I picked up an Akai 4000ds reel to reel deck some time back for $20 and made the transport functions work. Sweet.
Then I went to set the input/output calibration with the little pots on the bottom of the preamp board, and just like in one of those nightmares, the screwdriver slipped, shorted something on the board with a spark and now it's dead. No input or output.
Well heck, it was $20. But it's got components I can solder in. I had it working, so it would be nice to fix.
Could anyone explain to me how to divide and conquer checking most likely culprit components? Was it likely an op-amp or something? Should I pull transistors and try to test them with a multimeter - can they be tested in circuit? What's the quickest way to figure out what blew? It has four fuses and none of them blew. Any help appreciated.
Then I went to set the input/output calibration with the little pots on the bottom of the preamp board, and just like in one of those nightmares, the screwdriver slipped, shorted something on the board with a spark and now it's dead. No input or output.
Well heck, it was $20. But it's got components I can solder in. I had it working, so it would be nice to fix.
Could anyone explain to me how to divide and conquer checking most likely culprit components? Was it likely an op-amp or something? Should I pull transistors and try to test them with a multimeter - can they be tested in circuit? What's the quickest way to figure out what blew? It has four fuses and none of them blew. Any help appreciated.
Odds are it isn't too badly damaged.
Check the power supply DC voltages on the board first.
Use your DVM to in-circuit diode check all p-n junctions in transistors and diodes. No need to remove them.
But there are op amps as well.
No scope?
Check the power supply DC voltages on the board first.
Use your DVM to in-circuit diode check all p-n junctions in transistors and diodes. No need to remove them.
But there are op amps as well.
No scope?
Check - hopefully one is off since the accident and you can then go after the power supply.
Roger that, will do and report back. I'm working with the attached from the Service Manual - I'm not too good with schematics.
I will check the transistors and diodes according to this process, I suppose:
https://www.electronics-notes.com/articles/test-methods/meters/multimeter-diode-transistor-test.php
Many thanks!
Your DVM should have a range marked with a diode symbol.
That's the one to use, it should beep if the junction is good.
Don;t try to read the resistance, just use the diode symbol range.
Try testing a known good diode first to see what the DVM should do.
Some DVMs will only show a low value of ohms reading though.
That's the one to use, it should beep if the junction is good.
Don;t try to read the resistance, just use the diode symbol range.
Try testing a known good diode first to see what the DVM should do.
Some DVMs will only show a low value of ohms reading though.
The board looks relativelly large. Did you see where was the accident? If there was a spark, probably some semiconductor may be exploded and you look its burned intestines and usualy also smell ugly.
This may help to reduce the amount of elements damaged to a smaller area. Also, if neither of the chanels is OK, thus I would start searching between those common to both: power lines, dropping resistors,, a shorted zener, a melted track of the PCB...
This may help to reduce the amount of elements damaged to a smaller area. Also, if neither of the chanels is OK, thus I would start searching between those common to both: power lines, dropping resistors,, a shorted zener, a melted track of the PCB...
Okay thanks. My meter doesn't beep, but on the diode mode it shows me a voltage number, eg 0.957v. What should I be looking for? I'm new to solid state electronics.
I'm trying to figure out where to read the power supply voltage. I'm getting zero DC volts on all those leads terminating at the board.
The machine's VUs light up and the motor spins and operates normally.
But I am getting no signal out or in the preamp board. No sign of hiss from the headphone jack or line outs. So I think it is something common to the whole thing.
Unfortunately I didn't take note of exactly where it was, but I believe it was on the section marked PB (playback) Right on this board, or possibly the one to the right of it, Rec Right. The PB right pot actually floats on the bottom side and maybe it pushed down and shorted to the lead under it.
Unfortunately for my diagnosis, nothing looks burned on the board, as far as I can tell. I think you're right that it is something common to the whole board, because there is no input or output.
The diode test is done unpowered. The reading isn't critical, but the junctions should be roughly similar.
Okay. I tested a couple of those diodes on the preamp board - unpowered of course - and the VU jumped when I did, which alarmed me a little, because I thought I must have it plugged in for a moment. I guess the meter feeds a voltage.
