ground on one channel and virtually open circuit
on the other channel.
I think that seems normal. I'll checkout the output devices also.



  #11   Report Post  
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jakdedert
 
Posts: n/a
Default How to test how good my PSU electrolytics are in an old amp?

royalmp2001 wrote:
Thanks Jak,
This design is a dual monoblock config...separate psu transformers, etc
per channel.
Resistance of +ve line to ground was about 68K on either channel, and
was 600K from -ve to ground on one channel and virtually open circuit
on the other channel.
I think that seems normal. I'll checkout the output devices also.


Nope...you've got a problem. Assuming identical left and right
channels, one would have to assume identical readings from channel to
channel.

jak

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