Parasound A21 assistance

[Paul W]

One of my Parasound A21's developed a problem and, without a schematic, I'm not confident in fixing it...nor do I want to send it back to CA.

Problem description: Amp turns on okay, with the "P" lighting normally. However, the two blue power lights do not change intensity, nor does the relay click. Nothing seems burned so it appears to be simple component failure.

Does anyone have an A21 schematic? Suggestions?

 

[Bensen]

Hi, schematic can be found for example here: https://www.diyaudio.com/community/...plifier-part-iii.318975/page-645#post-5675428

More complete schematics can be found on this forum.

 

[Paul W]

I've tried searching for a more complete schematic, but no luck. Are you able to point to one with a link?

Thanks!

 

[rayma]

Here you go.

 

[Paul W]

Thank you!

 

[rayma]

Good luck, post your progress and photos here.

 

[mark_1]

Hello. I was going to start a new thread but this one came up as an option.

My A21+ needs left & right bias adjusted; with an infrared thermometer, the left heatsink is 63-64 degrees C while the right is 73-74, measured 2 1/2 inches away. I would like to know if the A21 bias adjustment in the attachment provided by rayma is or could be the same for the A21+.

Is there any chance someone could please share the A21+ schematic and bias adjustment procedure if different from the A21? I suspect there are differences, and I really do not want to end up with a door stop. (It's just a month out of the 2 year Canadian warranty and is my chosen end-game amp).

Cheers,
Mark

 

[tomchr]

When I needed a schematic for an A23 I just contacted Parasound and they sent me the schematic. Maybe try the obvious source first. 🙂

Tom

 

[mark_1]

I was going to contact Parasound, but their website suggested checking forums first for out of warranty questions or help. Not that they won't help because I have read that people do find them helpful. I just thought it didn't hurt to ask the good folks here. 🙂

 

[cumbb]

My A21+ needs left & right bias adjusted; with an infrared thermometer, the left heatsink is 63-64 degrees C while the right is 73-74, measured 2 1/2 inches away. I would like to know if the A21 bias adjustment in the attachment provided by rayma is or could be the same for the A21+.

I would take a relaxed view: simply reduce the bias until the heat sinks cool within a reasonable range. Hardly or not at all audible.

 

[mark_1]

Thanks for your encouragement cumbb. I hope I'm not sounding desperate, I just want to tread carefully. And to be honest, it bugs me they are different.

 

[cumbb]

We have the idea that amplifiers, for example, are designed down to the last detail intentionally. This is not the case. And a bias setting is variable. You can also correct it as a developer, e.g. if you notice that the housings are not cooling adequately, or the transformer is a bit warmer than expected, or whatever - nobody watches how things are cooked in the kitchen;-) The specified power values are also quasi arbitrary. Everyone measures something different or sets the parameters as required.
And so on;-)

 

[Michael Chua]

My A21+ needs left & right bias adjusted; with an infrared thermometer, the left heatsink is 63-64 degrees C while the right is 73-74, measured 2 1/2 inches away.

That seems very hot for a class A/B amp.
When biased to 20mV, it's only 90mA per output transistors.

 

[mark_1]

So, I did pop the cover and checked and adjusted the bias ... now they're set about 16.5 mv each or about 75 ma per transistor. And I found the settings were 12.4 and 13.5, realistically, not too different. Heatsink temps didn't change a whole lot either. Go figure. 🙄

I noticed the output transistors are 2SA1295 and 2SC3264 vs 2SA1216 and 2SC2922 as shown on the schematic. Not too sure if the circuit changed much with A21+ because the component numbering looked to be the same, but component values may be different.

I'm babbling now, sorry "bout that. When younger and actually being a tech, I did pcb layouts, built and tested accelerometer and filter circuits, so I got to like symmetry and matching components as best as possible. I guess that's why I would like this amp to be balanced.

If I happen to get the A21+ schematics, I'll try to post them here. Thanks for all of your thoughts!

 

[cumbb]

Have you measured the transistors individually? Maybe there is one or two that are deviating and making a lot of steam.