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DBX-BX3 - Help!

S

special_20

Active Member
I have a DBX-BX3 that seems to be in melt down mode!

Background: I bought amp awhile back and had it sitting in the pile project. Decided to dust it off and clean it up. It had what seemed to be classic issues with dirty controls and dirty relays making use somewhat intermittent and unreliable. Took unit apart, cleaned up insides and cleaned front switches multiple times with deoxit but it was still acting up so I decided to just bite bullet and replace the relays

  • I replaced all 4 speaker output relays.
  • Fixed a faulty LED on the front panel that has caused some other users grief.
  • During testing I did accidentally short out Transistor Q7 on the protection board (original was a 2sc2705; replaced with a ksc2383).
  • Confirmed resistors R28 and R29 were still good

So,after replacing the four channel specific relays and Q7, I reassembled the back panel and brought it up on a dim bulb tester. No shorts I then plugged direct to outlet to check bias and offset. I started seeing smoke. I have burned resistors R31, R33, and R34 on the front right amp board. Same resistors are showing the start of scorch marks on the back left amp board. Front left board was running hot. Manual on this unit is less than stellar and doesn't have voltage listing or great diagrams. I'm a bit at a loss on how to trouble shoot. .

My thoughts were to pull the board and check Q12, Q13, Q14, and Q15. Curious if I should be suspect of something else though given problem is manifesting on multiple boards (power supply, protections cirucuit etc).?
 
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Adjust your bias adjustment pot for zero bias usually fully ccw. Before powering it up again.
 
special_20 said:
My thoughts were to pull the board and check Q12, Q13, Q14, and Q15
Click to expand...
Probably a good idea. Have a good look at associated resistors, besides the smoked ones,for slight discoloration. Or pull leads and verify they are sill within tolerance.
 
and another short spotted - note to self, watch your boards! There is an impedance selection switch on the back. It appears I may have shorted across this when it touched some case work while the back panel was disconnected. This switch is identified as BF70. It appears that this is connected directly to the transformer and carries some significant voltage. I can't find it on the copy of the schematic I have. Hooks in off orange and red leads from transformer ahead of entering the protection circuit board

Could shorting this switch damaged the transformer or power supply and should I be backing it all the way up and disconnecting the amp boards and checking my transformer? Unit did power on and lights and vu meters were working with a signal input. I didn't get chance to confirm actual speaker or amp board outputs.
 
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well confirmed that I have some shorted transistors in addition to resistors. Putting in a parts order this weekend. Since I'm rebuilding boards i'm going to go ahead and hit all four so they stay matched and replace small value caps on amp boards and replace the trim pots. Should be fun (sigh). Anything else I should be looking at for root cause or would a short (protection circuit) or short/arc (speaker impedance selector) be enough to cause some cascading failures like this?
 
ok, I had a chance to sit down with some pin out diagrams and a transistor tester tutorial to confirm my initial readings. I tested in diode mode on the DMM. Q14 Tested good; Q11, Q12, Q13 and Q15 all tested bad.

I'm assuming that my arc/short across the speaker selector switch is my root cause. This might have shorted a 69v tap off the transformer to chassis. Should I be concerned about components that see the 69 volts coming off the transformer (i.e. Q3, Q4, Q18, Q17, ZD1 etc.)? Should I be taking testing one step beyond to point where I can confirm some adjacent components are good?
 
It wouldn't hurt to check them. I'd also check Q16 & Q19. Emitter resistors R39 - R42( .22Ω) .Also base resistors R35 -R38( 4.7Ω)
 
And here I was just hoping for "3-mile island". Then again, throwing 69v headed the wrong direction... Is it also then safe to assume that if one board melted down, they probably all did in similar fashion? Should I disconnect all power (i.e. pull the red, orange and purle leads off the amp board and pre-amp cirucit) and check my transformer voltages to make sure this is still a viable unit (saying that makes me sad...)
 
special_20 said:
Should I disconnect all power (i.e. pull the red, orange and purle leads off the amp board and pre-amp cirucit) and check my transformer voltages to make sure this is still a viable unit
Click to expand...
Probably not required. The resistors that smoked acted like slow blow fuses. Just make sure you find all the bad diodes , transistors and damaged resistors.
 
