billm256

Hello,

?

Please pardon me for submitting this non-Tektronix question. However, I know there's a lot of general electronic expertise in this group, and I really need help (and I am using my Tek 2245 to troubleshoot this problem).

?

Issue is with my Hafler DH-500 stereo power amp, which has picked up a raspy hum on the output. I'm hoping someone with more power supply experience than I have will be able to help.

?

General description of the power supply circuit.

1.?????? Classic linear power supply.

2.?????? Step-up xformer.

3.?????? Bridge rectifier.

4.?????? On the output of the bridge, 2, 20,000 mfd, 100 V electrolytic filter caps between ground and each?of the +/- 90 VDC rails.

5.?????? There's a .01 mfd, 1000 V disc cap across the xformer connections to the bridge. I mention this because it's one of very few things the rails share.

?

General symptoms:

1.?????? There is a ~.35 V peak to peak, 120 Hz distorted saw-tooth riding the plus and minus 90 VDC rails. That its frequency is 120 Hz suggests to me it originates after rectification, i.e., the rectifier's output is 120 Hz.

2.?????? The saw-tooth is essentially identical on each rail except they are 180 degrees out of phase.

3.?????? The saw-tooth varies ~.05 V in amplitude yet triggers the scope reliably and presents the same sweep repeatedly. I.e., displaying 6 cycles of saw-tooth on the scope, amplitude variations repeat on the same cycles sweep after sweep. (Seems very strange to me.)

4.?????? Amplitude of the output hum does not vary with the input signal.

5.?????? Amplifier continues to function other than the hum.

?

A couple of symptoms lead me to suspect the power supply (as opposed to a downstream component putting noise on a rail).

1.?????? Reversing the power plug changes the amplitude of the output?hum.

2.?????? Changing the setting of an SCR light dimmer on the same house circuit changes harmonics on the hum. I.e., AC distortion from the dimmer is getting thru the power supply.

3.?????? Both rails equally affected.

?

One final observation that seems odd to me: When I hooked ground on the scope to circuit ground on the amp, the chassis of the amp became very sensitive to being touched by me in that touching it produced significant 60 Hz sine wave hum on the output. The scope is isolated from ground at the outlet as is the amplifier.

?

In conclusion, I expected to find one of the electrolytics deteriorated, and to see AC riding one rail only; that it's showing up equally on both rails is mystifying to me. That it's a saw-tooth vs. a sine wave is also indicative of something. That it's 180 degrees out of phase and 120 Hz seem to pin its origin to the rectification process. Again, the small disc cap across the bridge's input may be of interest, but its function is not apparent to me. (I'm putting a replacement cap on order and should have it installed before end of the week.)

?

Thank you for wading thru this lengthy description. Any help will be greatly appreciated!

?

Best regards,

?

Bill McDonald

?

#78474

开云体育

Suggest you either test or replace both 20K ufd 100 VDC electrolytics.

?

Hope this helps.

?

Joe

?

Show quoted text

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On Behalf Of billm256
Sent: Sunday, July 22, 2012 6:56 PM
To: TekScopes@...
Subject: [TekScopes] Stereo Power Amp Problem

?

Hello,

?

Please pardon me for submitting this non-Tektronix question. However, I know there's a lot of general electronic expertise in this group, and I really need help (and I am using my Tek 2245 to troubleshoot this problem).

?

Issue is with my Hafler DH-500 stereo power amp, which has picked up a raspy hum on the output. I'm hoping someone with more power supply experience than I have will be able to help.

?

General description of the power supply circuit.

1.?????? Classic linear power supply.

2.?????? Step-up xformer.

3.?????? Bridge rectifier.

4.?????? On the output of the bridge, 2, 20,000 mfd, 100 V electrolytic filter caps between ground and each?of the +/- 90 VDC rails.

5.?????? There's a .01 mfd, 1000 V disc cap across the xformer connections to the bridge. I mention this because it's one of very few things the rails share.

?

General symptoms:

1.?????? There is a ~.35 V peak to peak, 120 Hz distorted saw-tooth riding the plus and minus 90 VDC rails. That its frequency is 120 Hz suggests to me it originates after rectification, i.e., the rectifier's output is 120 Hz.

2.?????? The saw-tooth is essentially identical on each rail except they are 180 degrees out of phase.

3.?????? The saw-tooth varies ~.05 V in amplitude yet triggers the scope reliably and presents the same sweep repeatedly. I.e., displaying 6 cycles of saw-tooth on the scope, amplitude variations repeat on the same cycles sweep after sweep. (Seems very strange to me.)

4.?????? Amplitude of the output hum does not vary with the input signal.

5.?????? Amplifier continues to function other than the hum.

?

A couple of symptoms lead me to suspect the power supply (as opposed to a downstream component putting noise on a rail).

1.?????? Reversing the power plug changes the amplitude of the output?hum.

2.?????? Changing the setting of an SCR light dimmer on the same house circuit changes harmonics on the hum. I.e., AC distortion from the dimmer is getting thru the power supply.

