SAMSON DB500A Repair.

[capwizard]

A very nice PRO PA Amp: SAMSON DB500A is power on not in the protection mode.

PS: The problem is I forget what was the problem?

PS: I called him, he said " no sound from iphone input"....I said " Bring me the input cable, I need to check it."

Hartke Systems: SAMSON DB500A PCB is like a piece of art, so beautiful.
http://www.samsontech.com/hartke/

LINE Combo Balanced Input
For added convenience, the dB500a employs a Combo connector that accepts a standard XLR mic cable for balanced line level
signals, or a standard 1/4" instrument cable for either balanced (TRS – TIP/RING/SLEEVE) or unbalanced (TS – TIP/SLEEVE) line level
signals. The LINE input can work simultaneously with the Mic input so it is possible to use a microphone while playing alone with a
keyboard that is plugged into the dB500a’s LINE input. You can control the LINE input by using the LINE LEVEL control as described
in the previous page. For more information on cable and wiring, see page 13 of this manual for a detailed wiring diagrams

[capwizard]

Re: SAMSON DB500A Repair.

Testing; A standard 1/4" instrument cable with cellphone stereo adapter inputs to the Amp, it has no sound! Relay makes click click noise. I thought was input cable problem. The fact is that the amplifier is the issue.

[capwizard]

Re: SAMSON DB500A Repair.

Help!....The amp starts at 0.5ma then in 3 minutes go up to 3.5 A gradually , the AMP MOSFETs burn my finger, it stops until overheat relay clicks and clicks again. Isolate the preamplifier is still the same thing!

PS: Can't find any shorted diodes or Trs.

[budm]

Re: SAMSON DB500A Repair.

"the AMP MOSFETs burn my finge" What AMP MOSFET? Those 4 (+2 small transistors) attached to the heatsink that you can barely put the fingers on?

[capwizard]

Re: SAMSON DB500A Repair.

Maybe they are NPN PNP, they are upside down, I can't see the parts No. Looks like amplifier noise harmonic vibration but current goes up gradually.

PS: Should I take out that 4 TRs and test again.

PS: They are 2SC5200 and 2SA1943, I cut their legs. Now I have time to test DC voltages are +50V and -50v.
The Current still go up gradually.

PS: I cut the last two power TRs legs, current stop at 0.246A, and the two TO-202 still get hot.

[budm]

Re: SAMSON DB500A Repair.

https://www.badcaps.net/forum/attach...5&d=1487194391
There are 8 large Transistors, which one are getting hot? The output of the power amp showing DC >1V?

[capwizard]

Re: SAMSON DB500A Repair.

This is like heart bypass surgery! I found the transistor Q10 2SC3202 produce a lots heat.

PS: take out Q10 test it is good. Amp still has current at 0.147A, and I can smell something is heating up it has new plastic smell. New I need a infrared camera instead of my fingers.

PS: I bet you it is A leaking diode.

"the power amp showing DC >1V?" They have speaker protection relay.

PS: Check nearby capacitors are OK.

At front two are MOSFETs as voltage regulators for DC voltages +50V and -50v (tested OK). The rest PNPs and NPNs all get hot to untouchable.

[capwizard]

Re: SAMSON DB500A Repair.

New discovery: If I put the Daughter-board back to amp the 7915ct -15v is at -8.45v, 7815ct is ok +15v. The source input voltage is wrong.

PS: At 5:00 I need to fix Sprinklers, See your guys tomorrow.

[CapLeaker]

Re: SAMSON DB500A Repair.

so either the -15 volt regulator is bad, or something is dragging the -15V rail down. Like a bad op amp.

[capwizard]

Re: SAMSON DB500A Repair.

The + - 25.4v are from AC transformer AC 18v then regulate to + - 15v DC which have problem.

[budm]

Re: SAMSON DB500A Repair.

So the - 25V is still at -25V when the output of the -15V is at -8V?
Is the -15V reg getting quite hot?
If the output of the -15V is only 8V then the Vdrops on the -15V reg is 17V (25V -8V) instead of 10V Vdrops so it should get quite hot.
You can try replacing the Reg IC or isolate loads to see which is loading down or it can be bad reg IC.

[capwizard]

Re: SAMSON DB500A Repair.

If I open J4-A then - 15 is OK.

J4 is for pop cancel circuit U12 TA7317P but they modified to this(pic3).

PS: If I take out this Add-on board, the Amp still get hot then I cut the all the TRs legs.

[capwizard]

Re: SAMSON DB500A Repair.

Keep on Troubleshooting.......

[budm]

Re: SAMSON DB500A Repair.

So U12 speaker protection is not used in this unit?
That U12 does the delay turned on and also shutdown the speaker relays if it detects DC on the output of the amplifiers, very old IC that has been around for a long long time, it is used a lots in the stereos.
So the speaker relays are coming ON?
I still do get the answer on the DCV at the outputs of the amplifiers (it is Bi-AMP system).

[capwizard]

Re: SAMSON DB500A Repair.

Unbelievable this Amp use DC200v. Isn't this the OCL amp.

PS: i can't measure the DC voltage of the Amp output After I was scared and I cut the TRs legs. Beside this amp has a protection relay or I can still measure the output dc voltage?

PS: Do you mean the High-out and Low-out over the relay? I put the Add-on board and measure the relays are 0v.
AC current at 0.145A. If I put back Q10 will get hot and AC current is at 0.188A.

[budm]

Re: SAMSON DB500A Repair.

Unbelievable this Amp use DC200v. Isn't this the OCL amp.

PS: i can't measure the DC voltage of the Amp output After I was scared and I cut the TRs legs. Beside this amp has a protection relay or I can still measure the output dc voltage?

The Low frequency amp section is Class G, the high voltage is switched in by Q49, Q50 supply to the output section when high audio output needs to go high OUTPUT for short period of time, otherwise it runs on 55V.
HITACHI came out with that topology in the 70S.

[capwizard]

Re: SAMSON DB500A Repair.

I think we found the problem.The Q10 is over here(pic1). I took out the Q10 it tested OK!

The voltage of Q10 (2SC3206) is B - C - E : 6.9v - 55v - 6v When no input the TR Q10 leg E should be 0V.

PNP Q20 (2SA1024) B C E is at -6.7v -55v -7.3v


Compared to other side of Amp Q36 B C E is 1.2v 54v 1.8v...................... Q37 B C E is -1.2v -54v -1.8v

Q31 (2SC3902) B C E at -1.3v 1.8v -1.9v Q22 B C E at -4.8v 7.3v -0.1v

Q9 B C E at 6.5v 54v 6.1v Q11 B C E at -5.9v -54v -6.2v (Both are Hot)
Q38 B C E at 1.1v 54v 0.5v Q39 B C E at -1.2v -54v -1.2v

[budm]

Re: SAMSON DB500A Repair.

As I asked, what DCV do you have at the output of both amplifiers (low freq and high freq)? I have a feeling that one of the amp has high DC OFFSET right now.
Q22, Q31 are part of the active Bias for the output transistors.
"When no input the TR Q10 leg E should be 0V" what do mean by NO INPUT?
Q10, 20, 36, 37 should have E-B forward junction Voltage of around 0.60V.

[capwizard]

Re: SAMSON DB500A Repair.

what DCV do you have at the output of both amplifiers, Both channel DCV are 0.0v.

[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?