Cadence TXA-750D? Amp

[mbates14]

I cant remember the exact model number as its not in front of me right now. I think its the 750D But its not the 1000D, its a little bit smaller, only 1 power toroid. But its the same body style, and its a monoblock.

Anyway, it works fine if you use an MP3 player or a PC for a source. But no sound if you use a head unit. You get a good spark when plugging in the RCAs.

So i checked that, there is 45V between the RCA ground and Amp Ground.

Checked the RCA ground on the inside and its 0 ohms grounded to the internal amplifier ground reference, basically the secondary of the toroid. But there is a 45V difference between secondary ground and primary ground.

any ideas? Amplifier works.

 

[Perry Babin]

One of the rectifiers, regulators or one of the output transistors shorted/leaking to the heatsink?

 

[MatthewS]

This has been discussed a few times in the past here. I'm not sure of a great keyword combination to give you for the future though. I would also make it habit to check this prior to plugging sources in if you value your sources.

Have you checked for this voltage presence in the chassis too? or just at the jacks? If it's in the chassis it takes you in one direction to focus on (what perry said with sink mounted devices), if it's not I would check the secondary of the torridal for shorts. If it's already out of the chassis, then start at the later.

 

[mbates14]

Its possible one of the output devices is shorted to the heatsink, BUT if that were the case, Then why does the amp actually still work? Because the chassis is technically ground, and if its ground, then itll crowbar a rail, or am i wrong?

 

[MatthewS]

Because the chassis is technically ground, and if its ground

The chassis is only technically ground if the chassis grounding point is technically still alive.. LOL.. i'd check that while you're in there.

 

[mbates14]

Yea well thats true, I do need to check everything I guess. Someone was in it before me, lost half the screws, and they hooked it up backwards blowing the protection diode in half Which is what its original problem was. Replacing that diode and hooking it up CORRECTLY, it seemed to work but stuck with this issue.

 

[mbates14]

I fixed the screwed head unit afterwords, which I could have avoided. BUT usually i never ever run into this issue. I only ran into it 2 times. 1 time was a PPI with a plexiglass silkscreened cover that would explode when hooked to a headunit and work fine with an MP3 player.

Other time it was just a broken ground on the RCA causing a floating DC-offset

 

[Perry Babin]

You need to protect the head unit's shield with a current limiter (12v lamp, resistors or fuses). Shorted transformers are fairly common and you'll waste a lot of time repairing head units if you don't insert some sort of protection.

 

[mbates14]

Yea, shorted transformers can be a B!tch. I had to junk an Orion beast because of a faulty toroid. Didnt want to put all the time and labor into attempting a rewind considering trying to get it as perfect as the original is next to nill.

 

[MatthewS]

I had to junk an Orion beast because of a faulty toroid.

Next time i'll pay shipping.. send it this way..

 

[mbates14]

Checked heatsink to tab shorts, they are fine. no shorts between main ground and RCA ground. its open. no resistance at all.

The heatsink also appears to be isolated, its not connected to either battery ground, or circuit ground.

 

[mbates14]

Strange. with all the covers removed top and bottom, the 40V goes away.

So, i slipped some insulators under the 3 screwholes toward the front input jacks and pots, and so far its ok.