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PA3004 chip PCB substitute

markthefixer said:
I'm concerned about pin 7 the AC input. It has a definite current draw that affects any voltage divider used with it. The sx-780 for example has a pair of 1 meg resistors set up as a voltage divider. The resultant amplitude of AC at pin 7 strongly suggests that the input impedance of that pin is 10,000 ohms. Changing directly from a PA3004 IC to your substitute is probably going to have excessive AC voltage at that pin. The simple addition of a 10k resistor to ground on the subsititute circuit would seem to be the best cure.

IIAC this should be tested on the chip version first. I discovered this helping someone with their AC detect on the chip.

and, yes I see the diode. but what about negative voltages.
Click to expand...

@merlynski and I discussed Pin 7 quite a bit during the design process! The diode and the NPN should clamp the VAC at ~0.7VAC, no? We worried about an insufficient voltage divider in front of it, but were unable to find any examples in existing schematics. Regardless, we should be able to squeeze another 10K resistor in there if we want to replicate that supply current... since it's labeled MAX, might it just be a max leakage current spec for whatever analog to a diode/NPN combo they were able to get onto a chip back in 1982?

I tested the original chip by putting a 24VAC signal (from a wall wart adapter) on a spare 50K pot I had lying around, and adjusted it until I saw it throw a fault on Pin 3 - I consistently saw it throwing the fault when VAC<1.5Vrms

I had actually only seen the block diagram until now - the waveforms and pin functions table would have been helpful, but I was able to figure out most of the info by testing the chip. I still have it sitting here if more testing is needed!
 
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markthefixer said:
I'm concerned about pin 7 the AC input. It has a definite current draw that affects any voltage divider used with it. The sx-780 for example has a pair of 1 meg resistors set up as a voltage divider. The resultant amplitude of AC at pin 7 strongly suggests that the input impedance of that pin is 10,000 ohms. Changing directly from a PA3004 IC to your substitute is probably going to have excessive AC voltage at that pin. The simple addition of a 10k resistor to ground on the subsititute circuit would seem to be the best cure.

IIAC this should be tested on the chip version first. I discovered this helping someone with their AC detect on the chip.

and, yes I see the diode. but what about negative voltages.
Click to expand...

I appreciate your input. If we need to change something to make it a better 'drop-in' replacement, we should do so.

The transistor BE junction limits the + voltage at that pin. It seemed to me that the 1MΩ resistor on the SX-780, to ground, was to discharge the junction stored charge quickly when the AC vanished. I don't really think a 60Hz (or 50Hz) signal will have a problem with that, but . . .
The diode is there to clip negative half cycles so as not to create a high reverse voltage across the transistor BE junction. I did not see any PA3004 applications that had a negative dc voltage input, instead of AC, did I miss one? Please tell me what unit that is. I did notice that the Pioneer discrete protection circuits used the negative half cycle, half-wave rectified, to detect AC presence/loss, but did not see it as a requirement for the PA3004.
I looked over the schematics I had on hand and deliberately left that pin direct to junctions because of the high resistance in some of the voltage dividers coming into that input.
Can you post a link to a thread that discusses that AC detect issue, please?
I do not know all the units (yet) that use the PA3004, but we are collecting that info in this thread. I guess this is our 'coming-out' party.
 
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Strgazr1967 said:
Thanks to Alex and the others helping me out IT WORKS in my 780.

What I did notice is because of the location of the relay in the 780 in relation to the 3004 the Lin’s could be longer. I had to put a funky bend in them to get them to seat in the board. I am contemplating using small gauge wire to extend the pins and using some form of insulating mount, perhaps under the main board to make soldering easy. I REALLY had to push to get it to clear. But that was a small price to pay for having my 780 playing the blues again! Thanks again Alex! I will be in touch.
Click to expand...

Amazing! I'm so glad it worked - definitely want to see pics too! There is actually an alternative mounting solution - I took some pics to illustrate.

Pic 1 shows what I would consider the "normal" mounting configuration - you push the 8-pin connector in until the plastic is up against the board, and solder the other side. The pins end up being offset from the plane of the board about 7mm in this case.

You can also install the connector backwards, as shown in Pic 2. Once the pins are all soldered, you can then pull off the black plastic part, and have the pins sitting almost exactly in the plane of the board!

You can also install a header such as Mouser # 571-829265-8

Sadly, the original PA3004 chip doesn't fit in these headers, so it's not as useful as I had originally hoped, but it does give you a little vertical clearance, which I think is what you were proposing, @Strgazr1967 ? It's the solution I chose, since I've been swapping these things in and out of my amp for a month!
 

