Zapco Z400C4-SL... Not Again! - Page 2 - diyAudio
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Old 25th August 2012, 10:26 PM   #11
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that s/sx looks out of place, wink, wink....... and i'm in need of a 4th.
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Old 26th August 2012, 01:47 AM   #12
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All sold except one 405 and one engineering sample 405 thats never been installed and does work...
OK...back to the Crapco...oooppps....Zapco
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Old 26th August 2012, 06:38 PM   #13
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During some testing I replaced, one at a time, some opamps and after replacing one of them, the amp powers up. I found that if I put audio in the front input and selected 4 channel, I could only get sound from the left rear channel... Front channels work fine. When I switch it to 2 channel input, both channels on the rear work. I kept replacing the opamps until the rear input worked on both rear channels. Also I found that when its in 4 channel mode and I put the crossover to high pass on the rear and select 24dB slope the right rear channel cuts off. I suspect that the opamps on the crossover are flakey also. I also noticed when I was replacing opamps, the dc voltage on the outputs would change. So as it sits, the DC on the front channels is .001 or less. The rear had .44 on the right and .001 on the left unless I swap an opamp the it would change. There is a little static on the rear left at idle. Idle current is about 2.5A. All the op amps I used were from the previous zapco I fixed so some of them are questionable. It does plays very clear but I know I need to change the opamps with new ones.
So next is to order opamps and replace them all as I think someone really gave the inputs to this amp hell. I can't imagine how it had been hooked up, speaker outputs from the factory radio jammed into the the inputs? I just don't know.
Will let yall know the outcome.
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Old 26th August 2012, 08:05 PM   #14
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Also I'm changing the NAND gates, there are two of them and there are four TL072s on the crossover module as well as three 5532 opamps.
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Old 27th August 2012, 12:28 AM   #15
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Side note...
When I first got the amp to stay on, the left rear speaker was moving, low rumble with static. Thats what bad opamps seem to do on these Zapcos. And I dont know how accurate my HP power supply current meter is but it show about 3.5 Amps idle, kinda high I think. The owners manual says 2Amp idle so I dont know. I did change a couple more opamps and there is .000 to .001 VDC on any of the speaker terminals so thats good but I didn't change any on the crossover yet. I think the noise is coming from them.
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Old 27th August 2012, 02:55 PM   #16
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I noticed on the last Z400 that I worked on, the idle current was little over 2 amps using the current meter on my HP power supply. This one is 3.5 to 4 amps idle. So my question is- I don't know if someone messed with the bias pots on this amp. Is there an effect of bias adjustment on idle current draw? I have not touched them yet...
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Old 27th August 2012, 05:36 PM   #17
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yes, there can be a great effect. i have had little 250watt amps peg out 15amps off bias adjustment. too little, and you get bad signal distortion, mostly at lower volume. have you adjusted bias pots before? come to think of it, deviations between components being swapped out can effect the bias, as well.
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Old 27th August 2012, 07:52 PM   #18
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I did not change any of the pre or finals. We talked about it at work and I'll measure the bias on the base first. I think I can turn them down all the way, inject a 1000hz signal to the input, look at the output with my scope and turn them up until the sine-wave smooths out. I know the bias should be about .6 or .7 volts for class A, if I remember correctly. I am going to wait untill I install new op-amps be for I do this.

Someone tell me if I'm wrong

Last edited by 90scaraudio; 27th August 2012 at 08:17 PM.
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Old 27th August 2012, 10:25 PM   #19
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I suggest you start at 100 micro-volts as measured across the emitter resistor pairs for each set of outputs as simple voltage drop. 0.001 DC Volts should be adequate and is the most common bias settings for PPI, ZAPCO, PG, RF, etc...most Japanese made amps like Kenwood, Alpine, Pioneer, etc... were using 2 to 4 milli-volts DC voltage measured the same way. These are "Old school" settings from memory but as Rockford Fosgate says in their final test doc's "don't kill the music " right in their bias settings remarks
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Old 27th August 2012, 11:17 PM   #20
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K Thanks 1moreamp. So about 1mv like the soundstreams... See pic...
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