3.?????? Both rails equally affected.

?

One final observation that seems odd to me: When I hooked ground on the scope to circuit ground on the amp, the chassis of the amp became very sensitive to being touched by me in that touching it produced significant 60 Hz sine wave hum on the output. The scope is isolated from ground at the outlet as is the amplifier.

?

In conclusion, I expected to find one of the electrolytics deteriorated, and to see AC riding one rail only; that it's showing up equally on both rails is mystifying to me. That it's a saw-tooth vs. a sine wave is also indicative of something. That it's 180 degrees out of phase and 120 Hz seem to pin its origin to the rectification process. Again, the small disc cap across the bridge's input may be of interest, but its function is not apparent to me. (I'm putting a replacement cap on order and should have it installed before end of the week.)

?

Thank you for wading thru this lengthy description. Any help will be greatly appreciated!

?

Best regards,

?

Bill McDonald

?

Brad Thompson

#78477

On 7/22/2012 7:55 PM, billm256 wrote:
Hello,

Please pardon me for submitting this non-Tektronix question. However, I
know there's a lot of general electronic expertise in this group, and I
really need help (and I am using my Tek 2245 to troubleshoot this problem).

Issue is with my Hafler DH-500 stereo power amp, which has picked up a
raspy hum on the output. I'm hoping someone with more power supply
experience than I have will be able to help.<snip>

Hello--

You might try adding a small disk-ceramic capacitor across each
of the bridge rectifier's diodes, keeping the capacitors' leads as
short as possible.

Capacitors in the range of 1000 pF to 0.01 uF may make a difference--
their voltage ratings should be two to three times the peak inverse
voltage developed across each diode (600 V to 1 KV will provide a
generous safety margin).

Garden-variety silicon diodes manifest what's called " reverse-recovery
time"...



...During which a diode continues conducting, even though the applied
voltage across the diode has reversed polarity. When stored charge within the diode has dissipated, the diode abruptly switches off,
creating a harmonic-rich edge waveform that can radiate via
power-supply wiring and get picked up by low-level signal wiring.

Diagnosis at a distance is always risky, so this approach may not
make any difference in resolving your problem.

73--

Brad  AA1IP

#78478

I doubt the problem is in the supply. More info would help:

1. Did the problem coincide with any equipment or hookup changes in the audio system?

2. Is the problem in both channels, or only one?

Without knowing more about the circuit, I'd suggest looking at the smaller electrolytic filter caps within the amplifier circuits - look for any that seem discolored, leaky, or puffy. Also, look for any cooked parts like resistors or board-mounted power transistors or regulators, and component or board discoloration - these are common in consumer A/V equipment. Finally, look at the internal wiring and connections for any breaks.

Ed

Show quoted text

--- In TekScopes@..., "billm256" <BillM256@...> wrote:

Hello,

Please pardon me for submitting this non-Tektronix question. However, I
know there's a lot of general electronic expertise in this group,
and I really need help (and I am using my Tek 2245 to troubleshoot this
problem).

Issue is with my Hafler DH-500 stereo power amp, which has picked up a
raspy hum on the output. I'm hoping someone with more power supply
experience than I have will be able to help.

General description of the power supply circuit.

1.       Classic linear power supply.

2.       Step-up xformer.

3.       Bridge rectifier.

4.       On the output of the bridge, 2, 20,000 mfd, 100 V electrolytic
filter caps between ground and each of the +/- 90 VDC rails.

5.       There's a .01 mfd, 1000 V disc cap across the xformer
connections to the bridge. I mention this because it's one of very
few things the rails share.

General symptoms:

1.       There is a ~.35 V peak to peak, 120 Hz distorted saw-tooth
riding the plus and minus 90 VDC rails. That its frequency is 120 Hz
suggests to me it originates after rectification, i.e., the
rectifier's output is 120 Hz.

2.       The saw-tooth is essentially identical on each rail except they
are 180 degrees out of phase.

3.       The saw-tooth varies ~.05 V in amplitude yet triggers the scope
reliably and presents the same sweep repeatedly. I.e., displaying 6
cycles of saw-tooth on the scope, amplitude variations repeat on the
same cycles sweep after sweep. (Seems very strange to me.)

4.       Amplitude of the output hum does not vary with the input
signal.

5.       Amplifier continues to function other than the hum.

A couple of symptoms lead me to suspect the power supply (as opposed to
a downstream component putting noise on a rail).

1.       Reversing the power plug changes the amplitude of the output
hum.

2.       Changing the setting of an SCR light dimmer on the same house
circuit changes harmonics on the hum. I.e., AC distortion from the
dimmer is getting thru the power supply.

3.       Both rails equally affected.

One final observation that seems odd to me: When I hooked ground on the
scope to circuit ground on the amp, the chassis of the amp became very
sensitive to being touched by me in that touching it produced
significant 60 Hz sine wave hum on the output. The scope is isolated
from ground at the outlet as is the amplifier.

