Quad 44 quite warm to touch

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07-Jun-2019 04:29 PM

jonintaiwan

jonintaiwan

Posts: 10

Just finished revision of Quad 44, mostly seems okay now, although I have one or two faults in the input stage, such as disc input only working on one channel. I shall probably have to take the lid off and check my soldering.

Anyway, the Quad 44 seems to get quite warm to touch, which surprises me, as both the Quad 33 and 34 stay as cool as a cucumber when I use them. So I just wondered if this is cause for concern, or normal for the 44?

 Thanks

Jon

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08-Jun-2019 08:57 AM

EJP

EJP

Posts: 1638

No it is not normal. Sounds like you have a DC short somewhere. Try removing the disc board that doesn't work and see if that fixes the heat problem. If it doesn't, try removing the others one by one. If one of those removals fixes it, there's your culprit, otherwise it is on the motherboard or the tone board. Also measure the DC rail voltages, should be about 15V each.
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08-Jun-2019 11:05 AM

jonintaiwan

jonintaiwan

Posts: 10

Thanks, that's useful to know!

Time to remove the lid again...

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18-Jul-2019 05:46 PM

jonintaiwan

jonintaiwan

Posts: 10

Eventually I asked a local (Taiwanese) professional to check the quad 44, and I think he replaced a selector chip, I think he said the selection wasn't working properly (although my Chinese might not be good enough to verify this). Anyway, all channels are working nicely now. So that solves the input problem.

Concerning the heat coming from the motherboard, I didn't get any response, and maybe the temperature is normal - I should note that ambient temperature here is now around 30 degrees C.

Anyway, ignorant as I am, I opened it up to see where the warmth seems to be coming from. I noticed that R406 and R407 seem to be hot to touch, so I wonder if this is normal considering the warm climate here? DC rails measured 14.7 volts, plus and minus, so that seems about right.

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19-Jul-2019 02:41 AM

EJP

EJP

Posts: 1638

Aha, that narrows it down considerably. I think it is one or more of the four 4066 chips on the motherboard. Sounds like your guy already replaced one of them. See if any of them is warm to the touch, carefully, or go back to him and ask him to check them all, and their sub-rail voltages, which should be +/-7.5V. Probably he just needs to replace another one, or maybe all three remaining ones. Also get him to check the 7.5V zeners in that subrail supply.
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19-Jul-2019 05:10 PM

jonintaiwan

jonintaiwan

Posts: 10

Okay, so those resistors and the diodes seem to be pretty hot to touch, but not the ICs. I will take it back to the shop for another check.

Thanks for the advice so far!

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21-Jul-2019 07:25 AM

EJP

EJP

Posts: 1638

Sounding like one or both of the 7V5 zeners in that case.
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25-Jul-2019 03:33 AM

jonintaiwan

jonintaiwan

Posts: 10

Thanks, I have returned the Quad 44 to the repair guy. He was a bit sceptical about the heat scare, saying that because it's the power supply for this section, some warmth is normal, but on my request, he promised to check the zener diodes anyway, and anything else which might need checking. I'll update when I get it back in due course.
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