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ZX500 going into overload
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Author:  Silverhorse [ Tue Jul 16, 2019 12:14 am ]
Post subject:  ZX500 going into overload

Hi everyone

my ZX500 is in overload mode (red led is on) ventilator is working.
the 2 thermistors are ok, no short in the BJT's and FET's.
It drives me crazy. Any ideas to help me further?

Author:  lilviper [ Tue Jul 16, 2019 9:35 am ]
Post subject:  Re: ZX500 going into thermal

Check the paths, i read something on how their system works on here. Somewhere.

Sent from my SM-G960U using Tapatalk

Author:  Jacampb2 [ Tue Aug 06, 2019 3:05 pm ]
Post subject:  Re: ZX500 going into thermal

The two temperature sensor should be very low resistance at ambient temperature. They each are directly connected to ground in one end and iirc summed via a pair of 1n4148 diodes before the lm339 comparator which handles protection.

Is the amp actually in thermal protection, or is the led just lit? If it's truly in thermal the fan will run at it's maximum speed and the Poweramp should be muted. If it's just the led lit, Q6 on the PS card is probably shorted. If it's truly in thermal, first check that the trace from each thermal sensor is on tact all the way back to the PS card. Check all the way to the card because the solder joints to the right angle header can crack. I've also seen amps with the trace broken where it runs along the edge of the board. Other possibilities are that the lm339 is faulty, but I believe it handles o/l protection as well, so if it's not stuck in over load, it's unlikely that the comparator is only partially damaged.

The temperature sensors are similar to a PTC thermistor, but they have more in common with a polyswitch then an actual thermistor. They have a very low resistance and will actually exhibit a bit of NTC behavior up to their "knee temperature". At the knee temperature, they abruptly transition to a very high resistance. They have bit of hysteresis near their knee temperature, so they have to cool back off well below their knee temp before they will switch back to their low resistance state. So basically, any high resistance path between the sensor and the comparator will cause the thermal protection to engage, it doesn't matter if it's a broken trace or a cracked solder joint, open and high resistance are the same for all intents and purposes in this scenario.

Good luck,
Jason

Author:  Jacampb2 [ Wed Aug 07, 2019 3:46 am ]
Post subject:  Re: ZX500 going into thermal

Also, I just realized that you said the red led is on. The red led is the overload indicator. The overload protection is based directly on output current flowing through the outputs.

If it's actually showing an overload fault, then you likely have some shorted outputs. There are other reasons it could happen, like problems further upstream in the drivers and pre-drivers, but more than likely it's the outputs.

Good luck,
Jason

Author:  Silverhorse [ Wed Aug 07, 2019 10:34 am ]
Post subject:  Re: ZX500 going into thermal

Hello Jason!
You are the man. I did a quick test on the output's but non of them have a short. The amp puls almost no current just the red led is on. This weekend I continue with the repair.
something else : do you know a good replacement for the 2SC2389 BJT?

Author:  Jacampb2 [ Fri Aug 09, 2019 4:19 am ]
Post subject:  Re: ZX500 going into thermal

Silverhorse wrote:
Hello Jason!
You are the man. I did a quick test on the output's but non of them have a short. The amp puls almost no current just the red led is on. This weekend I continue with the repair.
something else : do you know a good replacement for the 2SC2389 BJT?


KSC2316/KSA916 are what I use. You'll have to replace the compliment as well if it's a problem with the differential input pair.

Make sure the rails are actually coming up. I've seen amps do this when they have a badly blown power supply. All the protection circuitry, fan, etc are powered by the main 12v input, so you can get lights and signs of life, but the power supply could be blown.

Author:  Silverhorse [ Sat Aug 10, 2019 7:31 am ]
Post subject:  Re: ZX500 going into thermal

Hi Jason

thank you for the info. I work on 2 project's now.
the 2SC2389 part if for another repair (ZPA0.5). The problem is that the guy who repaired this did not change the counter part, some diodes are blown on the power supply board. D'ont know the damage yet...

did some more testing on the zx500 today: so the red LED is on and the fan is working. If this is the case would that mean that the power supply is ok?
The input RCA is shorted on both channels. So I thought there could be a problem inside the RCA connector. After desoldering it the connector is 100% ok. The 4 through holes of the connector al all shorted on the board. Could it be that the input signal was to high? If so how can I find this out?

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Author:  Jacampb2 [ Sat Aug 10, 2019 4:07 pm ]
Post subject:  Re: ZX500 going into thermal

Not necessarily, the fan and all the protection circuitry all runs off 12v DC. Normally when the power supply fails all the mosfets short drain to source. The problem comes when someone has it fail and the amp was not fussed or not properly fused. In this case the mosfets can blow open after failing shorted. In that scenario, the amp will not draw excessive current and the protection LEDs and fans etc will continue to appear to be working. The only way to know for certain is to measure the rail voltages.

As for the inputs being being shorted, I've never seen that happen and I don't really know a plausible scenario where that could happen. The shield of the RCA connectors should be connected to secondary ground via a 100R ⅛th watt resistor. The signal goes via resistor and coupling capacitor to an op-amp. It's possible that the op-amp is shorted. The zx500s do have a electrolytic cap across signal to shield. Reverse your test leads and make sure that your not just reading the cap across inputs.

Author:  Silverhorse [ Sun Aug 11, 2019 5:08 am ]
Post subject:  Re: ZX500 going into thermal

let's start from the beginning.
when I power up and measure the rail caps I have 0 volts (13.6v on the PSU caps). So I decided to take out the 8 PS FET's. They are type P55N06 with 33 ohm gate resistors. I tested the switch function of the FET's once they where out and they al seem to work (tested with multi meter) The gate resistors measure all 33 ohm so no open resistors.
When I measure now (no FET's installed) between drain and gate I have 13.6Vdc. When I measure between drain and source I have also 13.6Vdc. My power supply is also 13.6Vdc

did some research and find out that the sg3525 could be the problem. I have no voltage on the output A and B of the sg3525. On VCC and ground I have 13.6vdc.
The first thing will be changing the sg3525 controller. Is it possible that the sg3525 blows up before there is damage to the FET's?

about the RCA input: it stay's a short circuit even when I'm changing the + and the - pen and hold it for a while.
problem solved: I measured the output RCA's not the input RCA's :oops:

Author:  Silverhorse [ Thu Aug 15, 2019 12:27 pm ]
Post subject:  Re: ZX500 going into thermal

Did change the SG3525 but still the same problem. Only difference now is the green LED is flashing...
If the SG3525 gets powered up it should send 6 volts to output A and output B not? I have no power on output A and B...

Author:  Silverhorse [ Sat Aug 17, 2019 11:34 am ]
Post subject:  Re: ZX500 going into thermal

lilviper wrote:
Check the paths, i read something on how their system works on here. Somewhere.

Sent from my SM-G960U using Tapatalk


yes, I read in the old post that there was a problem with the thermistors but these are ok...

Author:  Silverhorse [ Sat Aug 17, 2019 11:39 am ]
Post subject:  Re: ZX500 going into overload

what about the transformer: is it normal that when the amp is not powered up the windings are shorted?
Below is a picture, the red = shorted

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Author:  Silverhorse [ Sun Aug 18, 2019 2:28 am ]
Post subject:  Re: ZX500 going into overload

Today I removed the 2 diodes that separate the power supply and the outputs. The 2 diodes test ok.
When I power up nothing changed (fan is running and red LED burning) so I think that the problem is in the power supply section.

I changed the SG3525 controller because I have no 6V on the gates of the fet's and changed the FET's with IRFZ44 FET's (33 Ohm resistors).
But still no power on the output of the controller.

Is it possible that somehow the shutdown function of the SG3525 is activated?

pffff this is a hard one for me :?

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Author:  Silverhorse [ Sun Aug 18, 2019 8:58 am ]
Post subject:  Re: ZX500 going into overload

Think I find the problem.

Lilviper you where right!

Because I knew it was in the power supply region I checked all the paths wit a magnifying glass and guess what. Few inches away from the sg3525 there was a place where 2 diodes, 2 resistors and 2 caps that had bad connections to the board. Because it is just next to the vertical board it was very difficult to see. I removed these and replaced them with new ones. Some through holes were oxidized badly. Does somebody know what could caused this? Is this due to excessive heat? When I power up the red led goes out and the green led lights up.

