Buzzing/Distorted Sound on Technics SX PR603 Electric Piano

Thread Starter

soundmasterdee

Joined Dec 28, 2019
24
Hi, I have a Technics SX PR603 electric piano which has started giving me a bit of a headache. The sound is fine when I plug in headphones but its buzzy and distorted through the speakers. Obviously, this suggests that the pre-amp section of the board is fine, and the problem lies with the amp side. Its as if that side is not receiving sufficient power to drive the amp. I have tested the speakers and the tweeters separately and they are working fine. I was wondering whether anyone has come across a similar scenario. Any ideas about the possible culprit (capacitor, transistor, resistor, etc) will be great. Thanks.

Regards
SoundMaster
 

dendad

Joined Feb 20, 2016
3,261
Check the power supplies. Look for failed caps. Headphones require a lot less driving power than the speakers, so may not be loading the supply down.
Also, look for a shorted power transistor in the amp output.
The headphone output may not be driven by the power amp.
Do you have a circuit, and oscilliscope?
 

Thread Starter

soundmasterdee

Joined Dec 28, 2019
24
Check the power supplies. Look for failed caps. Headphones require a lot less driving power than the speakers, so may not be loading the supply down.
Also, look for a shorted power transistor in the amp output.
The headphone output may not be driven by the power amp.
Do you have a circuit, and oscilliscope?
Hi Dendad, a million thanks for the quick response. Your pointers are very helpful.
It appears that the preamp is working fine, hence the reason why the sound is ok via headphones.
Unfortunately, I'm unable to locate a circuit diagram for the board at the moment.
Moreover, I only have a multimeter as my main testing tool so my diagnosis options are rather limited.
Nonetheless, I'll check the power supply section as recommended to see if I can locate any failed caps or shortened transistors.
 

Thread Starter

soundmasterdee

Joined Dec 28, 2019
24
Wow! That's very timely and extremely helpful indeed.
I'll check the capacitors and transistors to see if there's any dead cap or shortened transistor on the board.
Your pointer for the service manual is particularly helpful.
I'll also try to get the service manual immediately.
You're indeed a saviour.
 

Ylli

Joined Nov 13, 2015
879
Found a schematic for the '602 on Electrotanya, don't know how similar that is to the '603. Speaker amp is Class D, headphone drive seems to come through an entirely different path. Some of the strangest circuitry I have run across.
 

Thread Starter

soundmasterdee

Joined Dec 28, 2019
24
Hi Ylli, thanks for the pointer. I've downloaded the service manual for the SX PR602 from Electrotanya (https://elektrotanya.com/technics_sx-pr602_sx-pr602m.pdf/download.html#dl).

There seems to be lots of similarities between the SX PR602 and my SX PR603, although the circuitry is rather confusing. I'm studying the board now to see if I can identify the root cause. All the same, I'll be grateful to receive some guidance from anyone with knowledge about these Technics PCBs.
 

Alec_t

Joined Sep 17, 2013
10,745
It might be worth re-making the solder joints on the legs of those 4 big power resistors on the right-hand side of your pic. Dry joints there could be problematic.
 

Chris65536

Joined Nov 11, 2019
160
It might be worth re-making the solder joints on the legs of those 4 big power resistors on the right-hand side of your pic. Dry joints there could be problematic.
This was the recommendation from the google search I did on this electric piano. Apparently, these were from the early days of lead-free solder, and they had some issues.
 

Thread Starter

soundmasterdee

Joined Dec 28, 2019
24
Hi Alec, thanks for the tip. I did inspect those resistors and they looked ok from the naked eye. All the same, I'll re-inspect that section and redo the solder as recommended. Thanks.
 

MrChips

Joined Oct 2, 2009
19,932
Technics SX PR603 Keyboard AS speaker output.jpgSchematics for PR602 is the same for PR603.

Can you describe the buzzing sound? Is it a low frequency hum or a higher frequency?
Is it loud and over powers the piano notes?
Is it coming from both sets of speakers or only from one side, left or right.

This is not going to be easy to diagnose. Do you have access to an oscilloscope?
 

Thread Starter

soundmasterdee

Joined Dec 28, 2019
24
Hi Mr Chips, a million thanks for your interest in helping out with this issue. The sound is kind of a blend between a buzz and a distortion. Its happening on both the main speakers and the tweeters. Its as if the cones of the speakers are torn but speakers themselves are fine when I test them independently with a different sound source. Its also happening on both channels. However, its fine when I plug in headphones, implying that the pre-amp side is ok. I've tried to attach a sample of the sound to this thread but I've been unsuccessful. I can send a sample to you if possible.
 

MrChips

Joined Oct 2, 2009
19,932
No need to send audio samples.
However, tracing with an oscilloscope would be quicker in finding the problem.
Normally, the first thing we would check is the quality of the power supplies. I suspect that the power supply is OK.
Technics SX PR603 Keyboard AS power supply 15V.jpg

There is a MUTE circuitry which is common to the LEFT and RIGHT channels. This could be flaky.
 

MrChips

Joined Oct 2, 2009
19,932
Whoa! Wait a minute. Your +Vcc and -Vcc are twice what it should be.
Are you sure that this piano worked previously?
Are you sure that the power input is set for 230VAC and not 120VAC?
 

dendad

Joined Feb 20, 2016
3,261
Whoa! Wait a minute. Your +Vcc and -Vcc are twice what it should be.
Are you sure that this piano worked previously?
Are you sure that the power input is set for 230VAC and not 120VAC?
That is a good find. If it is per the circuit, the volts should be about 28V each.
I'll be interested to see if that is the problem. And if it still works, it must be well designed!
 
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