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I am not seeing an issue, as D3 and Q5 clamp the AC to ~ +/-0.7V. I would have thought that some resistance in series could have been used as some form of input protection (for ESD perhaps)
The abs max Ib on the fjv1845 is 10mA so if it has a 1M source R, it will allow a how much supply V before she blows?

I am using these on my SIP adapters
https://www.digikey.ca/en/products/...8ArqpKRyUkIQQGjkEGPEhrziCYuOgAJqg86AC2uNiMokA
 
rcs16 said:
I am not seeing an issue, as D3 and Q5 clamp the AC to ~ +/-0.7V. I would have thought that some resistance in series could have been used as some form of input protection (for ESD perhaps)
The abs max Ib on the fjv1845 is 10mA so if it has a 1M source R, it will allow a how much supply V before she blows?

I am using these on my SIP adapters
https://www.digikey.ca/en/products/...8ArqpKRyUkIQQGjkEGPEhrziCYuOgAJqg86AC2uNiMokA
Click to expand...

Thanks for the link, very timely, I had been thinking of something like that after I saw Strgazr1967's comment about fitment.

I thought about the ESD issue as well. My conclusion was that 1: two back to back PN junctions pretty much protect each other from excess reverse voltage, BJTs are not particularly susceptible, and they are not FETs, and 2: components usually arrive in static safe packaging, so people should ( :rolleyes: ) know to use proper handling practices. If it is produced (and assembled) by an outside vendor it will be up to them to package it properly. Our goal was just like yours, produce open source information and let DIY people benefit from it.
 
Thanks to @johnhoef , we have confirmed that this board works properly on an SX-1280! There are some pics and a report on the website linked from the first post in the thread. An SMD version was used for this test, but from the photos it looks like the thru version would fit as well!
 
Hello. I am currently working on a SX1280 that needs the PA3004 which I just ordered from China for $37.00 including shipping. I would love to construct this board be it SMD or through hole but I don't have any experience in creating my own PC board from scratch, except for once a long time ago in electronics school, but that is primitive compared to boards today. I don't know gerbers from eagles. Could someone point me in the right direction to obtain knowledge in doing this? Going forward it appears the skill to do this will be necessary as replacement parts become very expensive or obsolete. Thanks for any feedback. Tom
 
Hi Tom,

Follow the link in the first post.In that link there are the two links to the zip files that you send one of them to pcb fab in this case, jlcpcb. You will need an account at jlcpcb in order to up load the zip file you want made. Use the quote system to upload the zip file which contains the fab data. Use the pcb default settings and order/ pay ship
 
Hi Tom - if you just want one board to try building the circuit for your 1280, either @merlynski or I can get you one - I got plenty of spares the last time I ordered them! We've verified that the board works on the 1280 so you should be good to go - just let us know if you have SMD soldering capability or not and we can get you the appropriate board. The BOMs for each board are available with mouser part #s on the first post in this thread.

--Alex


Tom A. said:
Hello. I am currently working on a SX1280 that needs the PA3004 which I just ordered from China for $37.00 including shipping. I would love to construct this board be it SMD or through hole but I don't have any experience in creating my own PC board from scratch, except for once a long time ago in electronics school, but that is primitive compared to boards today. I don't know gerbers from eagles. Could someone point me in the right direction to obtain knowledge in doing this? Going forward it appears the skill to do this will be necessary as replacement parts become very expensive or obsolete. Thanks for any feedback. Tom
Click to expand...
 
rcs16 said:
Hi Tom,

Follow the link in the first post.In that link there are the two links to the zip files that you send one of them to pcb fab in this case, jlcpcb. You will need an account at jlcpcb in order to up load the zip file you want made. Use the quote system to upload the zip file which contains the fab data. Use the pcb default settings and order/ pay ship
Click to expand...
Thank you Alex, I appreciate the information.
 
alexhastings said:
Hi Tom - if you just want one board to try building the circuit for your 1280, either @merlynski or I can get you one - I got plenty of spares the last time I ordered them! We've verified that the board works on the 1280 so you should be good to go - just let us know if you have SMD soldering capability or not and we can get you the appropriate board. The BOMs for each board are available with mouser part #s on the first post in this thread.

--Alex
Click to expand...
Thanks Alex for the offer. This aspect of electronic repair is new to me. I'm pretty good at tracing out defective components, but I'm not an engineer. I am grateful there are people like yourself that can make it possible to keep these units working. I'd like to get familiar with the pcb making process so I'll send the files to JLC. If I run into trouble I'll be asking for help. Tom
 
Well done, sir!

I'd be interested in a through-hole board if you have any left. The OEM unit in my 980 is working OK, but you never know...

- Jim
 
Thanks to Alex for information. i work in my pa3004PCB for a SX780. and i have a question : in the position Q9 is a NPN 512-KSC1845FTA is correct? thanks and regards Monterrey Mexico.
 
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