In conclusion, I expected to find one of the electrolytics deteriorated,
and to see AC riding one rail only; that it's showing up equally on
both rails is mystifying to me. That it's a saw-tooth vs. a sine wave is
also indicative of something. That it's 180 degrees out of phase and
120 Hz seem to pin its origin to the rectification process. Again, the
small disc cap across the bridge's input may be of interest, but its
function is not apparent to me. (I'm putting a replacement cap on
order and should have it installed before end of the week.)

Thank you for wading thru this lengthy description. Any help will be
greatly appreciated!

Best regards,

Bill McDonald

#78479

Does the hum/noise go away when the inputs are grounded?
-ls-

#78483

I second that suggestion. On one of the amplifiers I deal with I routinely add 470nF X2 capacitors across the PTX secondaries, for the same reason. My tape-recording life was plagued by fridge motor startup noise before that..

Does it happen when the light on the dimmer is off?

EJP
Dada Eectronics Australia

Show quoted text

--- In TekScopes@..., Brad Thompson <brad.thompson@...> wrote:

On 7/22/2012 7:55 PM, billm256 wrote:
Hello,

Please pardon me for submitting this non-Tektronix question. However, I
know there's a lot of general electronic expertise in this group, and I
really need help (and I am using my Tek 2245 to troubleshoot this problem).

Issue is with my Hafler DH-500 stereo power amp, which has picked up a
raspy hum on the output. I'm hoping someone with more power supply
experience than I have will be able to help.<snip>

Hello--

You might try adding a small disk-ceramic capacitor across each
of the bridge rectifier's diodes, keeping the capacitors' leads as
short as possible.

Capacitors in the range of 1000 pF to 0.01 uF may make a difference--
their voltage ratings should be two to three times the peak inverse
voltage developed across each diode (600 V to 1 KV will provide a
generous safety margin).

Garden-variety silicon diodes manifest what's called " reverse-recovery
time"...



...During which a diode continues conducting, even though the applied
voltage across the diode has reversed polarity. When stored charge
within the diode has dissipated, the diode abruptly switches off,
creating a harmonic-rich edge waveform that can radiate via
power-supply wiring and get picked up by low-level signal wiring.

Diagnosis at a distance is always risky, so this approach may not
make any difference in resolving your problem.

73--

Brad  AA1IP

Rob

#78484

W/o a schematic or more input hard to say. Back in the day I did a lot of
audio work but that was 20 years ago or so. 

Anyway from memory that dated.... Some items to check verify.
1.) The power supply decoupling capacitors as already mentioned
2.) Verify no DC on the speaker lines. i.e. The amplifier is not offset.
	Without knowing the circuit hard to explain what is and isn't normal
for offset, Depends if the outputs drive a transformer. Coupled to the
speakers via Caps, etc. However a large proportion of audio amps are/were
"push/pull" and so the output is balanced and therefore DC needs to be 0... 
3.) Verify the grounds between amp and power supply is intact. 
4.) Preamp problems can cause humming in the amplifier so make sure the
preamp is not offset as well. Noisy transistors/IC's in this stage can cause
issues as well.  In addition if the feedback is broken the preamp can
oscillate and cause humming as well. Without proper feedback gain goes
towards infinity. In addition in a lot of cases the preamp is a single
package so it can cause issues in both channels. 

anyway, all of that a shot in the dark but hopefully helpful.
Rob

Show quoted text

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On Behalf
Of larrys@...
Sent: Sunday, July 22, 2012 8:18 PM
To: TekScopes@...
Subject: Re: [TekScopes] Stereo Power Amp Problem

Does the hum/noise go away when the inputs are grounded?
-ls-



------------------------------------

Yahoo! Groups Links

#78488

I'll tell you what to look for.? Coupling caps on the order of 0.1 to 10 uf electrolytics.? Especially look at the symmetry of the output waveform.

What does the hum at the speaker look like with no inputs plugged in?? Is there any DC at the speaker terminals?? The one or both channel question is a good one.

See if the hum is present with all of the audio inputs unplugged.

Show quoted text

--- On Sun, 7/22/12, Ed Breya wrote:

From: Ed Breya
Subject: [TekScopes] Re: Stereo Power Amp Problem
To: TekScopes@...
Date: Sunday, July 22, 2012, 9:15 PM

?

I doubt the problem is in the supply. More info would help:

1. Did the problem coincide with any equipment or hookup changes in the audio system?

2. Is the problem in both channels, or only one?

Without knowing more about the circuit, I'd suggest looking at the smaller electrolytic filter caps within the amplifier circuits - look for any that seem discolored, leaky, or puffy. Also, look for any cooked parts like resistors or board-mounted power transistors or regulators, and component or board discoloration - these are common in consumer A/V equipment. Finally, look at the internal wiring and connections for any breaks.

