Amiga 4000 audio problem (one for castellen)

[countzero]

I got this A4000 mobo with audio problem (no sound from both channels) off ebay some time ago (as-is, grrr). Recently I got some time to fiddle with this baby and see what's wrong with the audio. First I started with the caps, and I desoldered all caps and fit in new ones. unfortunately this didn't solve my problem, there was still no audio. So I proceeded to the next step and measured the voltages on some components as explained by castellen's excellent repair guide here. Unfortunately, I got some very strange readings. I measure -9v where I should have +12. Most other levels are screwed too. I'ld be glad if someone could give me an advice what to do next. Are u402 and u403 dead ? or do I have some short somewhere which messes with the 12v line ?

here are my readings,
U402
1- -8
2- -8
3- -6
4- -9
5- -6
6- -8
7- -8
8- -8
9- -1
10- +2.5
11- -12
12- +2.5
13- -1
14- -8

U403
1- -8
2- -8
3- 0
4- -12
5- 0
6- -8
7- -8
8- -9

PSU seems to be fine.

[Zac67]

Check power on U402 & U403 - U402 pin 4/U403 pin 8 should be +12V, U402 pin 11/U403 pin4 -12V.

Not sure about the exact voltages, but should be >= +-9V - and must be symmetric.

If the power is OK, you can trace the audio signal originating from Paula with a headphone: connect the sleeve to ground with a crocodile clip and touch the tip to the contact you want to check.

PS: hadn't read your post properly - seems you've got a short to -12V. If C405-408 are AOK, check R403 - may be hot (a short somewhere else) or high (doesn't feed the +12V). The schematics don't tell the value, but it should be the same as R404 (I guess ~47Ω). Oh yes, the voltage differential over R403 shouldn't be much (<2-3V).

[rkauer]

 Seems like the cable coming from the mobo to the audio port is inverted!

 Check it out.

[Castellen]

@countzero:
Thanks for the Email, I usually read A.org every day, but I was away at Jeff Wayne's War of the Worlds live orchestra at the time of this thread.  Most impressive thing I've ever heard, pity they didn't make a better job of the official website though....

Anyway, your audio problem looks simple enough.  Either there is no +12V actually coming into the motherboard, or one of the PCB tracks around the audio area has been corroded open circuit.

You can check for motherboard +12V at pin 9 on the serial port.  Though the corrosion problem is more likely if you've had leaking capacitor issues.

The 12V track in question runs underneath U402 and U403, plus through several vias which are right beside, and under, the capacitors which often leak.
If this is the case, you really need to unsolder both op-amps to trace and repair the problem.  Plus you'd obviously need to replace all of the dodgy capacitors.  Chances are the op-amps need replacement anyway if they've been subjected to electrolyte damage.

If you can't manage the SMD soldering yourself, I'd be happy to take a look for you.  Amiga repair turnaround times are approx 3-5 days at the moment, excluding return shipping time to New Zealand.


Cheers,
Anthony.

[countzero]

I've solved this issue a while ago, it was missing 12v as castellen put it. Strangely, problem was a fix done by the previous owner that escaped my attention, near the power socket (totally unrelated to audio area). Appearantly 12v track along with a diode burned out and the previous owner replaced it, but the solder was dead on one end, it wasn't inducting. 12v was missing from the whole board. After a quick fix, my a4000 blasted my speakers with pure joy after god knows how many years

now I'm unto another problematic a4000 board which has a mouse port that doesn't recognise horizontal movement. Sounds easy enough but I couldn't locate a problematic part at the first look. might bug you people again in a short while...

many thanks for all the replies.