DAP AUDIO QI-4400 Goes In To Protection Mode - Blown Cap

[RetroComputingGrotto]

I've got a DAP AUDIO QI-4400 Amplifier on my workbench. When you turn it on, it goes in to a protection mode that a lot of these bits of kit seem to have. On opening up the unit I could immediately see a blown cap, so I replaced it and powered it back on. It still went in to protected mode (so there's still something wrong with it) and after about 10 minutes the cap I replaced went bang! Just in case you can't see it, it's the one I've drawn a black cross on so I dont forget which one it is!!





You have to remove the metal housing in the middle as the PCB the cap is on is the whole width of the device and is a pain to remove as there are lots of cables plugged in to it all over the place.

My only experience of Amplifiers is the tiny ones you get in powered speakers so any pointers on where to start testing/looking are greatly appreciated. I'm on a Learning Curve with this one so please bear with me!

[CapLeaker]

Whatever it is, it must have drawn some current to melt the connector next to the cap. The cap exploded, because it was in backwards or the output voltage of the PSU is larger than the cap is rated for.
Protection mode usually kicks in when there is DC detected on the output. That means that pre driver, driver, and finals are suspect.

[RetroComputingGrotto]

Whatever it is, it must have drawn some current to melt the connector next to the cap. The cap exploded, because it was in backwards or the output voltage of the PSU is larger than the cap is rated for.
Protection mode usually kicks in when there is DC detected on the output. That means that pre driver, driver, and finals are suspect.

Thanks for the reply. It's difficult to see in the photo but the connector isnt melted, it's just covered in glue and the reason you cannot see any glue on the connector to the left is I managed to get it off on that one but not the other one - all of the small connectors on this board have some sort of glue on them - it's only the large power connector on the right that doesnt but that has a clip on it to stop it coming loose.



I have disconnected the power connectors that go from the bottom board to the top and (assuming 89V instead of 80V is OK) they all seem OK?



-65V
-89V
-89V
0
89V
89V

11.7V
0V
11.9V

23.9
-23.9
0
24.3

Unless someone tells me otherwise, the voltages going to the top board appear to be OK.

[RetroComputingGrotto]

I've just noticed that quite a few surface mount resitors have seen better days!




Bearing in mind I work in a School and the speakers I have recently been repairing were pretty much damaged by things (and I kid you not) like nails being inserted through the bass hole causing components to short out and go bang, do you think a power surge has blown these resistors or could it be liquid damage through the holes at the back? Around the time this stopped working we were having some work being done on the whole sites electricity supply (they were going around testing the fuse boards by turning then off/on for whatever reason) so could that have blown them do you think?


Are these the correct replacements for the above SMD R050 resistors?

https://uk.farnell.com/vishay/wsl251...3ljGpzh5ekF7Mq

[stj]

scope the power rails, i think some AC is getting through a diode.
that or a voltage is spiking above what the cap can handle
and obviously check all the output silicon for breakdowns.

[CapLeaker]

I wonder what the cap is rated at?
some of the surface mount resistors need to be replaced (the ones on top) the others, just need to be taken off, area cleaned and with fresh solder and flux back to the board (the ones one the bottom). Haven't had time to look for a service manual for this amp yet…
I am not sure what caused the amp to malfunction. At least not in this stage. I don't think it's liquid damage, almost looks like heat and not enough ventilation. I used to cool my amps with a big fan blowing from top down back in the hey day. Also it looks like someone tried to repair it and did a miserable job (the resistors on top),
I see may fair share of weird things myself that I can absolutely not explain like oil mist getting into a power supply backwards of the flow of a quite speedy fan etc.

Measure those transistors on that large heatsink. Hmm… quite small footprint for an amp with 4x 400w power,,, must be class d.

[RetroComputingGrotto]

Nobody has said a repair has been previously attempted as I'm the only person in the whole company who is authorised to attempy any kind of electronic repair - so I have (perhaps naively) just assumes the solder blobs on those resistors were due to heat from whatever fault occurred - could it have been a power surge from a thunderstorm for example?

I'm going to remove them with hot ait shortly to see if theres any damage to the board underneath.

[RetroComputingGrotto]

I had to use both both hot air and my iron on my rework station to remove those resistors as the board was really sucking the heat away and I am now 100% convinced someone has previously attempted a (bad) repair of this board! A couple of the resistors are so badly blown, they have split in half and the solder used for the attempted repair has a different melting point to what was used in the manufacturer of the PCB.

