# Longshot: Yorkville AP1020 Power Amp Trouble



## sammyr (May 7, 2013)

Hi everyone, 

This is a longshot but I'm curious as to whether or not anyone can help me here. I've got a friends Yorkville ap1020 power amp (http://yorkville.com/legacy/power_amplifiers/product/ap1020/). One channel works just fine but the other other channel has a fault and is frying speakers. 

The clip led is always on the faulty channel and when I opened it up I noticed one of the tantalum resistors was fried (r45, 6190 ohms tantalum I think). I've marked the resistor on the schematic. My knowledge of solid state devices is pretty limited, does anyone have an idea why that resistor would be fried? I'm going to order replacements but am curious to get some insight before I replace it (in case it just refries itself). 

The service manual is available on the yorkville website and I've attached a schematic with the suspicious area marked in red. If anyone can advise you would be my hero.


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## Cups (Jan 5, 2010)

I have limited knowledge as well but it looks like it helps bias a part of the power section. There's not a lot of volts going through it but there is probably a good amount of current. 
There is no such thing as a tantulam resistor. There are tant caps though.


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## jb welder (Sep 14, 2010)

Resistors don't fry themselves, there has to be something else wrong. These are not simple amps and beyond repair by novices. You may end up causing more damage by starting it up if you only change the resistor rather than finding the source of the problem.


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## sammyr (May 7, 2013)

yeah I'm most definitely in over my head here. I shotgunned the bjt's electrolytics burnt precise resistor some shorted diodes and burnt resistors in the zobel network but I've yet to fire it up. I don't have a variac that can handle it so it's likely going to go off to a friend who has a better understanding of this than I do.


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## dtsaudio (Apr 15, 2009)

This is a symptom of a much larger problem. It is part of the bias circuit and is most likely a sign of at least a blown driver transistor, and quite likely a blown output transistor(s). 
* JB is correct, this is not a repair for a novice*. Don't even try to replace that resistor. Doing so, when turning on will almost certainly burn it again, and will probably cause more damage.
FYI it is a metal film resistor.


Cups said:


> There is no such thing as a tantulam resistor


Yes there are tantalum resistors. Rare and expensive and not used in your average pro or consumer gear.
http://www.hificollective.co.uk/components/shinkoh_resistors.html


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## Cups (Jan 5, 2010)

I stand corrected. Learn something all the time


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## sammyr (May 7, 2013)

the tantalum resistor thing was new to me to, I'd never heard of them but they came up when I googled a 6190 ohm resistor. I'll have to take this in to someone qualified.


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## dtsaudio (Apr 15, 2009)

That is a weird size. Not standard but you can buy all kinds of laser trimmed resistors these days. Chances are you can get away with 6200 ohm with no issues.


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## jb welder (Sep 14, 2010)

I would think if you take it to L&M the will send it straight to the factory for repair. I'd expect the price to be reasonable and think they should know as much about them as anyone.


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## sammyr (May 7, 2013)

I'm sure l&m can fix it but I prefer to take things in elsewhere when I can. I find L&M usually takes forever and its been discontinued/not under warranty so that can't help either. I've got it over at a friends who understands this stuff better than me to see if he can figure it out. I'd rather support the little guy.


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## sammyr (May 7, 2013)

thanks for the input guys I appreciate it.


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## jb welder (Sep 14, 2010)

I mentioned L&M under the assumption that they do not repair them on site, but send them straight to the Yorkville factory. That is how they used to do it (for GTA stores), maybe things have changed. The repairs were not supposed to be big money makers, but as a support service of Yorkville product. 
If your friend has experience repairing solid-state power amps, he should be fine with it. From your description of the damage, it doesn't sound like it has a smoked board or anything.


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## sammyr (May 7, 2013)

You're right on the money jb, they send all their electronics to yorkville and do the guitar repairs in the store. I know at least for the toront0 location that's the case. I just prefer to avoid using long and mcquade if I can. I like to be a little more involved and know what's going on. The head of the service dept at Yorkville is Guy Beresford (of GBX amps if you remember those) he's a nice fellow. I was able to pick his brain a few weeks ago when I had a gbx amp came through and had some questions about the schematic. Yorkville and their people are great I have no qualms about them I just find its a pain in the ass dealing with the l and m store in toronto.


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## nonreverb (Sep 19, 2006)

I strongly agree with JB on this. I've worked on these in the past and they're very unforgiving. If you don't find all the bad components and there usually are a few, you may be spending an inordinate amount of time and money trying to solve it.


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