21st June 2012
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#1 | | Lives for gear
Joined: Oct 2006
Posts: 557
Thread Starter  | HELP! Recapping Killed my Consoles Supply
Hello Friends,
This has always been the place for me to learn, get inspired, ask questions, get motivation for building more and more compressors….
Now it´s different because i have to ask for help.
I messed up something with my Tascam M600 PSU. I feel horrible now.
Schematic is attached, the part of interest is on the top with U001 7815, U002 7915 and the transistors shown on the bottom (mounted on heatsink) Q009 2sd845 and q10 2sb755
I recapped it, also changing the sice of some caps around the 78/7915 regulators. I got strange oscillation, started undoing changes i had made.
Anyhow, i accidentally hooked Q9 from the neg supply to where Q10 should have been.
After reversing that the unit worked, but only for about half a minute.
Then neg supply started to drop and a huge hum was on my monitors. I expect my missplugging killed the power transistor.
I Exchanged Q9 from a 2sd845 to a 2SC3264. This part had been recommended elsewhere as a replacement.
I also swapped the 78/7915 regulators for new ones.
It is still the same. After some seconds the 7915 gets really hot (7815 is cool) and voltage drops from -16.2v to around -15v. Loud hum.
BTW, the part in the schematic below the -15 section shows a +8v supply that seems to be only for the fan and the meter lamps. I was surprised that this is a regulated supply also.
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22nd June 2012
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#2 | | Lives for gear
Joined: Oct 2006 Location: Toronto, Canada
Posts: 831
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When you changed the regulators, did you remember to put in the isolating pads so they are not shorted to the heat sink?
Best,
Ike
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23rd June 2012
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#3 | | Lives for gear
Joined: Oct 2006
Posts: 557
Thread Starter |
Yes, i double checked.
All Caps are in the right place, Transistors are isolated, regulators are installes the right way.
On Q9 i measure
Base -28,5
collector -16,2
Emitter -28.2
Q10 looks just the same, but pos voltage...
It seems that Q9 is not taking it´s share of the current. So it works up untill 7915 gets to hot.
COuld it be that i just picked the wrong transistor?
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23rd June 2012
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#4 | | Lives for gear
Joined: Nov 2006 Location: Hickory, MS
Posts: 2,599
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Q02 may be current limiting .. the problem could be current draw by the load.
Look at voltage base-emitter on Q02 ... 0.5V/.11 ohm is around 5A.
If one of the two .22ohm resistors is open, the current limiting will happen at 2.5A.
JR
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24th June 2012
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#5 | | Lives for gear
Joined: Oct 2006
Posts: 557
Thread Starter |
Thank you. I was playing today, so no further working on the psu. But i got a bit of time tomorrow.
Base emitter of q2 - should i measure this voltage with or without the desc connected to the psu?
If you say this transistor might be limiting current, do you meen it´s bad and needs to be replaced?
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24th June 2012
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#6 | | Lives for gear
Joined: Nov 2006 Location: Hickory, MS
Posts: 2,599
| Quote:
Originally Posted by jrp  Thank you. I was playing today, so no further working on the psu. But i got a bit of time tomorrow.
Base emitter of q2 - should i measure this voltage with or without the desc connected to the psu?
If you say this transistor might be limiting current, do you meen it´s bad and needs to be replaced? | No... what I said was "the problem could be current draw by the load" . You mention that the 7915 gets really hot... I ASSume this is under load, without load, that is very very wrong.
With no load connected there isn't enough current flowing through the 82 ohm resistor in series with regulator to turn on the helper transistor... The current limit circuit protects the helper transistors, and whatever is shorted.
If the regulator is dropping out of regulation as you suggest, the helper circuit is not helping enough, which could occur if a problem with the load was drawing more current than it is designed to allow, or the boost/current limit circuit is not operating properly.
Something as simple as an open solder connection could cause that, or a fault exists elsewhere causing too much current draw, and the PS is just protecting itself.
JR
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24th June 2012
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#7 | | Lives for gear
Joined: Oct 2006
Posts: 557
Thread Starter |
this sounds very reasonable! i am learning from this!
I was not considering the desc to be broken, everything was running untill i disconnected the psu and started recapping it.
I did quite some soldering to it. But i have looked so many times, i don´t see any shorts, no open connections.
I thought maybe i should test the PSU with some kind of dummy load?
A 4R 100W resistor would draw 4A.
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25th June 2012
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#8 | | Lives for gear
Joined: Oct 2006 Location: Toronto, Canada
Posts: 831
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+1 to everything that John said. I would add that excessive heating is often a sign of a short, and in this case, a short that bypasses any protection circuitry. But, the short, if there is one is probably not directly on the 7915, as it would self protect and put out around -1.5 volts. Chances are very high that something you did to the circuit board, either a solder bridge, or a broken pad, lifted trace or pulled plate-through (via) is causing the problem. JR mentioned an open circuit on one of .22 ohm resistors, and it could be that you have either defeated or bypassed the pass transistor circuit, so the 7915 is doing all of the work. Solder bridges can look exactly like solder joints that are meant to be there. If you have good de-soldering equipment, you could start by going through and de-soldering one joint at a time...sometimes the solder comes up and you see that what looked like two pads joined together is actually two separate pads. Another trick for finding open circuits caused by board damage is to go through the schematic and measure continuity from the leads of one device to the next one in the chain, so, from Pin-2 on U-002 to the collector of Q-002 and the base of Q-009 for example. Measuring from the leads of the devices verifies all points in between on the circuit board.
Good luck!
Ike
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25th June 2012
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#9 | | Lives for gear
Joined: Oct 2006
Posts: 557
Thread Starter |
YES! I had it running for 15min now, 7915 not getting hot, everything working.
I did as you suggested and tested all connections with a dmm.
The 82r resistor going to the collector of Q2 was reading 1.2 ohm.
Unsoldered the resistor, it read ok.
Then I replaced Q2. For now it´s running!
I am still a bit unsure as i replaced Q2 2SC2655 with a BD319 i had laying around. It sure is stronger, but since the 2655 can handle 900mw and all my bc550s can only take 500mw i thought i better pick the next stronger Transistor i have and give it a try.
Do you think it´ll blow eventually?
They have the same pinout, but specs are a bit different.
2sc2655 http://www.toshiba.com/taec/componen...nc/66/7733.pdf
collector power dissaption 900mw
Collector current 2A
collector cutoff current 1uA
emitter cutoff current 1uA
DC Current gain hfe1 min 70 max 250, hfe2 40
Base-Emitter Saturation 1.2v
BD139 http://www.fairchildsemi.com/ds/BD/BD139.pdf
collector power dissaption 12.5w
Collector current 1.5A
collector cutoff current 0.1uA
emitter cutoff current 10uA
Base-Emitter on 1v
DC Current gain min40 max160
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