Not got around to testing any other components just yet as my CPC delivery turned up and I have been soldering together a IR Remote Tester Kit as we have to use our mobile phones at the moment to check if a remote control is working or not which is less than ideal.

[stj]

if something is effected by a thunderstorm you will know it!!
you will have plasma burns and metal oxide deposits all over the psu area!!!

[RetroComputingGrotto]

The transistors are A92 B331 but they dont appear to readily available so I assume they have been replaced with something else - what would be the correct ones to purchse from the likes of CPC or RS Components that would be a good replacement if any are not functioning as they should?

[CapLeaker]

Are the final output transistors damaged (shorted, open etc)? Maybe look at MJEsomething transistors.

[RetroComputingGrotto]

Are the final output transistors damaged (shorted, open etc)? Maybe look at MJEsomething transistors.

Not had a chance to check yet, this is a side project as the unit has been replaced with a new one so I'm dipping in to trying to repair this inbetween fixing other things. I've got to order some of the resistors that have blown so thought I would order a couple of transistors in case I needed them as I hate having to pay £6.99 delivery on some resistors that cost a lot less than that and you get free delivery with orders over £20 with CPC and £50 with RS Components - I'd hate to order some of the resistors and pay delivery to then order some transistors and pay delivery costs again that's all.

[RetroComputingGrotto]

Finally (after waiting ages for our Finance department to process my Purchase Order Request) the parts I ordered have been delivered and I've had a go at using solder paste for the first time - the SMD resistors are a little wonky in places but they are soldered securely to the PCB and test OK on the multimeter so I'm happy with that.

Good call on the transistors as the three by the blown components were indeed open circuit so all have now been replaced.



I plugged the amp in to my isolating transformer ready for testing...

KABOOM!

OK, not quite that bad but a thermister on the power board decided to go pop! Everything else looks good so I guess I'll have to do some testing and get a thermister ordered.



UPDATE ON ORIGINAL FAULT: After speaking to a few people it turned out something was incorrectly plugged in to this amp and another amp at the same time and thats when this one went pop and stopped working.


Even if I dont get this back up and running it's been a great learning opportunity as I've now got a very small amount of soldering experience using solder paste for use on SMD resistors.

[R_J]

Check if the relay contacts are across the NTC
The ntc is for inrush protection, after a few seconds, the relay should engage and short out the ntc, I suspect either that circuit is not working or the relay contacts are burnt and not shorting the ntc

[RetroComputingGrotto]

Check if the relay contacts are across the NTC
The ntc is for inrush protection, after a few seconds, the relay should engage and short out the ntc, I suspect either that circuit is not working or the relay contacts are burnt and not shorting the ntc

I've never had to replace a relay before so hope you dont mind if I ask a silly question... it says 12VDC, 16A and 250VAC on it (JQX-11 F is the code on it), is the one I've found pretty much the same thing as I can't find anything with JQX-11 F?

https://uk.farnell.com/omron/g2rl-1a...16a/dp/1695877

[RetroComputingGrotto]

Check if the relay contacts are across the NTC
The ntc is for inrush protection, after a few seconds, the relay should engage and short out the ntc, I suspect either that circuit is not working or the relay contacts are burnt and not shorting the ntc

I have removed the relay from the PCB and tested it as follows:



I believe the 370ohm reading is correct for the coil end and if I apply 12V to it I can hear the relay click. I'm unsire if the 0ohm short circuited switch is correct or not for this relay, I was kind of expecting them to be open when there was no 12V voltage on the coil and to only be shorted when 12V was applied but like I say this is my first time ever coming across a relay in a circuit!

[RetroComputingGrotto]

UPDATE:

I've stolen a couple of crocodile clips from our Science department so I have been able to apply 12V to the coil and test the switch part of the relay for continuity and regardless of whether 12V is applied or not, the relay switch stays closed.

I shall replace the relay and the NTC - is there anything else I ought to check/replace before testing it after I've replaced these two components? Many thanks for all your guidance/help so far - I'm learning loads.

[stj]

maybe you should dim-bulb it next time.

[RetroComputingGrotto]

maybe you should dim-bulb it next time.

Good idea! Bit of a headache sourcing a suitable light bulb as when you try and search for a correct one thousands of LED ones come back which are no use! It's company policy not to buy from the likes of eBay so I'm stuck with CPC, Farnell, Amazon and RS Components who all have thousands of light bulbs but for the life of me I can't find a suitable incandescent one!