Blown Amp/Mixer

[Trinite]

Re: Blown Amp/Mixer

Makes me concerned that when I fix and sell this thing, I will be getting a complaint that it blew after a few months use...

[Scenic]

Re: Blown Amp/Mixer

i'm starting to wonder how the hell this thing lasted as long as it did...?

40V on that 33V zener as a half-assed attempt at limiting the voltage going to the 7815...
zener took a crap, 7815 got the full 40-ish volts and decided to 'splode a little..

[Trinite]

Re: Blown Amp/Mixer

This just gets weirder. The voltage stored in the capacitors are as follows: 80V for the primary rail and 40V for the secondary rail. As the transformer uses a split tap, the specified voltages apparently only reference one side of the winding to center tap, and not across both. So the unit is running at double the voltage than what is specified on the transformer output.

This is the problem: The 7815 is rated for a maximum of 30V. The input is 40V. It is connected to ground in series with a 1N4752A zener diode (the number I posted previously was a typo). The diode has a reverse-breakdown voltage of 33V. I'm assuming this is some lousy attempt to limit the voltage going through the 7815 VR?

[Th3_uN1Qu3]

Re: Blown Amp/Mixer

1N4007? Man, talk about cheap.

[Trinite]

Re: Blown Amp/Mixer

Pulled the HS off, looked at it more closely. Here is what I found: The VR failed shorted (input to common pin) and that in turn caused the nearby zener diode (1N4572A) to also fail shorted, shorting the common pin to ground.

I hooked it up to power again, only to find that there was another short. One of the diodes (1N4007) on the 18V rectifier failed shorted and was shunting AC through the filter capacitors. Sounded like one of the caps vented too, but I'm replacing those anyway. Quite the catastrophic failure, apparently. After removing the offending diode, everything worked (but I didn't keep on for long, probably not good to do on just half-wave rectification).

All that from a VR failure...

[Trinite]

Re: Blown Amp/Mixer

Likely going to resell it. The previous owner was getting rid of it. I'll probably stick in a ground lift switch (was thinking the same thing you were) and sell it as a "modded" unit.

[Th3_uN1Qu3]

Re: Blown Amp/Mixer

Well what are your plans with this thing? If it were for personal use i'd ground the center tap for normal use and add the appropriate ground lift switch for when it's needed (it is needed sometimes).

[Trinite]

Re: Blown Amp/Mixer

Built-in ground lift! Woohoo!

There is a 75V potential between the transformer center tap and ground

Although I would assume it is not very high current.

[Th3_uN1Qu3]

Re: Blown Amp/Mixer

The power switch is located on the neutral side, leaving everything before it live, even when off.

More common than you might imagine. Here in Europe the mains plug isn't polarized so it can be inserted either way. It's a 50/50 chance which end goes to the power switch since no one bothers with double pole switches.

Also, there is nothing attached to the ground connector.

Built-in ground lift! Woohoo!

[Trinite]

Re: Blown Amp/Mixer

And a few other deficiencies I've noticed. The power switch is located on the neutral side, leaving everything before it live, even when off. Also, there is nothing attached to the ground connector.

[Th3_uN1Qu3]

Re: Blown Amp/Mixer

It looks like it's been cut with a hacksaw.

[Trinite]

Re: Blown Amp/Mixer

From what I can see, all the traces running out of the VR terminate in resistors, the lowest of which are around 460 ohms. Hoping then that it was just a defective part.

As for build quality, this is laughable. I dug a bit deeper into the unit and found an improvised connector cut out of a larger part (great way to cut corners...literally). Check the photos below:

[Scenic]

Re: Blown Amp/Mixer

no idea.. i've never seen a TO92 78Lxx where the short circuit protection actually worked.. it always blew them ._.

the normal 78xx's just shut down and continue working after a soft restart (if the short is gone obviously..)

[Th3_uN1Qu3]

Re: Blown Amp/Mixer

Then why does everyone list it in their datasheets in big bold text? http://www.datasheetcatalog.com/data.../0/78L05.shtml

[Scenic]

Re: Blown Amp/Mixer

the 78Lxx's don't have any protections against short circuits or too high temperature.

all the others do though..
- the 78Mxx (500mA)
- the "normal" 78xx (1A ; the ST 78xxCV up to 1.5A)
- the 78Sxx (2A)
- and the pretty rare 78Txx (3A) in TO3

[Th3_uN1Qu3]

Re: Blown Amp/Mixer

Never seen a reg blow due to input ripple. It might just be the occasional dud part - for example, the 78L05 supposedly has short circuit protection. I tried it out and it left a hole.

[Trinite]

Re: Blown Amp/Mixer

Regulator tend to die by voltage falling off or shorted, and should had fuse on output or 1 ohm resistor just in case. Exploded regulator means output might had shorted out.

Tested the output path, no obvious shorts. Seems that it provides the selection voltage for switching between 12V and 48V phantom power on the mic connections. So no major load on it, apparently. Odd that it would blow. However, it does seem that it draws directly from both 18V rectifiers, one supplying +18V to the input pin and the other supplying +18V to the common pin. Perhaps too much dirty power for it handle? There is a spot on the PCB to add a filter cap, so I ordered one for that.

[Trinite]

Re: Blown Amp/Mixer

Couldn't find any specs on the transformer, but going by weight (2.2kg), it would seem to be comparable to a 250VA toroid. So, more like 250W combined max output for the entire unit - and even that is probably stretching it.

[Wizard]

Re: Blown Amp/Mixer

And really poorly designed!

All the regulators should be on one big plate or finned heatsink (insulated as necessary) and arranged properly. Not willy nilly. And that 4 transistors does not looks like can do 250W per speaker. Not at all. Regulator tend to die by voltage falling off or shorted, and should had fuse on output or 1 ohm resistor just in case. Exploded regulator means output might had shorted out.

FYI: good linear regulators have thermal overload built in, this one is fairchild brand.

Good one tend to have 8 transistors and two even four transformers for that claimed wattage much and 1KW from that transformer. Impossible.

PS: be really wary if the amp transistors are true copy, there were plenty of fakes out there on that particular part numbers were popular ones to copy using really underrated semiconductors. Make sure you get these from good supplier.

Cheers, Wizard

[Trinite]

Re: Blown Amp/Mixer

And they're pushing it on regulator input voltage as well. If the reg is indeed fed from the main rail, there is zero headroom. Check that before you replace it.

The main transformer has two windings. One for the 40V output and another for 18V output. The VR draws from the 18V rail, so it is within spec. It has no heatsink, but there are 3 other VRs that do, so assume the one that blew provided power to a relatively light load (or should at least, anyway).