Ed

--- In TekScopes@..., "billm256" wrote:
>
>
> Hello,
>
>
>
> Please pardon me for submitting this non-Tektronix question. However, I
> know there's a lot of general electronic expertise in this group,
> and I really need help (and I am using my Tek 2245 to troubleshoot this
> problem).
>
>
>
> Issue is with my Hafler DH-500 stereo power amp, which has picked up a
> raspy hum on the output. I'm hoping someone with more power supply
> experience than I have will be able to help.
>
>
>
> General description of the power supply circuit.
>
> 1. Classic linear power supply.
>
> 2. Step-up xformer.
>
> 3. Bridge rectifier.
>
> 4. On the output of the bridge, 2, 20,000 mfd, 100 V electrolytic
> filter caps between ground and each of the +/- 90 VDC rails.
>
> 5. There's a .01 mfd, 1000 V disc cap across the xformer
> connections to the bridge. I mention this because it's one of very
> few things the rails share.
>
>
>
> General symptoms:
>
> 1. There is a ~.35 V peak to peak, 120 Hz distorted saw-tooth
> riding the plus and minus 90 VDC rails. That its frequency is 120 Hz
> suggests to me it originates after rectification, i.e., the
> rectifier's output is 120 Hz.
>
> 2. The saw-tooth is essentially identical on each rail except they
> are 180 degrees out of phase.
>
> 3. The saw-tooth varies ~.05 V in amplitude yet triggers the scope
> reliably and presents the same sweep repeatedly. I.e., displaying 6
> cycles of saw-tooth on the scope, amplitude variations repeat on the
> same cycles sweep after sweep. (Seems very strange to me.)
>
> 4. Amplitude of the output hum does not vary with the input
> signal.
>
> 5. Amplifier continues to function other than the hum.
>
>
>
> A couple of symptoms lead me to suspect the power supply (as opposed to
> a downstream component putting noise on a rail).
>
> 1. Reversing the power plug changes the amplitude of the output
> hum.
>
> 2. Changing the setting of an SCR light dimmer on the same house
> circuit changes harmonics on the hum. I.e., AC distortion from the
> dimmer is getting thru the power supply.
>
> 3. Both rails equally affected.
>
>
>
> One final observation that seems odd to me: When I hooked ground on the
> scope to circuit ground on the amp, the chassis of the amp became very
> sensitive to being touched by me in that touching it produced
> significant 60 Hz sine wave hum on the output. The scope is isolated
> from ground at the outlet as is the amplifier.
>
>
>
> In conclusion, I expected to find one of the electrolytics deteriorated,
> and to see AC riding one rail only; that it's showing up equally on
> both rails is mystifying to me. That it's a saw-tooth vs. a sine wave is
> also indicative of something. That it's 180 degrees out of phase and
> 120 Hz seem to pin its origin to the rectification process. Again, the
> small disc cap across the bridge's input may be of interest, but its
> function is not apparent to me. (I'm putting a replacement cap on
> order and should have it installed before end of the week.)
>
>
>
> Thank you for wading thru this lengthy description. Any help will be
> greatly appreciated!
>
>
>
> Best regards,
>
>
>
> Bill McDonald
>

Bill McDonald

#78508

开云体育

All,

?

Many thanks for your thoughtful comments, suggestions and questions; my responses, in no particular order, are below.

1.?????? Hum is equally on both channels.

2.?????? Happens when amp is connected to preamp or disconnected.

3.?????? Happens when light dimmer is off or amp is on a different circuit.

4.?????? Happened while installing new speakers. During the process there was an accidental power up that may have popped something.

5.?????? Speaker output: Approximate sine wave at 120 Hz, ~.1 v p-p. Very short duration spikes at each zero crossover, several volts in amplitude. One channel has a dc offset of ~ -.1 volt, the other channel is ~ -.05 volts.

6.?????? Grounds confirmed between power supply, amp boards and speaker outs.

7.?????? I see no physical indication of component problems on the amp boards or relay board.

8.?????? After reading your comments, and doing further research of my own, I start to suspect the saw-tooth riding the +/- 90 rails may be normal and the failure is downstream, maybe decoupling in the amp. However, this would require a common failure on both channels. But considering #4, that’s a possibility.

9.?????? I’ve attached the manual, with schematic on pg. 16. If attachment doesn’t work, the manual is available at .

10.?? Also worth mentioning, the amp has run perfectly for more than a decade, so it’s not a defect in design or a new part, e.g., Brad’s point about rectifier reverse recovery time.

?

Thanks again!

?

Bill

Bill McDonald

#78514

开云体育

A correction and an addition in red below.

?

Show quoted text

From: Bill McDonald [mailto:BillM256@...]
Sent: Monday, July 23, 2012 11:23 AM
To: 'TekScopes@...'
Subject: Re: Stereo Power Amp Problem

?

All,

?

Many thanks for your thoughtful comments, suggestions and questions; my responses, in no particular order, are below.

1.?????? Hum is equally on both channels.

2.?????? Happens when amp is connected to preamp or disconnected.

3.?????? Happens when light dimmer is off or amp is on a different circuit.

4.?????? Happened while installing new speakers. During the process there was an accidental power up that may have popped something. Installation also involved moving line-level inputs around, and this is where we may have caused the problem. I’m increasingly thinking input stages of each channel.

