Technics SL-P10 Audio Issue

[Chris Rist]

Hello everyone!

I recently rescued a Technics SL-P10 from a thrift store and managed to get it working again after addressing a few dry joints near the heatsink. While the player technically functions well, I've encountered an audible issue that I couldn't find much information about online. So, I thought I'd reach out to the community for some advice.

During playback, there's a noticeable noise floor or jitter that becomes apparent, particularly during quiet and subtle parts, especially in more dynamic music like orchestras or audio with long release/sustain (such as reverb). This noise is only present during playback, specifically when the audio circuits are switched on after the relay click. The noise is akin to artifacting, almost like the noise floor on a vinyl record. It seems to worsen the longer the player is powered on, though it's not dependent on the type of disc or the playback position (beginning or very end). The quieter the audio, the louder the noise. Importantly, the noise is not present during silent parts or between tracks.

I've taken a series of troubleshooting steps thus far, covering various components. This includes cleaning the transport and pickup/lens, reseating all connections, and meticulously inspecting solder joints on the power/audio/amp board, servo board, and digital board. Despite the original Panasonic electrolytics, all filter caps have tested well with low ESR.

During my investigation, I identified a broken trace on the power board connecting the collector of Regulator Q852. A resistor (R853) runs in tandem with this trace leading to pin 8 of the RCH and LCH daughter boards. Additionally, I noticed some additional caps soldered on the back of the digital board, likely added during the factory assembly. I addressed these concerns by retouching the pads and cleaning the board of flux and old glue.

However, these efforts have yielded little to no improvement in the observed symptoms. I'm left with a few possible explanations.

The symptoms might be indicative of a failing transport, issues with the drop-out board, problematic transistors or regulators, faulty amplification or switching components, or a concern originating from the digital board. Alternatively, it's possible that I could be overlooking something on the power supply board. Although the caps tested good capacitance with low Vloss and ESR, should I still consider replacing them, given the symptoms persist?

I was wondering if anyone has experienced something similar or could offer advice on where to look next. Your help would be highly appreciated!

 

[redk9258]

Some CD players use transistors to mute the output during silence. Maybe they are causing trouble? You might need a scope to troubleshoot this one.

 

[Chris Rist]

Thanks for the feedback. I got a 2 Channel Scope on loan for this one. I'll check the muting transistors and see if the service manual can point them out, do they usually run before or after the digital side, I'm assuming this would be after D/A conversation. I'll also check to make sure we got a good RF signal to begin with which should help narrow down the issue. Voltage rails on the power side and boards test fine so I'll hold off on recapping for now and address the issue first.

 

[Chris Rist]

I think I've identified the muting transistors. It seems that the mute signal is routed from pin 1 on the digital board to Q853, and then it passes through Q851. At Q851, the signal is utilized to activate relay RL851, resulting in the creation of the mute relay drive. This, in turn, mutes the output during track skips and idle states. I'm considering bypassing the switch entirely to quickly assess the situation.

 

[Chris Rist]

Update: After thorough investigation, it has been determined that the issue is not related to the muting circuits. The P10 employs a relay system for muting the output, and testing around the regulators and mute signal indicates that it is functioning correctly. To rule out any potential problems with the relay, I temporarily bypassed it, but the issue persists.

Upon further examination using an oscilloscope, it was observed that the signal chain is noisy throughout, extending all the way to the digital board. Upon closer inspection, a malfunction in the 5-volt rail from regulator D638-R was identified. This regulator supplies power to the Switching and DAC circuits. The cause of the regulator's failure was traced back to a dead short on a 5.6V zener diode, likely resulting from accidental contact with a probe or a similar incident during a previous repair attempt.

A replacement for the faulty regulator, D638-R, is on its way, and I plan to replace the damaged zener diode as well. Once these components are replaced, I will reassess the system to determine if the issue has been resolved. Further updates will follow after the replacement components are installed.

 

[Chris Rist]

[Resolved]

I thought I'd share a final update, which might be useful for others dealing with a similar problem. The issue was traced to IC804 on the digital board. The culprit turned out to be an operational amplifier, specifically the TL082CP, which had become noisy, likely due to a manufacturing defect. As far as I can gather, this op-amp is part of the de-glitch circuit preceding the LCH and RCH boards, making its proper functioning crucial for the PAM signal. Replacing the chip has successfully resolved the problem.

 

[Gotrah]

A bit late, but thanx for the update!