ZPA 0.5 control board problem

[Silverhorse]

Long time ago I bought this amp on a second hand site. It's a standard V2 version.The outside of this amp is like in new condition. Also the plexiglas is in top condition. This is what convinced me to buy this amp. I payed the top price for this amp. When the outside is in top condition after al these years the inside would be the same. Well... this time I couldn't be more wrong... . Inside this is the most fucked up amp I have ever seen or had. The guy changed the psu caps and the 2 little rail caps. One side of the power supply fet's where partly changed. Two fet's had burnt the board badly taken the pads with them. They removed the control board and messed up all throug holes. Several diodes were changed and also a transistor just underneath the fan connector on the left side. The SG3525 was also changed. Man, this amp was a mess.
Very disappointed I started the Restoration of this amp. The amount of work I spend already on this amp is unbelievable. Until now I have changed the 12 psu caps, the 16 fet's with the gate resistors and the 4 rail caps. I repaired all the 20 through holes of the control board (man this is hell to do). Changed the SG3525 and restored the bad solder work.

So after all this work I powered up the amp. I just powered it up few seconds to see if something bad happend and yes there was life!
Al the 4 red led's were powered up and also the 2 green lights. Man I was happy... yes yes now it is coming... the second time I powered it up. All the led's stayed of but the fans are working. The amp is dead now.

I took out the fan board and the control board. What slipped my first repair attempt is that the transistor Q21 was changed with another one than the original C2389. I started looking for a replacement but could not find a good one. So I asked the question on the forum and Jason (thank you man!) gave me a good replacement. The counter part (Q20) is also not replaced (it's still the A1035).
Other components that are smoked are: CR13 /CR14 on the fan board, CR21 CR22 / C23. R51 is cooked on the control board. What could have caused this?
I can change all these parts but I'm afraid that when I power the amp up again and the cause of this problem is not solved I can start over.

so please some advise of the experts....

[ttocs]

do you have the switch to tell if the plexi is in bypassed?

[Silverhorse]

hello Ttocs!

the switch is not in it's bypassed...

[Silverhorse]

some pictures when the amp just arrived...

[Silverhorse]

starting restoration...

[Jacampb2]

R29 and R32 burned most likely due to the main rectifiers being out of circuit and the amp being powered up. A lot of power gets dissipated across them if the main rails are down. They normally only see a potential of ~10V, with the main rails down they see about 70v.

CR13 and CR14 are there zener diodes which provide regulated +-15V rails. They typically run hot, but shouldn't burn. You might have an excessive load on your 15v supply. Also check their ballast resistors. R35 and R36, these power resistors get messed with when people try to "v2" the amps. If the ammo was powered with those resistors out of circuit, that could explain the zeners burning. Get rid of those idiotic oversized power resistors. Use the recommended v2 parts, not the biggest you can find. They will screw you being unsupported hanging off a vertical board. I don't know who started this trend, but it's insane. I have ti remove this crap from so many amps. Not only does it look terrible, but the joints will crack or the leads will break if the amp is actually used.

CR21, CR22, R51, and C23 are more troublesome. All are part of the O/L protection. My guess would be that one of the diodes failed shorted as is common, and took out the rest. Other than that there is potentially an issue in the poweramp or the circuit in the poweramp that triggers overload. This is one of the few PG amps which do not have all of the protection circuits opto-isolated. This means a fault can potentially drive full rail voltage into a circuit designed for much lower voltages. The LM339 on that card is there comparator that handles each protection circuit and HV/HC rail switching. I would expect there comparator to be damaged as well if that was the case.

The transistor you had to replace is partly responsible for the rail switching. I can't think of a plausible scenario for it to fail. I would check though that the high side rail isn't stuck in HV or HC mode.

I have cloned both the PS and fan control boards and put their schematics in the FB group. I'll try to post them here in a moment.

The only other thing I'd say is be careful with the eyelets, especially on higher density locations like there daughter cards. I personally do not like and will not use eyelets unless there is no other possible way to repair it. I've seen many failed installations because people lacked the proper tools or skills, and once you've drilled oversized holes in the board, you are pretty well committed. I'm nor saying you did a bad job, I've just seen a lot more failed attempts than successful ones and I don't want someone stumbling in this post whom had no clue what they are doing and attempt it.

I'll try to post schematics in a moment.

Later,
Jason

[Jacampb2]

I hope these work, I'm on my phone and could only find the pdf for one of them, the other is a screenshot.

I'm also not sure I'd they are my final revisions, but I'll try to get them from my PC when I'm back to the shop.

[Jacampb2]

I found a screenshot for the final draft of my modified power supply card. The pdf attached in the last post may be more useful since the attached here has several deviations from the stock design including a switch to lock the amp in HV mode and buffers for the sg3525 to allow the amp to drive modern very high current power supply mosfets. Other than that, component designations should be consistent with the factory card even though the layout is fairly different to accommodate the changes.

[Silverhorse]

Hi Jason!

thank you so mush for this info. Now I can start finding the problem.
The vertical boards are original V2 versions, I don't know why the first owner replaced so much components, I guess he handled the trial and error route.
Do you have a link to the FB page? I did a search but cant' find it (I'm not a intensive FB user)

[ttocs]

https://www.facebook.com/groups/1473298426288964/

[Jacampb2]

Here's a link to my general tech group on FB as well.

https://www.facebook.com/groups/149769529274064/

[Silverhorse]

https://www.facebook.com/groups/1473298426288964/

Hi ttocs, I tried several times to become member of this group but it doesn't work. Status says "in treatment"

[ttocs]

what is your name on FB? Andres can be a little tight on letting people in for some reason. I will see if I vouch for you if it speeds it up, or cancels it all together.