5.?????? Speaker output: Approximate sine wave at ~~120~~ 60 Hz, ~.1 v p-p. Very short duration spikes at each zero crossover, several volts in amplitude. One channel has a dc offset of ~ -.1 volt, the other channel is ~ -.05 volts.

6.?????? Grounds confirmed between power supply, amp boards and speaker outs.

7.?????? I see no physical indication of component problems on the amp boards or relay board.

8.?????? After reading your comments, and doing further research of my own, I start to suspect the saw-tooth riding the +/- 90 rails may be normal and the failure is downstream, maybe decoupling in the amp. However, this would require a common failure on both channels. But considering #4, that’s a possibility.

9.?????? I’ve attached the manual, with schematic on pg. 16. If attachment doesn’t work, the manual is available at .

10.?? Also worth mentioning, the amp has run perfectly for more than a decade, so it’s not a defect in design or a new part, e.g., Brad’s point about rectifier reverse recovery time.

?

Thanks again!

?

Bill

G?ran Krusell

#78517

Hi Bill,

Point #5 is interesting, "very short duration spikes". It shows, I believe, that the feedback is not working. No proper feedback and the amplifier channels can not reduce the 120 Hz hum to zero. I think you have broken transistors in the output stage for both channels. What you can do now is to short the inputs and then measure all dc levels
and also all base-emitter voltages. This might give you some information.

Carry on
G?ran

#78521

Bill,

? The information you provided about the amlifier power rails sounds normal.? I don't think you have an amplifier problem but a grounding issue.? Your comment about the chassis being 'touch sensitive' when the scope was connected to the amplifier circuit ground and the sensitivity to power plug polarity sounds like a floating circuit issue with the amplifier and scope chassis/cases acting as an antenna.?

?

? As I read the schematic the amplifier circuit is floating (not connected to an external ground).? Power line noises (SCR light dimmers produce high frequency transients locked into the AC line frequency.? High frequencies couple through stray capacitances of the circuit wiring and?power transformer in the?floating amplifier circuit.? Normally, the amplifier chassis is grounded to power?green wire ground (the 'U' shaped power plug pin)?but with unbalanced audio lines you don't want multiple ground connections (typically the lowest signal level point is grounded and everything else uses that ground)? so the preamp is normally the ground connection.

?

? You state that this has worked for a long time, SO something has changed, please provide more details:

?

* What have you done since the time it worked properly (you mention new speakers)

?

* Confirm that the scope is floating (the power plug ground pin is not connected to power green wire ground terminal)

?

* Is the chassis 'touch sensitive' without the scope connected

?

* As another commenter questioned, what happens if you short each input connector to ground (solder a jumper wire between the RCA plug tip and shell)

?

* Describe more details of the inadvertant power up of the amp, how long was it on, what was connected, any shorted leads

?

* Where does your audio system connect to ground (power line green wire ground)

?

? You mention moving the line input leads as well as trying new speakers which does raise input connection issues, some things to try:

* Create a shorting input plugs (RCA plug with jumpered input) for each input?- what is the hum output level

?

*?Measure the resistance from the RCA jack shell and the chassis while trying to move the jack?- should be zero ohms

?

* Install a temporary grounding path from chassis to?power 'U' pin with a 1000 ohm resistor (measure the AC voltage across the resistor).? Ideally there should not be any voltage present.? Report the voltage, hum output level, and if the shorting input plugs have any effect (also the effect of having the scope ground clip connected the amplifier)

?

Denis

?

Show quoted text

From: billm256
To: TekScopes@...
Sent: Sunday, July 22, 2012 4:55 PM
Subject: [TekScopes] Stereo Power Amp Problem

Hello,

?

Please pardon me for submitting this non-Tektronix question. However, I know there's a lot of general electronic expertise in this group, and I really need help (and I am using my Tek 2245 to troubleshoot this problem).

?

Issue is with my Hafler DH-500 stereo power amp, which has picked up a raspy hum on the output. I'm hoping someone with more power supply experience than I have will be able to help.

?

General description of the power supply circuit.

1.?????? Classic linear power supply.

2.?????? Step-up xformer.

3.?????? Bridge rectifier.

4.?????? On the output of the bridge, 2, 20,000 mfd, 100 V electrolytic filter caps between ground and each?of the +/- 90 VDC rails.

5.?????? There's a .01 mfd, 1000 V disc cap across the xformer connections to the bridge. I mention this because it's one of very few things the rails share.

?

General symptoms:

1.?????? There is a ~.35 V peak to peak, 120 Hz distorted saw-tooth riding the plus and minus 90 VDC rails. That its frequency is 120 Hz suggests to me it originates after rectification, i.e., the rectifier's output is 120 Hz.

2.?????? The saw-tooth is essentially identical on each rail except they are 180 degrees out of phase.

3.?????? The saw-tooth varies ~.05 V in amplitude yet triggers the scope reliably and presents the same sweep repeatedly. I.e., displaying 6 cycles of saw-tooth on the scope, amplitude variations repeat on the same cycles sweep after sweep. (Seems very strange to me.)

