How fast a 2520 can be destroyed ?

Hi,

I'm building API 312 clone on a prototyping board (which is very painful).
I'm following this schematic :



I use a Cinemag CM-75101APC with a RL of 150k. (I omitted R4, C5, C7 and RL is between (4) and the IN+ of the 2520.)
And a CMOQ-2S for the output tx.

I use the 2520 clone from Peter Purpose under +/- 16.65 volts.
(It's the 2520 which is under voltage, not Peter... :green: )

As usual, I'm "diy-ing" when I have time... the night ! :wink:
I was tired, so I made a few "mistakes" :

1) I forgot to link the ground pin of the 2520 to the ground. :roll:
2) I didn't use the C3 at the beginning.
3) Ah, yes, probably the worth, I had a 68 Ohms between the HI and the LOW of the output TX. It was a part of a attenuator I forgot to disconnect.

Before fixing the problems above (I've never be able to get make the preamp work), I noticed there was a transistor (the metallic one above the out pin) which was getting VERY hot in 15-20 seconds. I noticed it very quickly, so I swiched off the PSU. So the 2520 has never been under power more than 30 seconds.

I've checked, re-cheked and re-re-re-rechecked everything, corrected my mistakes and now, I'm testing it again :

At the IN+ I have between 150mV and 350 mV when I speak in front of a SM57.
At the output of the CMOQ-2S, the sound is thin and distorted.

The gain doesn't work at all. It makes the sound of a "blurry" buzz when I turn it, but that sound goes away when I stop turning it. (Do you follow me ? :green: ) I use a 20k rev log from JLM with a R2=270 Ohms

And again the transistor (btw, is it a transistor ?), the metallic one above the out pin, is getting very hot in 15-20 seconds.

Is it dead ? There is no external sign of burning and it doesn't smell like roasted plastic.
How fast is it possible to destroy a 2520 ?
Is there any simple way to test it ?

Thank you,

eD)))

does JLM sell 20k Rev log? you should buy a kit! :green: :idea:

Actually, it's a a dual 10 k.
Wired in series, you get a 20k !

Count about 3 weeks for the shipping to Montreal.

eD))

Disconnect the transformer primary.

Does it still get hot?

If not, you've possibly been passing LOTS of DC through the transformer primary... with possibly negative effects on the transformer...

Do you have a 'scope?

Keith

[quote author="SSLtech"]Disconnect the transformer primary.

Does it still get hot?

If not, you've possibly been passing LOTS of DC through the transformer primary... with possibly negative effects on the transformer...

Do you have a 'scope?

Keith[/quote]

Keith, thanks for your answer.

How can I disconnect the transformer primary ? By unlinking the points (9) and (10) on the schematics ?

No, I don't have a scope...

eD

I've got -5.67 Volts DC at the output pin of the 2520. :shock:

Connected to the transformer primary, that's a LOT of current flowing. Did you measure that WITH the transformer primary still connected or disconnected? -If it was connected, you'll see that number rise... a lot... when you remove the primary.


-You disconnect the primary by just un-soldering either the brown or the red wire and leaving it 'hanging in the air' -in other words, not connected to anything...

Keith

Ok, I disconnected the brown wire.

The transistor above the PIN OUT is not getting hot anymore...
But I'm measuring -15.78 Volts DC at the PIN OUT :shock:

Yep. You've been sinking massive current into the transformer. one transistor in the output pair is either turned fully on, or short circuit. -With the op-amp out of circuit, do you measure a low resistance between the -ve volts pin and the output pin?

Keith

23M Ohm !

EDIT : my second 2520 gives me almost 27M Ohm...

it is bad ? :roll:

eD

[quote author="vertiges"]23M Ohm !
it is bad ? :roll:
[/quote]

Probably not. What voltage are you seeing at the opamp inputs?

By the way, if you are going to test your opamp again with the transformer in place and aren't sure, I'd maybe add a low value resistor between the output and the primary (say 50 Ohms or so) or maybe a suitably large de-coupling cap. That'll stop you magnetising/frying your transformer and cooking your opamp.

rodabod,

Thank you for joining my "burning party". :wink:

I did the measurements you suggested :
(the brown wire is still disconnected)

IN + : -800mV (not stable, moving all the time)
IN - : -6.275 Volts
OUT : -15.77 Volts
V+ : 16.38 Volts
V- : -16.41 Volts

EDITED : BTW, I unplugged my SM57, I don't know if it's make a difference.

R2 270K !! it should be 200 ohms :shock:
Thats just a "limit" resistor for the gain control.

MM.

Marty,

Actually, it's 270 Ohm. I'm gonna correct it in my first post.

I read somewhere to increase to 270 Ohm when using the 20k JLM pot.

Thanks,

eD

Ed, we've all been there. Rushing to get some sound out of the creation.
I'm guessing the pnp output transistor is fried, but some other stuff may have been taken along with it.
Grab another BC161-16 and try swaping it .
Don't fire it up again until absolutely sure that your circuit is correct.

peter

Not so fast there Peter...

I think that if the PNP output was fried, there'd be a hard-short from the output to the power rail, measurable with the op-amp out of circuit.

If one power rail was not connected, the output would slew heavily towards the other rail... if there was no DC path to either inverting or noninverting input, the output would also typically slew to one or other rail...

The point here is to PROTECT the transformer from DC... the core may already have sustained some magnetisation damage, but we'll get to that later...

DO NOT reconnect the transformer until you have lost the several-volt offset at the output!!!

Now...measure DC voltage from ground at each input, also at each power pin of the op-amp, and then report back.

Keith

[quote author="peter purpose"]Ed, we've all been there. Rushing to get some sound out of the creation.
I'm guessing the pnp output transistor is fried, but some other stuff may have been taken along with it.
Grab another BC161-16 and try swaping it .
Don't fire it up again until absolutely sure that your circuit is correct.

peter[/quote]

:shock: so... you're telling me they are dead...

There are two BC161-16 per board right ?
So maybe I could try to make one board working with the components of two boards...
I'll try, even if these boards are very small and my hands skakes because of the cafeine.

Do you think the fact I accidentally shunt the secondary with a 68R could have caused that ?
Or the missing of the C3 ?

Hey Peter, do you still have some in stock ? :wink:

Thanks for the helps guys ! :thumb:

eD)))

[quote author="SSLtech"]
Now...measure DC voltage from ground at each input, also at each power pin of the op-amp, and then report back.

Keith[/quote]

That's what I did :

IN + : -800mV (not stable, moving all the time)
IN - : -6.275 Volts
OUT : -15.77 Volts
V+ : 16.38 Volts
V- : -16.41 Volts

Thanks,

eD))

You should be able to mock-up a 'test-check' unit to check the rest of the circuit... I stress this because you're not using a PCB butusing strip board or something similar, so there are a few unknowns here, and the possibility of overlooking a simple thing like a power rail connection is all-too-easy... (ask me how I know!!! )

You can use a 5534 or similar as a simple substitution. -It won't drive high levels into the transformer, but if that too slews wildly to one rail or other at the output, your circuit is dead... if it looks good, the 2520 is likely cooked.

Keith

Listen to Keef.. :thumb: