Akai GX-280D-SS playback speed issue

[Derek_Ronin]

Hi there, been lurking around and digging through posts to hopefully get my unit back and running again. I've owned the units for a few months and replaced and recap the relay boards, previously it was failing in playback. I was able to enjoy it for about a month until recently the unit started having a problem with the playback speed. Everything sounds like chipmunks, I've replaced TR209 & TR252 (2SDS234) and while I was replacing those, I noticed another transistor with a small crack in it (2SC1226A), replaced that as well. But could not find where it belongs on the schematic, looks like it was previously serviced or attempted. Even after replacing the usual suspected parts, the unit still fails to play back at the correct speed. Thanks in advance, any help or suggestions will be greatly appreciated.

 

[quadsearcher]

That cracked transistor is not original and might be a poor or wrong substitution, need to find out what it should be.
Can't just put the same unknown sub in there.

Is this the servo control amp pcb KD-1096? If so, check if +B1, 24vdc is present. If not, follow traces to see if that transistor is TR252, get the correct part in there, then troubleshoot that portion. If 24v had gone high, unregulated, there could be problems down the line.
If the correct part fails again there is likely a problem in the servo dragging the supply.

If that is not the servo control bd, nevermind. I'm not familiar with this model.

 

[Derek_Ronin]

Thanks for the advice, it is KD-1096 Servo board. I'll run the probe and check for the 24vdc on B+. It's the transistor right behind 2SDS234. I've attached some images for clarity on it position.



Would the original transistor be (2SC1014)?

 

[Derek_Ronin]

Ran the probe on the B+ and appears that as you suspected, the voltage has climbed up to about 36V. I have not replace the original cap for the B+ yet.

 

[quadsearcher]

Yes, I'd confirm by following traces, but if the other one is D234 (TR252), then it probably is C1014 (TR251). It looks like with your replacement, you have corrected the "crossed leads" thing they had going on? C1014 is CBE and the C1226 they had was BCE.

Check the 9v Zener, location looks like D253. I suspect it is open. Or TR253 or even the trimpot VR251. Spray some deoxit on the trimmer. I just replaced some of that style that were bias controls in an amp. No good when they open up.
Maybe break the +B1 line (and unplug P11) while testing until it is regulating OK. It should regulate without a load.

 

[Derek_Ronin]

Yes, I'd confirm by following traces, but if the other one is D234 (TR252), then it probably is C1014 (TR251). It looks like with your replacement, you have corrected the "crossed leads" thing they had going on? C1014 is CBE and the C1226 they had was BCE.

Check the 9v Zener, location looks like D253. I suspect it is open. Or TR253 or even the trimpot VR251. Spray some deoxit on the trimmer. I just replaced some of that style that were bias controls in an amp. No good when they open up.
Maybe break the +B1 line (and unplug P11) while testing until it is regulating OK. It should regulate without a load.

Zener diodes read .353v with the probes one way and .726v with the probes switched the other way.

TR253 readings are :
1.95v and 1.42v also ran HFe and picked up measurements.

Will unsolder the +B1 line and unplug P11 for measurements next. Thanks for the guidance, I was so bummed with this unit on my bench for a month now. Getting closer, can't wait to hear it again.

 

[quadsearcher]

With those readings I'm suspecting the trimpot now.
What did you use for TR251?

 

[Derek_Ronin]

With those readings I'm suspecting the trimpot now.
What did you use for TR251?

cleaned out the trim pot, which seems to have no effect with deoxit. For TR251, I had gone with C1226. With CBE configured. Unsolder B+ and P11 unplugged, how do you recommend I should proceed?

 

[Derek_Ronin]

Got a chance to test the resistance across the trimpot, with it at fully counter clocked, reading at 184 ohm. Fully adjusted clock wise, got a reading of .856 ohms. (Powered off)

 

[Derek_Ronin]

The obvious was mentioned, "trimpot", but of course me being me, I did not catch that. So I went ahead and swap out the trim out with a temporary one I had from my old guitar amp builds (10k), I didn't have the proper 5k on hand. Mess with the potentiometer and took some reading, with 10K B+ dropped to 18v and 11v (out of spec of course, but I may be getting closers) . Ordered a proper 5k trim pot and will see if I could get it to the target 24vdc. Fingers crossed.

 

[quadsearcher]

The schematic says VR251 is 1KB (B taper not that it should matter much), the parts list says 10K. I'm at a loss on that one.
I'd go with 1K. But really, I don't know for sure when the manual says two different things.

Was the part in there marked 5K? Wait, I see VR202 is 5K but that's in a different part of the circuit.

 

[Derek_Ronin]

I think it is the 5k trimpot (VR202) that had fried, however good point let me test VR251 while I'm in there.

 

[Derek_Ronin]

Update*** replaced 5k trimpot tonight for VR202, B+ now down to 2v. A bit baffled now, on why 5k would be giving me a lower reading on the B+ compared to when I had the temporary there 10K pot. So close but no cigar, will continue to go over the schematic and see what I've missed.

 

[quadsearcher]

VR202 is in a different part of the circuit.
VR251 is the trimmer that sets the output of the regulator in question. Usually best to start with power supply voltages before going elsewhere.
Also check for 9v across the zener.

 

[Derek_Ronin]

Here's some images just to confirm the trimpot (VR251), that I should be looking at? Ordered a 1k replacement, it should arrive tomorrow.

 

[Derek_Ronin]

Getting a 2v reading on the zener diode, hrm.... What am I overlooking? Bridge Rectifier?

 

[quadsearcher]

Well, you could measure the voltage at the bridge!

 

[quadsearcher]

It is a series pass linear regulator, if you are not familiar with the design, there are technical guides. I don't mean this in a negative way, we all have to start somewhere and basic troubleshooting is only basic when the basics are understood.
Yes, look for "must-haves" . Nominal 34v across the bridge is a must-have.

 

[Derek_Ronin]

I appreciate the guidance, you've made this hobby a lot less frustrating. I would not have known where to being....

 

[Derek_Ronin]

Bridge rectifier read-out at 38v haha, I might be over my head here.