4.?????? Amplitude of the output hum does not vary with the input signal.

5.?????? Amplifier continues to function other than the hum.

?

A couple of symptoms lead me to suspect the power supply (as opposed to a downstream component putting noise on a rail).

1.?????? Reversing the power plug changes the amplitude of the output?hum.

2.?????? Changing the setting of an SCR light dimmer on the same house circuit changes harmonics on the hum. I.e., AC distortion from the dimmer is getting thru the power supply.

3.?????? Both rails equally affected.

?

One final observation that seems odd to me: When I hooked ground on the scope to circuit ground on the amp, the chassis of the amp became very sensitive to being touched by me in that touching it produced significant 60 Hz sine wave hum on the output. The scope is isolated from ground at the outlet as is the amplifier.

?

In conclusion, I expected to find one of the electrolytics deteriorated, and to see AC riding one rail only; that it's showing up equally on both rails is mystifying to me. That it's a saw-tooth vs. a sine wave is also indicative of something. That it's 180 degrees out of phase and 120 Hz seem to pin its origin to the rectification process. Again, the small disc cap across the bridge's input may be of interest, but its function is not apparent to me. (I'm putting a replacement cap on order and should have it installed before end of the week.)

?

Thank you for wading thru this lengthy description. Any help will be greatly appreciated!

?

Best regards,

?

Bill McDonald

?

Stefan Trethan

#78522

I also suspect a grounding issue.
Such questions will gladly be answered at Electronics_101 group, where they are on topic.

ST

Show quoted text

On Mon, Jul 23, 2012 at 8:44 PM, Denis <xyzzx_adv@...> wrote:

Bill,

? The information you provided about the amlifier power rails sounds normal.? I don't think you have an amplifier problem but a grounding issue.?

#78525

A few thoughts I have about this problem:

What is the volume setting? Does the noise level change when changing the volume level?

Is this in a noisy RF environment? Do you have flourescent lights on? I do telecom work, and whenever I run across a noisy phone with the speakerphone buzzing, I pull out my fox/hound tracer. With a quick push of a button, it will amplify any noisy environment in the AF range for the most part. I will wave it around the room monitoring the audio. Whenever I shutoff the flourescent lights, it will immediately quiet down. Another great way to use it is to trace the problem in the amplifier. It will especially work well because it is AF, not RF. It is very sensitive too. I have placed a signal from the fox signal generator on the pre-in and quickly traced it thru the amplifier by just placing the tip near the amplifying components. Just don't place it on the 120VAC directly, I did that once and blew the first stage amplifier out. Just a small transistor and easy fix. I also use it to see if slow processors are active. They make noise too. It is a very smart investment for troubleshooting. You can use it to judge the amp stages by the loudness in/out.

I would sub out the power supply too if it can be easily done. But I would say most of all ensure of the bonding. ground out the chassis to earth ground.

I have been reading up on my NEETS Basic Electronics manual and have just been studying power supplies. A very good book and free too. It might be dated, but so is your P/S. Just google NEETS, and you can get the whole series too.

Place the amplifier in a completely different environment. Did you plug in some other device in the house that might be backfeeding on the wall outlet?

And of course, isolate everything.

In telecom, whenever I troubleshoot a noisy circuit/wire pair, It is almost always an unbalanced cable pair. Whether it is due to a wet cable changing the capacitance between conductors, or one of the conductors has a lower resistance to ground thru a damaged cable. Make sure your pre-amp signal cable is OK. Also ground/jumper out the preamp in center pin of the RCA connector. If it is picking up noisy environment, this should quiet it down.

Make sure you did not break the solder trace at the preamp in RCA. If it did on the ground, it will howl!

Just looking at the specs of this amp, it is very impressive!

Nick

Show quoted text

--- In TekScopes@..., "Bill McDonald" <BillM256@...> wrote:

A correction and an addition in red below.

From: Bill McDonald [mailto:BillM256@...]
Sent: Monday, July 23, 2012 11:23 AM
To: 'TekScopes@...'
Subject: Re: Stereo Power Amp Problem

All,

Many thanks for your thoughtful comments, suggestions and questions; my
responses, in no particular order, are below.

1.       Hum is equally on both channels.

2.       Happens when amp is connected to preamp or disconnected.

3.       Happens when light dimmer is off or amp is on a different circuit.

4.       Happened while installing new speakers. During the process there
was an accidental power up that may have popped something. Installation also
involved moving line-level inputs around, and this is where we may have
caused the problem. I'm increasingly thinking input stages of each channel.

5.       Speaker output: Approximate sine wave at 120 60 Hz, ~.1 v p-p. Very
short duration spikes at each zero crossover, several volts in amplitude.
One channel has a dc offset of ~ -.1 volt, the other channel is ~ -.05
volts.