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When Yamaha M-xx amps do this. I usually replace all the diodes and transistors and damaged resistors. In the effected amplifier circuit.
 
The rest of the transistors should be available at B+D Enterprise.
Your amp uses alot of the same transistors as Yamaha's.
 
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Avionics, here is to hoping you subscribed to the thread. Sorry for the slow progress but it’s been a winter of failing appliances.


When last I posted, amp had gone down into meltdown mode. I bought the amp but it had been a problematic performer and at low volume relays were cycling a bit and one channel was touch. Per recommendations from this board, I had replaced all four speaker relays and cleaned the crap out of the front switches and fixed a dodgy LED connection.


I then made a boo boo and shifted amp around without securing my boards. The protection board (main protection board and left switch bank) and then a switch (voltage selector switch) made contact with the case shorting out and making for a bit of a light and smoke show.


  • I had the protection board out and replaced trasnsistor Q7 (2sc2705; replaced with a ksc2383) and confirmed R28 and ZD1 were still good.
  • I then demounted most burned board and confirmed smoked parts. I figured I go off worst case and order parts assuming any transistor or diode that directly saw voltage would be bad.
    • Component ID: original - > replacement
    • Q12: 2sc2705 -> Ksc2383
    • Q13: 2sa1145 -> KSA1013
    • Q14: 2sc3298 -> 2sc4793
    • Q15: 2sa1306 -> 2sa1837
    • Q16, Q18: 2sc3281 -> 2sc5200n
    • Q17, Q19: 2sa1302 -> 2sa1943
    • ZD1: (amp) 05z12x -> 1N4742A - 12V 1W
    • Fusible resistors were all replaced with equivalent value wire wound resistors
    • Emitter resistors were all replaced with equivalent [edit] 0.22 ohm 5w resistors
I demounted boards, removed parts and soldered in replacement parts 1 board per weekend. I finally finished up last week. I buttoned everything back up and put it on my dim bulb tester. I pushed the power button. Everything lit up. I hear the main power relay click. The bulb on the tester dims. However, I do not hear the individual speaker relays clicking. The red light on the front below the power button is blinking. I imagine this is some protection warning, but the manual does not speak to this. Needless to say, things ain’t working nominally…


I then went to measure and adjust DC bias in the hopes that maybe some adjustments could bring things into spec. Left Front, Left Rear and Right Rear amp boards were all able to have DC current measured and adjusted per manual to ~3 mv. I could not get a DC mv reading on the right front board and the variable resistor is making no changes to the value of zero (0) mv at the test point.


Below are emmiter voltages (VDC) off the output transistors, inductor and the test point terminals. I also checked the VDC at the positive terminals of my main capacitors.

  • Left front
    • Q16: 2.17
    • Q17: 2.16
    • Q18: 2.17
    • Q19: 2.16
    • Inductor: 87
    • T: 4 (VR able to adjust v)
  • Left Rear
    • Q16: -76.5
    • Q17: -82.7
    • Q18: -75.8
    • Q19: - 82.6
    • Inductor: -103
    • T: 3.4 (vr able to adjust v)
  • Right Front
    • Q16: 37.2
    • Q17: 37.1
    • Q18: 37.1
    • Q19: 37.2
    • Inductor: -90
    • T: 0 (VR doesn’t affect V)
  • Right Rear
    • Q16: 50.8
    • Q17: 43.1
    • Q18: 50.9
    • Q19: 43.2
    • Inductor: -90
    • T: 3.7 (VR able to adjust V)
  • Filter Caps (at positive terminal)
    • 1: 47.7
    • 2: -56

Any suggestions on next steps here? I did not pull or check small value caps on board. Should that be my next point of inspection? Any other suspects based on what we’re seeing here from the voltages – which just seem weird and make me question my cheap harbor freight DMM.
 
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22 Ohm OR .22 Ohm 5 Watt emitter resistors? I think I would have used some metal film resistors in place of the fusistors.

Craig
 
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