Next is putting the diodes back in and testing...

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Author:  lilviper [ Sun Aug 18, 2019 11:56 am ]
Post subject:  Re: ZX500 going into overload

Looks like it got wet at some point. One of my amps had some corrosion due to a bad trunk seal.

Sent from my SM-G960U using Tapatalk

Author:  Jacampb2 [ Sun Aug 18, 2019 7:08 pm ]
Post subject:  Re: ZX500 going into overload

Sorry for the long delay, I was out of town. Cr22 and cr40 are half of the diodes which make up the rectifiers for the auxiliary rails. These diodes as well as their associated snubber networks (R120-123 & C42-43 & C63-64) will get extremely hot if the amp is run with the main rectifiers out of circuit for any extended length of time. It's odd that cr22 & 41 did not overheat as well. Even with these completely out of circuit, I don't think it should yep the overload protection circuitry, but I've never actually removed them to test that.

Anyhow, the point is, there is virtually no load on the auxiliary rails. They are there only to provide a voltage source that's far enough above the main rails to allow the power amp to swing "rail to rail". They don't supply any meaningful current, they just need to be far enough above and below the main rails to fully switch the transistors on. They really shouldn't fail and I've never actually seen them fail before.

I hope you've got it sorted out. To answer your earlier question, yes, the sg3525 is held in "shutdown" state when an o/l condition exists

Good luck,
Jason

Author:  Silverhorse [ Sun Aug 25, 2019 7:43 am ]
Post subject:  Re: ZX500 going into overload

The corrosion is caused by leaking PSU caps. Half of the pins of the vertical pcb are also very bad corroded. So I took out the vertical pcb and cleaned the pins and through holes. After putting everything back the power supply is working! By doing this I damaged the flat cable between X-over and main board :( .
finding a new one will be something else...

Author:  Eric D [ Sun Aug 25, 2019 9:50 am ]
Post subject:  Re: ZX500 going into overload

If you have access to old obsolete computers, a floppy cable, or IDE33 data cable make good replacements for the X-over ribbon cable. As a bonus they are more flexible than the original equipment.

Author:  Jacampb2 [ Sun Aug 25, 2019 10:24 am ]
Post subject:  Re: ZX500 going into overload

This is the part number for the cables in the newer zxti and ti amps. They make them in both plastic and paper. The plastic ones are a little more robust, but if you don't take the amp apart a lot, the paper ones are cheaper and work fine. These are 10 conductor ribbons. If memory serves the newer amps were only 7 conductor. I just trim off what I don't need. The 10 conductor ones are more common, so a bit cheaper then the 7 place ones.

Double check the length and number of pins. Digikey has most variants in stock.

In the past I've done the IDC cable that Eric suggests. It does work well, but I found it to be a total pita stripping each stranded conductor and getting them all fed through the board without fraying or breaking again.

https://www.digikey.com/product-detail/ ... -ND/137494

Good luck,
Jason

Author:  Silverhorse [ Sun Aug 25, 2019 11:06 am ]
Post subject:  Re: ZX500 going into overload

Hello Eric! Thank you for the info but I want to keep the repairs as close as possible to the original. Thank you Jason for the part number, this saves me a lot of time with the search module.

Author:  Silverhorse [ Sun Sep 01, 2019 3:20 am ]
Post subject:  Re: ZX500 going into overload

The parts from Digikey arrived and the flat cable is replaced. Today I powerd up the amp and it plays wonderful!
Just the sensitivity pot is giving a lot of distortion when you turn it up or down. Once it is set correctly it works fine.
There is another post from zztunnell who has the same problem so I'm following his progress.
Thank you all for helping me out!!

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Author:  Silverhorse [ Mon Sep 02, 2019 9:59 am ]
Post subject:  Re: ZX500 going into overload

found the tutorial from Eric about the pot meters.
Thank you Eric for your countless contributions to this forum.
It's a wonder that even after 8 years this info is still here.

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