Moving on to the power supply board, which is much harder to take out, I tested as many of those components in-circuit as I could. All the resistors highlighted here check out, and the diodes or transistors all show some voltage number on the meter in diode mode except for the one called D3 on the left hand side.
I don't have a capacitor check mode on the meter.
Checking the voltages on the bottom row there, I am getting 29.5v AC in from the transformer, but I'm not showing any DC (or AC) voltage going out to the preamp board. I might be missing something.
The D3 is a 24V Zener diode, but should test OK as a regular diode.
If not, remove and test it again. If it's bad, you have to replace it.
It's the voltage reference for the series pass voltage regulator.
If there's no DC voltage on the preamp board, that could be why.
But be sure that the related pass transistor TR1 is still good, along with D1 and D2.
The AC line fuse did not blow?
If not, remove and test it again. If it's bad, you have to replace it.
It's the voltage reference for the series pass voltage regulator.
If there's no DC voltage on the preamp board, that could be why.
But be sure that the related pass transistor TR1 is still good, along with D1 and D2.
The AC line fuse did not blow?
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Thanks that's helpful.
I'm not sure I'm doing this right, is this transistor Base - top / Emitter middle / Collector - bottom?
If so, when I measure TR1 for resistance, I get
base>emitter 1.3k ohm
base> collector 8-10k ohm and rising
collector>emitter similar 10-12k and rising
The unit has four fuses, and none of them blew. I guess if the power supply board blew, there is no guarantee that components on the preamp board did not also go. I'm not sure what voltage I should be seeing to the preamp but there is zero.
Thanks for your patience.
On a transistor, measure both the BE junction and BC junctions, in both ways.
For an NPN type, with the red probe on the base, you should get similar, lowish readings to either the E or C.
With the black probe on the base, readings to either the E or C should be open.
For a PNP type, with the black probe on the base, you should get similar, lowish readings to either the E or C.
With the red probe on the base, readings to either the E or C should be open.
For an NPN type, with the red probe on the base, you should get similar, lowish readings to either the E or C.
With the black probe on the base, readings to either the E or C should be open.
For a PNP type, with the black probe on the base, you should get similar, lowish readings to either the E or C.
With the red probe on the base, readings to either the E or C should be open.
There should be 24VDC at the positive power connection of the preamp board.
There's a chance it's just the D3 that's bad, but there could be more bad parts as well.
I was thinking about whether I could apply that DC voltage (the 24v) from another source to test the preamp board, before I go on. I am a little wary of soldering out and into the board, now that I look at it. I've done handwired tube amps but not messing up traces on boards is a challenge. If the preamp board is fine it would be worth buying a power supply board. If not, I might be in too deep. Would hate to part this machine out but it might be worth more to someone who needs the transport functions.
What would be the lowish resistance readings I should see on transistors? That article said it should show open circuit, but I'm getting the figures I posted above. I don't know which type these are yet.
It might be easier to just wire this to take ins and outs from my interface's preamp to the heads........
Initially ignore capacitors. They usually don't blown as a consequence of a shortcircuit so quickly. They degrade sloooooooooowly across several years (lytics mainly).
Regarding fuses and given anorher tread regarding Murphy's laws, one of them says rougly that "an expemsive transistor always protect a cheap fuse". So don't alarm if the device is completely carbonized and fuses are all OK.
Regarding fuses and given anorher tread regarding Murphy's laws, one of them says rougly that "an expemsive transistor always protect a cheap fuse". So don't alarm if the device is completely carbonized and fuses are all OK.
From experience, I would restore the power supply first.
That makes sense.
The transistors seem to have part numbers that are specific to Akai - for example 2SCI096. So I think I can't find these specific transistors / diodes as single parts.
First check if any of them is definitively destroyed. In those PSU, there is no need for exact parts. PSU are usualy not too sensitive to parts characteristics as the are strongly negative feed back'ed and thus any part that passes the desired current and can sustain the voltage safely may do the job.