6.       Grounds confirmed between power supply, amp boards and speaker
outs.

7.       I see no physical indication of component problems on the amp
boards or relay board.

8.       After reading your comments, and doing further research of my own,
I start to suspect the saw-tooth riding the +/- 90 rails may be normal and
the failure is downstream, maybe decoupling in the amp. However, this would
require a common failure on both channels. But considering #4, that's a
possibility.

9.       I've attached the manual, with schematic on pg. 16. If attachment
doesn't work, the manual is available at
.

10.   Also worth mentioning, the amp has run perfectly for more than a
decade, so it's not a defect in design or a new part, e.g., Brad's point
about rectifier reverse recovery time.

Thanks again!

Bill

#78530

开云体育

Please take a look at the power supply fuses and be certain you have good fuses in all four positions. ?It is abnormal for this amp to have excessive offset on the outputs.? You state that you have .1V offset on one channel, that’s wrong and is where you should begin to look.? Can you put a low level 300mV sine wave on the amp’s inputs and look at the waveform on the output?? If you don’t have a complete sine wave then tell us what you see and we can advise further.? Many of the transistors in this amp are out of print so start with the fuses and proceed with caution.

?

Kevin

?

Kevin Collier

Director of Engineering

Warner Bros Studio Facilities, PPS

voice-818-954-1725

mobile-818-652-6030

?

#78554

I have enough experience with this sort of issue so I can sort the wheat from the chaff, AND THERE'S A WHOLE LOTTA CHAFF BLOWIN' AROUND HERE! ?First, this has nothing to do with rectifier recovery times. ?Similar diodes are used everywhere, and only in recent years have the audio-fools gotten the hots for fast-recovery diodes for 60 Hz power. ?(Tek never went for this nonsense.) ?It probably is not electrolytic caps, either. ?Except in some SMPS and TV applications where they are highly stressed, they last 50 years or more (except in many Tek scopes ;-) ). ?Original post said amp worked, but this bears scrutiny. ?I always test them by running them into 4 or 8 ohm dummy loads, look for clean sine waves and symmetrical clipping, and measure THD; this could be AFU and you might still get an OK sounding watt into a speaker. ?In which case, you have the usual blown amp repair situation. ?You may also have an open filter cap or bad regulator; this is not typical and would produce high AC ripple with the output. ?From what I've read so far, none of this has been?exonerated. ?So I'll just have to shoot in the dark like everyone else. ?This looks like a grounding problem to me, which almost never shows up with simple ohmmeter testing. ?First off, don't use a 3rd pin power cord ground, IMHSHO, it's just asking for hum troubles. ?(Although it sounds like you have more of a "buzz" problem, i.e. high-harmonics/spikes.) ?The 120 Hz on the output seems to me to be the current spikes charging the main filter caps somehow leaking into the signal path. ?When you touch the chassis and have an audible effect is a dead gave away. ?The first thing I'd do is tighten the mounting screws on the PCB's, filter caps (if any), input jacks, and anywhere else. ?Then I'd carefully inspect the grounding strap between the main filter caps; this is usually short and thick, with the power transformer CT connected to it. ?It may be made of thick copper soldered together, and the solder could be cracked or simply a cold joint, esp if it was built from a kit. ?If it still isn't fixed, then there's a couple more tricks. ?Take a clip lead, and try connecting various ground points together; the most important are the input grounds and the above mentioned point. ?You can also take a Tektronix scope with the chassis floating, connect the probe ground to the amp input ground, and go probing for any power line signal at other ground points. ?You probably want to use a 1X probe and max gain. ?You might also be dealing with outside interference. ?Are you next to an industrial welder or early warning radar? ?To check for this, take a portable AM radio, tune it between stations, see if you pick up any buzz and if you do, move the radio around to zero in on it. ?BTW, this is a quick and dirty way to track down nearly any source of radiated RFI. ?If you can get through all this, then the amp will be fixed. ?Or give up, and ship it to me!

Art

Jim Popwell Jr

#78561

are the amp;i and preamp plugged into the same ac outlet? ?you can get a ground loop by using two different wall outlets in a house very easily. one outlet could have a heacy load on it's "line" wheile the other may come from a completly different circuit with a light load…..then is the amp is plugged into one and the preamp the other you will have a voltage between them because of line losses. ? just plug the amp and preamp into the same outlet an see if there is a noticable change in the hum….

jim

Show quoted text

On Jul 23, 2012, at 11:44 AM, Denis wrote:

?

Bill,

? The information you provided about the amlifier power rails sounds normal.? I don't think you have an amplifier problem but a grounding issue.? Your comment about the chassis being 'touch sensitive' when the scope was connected to the amplifier circuit ground and the sensitivity to power plug polarity sounds like a floating circuit issue with the amplifier and scope chassis/cases acting as an antenna.?

?

? As I read the schematic the amplifier circuit is floating (not connected to an external ground).? Power line noises (SCR light dimmers produce high frequency transients locked into the AC line frequency.? High frequencies couple through stray capacitances of the circuit wiring and?power transformer in the?floating amplifier circuit.? Normally, the amplifier chassis is grounded to power?green wire ground (the 'U' shaped power plug pin)?but with unbalanced audio lines you don't want multiple ground connections (typically the lowest signal level point is grounded and everything else uses that ground)? so the preamp is normally the ground connection.

?

? You state that this has worked for a long time, SO something has changed, please provide more details:

?

* What have you done since the time it worked properly (you mention new speakers)

?

* Confirm that the scope is floating (the power plug ground pin is not connected to power green wire ground terminal)

?

* Is the chassis 'touch sensitive' without the scope connected

?

* As another commenter questioned, what happens if you short each input connector to ground (solder a jumper wire between the RCA plug tip and shell)

?

* Describe more details of the inadvertant power up of the amp, how long was it on, what was connected, any shorted leads

?

* Where does your audio system connect to ground (power line green wire ground)

?

? You mention moving the line input leads as well as trying new speakers which does raise input connection issues, some things to try:

* Create a shorting input plugs (RCA plug with jumpered input) for each input?- what is the hum output level

?

*?Measure the resistance from the RCA jack shell and the chassis while trying to move the jack?- should be zero ohms

?

* Install a temporary grounding path from chassis to?power 'U' pin with a 1000 ohm resistor (measure the AC voltage across the resistor).? Ideally there should not be any voltage present.? Report the voltage, hum output level, and if the shorting input plugs have any effect (also the effect of having the scope ground clip connected the amplifier)

?

Denis

?

From: billm256 <BillM256@...>
To: TekScopes@...
Sent: Sunday, July 22, 2012 4:55 PM
Subject: [TekScopes] Stereo Power Amp Problem

?

Hello,

?

Please pardon me for submitting this non-Tektronix question. However, I know there's a lot of general electronic expertise in this group, and I really need help (and I am using my Tek 2245 to troubleshoot this problem).

?

Issue is with my Hafler DH-500 stereo power amp, which has picked up a raspy hum on the output. I'm hoping someone with more power supply experience than I have will be able to help.

?

General description of the power supply circuit.

1.?????? Classic linear power supply.

2.?????? Step-up xformer.

3.?????? Bridge rectifier.

4.?????? On the output of the bridge, 2, 20,000 mfd, 100 V electrolytic filter caps between ground and each?of the +/- 90 VDC rails.

5.?????? There's a .01 mfd, 1000 V disc cap across the xformer connections to the bridge. I mention this because it's one of very few things the rails share.

?

General symptoms:

1.?????? There is a ~.35 V peak to peak, 120 Hz distorted saw-tooth riding the plus and minus 90 VDC rails. That its frequency is 120 Hz suggests to me it originates after rectification, i.e., the rectifier's output is 120 Hz.

2.?????? The saw-tooth is essentially identical on each rail except they are 180 degrees out of phase.

3.?????? The saw-tooth varies ~.05 V in amplitude yet triggers the scope reliably and presents the same sweep repeatedly. I.e., displaying 6 cycles of saw-tooth on the scope, amplitude variations repeat on the same cycles sweep after sweep. (Seems very strange to me.)

4.?????? Amplitude of the output hum does not vary with the input signal.

5.?????? Amplifier continues to function other than the hum.

?

A couple of symptoms lead me to suspect the power supply (as opposed to a downstream component putting noise on a rail).

1.?????? Reversing the power plug changes the amplitude of the output?hum.

2.?????? Changing the setting of an SCR light dimmer on the same house circuit changes harmonics on the hum. I.e., AC distortion from the dimmer is getting thru the power supply.

3.?????? Both rails equally affected.

?

One final observation that seems odd to me: When I hooked ground on the scope to circuit ground on the amp, the chassis of the amp became very sensitive to being touched by me in that touching it produced significant 60 Hz sine wave hum on the output. The scope is isolated from ground at the outlet as is the amplifier.

?

In conclusion, I expected to find one of the electrolytics deteriorated, and to see AC riding one rail only; that it's showing up equally on both rails is mystifying to me. That it's a saw-tooth vs. a sine wave is also indicative of something. That it's 180 degrees out of phase and 120 Hz seem to pin its origin to the rectification process. Again, the small disc cap across the bridge's input may be of interest, but its function is not apparent to me. (I'm putting a replacement cap on order and should have it installed before end of the week.)

?

Thank you for wading thru this lengthy description. Any help will be greatly appreciated!

?

Best regards,

?

Bill McDonald

?

Gordon

#78562

On 24/07/2012 05:19, Art Rivard wrote:
This looks like a grounding problem to me, which almost never shows up with simple ohmmeter testing.I had a not dissimilar problem recently on one of my own amps. Turned out to be the screws on the front panel not being tight enough.

Gordon

#78587

?

This is NEVER recommended and very dangerous. Check any Tek manual.

?

Show quoted text

----- Original Message -----

From: Denis

To: TekScopes@...

Sent: Monday, July 23, 2012 2:44 PM

Subject: Re: [TekScopes] Stereo Power Amp Problem

* Confirm that the scope is floating (the power plug ground pin is not connected to power green wire ground terminal)

?