[Silverhorse]

did find some time to continue with this repair.

I did change all the broken diodes and resistors + I changed the KA339.
Put the board back in the amp and powered up the amp.
result: the 4 bias red LED'sare working + the 2 fans but no green LED's on the vertical board. I measured the voltage over the LED's and I had 1.7v on the green ON LED and also 1.7v over the yellow LED.
In the dark you can see that the green and yellow led are slightly lichting up but they need 2v not 1.7v to work normal.

So I started all over again and measured a sort circuit over resistor R61 (10K) on the vertical PS board. I pulled out the resistor and still had a short. So I looked further and find a short on the NPN 2N4401 transistor (Q27).
Changed this transistor and now the yellow led is out and I measure 1.7v on the green high voltage status LED.

So now I have to find out why the voltage on the green LED's is to low but have other things to do...
to be continued...

[ttocs]

keep pluggin away at it.

[Silverhorse]

I have 3V where I should have 15v so something is pulling it down

[Silverhorse]

think I found the problem.
I was so focused on the control board but the problem is not on this board. The problem is on the fan board. The fans are working so that is why I skipped this at first.
There is also a LM393 on the fan board in the right corner that is getting the 15v from the same circuit as the control board. The fans are not connected with this power supply because of the optocouplers. The LM393 on the fan board is toast. So Jason you are right but I did not made the connection with the LM393 on the fan board at first.
So now I have to put the 2 boards back in and lets hope I've got this right.
so to be continued...

[Silverhorse]

changed the LM393 and the green led's are working but now the red overload LED is activated. The 4 bias LED's are working. I have no shorts in the output transistors but one IRFZ44 of the voltage regulators is shorted (Q32). Replaced al 4 of them. The red overload LED is still active. Pfff my motivation has a dip on this moment . Any suggestions? Yeah I know it can be anything. The hours I spend trying to fix this man... Next time I remove all the parts so I have only the naked PCB left and put all new pieces in

[Silverhorse]

is it usually so that the 2 green LED's and red LED are turned on the same time? The green on LED should be out as the red LED is turned on, not?

[Silverhorse]

the 2 bias pots where turned down all the way. I turned them up until the point I can see that there is a peak moment in amp use, the point that the BJT's are coming to live. Don't know a better way to explain this. Now the amp is pulling around 5A. But the overload LED is still on.
So what happens when I power up:
- the fans are starting to turn with higher speed and lower the speed after let's say 2 seconds
- the yellow thermal LED is turned on for 2 seconds and goes out after 2 seconds
- the red LED is coming up from the beginning but sometimes a bit later then the yellow LED
- the 4 bias LED's are coming up.
- the 2 green LED's are also light up the moment the power comes up.
- after the power is down the green High voltage LED is staying 3-4 seconds longer then the rest of the LED

tested al the resistors in the output section but nothing so far that is not normal.

don't know what else I can do...
How can I know that the problem is in the left or the right channel?

[Silverhorse]

Jason, You said that "fault can potentially drive full rail voltage into a circuit designed for much lower voltages"
Could it be that the overload detection circuit is damaged in the power amp section?

[Jacampb2]

Power on sequence should be just o/l led when power is applied. Switched to green power led when the poweramp comes out of mute. Fans should stay on low speed throughout. Thermal should never light. And the amp should start in high voltage mode and only switch to high current when the rail switching curriculum detects that the output current had home to high.

The irfz44s are not part of the 15v regulation, but are part of the rail switching control. Having one shorted is odd, but it seems you also had other parts in the rail switching circuit blown.

I would still be suspect of damaged traces and through hole playing. I see it all the time in these amps. I'd also start working backwards into the poweramp. Both the circuit to detect o/l condition and the one that senses output current to trigger rail switching are in the poweramp.

Good luck,
Jason

[Silverhorse]

think I've spend 6h to desolder and solder in again, with no result.
While doing the testing I also saw that after let's say 30 sec the 4 bias LED's are shutting down.
So the first thing that happens when you power up is the yellow thermal LED is lichting up for 2 seconds then the red overload LED is lighting up and stays up.
The 2 green LED's are also lighting up. After 30 sec the 4 bias LED's go out but the 2 green LED's stay on.
So first there is a detection of heat, but there is no heat? Then there is a detection of overload but the amp is not pulling excessive current?
Man, I think I'm gone give up on this one...

[Silverhorse]

there are 6 thermistor's with corresponding resistors and diodes. The resistors and diodes are ok.
I've putted a jumper over the 6 thermistors but the problem stays the same.
How does the mute functions works? To me it seems that the thermal LED is somehow connected with the mute function?

[Jacampb2]

Any time the amp is in thermal or o/l, the poweramp is muted. Q4 on the differential amp daughter boards is the mute transistor. Basically the way the power supply control is set up is that you should never be able to have a green "power on" led at the same time as either thermal or o/l. If the green power led is off, the poweramp is muted. Both red and yellow have to be off for green to be on. It's a direct result of the control circuitry design.

The amp does not have to be drawing high current to trip o/l. It is supposed to measure output current via voltage drop across the emitter resistors. Out of spec emitter resistors can cause odd behavior, but an issue anywhere in that measurement circuitry will put it in o/l. As for thermal, it's still a simple comparator driven detection circuit. Fault in the feedback, ±15v supplies, ground or faulty components could all contribute.