Akai AA-1020 burning resistor woe! - diyAudio
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Old 14th March 2016, 09:52 PM   #1
johnm is offline johnm  United Kingdom
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Default Akai AA-1020 burning resistor woe!

Hi folks!

Hoping somebody can help me to fix up my Akai AA-1020 which has been (hopefully temporarily) put out of action due to my own stupidity!

The amp had been recapped recently, adjusted, and was working beautifully. Sounded lovely. The only problem was the old headphone socket was cracked and resulted in intermittent connection with the jack inserted. Fortunately I sourced another that was an extremely close match. Did a test fit of the new socket to make sure… and forgot I'd left it there... loose! Yep (slaps forehead!).

Had just turned off the power (thankfully, or it might have been worse) after a listening session, and the socket decided to fall into the internals at that point, creating a spark and there was a loud pop. As I said the power was off, so it was either residual charge left in the reservoir cap, or from touching the live terminals of the on/off switch (probably the most likely). Obviously there must have been some short. The spark was large enough to burn off a corner of one of the solder tags on the new headphone socket as it fell to the chassis base!

Whenever the amp is turned on now, R16b (150R) starts smoking almost immediately. Replaced it with a new one and the same result, so obviously I've really screwed something up. This is alas out of my league really, so was hoping somebody might be able to look at the circuit diagram's below, and the PCB layout (smoking resistor circled in green) and spot a potential component to check and change. I'm hoping the transistors haven't blown from that short, but sod's law being what it is I'm guessing that's what's happened.

I can’t take any voltage readings on the amp board at the moment as that resistor starts heating up immediately. Have checked for any obvious burn marks on all PCBs (there aren’t) and there’s no sign of any blown components, at least not visually.

Main PSU and Amp PCB circuit (rest of the circuit not shown is for tone control and RIAA boards):

Click the image to open in full size.

Amplifier Board:

Click the image to open in full size.

What it looks like for real:

Click the image to open in full size.

Where to start?

Any help would be very gratefully received

Thanks.

- John

Last edited by johnm; 16th March 2016 at 11:51 PM.
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Old 14th March 2016, 09:55 PM   #2
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Check TR4, TR5 and TR6 plus surrounding resistors.
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Old 14th March 2016, 09:55 PM   #3
johnm is offline johnm  United Kingdom
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P.S. What looks like burned power resistors - the rectangular 'white' ones at the rear - are merely a stubborn coating of nicotine! The previous owner must have been a human chimney!
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Old 14th March 2016, 09:57 PM   #4
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Quote:
Originally Posted by JonSnell Electronic View Post
Check TR4, TR5 and TR6 plus surrounding resistors.
Jon, thanks for the very quick reply

How do I go about doing that if the resistor starts smoking right away? Do I need to desolder those transistors to test them? My testing knowledge is regrettably rather basic I'm afraid :/
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Old 15th March 2016, 02:10 PM   #5
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You can do some basic checks in-circuit using the trusty diode test function.
pn junction with >> 0.7 V? Probably open.
Looks like a dead short? Probably is.

Note that these VD1222 bias diodes (D1) can go intermittent (open) at times. A chain of three small-signal diodes in parallel may be used to troubleshoot such issues.

The output stage looks like it may benefit from a Baxandall diode... would require redoing bias though.

This amp probably is slower than it would have had to be since they skimped on an output inductor.
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Old 16th March 2016, 12:05 PM   #6
johnm is offline johnm  United Kingdom
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Thanks for your reply sgrossklass
Yes I'm going to check all the transistors. Have looked at a few youtube videos so think I know what to do now. I'm assuming they're all blown but fingers crossed anyway.
Have been Googling the VD1222 diode replacement, and it appears the common advise is to replace with a series pair of 1N4148s instead, so have ordered a batch of those. The original is deliberately angled very close to the heatsink, so will do the same with the 1N4148s but will insulate with a thin-walled heatshrink sleeving to prevent any shorts.
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Old 16th March 2016, 11:44 PM   #7
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Ok I've been testing the transistors and all but one of them I'm sure need replacing. I'm going to desolder them all tomorrow and check again just incase being in-circuit is affecting readings with my Amprobe 240 meter.

