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Old 22nd October 2012, 03:37 AM   #1
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Default Zapco Z100S2 Question

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?
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Old 22nd October 2012, 05:10 AM   #2
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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.
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Old 22nd October 2012, 11:06 PM   #3
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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!
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Old 23rd October 2012, 06:10 AM   #4
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I own a Z300 and measured the DC offset today.
Left channel is at 12mV, right channel at 42mV. So, is there something wrong with it? It sounds normal for me. I check the signals with a scope these days.

Last edited by DotCom36; 23rd October 2012 at 06:33 AM.
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Old 23rd October 2012, 09:00 AM   #5
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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...
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Old 24th October 2012, 02:52 AM   #6
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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?

Attached Images
File Type: jpg bias.jpg (168.8 KB, 73 views)
File Type: jpg 100_4308.jpg (631.3 KB, 74 views)
File Type: jpg 100_4309.jpg (558.6 KB, 69 views)
File Type: jpg 100_4310.jpg (565.0 KB, 62 views)
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Old 24th October 2012, 04:36 AM   #7
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Correct spot measure away... I don't see any possible other place to measure it at...
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Old 24th October 2012, 09:39 AM   #8
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Do you think two IRFZ44 are equal to four BUZ12?
Got to love engineers right.
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Old 25th October 2012, 03:58 AM   #9
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BUZ12 is a 42 amp fet so Z44 should work, or just jump to Z46 or Z48 The Z48 is a 72 amp as I recall and the Z46 is like a 50 amp rated but due to package limitations you can usually get something less... You will still need four Z44's if you use them.
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Old 25th October 2012, 04:20 AM   #10
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When I first opened the amp, it only had 2 IRFZ44 installed!
Should I put 2 more in it? The Gate res are there...
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