SAMSON DB500A Repair.

[capwizard]

Re: SAMSON DB500A Repair.

How to adjust the BIAS POT of amplifier.

Getting Bias Current of an Output Transistor of an Amplifier Circuit
http://ronelex.com/bias-current-outp...amplifier/156/

[capwizard]

Re: SAMSON DB500A Repair.

Thanks budm, You could fix it in 5 minutes, I fixed it but I am not happy, I could kill this amplifier by accident. AII THAT NEEDS TO DO IS ADJUSTED BIAS POTS. Who did that to me? It is so hard to troubleshooting nothing-wrong-amplifier........and It's not funny......

[capwizard]

Re: SAMSON DB500A Repair.

budm said "But did someone adjust the BIAS pots?" and I said " the BIAS pots still has red seal on it.", but after I adjust the pot all the current is normal. Who moved it?

[capwizard]

Re: SAMSON DB500A Repair.

Some voltage measurement. Q12 collector voltage is wrong!

PS: take out Q12 tested is OK?

[capwizard]

Re: SAMSON DB500A Repair.

"what happen to the -15V supply problem?" I resoldered back Q9 and Q11 then -15v is OK!

"As I indicated, you have two power amp: one power amp for the woofer and the power amp for the Tweeter/mid range that you are playing with, as I indicated, you should get sound from the woofer speaker."

I try to do the half-assed job and charge half the price to the customer, instead spend more time to troubleshooting the Amp .................

"Open Q10 Base is like open up the switch so no more current flow through Q10, Q9, Q19 due to no BIAS so no heat.. It is like turning off the power switch to the light bulb, the end result is the bulb does not get hot any more."

I thought that last night, the Current Chart shows slowly current went up is that A Miller Effect? or amplifier noise resonance problem or capacitor charges up??

[budm]

Re: SAMSON DB500A Repair.

Good news, I open the Q10 leg B. Now no more heat.

"That means those two output Transistors Q19 and Q21." I had removed them yesterday.

I need to go to fix Sprinklers (dig the dirt).

Open Q10 Base is like open up the switch so no more current flow through Q10, Q9, Q19 due to no BIAS so no heat.. It is like turning off the power switch to the light bulb, the end result is the bulb does not get hot any more.

[budm]

Re: SAMSON DB500A Repair.

BTW, what happen to the -15V supply problem?
As I indicated, you have two power amp: one power amp for the woofer and the power amp for the Tweeter/mid range that you are playing with, as I indicated, you should get sound from the woofer speaker. if there is no DC offset to shut off the relays.
So far you are lucky that you did not cause more damages when you remove that transistor in the bias control circuit.

[budm]

Re: SAMSON DB500A Repair.

http://sound.whsites.net/amp_design.htm

[capwizard]

Re: SAMSON DB500A Repair.

Good news, I open the Q10 leg B. Now no more heat.

"That means those two output Transistors Q19 and Q21." I had removed them yesterday.

I need to go to fix Sprinklers (dig the dirt).

[budm]

Re: SAMSON DB500A Repair.

Now I took the op Amp U7 out, the Q 10 still get hot. AC current at 0.288A

"you can tell if the BIAS has gone out off adjustment or not." It Is always at 0.001v

Ha, This is a joke, after I take out all the parts they still get hot, problem is in the PCB....

PS: Q10 B-E is at 6.1v and drop to 5.8v ....to 5.6v and it get hotter and hotter.

PS: took out Q22 , Q10 still get hot?

.001V on the Emitter resistors of the OUTPUT, are you sure? That means those two output Transistors not conducting.

"Q10 B-E is at 6.1v and drop to 5.8v ....to 5.6v;' How can E-B junction Voltage be at 6.1V!
"PS: took out Q22 , Q10 still get hot?" Take out the important Transistor for controlling the BIAS, that will cause the circuit to be over bias! I think you need to understand what each function of those transistors are for instead of removing them just because.

[capwizard]

Re: SAMSON DB500A Repair.

Now I took the op Amp U7 out, the Q 10 still get hot. AC current at 0.288A

"you can tell if the BIAS has gone out off adjustment or not." It Is always at 0.001v

Ha, This is a joke, after I take out all the parts they still get hot, problem is in the PCB....

PS: Q10 B-E is at 6.1v and drop to 5.8v ....to 5.6v and it get hotter and hotter.

PS: took out Q22 , Q10 still get hot?

[budm]

Re: SAMSON DB500A Repair.

If you check DCV on R92, R93 Emitter resistors in post 24 then you can tell if the BIAS has gone out off adjustment or not.

[capwizard]

Re: SAMSON DB500A Repair.

the BIAS pots still has red seal on it.

[budm]

Re: SAMSON DB500A Repair.

Checked RH1,2,3, all are OK. C65 and C67 are OK.

But did someone adjust the BIAS pots?

[capwizard]

Re: SAMSON DB500A Repair.

Checked RH1,2,3, all are OK. C65 and C67 are OK.

[budm]

Re: SAMSON DB500A Repair.

You have two amplifiers, one is for the woofer, one is for the high frequency speaker, so unless both of them are dead which is not likely so you should at least get sound from the woofer if the speaker relays are on, but that may be due to -15V power supply/load problem.
BTW, no one play with the bias adjustment pot VR5? What DCV do you have between two legs of R92, R93 Emitter resistors for the output Transistors?

[budm]

Re: SAMSON DB500A Repair.

Z1,2,3,4 all are 5.1v Zeners tested are OK

PS: R78: 0.02v other side is 6.1V the voltage went up gradually same as R88: 0.01v other side is -6.1V and went down.

MC4560?

OK, so the DC offset is basically 0V.

[budm]

Re: SAMSON DB500A Repair.

https://www.badcaps.net/forum/attach...5&d=1487270736
"Q9 B C E at 6.5v 54v 6.1v Q11 B C E at -5.9v -54v -6.2v (Both are Hot)"

That means the Vdrops on R65 (220 Ohms) is around 12V, so the current flow is 55mA (12V/220 Ohms = 55mA).
You have +/- 55V supplies, Vdrops on R65 is 12V so the Vdrops on each Q10, Q11 is (110V -12V)/2 =49V, so at 55mA, the power disipation for each one of Q10, Q11 = 49V x 55mA = 2.7W, no wonder they are hot.

[capwizard]

Re: SAMSON DB500A Repair.

Z1,2,3,4 all are 5.1v Zeners tested are OK

PS: R78: 0.02v other side is 6.1V the voltage went up gradually same as R88: 0.01v other side is -6.1V and went down.

MC4560?

[budm]

Re: SAMSON DB500A Repair.

https://www.badcaps.net/forum/attach...5&d=1487270736

You indicated Q10 Emitter (connected to R78; 220 Ohms) is 6V so what what is the DCV on the other end of R78?