Got a question….
I have a zapco z100s2 amp, it works and plays ok however…..
I replaced all the caps with low esr, audio grade and good filtering caps, it needed it.
The question I have is when I measure the dc on the outputs, the left channel measures about 5 mV and the right measures 60 mV.
I replaced all the A06/A56 and the 074 op amp. Also replace the two output trans on the right side.
Still have 60 mV?
Is this typical on these?
Also there is a very slight tick out of both speakers when it powers up.
Idle current is < .5 amps.
Is this typical on these?
I have a zapco z100s2 amp, it works and plays ok however…..
I replaced all the caps with low esr, audio grade and good filtering caps, it needed it.
The question I have is when I measure the dc on the outputs, the left channel measures about 5 mV and the right measures 60 mV.
I replaced all the A06/A56 and the 074 op amp. Also replace the two output trans on the right side.
Still have 60 mV?
Is this typical on these?
Also there is a very slight tick out of both speakers when it powers up.
Idle current is < .5 amps.
Is this typical on these?
Hi Scott, I would go by the golden rule of lower is better when measuring DC offset.
DC offset was not designed into the amp in the first place and some amps like PPI have adjustments to bring it under control.
Anything 5 mvdc or less is like new condition, and I have tested a number of amps new to verify this number. High DC offset is likely caused by damaged input diff pairs or bad current sources or possibly out of balance and leaky stages after the input pair, and even out or balance supply rails can cause this issue in some designs. SoundStream tested for less then 15mvdc on rebuilds as I recall in their spec's
The 60 mvdc is way bad and might cause the protection to trip on a ZAPCO and many other bands of car amps.
I would say something is leaking badly in the channel, and start with the input diff pair. Do you have some freeze spray or simple canned duster spray? Then use it to alter the temps of each transistor one at a time and monitor the DC offset. The likely culprit will show itself if you cool it down in many cases...hope this helps some old friend..C
PS some amp circuits are very temp sensitive and if you get confusing offsets with every transistor then don't rely on the cool spray to find the leaky device.
DC offset was not designed into the amp in the first place and some amps like PPI have adjustments to bring it under control.
Anything 5 mvdc or less is like new condition, and I have tested a number of amps new to verify this number. High DC offset is likely caused by damaged input diff pairs or bad current sources or possibly out of balance and leaky stages after the input pair, and even out or balance supply rails can cause this issue in some designs. SoundStream tested for less then 15mvdc on rebuilds as I recall in their spec's
The 60 mvdc is way bad and might cause the protection to trip on a ZAPCO and many other bands of car amps.
I would say something is leaking badly in the channel, and start with the input diff pair. Do you have some freeze spray or simple canned duster spray? Then use it to alter the temps of each transistor one at a time and monitor the DC offset. The likely culprit will show itself if you cool it down in many cases...hope this helps some old friend..C
PS some amp circuits are very temp sensitive and if you get confusing offsets with every transistor then don't rely on the cool spray to find the leaky device.
Thanks, I did check the dc before I replaced the small trans in the pre-amp stage and then the opamp. Still have the exact amount....60mV. So I changed the Rt channel finals but it still has 60mV. I did some reading on dc offset and a symptom is the slight pop during startup and the Rt channel sounds a little flat. The emitter resistor is good and I can adjust the bias to 1 mV across it. I need to check the rail voltage as you said and the +/- 15 on the opamp. Thats next. Thanks Cec for making me think!
Like new well designed and though out complimentary Class AB amps usually exhibit very low DC offset. With age and use and abuse I have found damaged transistors cause excessively high DC offsets. Zapco and many other amp makers used sense circuitry on the output of each channel and once the DC offset is bad enough it will trip the protection circuitry to shutdown the amp.
All amp makers that care about your speaker investment use this or some similar type of safety circuitry and DC voltage detection. Its the way they protect themselves from law suits and your expensive speaker from making smoke rings when an amp channel fails and tries to pass the entire DC rail supply out to your speakers. Shorted outputs can dump the amps full power supply of DC power into your speakers.
