Pioneer stereo (SX-550) What does higher emitter voltage mean? - Page 1
Forum section: electronics / repair

author: mkiijam
author_rank: Regular Contributor
timestamp: January 26, 2024, 12:06:52 am
content: I'm working on a Pioneer stereo / amp that had blown outputs. I've replaced them and have it running, but suspect it may be running too hot. One thing that I discovered was the voltage across the Left channels emitter resistor, which is also the test point for setting the bias, is about 20% higher than the Right side. Meaning: If I send it a 1v p-p sinewave and turn the volume up "half way" I will get 450mV across the right channel test point and 650mV across the left. What does that mean?**Also** This emitter voltage discrepancy is with the same RMS value on dummy loads, meaning the output appears to be the same on both channels, but one has this higher voltage.

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 26, 2024, 09:57:16 am
content: Resistor in question got a litte hot for some time, value drifted high?I assume that after replacing the output transistors you did re-adjust quescient current to specification or close to that of the other channel before dynamic comparison?

author: CaptDon
author_rank: Super Contributor
timestamp: January 26, 2024, 02:05:56 pm
content: Unless specifically stated in the instructions you don't measure bias voltage with signal present because you would have both an A.C. and D.C. component present and different meters would give different integrated values therefore no standard could be established unless you used exactly the same make and model meter as the manufacturer. Set the bias with no signal present.

author: mkiijam
author_rank: Regular Contributor
timestamp: January 26, 2024, 03:19:54 pm
content: Quote from: CaptDon on January 26, 2024, 02:05:56 pmThis is most coherent, thank you. However, I have switched to two meters I am using and the higher voltage follows the amp / side, not the meter.I'm not setting the bias, that's already done. I just noticed this discrepancy and thought it interesting. And, since I'm chasing a potential fault, everything looks suspect

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 26, 2024, 10:24:17 pm
content: Well at the risk of repeating myself.If you set bias current measuring DC voltage drop at emitter resistor then get higher AC voltage at emitter resistor on one channel compared to the other while having same output power to load it may be that the emitter resistor is higher in value.Other possibilities are DC offset, high frequency oscillations or current going elsewhere than into the load but apart from the capacitor if there's a Zobel network there shouldn't be many possibilities.

author: mkiijam
author_rank: Regular Contributor
timestamp: January 27, 2024, 02:48:00 am
content: This discrepancy is in DC. The sinewaves look they same.I left my two meters on the DC Bias setting test points when I ran audio into it and noticed the difference. The higher I ran the input / volume control the larger the difference would get. i.e. Ratio not offset. I swapped meters around to see if it was just the Fluke vs el-cheapo, but the issue didn't follow the meter.I'll check the Emitter resistors now.