Untouched 400A With Channel Blown --Easy Fix?

[Rob in Montana]

Hi All--

I reviewed all the "400A" threads and did not see this issue covered. So, I hope one or two experienced 400A lovers here will offer some guidance.

I just received a 400A on Wednesday. I hooked it up, and got a R channel but not a L channel. Rail fuse for L channel was blown, and I noticed it was a 6 amp not the 4 amp as called for. Replaced fuse, and it blew again. Previous owner says amp was working fine last time he used it, about 2 years ago. I think it possible that: a) something got stressed in shipping, b) condensation shorted something due to the amp still being cold from being outside when I tried it, or c) the L channel shorted from the #@##$ huge spade lugs on my speaker cables.

Upon inspection the amp looks pristine inside, as if it has never been opened before. The only visible problem is one "blown out" carbon resistor on the back of the output transistor that sits at the extreme front on the bottom row, just next to the power switch.

I would very much like to fix this myself. I have done a bit of DIY including building amps (from very good directions!) and am pretty good with an iron. But I am a parts swapper NOT a troubleshooter. I will also replace the electrolytics, maybe go "high end" on the coupling caps, and upgrade the inputs and outputs but other than that would like to keep this in stock, classic condition.

So here's my question:
What is likely damaged and needs to be replaced in order to get the left channel back, and how do I troubleshoot it to make sure?

[john65b]

Aha...you outbid me on that amp by a couple bucks...dang (it was on ebay a couple weeks go, correct?)

Anyway, if they are by the output devices, and looks to be a 5W, should be the emitter resistor (sorry, that's about as helpful I can be).

[GRollins]

It sounds as though the seller was, perhaps, a little less than honest in saying that the amp was in good condition. That 6A fuse didn't jump in there on its own. Whether this means you want to get your money back or not is up to you.
I'd expect one or more blown output devices. Begin your search with the one with the suspect resistor.
I don't believe the 400A used coupling caps.

Grey

[Rob in Montana]

So... sounds like the problem is typically confined to the resistor or the resistor and transistor. So guess the way forward is to test the transistor at the blown resistor. If that's not blown then good chance all it needs is to replace the resistor. And if that transistor is blown, then check a few more to make sure the problem is not spread beyond that one. Have I got it right?

[Rob in Montana]

Originally posted by john65b
Aha...you outbid me on that amp by a couple bucks...dang (it was on ebay a couple weeks go, correct?)

Yea, John. Probably was the same one. Sorry bout that!

[Nelson Pass]

Originally posted by Rob in Montana
So... sounds like the problem is typically confined to the resistor or the resistor and transistor. So guess the way forward is to test the transistor at the blown resistor. If that's not blown then good chance all it needs is to replace the resistor. And if that transistor is blown, then check a few more to make sure the problem is not spread beyond that one. Have I got it right?

Exactly what resistor was it, what value and location?

[OliverD]

Pictures would be helpful. They also entertain and motivate us to help

[john65b]

Yea, John. Probably was the same one. Sorry bout that!

Quite alright - sounds like you saved me a bit of aggravation anyway.

I am aggravated enough with my &#@$ Volvo... not the car they used to be. Mine seems to breaks down just sitting in my driveway.

[Rob in Montana]

Thanks for the reply Nelson. The only visible problem is one "blown out" carbon resistor on the back of the output transistor that sits at the extreme front on the bottom row, just next to the power switch. If I am reading the code right this is a .69 ohm? Its the same as all the others on the back of the transistors.

Ok doing my best to entertain and motivate here!! What you see in the picture is the bottom of the 400A with the bottom plate pulled back. For reference, note the power switch. In the center of the photo is the blown resistor and just to the right of that, the transistor it supports. To the left of the resistor is the bottom of the power switch.

[Rob in Montana]

Just need confirmation that it makes sense to simply replace the resistor. Or is it a sure thing other stuff is fried (for example, the transistor behind the resistor)?