DC offset can grow in level with the signal volume sometimes and this will cause a amp to trip offline when driven near, but not at full power. It all depends on what is causing the offset and how it reacts to signal input.
I use DC offset as a guide line indicator of how hard a amp has been run, and I use it to gauge weather or not my work is complete when I repair a amp. After a couple thousand amps go across your bench you sorta get a feel for what tells to quality of a repair and the overall condition of a used amp.
Can you hear DC offset? probably not in most cases. But once its bad enough you will hear the amp shutdown for no apparent reason and not at full power like it used to do before the issue became evident.
The link I made on all of this came back in the late 80's when i would get amps that would not produce full power without shutting off and turning back on. After much head scratching and testing I found the issue of non typically high DC offset on the speaker terminals caused the problem and once the damaged and leaking components were replaced the issue was resolved. Its just one of the many tells you pick up over the years of working on this gear. It has solved a many of issue for me so I use it as a standard test tool before and after a repair to gauge weather or not i have done my work properly.
Manufacturers like Zapco, PG, SoundStream etc.. all have test spec's they expect their rebuilds and new products to meet, and DC offset on the speaker terminals is on everyone of those companies lists to test for, So maybe they know more then I do, But then I agree with them and test for this issue.
Your high channel likely has some leaky transistors in it. Its still operating fine now but the issue can grow with age and use and abuse of the amp. Sound Stream would not pass a rebuild with more then 15 mvdc of offset on the speaker terminals. I don't know Zapco's spec as they rarely ever released any info to the public when they were a American company based in Modesto Ca.
If the amp was on my bench I would say it still broken, but I operate on my own standards of good business practices, and I chase the numbers I know the companies I was close to did. on my bench that 42 mvdc would have to be accounted for and repaired so both channels had similar operating and test spec's. but here again this is me and my way of thinking and doing things, and this does not apply to anyone but me and my clients and workmanship.
Anyway I hope this helps some..it has for me...
All amp makers that care about your speaker investment use this or some similar type of safety circuitry and DC voltage detection. Its the way they protect themselves from law suits and your expensive speaker from making smoke rings when an amp channel fails and tries to pass the entire DC rail supply out to your speakers. Shorted outputs can dump the amps full power supply of DC power into your speakers.
DC offset can grow in level with the signal volume sometimes and this will cause a amp to trip offline when driven near, but not at full power. It all depends on what is causing the offset and how it reacts to signal input.
I use DC offset as a guide line indicator of how hard a amp has been run, and I use it to gauge weather or not my work is complete when I repair a amp. After a couple thousand amps go across your bench you sorta get a feel for what tells to quality of a repair and the overall condition of a used amp.
Can you hear DC offset? probably not in most cases. But once its bad enough you will hear the amp shutdown for no apparent reason and not at full power like it used to do before the issue became evident.
The link I made on all of this came back in the late 80's when i would get amps that would not produce full power without shutting off and turning back on. After much head scratching and testing I found the issue of non typically high DC offset on the speaker terminals caused the problem and once the damaged and leaking components were replaced the issue was resolved. Its just one of the many tells you pick up over the years of working on this gear. It has solved a many of issue for me so I use it as a standard test tool before and after a repair to gauge weather or not i have done my work properly.
Manufacturers like Zapco, PG, SoundStream etc.. all have test spec's they expect their rebuilds and new products to meet, and DC offset on the speaker terminals is on everyone of those companies lists to test for, So maybe they know more then I do, But then I agree with them and test for this issue.
Your high channel likely has some leaky transistors in it. Its still operating fine now but the issue can grow with age and use and abuse of the amp. Sound Stream would not pass a rebuild with more then 15 mvdc of offset on the speaker terminals. I don't know Zapco's spec as they rarely ever released any info to the public when they were a American company based in Modesto Ca.
If the amp was on my bench I would say it still broken, but I operate on my own standards of good business practices, and I chase the numbers I know the companies I was close to did. on my bench that 42 mvdc would have to be accounted for and repaired so both channels had similar operating and test spec's. but here again this is me and my way of thinking and doing things, and this does not apply to anyone but me and my clients and workmanship.
Anyway I hope this helps some..it has for me...
OK, found a weak TIP142 on the Lt channel so I replaced all four finals. Replaced the two IRFZ44 and the bridge diodes.
In the pic I circled the bias adjust and where I think I mesure for 1mV, thought the 0.1 ohm resistor would be on the emitter side but??? Is this the correct measuring spot?
In the pic I circled the bias adjust and where I think I mesure for 1mV, thought the 0.1 ohm resistor would be on the emitter side but??? Is this the correct measuring spot?
Attachments
OK, Installing two more Z44 MOSFETs worked no problem.
Now for the DC off-set problem...
Was at 60 mV on the Rt speaker output.
After checking the res readings on the darlington pairs (finals) one was way high compared to the rest. Replaced them and now I'm down to about 30mV on the Rt channel. Next I looked at the MPSA06/A56 trans used and started checking the Hfe value on all my new spares and there is quite a spread in values. I picked out the closely matched ones and installed them and that balanced the differential amp much better. I'm next going to check the MPSU07/57 (next stage) and see if I can get it down further.
Question though....
I noticed that the +/- 15 coming out of the power supply is reading +15.5 and -16.02. There are zener diodes along with TIP31C/32C for that section of the power supply. On the differential amp section, I noticed that the MPSA06 pare use -16.02 fed through a 3.9k resistor and then to the emitters of that pair and the MPSA56 pair use the +15.5 through the 3.9k to emitters. SEE Part of schematic here.... diyAudio
Could this be an issue?
Now for the DC off-set problem...
Was at 60 mV on the Rt speaker output.
After checking the res readings on the darlington pairs (finals) one was way high compared to the rest. Replaced them and now I'm down to about 30mV on the Rt channel. Next I looked at the MPSA06/A56 trans used and started checking the Hfe value on all my new spares and there is quite a spread in values. I picked out the closely matched ones and installed them and that balanced the differential amp much better. I'm next going to check the MPSU07/57 (next stage) and see if I can get it down further.
Question though....
I noticed that the +/- 15 coming out of the power supply is reading +15.5 and -16.02. There are zener diodes along with TIP31C/32C for that section of the power supply. On the differential amp section, I noticed that the MPSA06 pare use -16.02 fed through a 3.9k resistor and then to the emitters of that pair and the MPSA56 pair use the +15.5 through the 3.9k to emitters. SEE Part of schematic here.... diyAudio
Could this be an issue?
Closed issue
The amp works fine
Special thanks to Perry and 1moreamp for skoolin me on dc off-sets and determining that the engineering on this amp was poor. I learned allot about audio amps on stuff I thought I knew. I felt like I was in first grade.
Its all buttoned up so know its off to re-cap the few Soundstreams I have.
Thanks all.
Oh, When I heat up Q20 (MPSA06) or Q17 (MPSA56), the offset starts going towards 0v from +40mV.
The amp works fine
Special thanks to Perry and 1moreamp for skoolin me on dc off-sets and determining that the engineering on this amp was poor. I learned allot about audio amps on stuff I thought I knew. I felt like I was in first grade
.Its all buttoned up so know its off to re-cap the few Soundstreams I have.
Thanks all.
Oh, When I heat up Q20 (MPSA06) or Q17 (MPSA56), the offset starts going towards 0v from +40mV.
Attachments
Last edited:
Quick up-date
So I also checked and had some ripple issues on the rails so I replaced the filter caps with some Panasonic's and put all the semi's back to stock (the originals that were in it when I got it). The ripple is gone and the dc offset is now 30mv on each channel. Sounds great.
So I also checked and had some ripple issues on the rails so I replaced the filter caps with some Panasonic's and put all the semi's back to stock (the originals that were in it when I got it). The ripple is gone and the dc offset is now 30mv on each channel. Sounds great.
Attachments
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.