Dynacord 600 burning transistors

[JohnF]

Hello everyone,

i have a dynacord 600. In the beginning the problem I had was that protect was staying on.
Now, after I change three burnt resistors and two transistors, everytime I put the power it keeps destroying the one transistor Q119.
The transistors I change are the Q119 with the same and Q121 (didn't have the 2N3904 and put the BC549C, hope that is correct) and the resistors are R173, R175, R179.
If anyone can help me with this I would be grateful.

Thank you in advance

John

[The Dude]

Welcome to the place!

Could you please post a schematic? Those circuit designation numbers mean little without one.

[Jazz P Bass]

Not too sure if you aware of it but the two transistors have 'opposite' pinouts.

Looking at the flat front, left to right:
2n3904: E B C
BC549C: C B E

[JohnF]

The Dude the schematic is here:
https://www.google.gr/url?sa=t&rct=j...52174688,d.bGs

Jazz P Bass didn't saw it was opposite, i'm sure i put it the wrong way. But to be sure I ordered yesterday 2N3904.


What else might cause the protection light to stay on all the time (except the parts I changed)?

[Jazz P Bass]

The protect circuit can be tripped by heat, high frequency & Volts DC on the output.

The transistors that failed appear to be a part of the output limiting circuit.
They usually fail when the main output devices fail.

Have you checked the main outputs for shorts (Collector to Emitter is a common short).

Do Not hook up a speaker or a load on the output jack until you have the amp fixed up.

Without a load attached I would look for Volts DC at the junction of D114 & D115.
That would indicate shorted output transistors.

Also, look up & build a 'Lamp Limiter'.
It will limit the overall current draw of the amp while you are attempting to figure out all that is blown.

[JohnF]

I received today the 2N3904 and replaced the wrong one with the correct, but it keeps burning the 2N3906 (q119) when powering on.
I tried twice to replace them and twice the same thing happened.

[R.G.]

You really need to stop using transistors as fuses and think for a bit. If you keep putting in new parts and they keep blowing, then logically the burned out part is a symptom, not an underlying cause. Something else is wrong that is making the protection devices burn up.

A really good first question is "how can that transistor get enough power/current to burn up?" There are only two places it can get current to overheat - through Q116 or Q123. In normal operation, the PNP is supposed to be able to clamp all the current coming through Q116 so that Q123 is starved for base current and the output devices do not get driven. That should not be enough to overheat Q119. This suggests that either Q116 or Q123 has a problem.

You don't say WHICH burnt parts you replaced. In solid state amps like this, it is common for a fault in the output stage to cause a chain of destruction leading back into the input side of the amp. Have you checked the output devices and the associated resistors and diodes to see if one of them is damaged, and telling the protection circuit to stay on all the time?