So planning ahead (if need be) I'm unsure whether to try and track down new old stock transistors for the following, or get modern equivalents. I'm not looking to 'improve' anything as I loved the sound of this amp as-is, so any advise on substitutions (or whether to stick with originals perhaps from a donor amp) would be great, thanks.

Transistors:

TR3 / 3b = 2SD438E (currently installed on PCB) (2SC1166O or Y is indicated on schematic).

TR4 / 4b = 2SB560D (currently installed on PCB) (2SA661O or Y is indicated on schematic).

TR5 / 5b / 6 / 6b = 2SD371Y (120-240 hFE) or 2SD371O (70-140hFE).

D1 & D1b (VD1222) actually both test fine, but will replace each with a series-connected pair of 1N4148s to be safe.

I think I'll replace the 5 watt 0.47R resistors R17, 17b, 18 & 18b as they all look a little sorry for themselves. Do I need to use the same brand/type (MPC71s) or would any 5W 0.47R power resistor do?

Thanks,

John.

Last edited by johnm; 16th March 2016 at 11:48 PM.
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Old 17th March 2016, 06:13 AM   #8
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Quote:
Originally Posted by johnm View Post
Ok I've been testing the transistors and all but one of them I'm sure need replacing. I'm going to desolder them all tomorrow and check again just incase being in-circuit is affecting readings with my Amprobe 240 meter.

So planning ahead (if need be) I'm unsure whether to try and track down new old stock transistors for the following, or get modern equivalents. I'm not looking to 'improve' anything as I loved the sound of this amp as-is, so any advise on substitutions (or whether to stick with originals perhaps from a donor amp) would be great, thanks.

Transistors:

TR3 / 3b = 2SD438E (currently installed on PCB) (2SC1166O or Y is indicated on schematic).

TR4 / 4b = 2SB560D (currently installed on PCB) (2SA661O or Y is indicated on schematic).

TR5 / 5b / 6 / 6b = 2SD371Y (120-240 hFE) or 2SD371O (70-140hFE).

D1 & D1b (VD1222) actually both test fine, but will replace each with a series-connected pair of 1N4148s to be safe.

I think I'll replace the 5 watt 0.47R resistors R17, 17b, 18 & 18b as they all look a little sorry for themselves. Do I need to use the same brand/type (MPC71s) or would any 5W 0.47R power resistor do?

Thanks,

John.
Touching those heat sink mounted diodes is asking for trouble. I would really leave them alone. As for nos transistors - there are none, plenty of fakes, though.
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Old 17th March 2016, 11:04 AM   #9
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For TR3/4, it looks like the trusty BC639 + BC640 should sub in fine.

As for the outputs, read the suggestions here. Swap out both pnp + npn.

If you need to take the board out, that might be a good opportunity to give it a good scrub with soapy (diskwashing detergent) water, sparing the trimpots as well as possible. Getting water back out of pots and inductors and other stuff like that tends to be a royal pain.

As for the VD1222 replacement, yes it's a double-edged sword. On the one hand and intermittent failure of these can cause the output stage to blow up like it's done here, though an oxidized bias pot could do the same (a check is definitely warranted). On the other hand it seems temperature coefficient with ordinary small-signal diodes may be different. I guess if all fails you could just rig up a Vbe multiplier instead, more flexible anyway.

I take it you do already have a bulb tester...?
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Old 17th March 2016, 12:14 PM   #10
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TR6 B-E junction together with R18 should clamp the voltage across the 150 ohm to less than 1 volt. So we can for certain that one or both these components is faulty (open) and/or print has been zapped.

There will be more wrong beside that but the above is a definite.

And use a bulb tester while fixing this.
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