ZX475ti amplifier

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megawatt
Posts: 11
Joined: Sat Sep 16, 2023 4:06 am
Location: Yarmouth, Nova Scotia, Canada

ZX475ti amplifier

Post by megawatt »

Hi, New here but have been trying to locate a service schematic for this old amplifier. There is a problem with the Front Left and Right Rear outputs, the rest of the amp works perfectly.
These outputs are low compared to the working section and the higher the volume the more distorted it becomes.

No indications of anything burnt, blown up or missing in the amp itself. All transistors tested fine and Op-amps are fine and the other components tested fine as well. These components were removed from the circuit board for testing to avoid any false readings.
I had the scope (Techtronic's) connected to it and the working outputs are showing a good sinewave (with a sinewave input to the amp) except one of the defective sections are showing a Sawtooth Waveform in one section. Thats where a schematic would be very very handy in identifying the section and the culprit causing the problem.
All the red LED's beside the transistors closest to the heat sink section are all working, the other row of red LED's are all off. The amp starts up fine, normal sequence and the blue LED's on the front cover all turn on.
When the amp is working normally on the good section, have it set up on the bench, operating at 12.5 VDC and can easily draw 10 amps of current with the volume turned up part way and no distortion or other problems noticed or heard.

Any assistance in the matter greatly appreciated.
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Jacampb2
Mad Scientist
Posts: 1562
Joined: Sat Apr 19, 2008 11:57 pm
Location: Beaverton 48612

Re: ZX475ti amplifier

Post by Jacampb2 »

If some of the steering LEDs are out, they are either faulty, which often happens with these, or you will have open fusible resistors in those channels. Normally in either case the output will not only be low, but also distorted.

There are fusible resistors scattered throughout every PG amp. They will show no outer signs of failure. You must meter check each one. The fusibles will typically be a bluish green color with an almost satin looking texture. They will also have an extra resistance band which will not make sense no matter which way you shake a stick at it!

And yes, some of us do still like our heads in, but sadly the phorum doesn't see the traffic it used to.

Lmk if you have more questions.

Later,
Jason
M: M100, M44 for a custom amp project
Zx: Zx500, Zx450, Black Zx350
ZxTi: 4 Zx600Ti's, 1 Zx400Ti
Ti: 5 800.1's & 900.7 for a custom amp project. 1 1200.1, 1 1000.2
Tantrum: 2 1200.1's, 1 600.4, 1 500.2
XS: XS6600
megawatt
Posts: 11
Joined: Sat Sep 16, 2023 4:06 am
Location: Yarmouth, Nova Scotia, Canada

Re: ZX475ti amplifier

Post by megawatt »

Hi Jason, Thank you for the reply, I have been talking with Perry and he was very helpful, I have the amp operating once again, finally! It has been a battle as everything I/we suspected and checked always checked as good. I did however, replace some defective electrolytic capacitors as I checked them when out of circuit and as an example, a 100Uf cap only measured 60Uf and other similar readings so just replaced them with new ones along the way. But the idle current problem kept evading me :idiot: , I am very familiar with the home type amps and guitar amps etc.... but not with car amplifiers, they are a different animal for sure but did not give up. I could have set up a small power supply to provide Bias voltage for these two channels but to me it was not the proper way to go and did not know what it may disturb further in the amp so decided to stay with it until I found the cause.
I finally found the actual problem, it was caused by two of the RED LED lights were defective, I had some in stock and replaced them, one per each channel affected, and the Bias was back again and able to be adjusted.... :clap: So Simple!
Now I need to set each channel to the proper idle current and apply some music and see how it sounds.
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Jacampb2
Mad Scientist
Posts: 1562
Joined: Sat Apr 19, 2008 11:57 pm
Location: Beaverton 48612

Re: ZX475ti amplifier

Post by Jacampb2 »

Jacampb2 wrote: Fri Sep 29, 2023 12:02 pm If some of the steering LEDs are out, they are either faulty, which often happens with these
megawatt wrote: Wed Oct 04, 2023 5:44 am I finally found the actual problem, it was caused by two of the RED LED lights were defective, I had some in stock and replaced them, one per each channel affected, and the Bias was back again and able to be adjusted.... :clap: So Simple!
Now I need to set each channel to the proper idle current and apply some music and see how it sounds.
:mrgreen: You don't say ;)

You must be the person Perry emailed me about... Sorry I didn't see your post when it first came through. You are always more than welcome to use the phorum email link to reach me. I haven't been on much social media lately, I'm trying to minimize the amount of my day I loose in the social media black hole.
M: M100, M44 for a custom amp project
Zx: Zx500, Zx450, Black Zx350
ZxTi: 4 Zx600Ti's, 1 Zx400Ti
Ti: 5 800.1's & 900.7 for a custom amp project. 1 1200.1, 1 1000.2
Tantrum: 2 1200.1's, 1 600.4, 1 500.2
XS: XS6600
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