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Yamaha PC1602 pro amp cleanup and test

terracode

terracode

New Member
Hi All,

I recently bought a used Yamaha PC1602 power amp. The history is that previous owner bought it new for a gig and only used for several hours before getting very ill and having to leave his job to recover. The amp sat many years and was given to his friend 4 months ago to sell.

1000007934.jpg

The amp overall is very clean and caps seem to be in good shape (no leaking or bulging), but didn't test the caps for issues. The unit produces sound, but I felt something amiss. The power button sticks in either on or off position when pressed.

The clip and signal LED don't light up (looks like LED soldered to board and not bulb. See pic). Maybe a relay is not working. I do get around 2mv from left channel when power is off via speaker output and almost zero from right channel. When turned on, the signal noise goes away. Voltage offset for right and left channel at speaker posts looks good when at idle with no input (less than 14mv on left and 2.5 on Right). No danger of speaker damage thankfully.

I cleaned the power button and it is now working fine. Boards and input/output connections on back panel were cleaned. A 1Khz Sine wave looks good on both channels.

Overall sounds good. Running the music through amp for a few hours.

Need to diagnose the signal and clip light issue. Any help on where to check would be welcomed.
 

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@bad_boo mentioned that his signal light only goes on intermittently when the gain is increased at higher input volumes. I turned up WiiM to 60% volume with gain at 3/4 turn and signal light flashes. Happy now. Will check with my speaker dummy load when it arrives later today or tomorrow.
 

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Did a listening session with a audiophile friend of mine. We agreed that there sounds like something is missing in a mid-range frequency and with a metallic type sound. High frequencies and low frequencies sound good but something is not quite right around vocals frequency range. Possibly one of the filter caps may not be behaving. I plan to do a frequency sweep from 1khz to 20KHz in 1khz increments to see if I can narrow this down.
 
terracode said:
Possibly one of the filter caps may not be behaving.
Click to expand...
Doubtful to be cause. Filters take a beating when playing bass intensive passages. If filters can handle such bass then they can easily handle mids...

terracode said:
I plan to do a frequency sweep from 1khz to 20KHz in 1khz increments to see if I can narrow this down.
Click to expand...
Worthwhile. If you have a scope then square wave test may/should show issue

edit ok, scope shown in post #1.
Try 400-1kHz square wave, first check at the amp input to see what the source is like then test the speaker outs.
Both channels the same?
 
mbz said:
Doubtful to be cause. Filters take a beating when playing bass intensive passages. If filters can handle such bass then they can easily handle mids...


Worthwhile. If you have a scope then square wave test may/should show issue

edit ok, scope shown in post #1.
Try 400-1kHz square wave, first check at the amp input to see what the source is like then test the speaker outs.
Both channels the same?
Click to expand...
Thanks for the recommendation. The sine wave at 1kHz is the same. Will need to check each channel with square wave to see if the possible issue is affecting only one channel. I am pretty new to oscilloscopes, so entering new territory for me.
 
Hi.. below are the oscilloscope readings with square wave. I did a sine wave at two frequencies as well. The left channel 10 and 20kHz waveforms look weird compared to right channel.

Right: Square Wave 400Hz, 800Hz, 1Khz, 2.5Khz, 5Khz, 10Khz, 20Khz and a Sine wave 400Hz
Right_Square_400Hz.jpgRight_Square_800Hz.jpgRight_Square_1kHz.jpgRight_Square_2_5kHz.jpgRight_Square_5kHz.jpgRight_Square_10kHz.jpgRight_Square_20kHz.jpgRight_Sine_400Hz.jpg
Right_Square_400Hz.jpgRight_Square_800Hz.jpgRight_Square_1kHz.jpgRight_Square_2_5kHz.jpgRight_Square_5kHz.jpgRight_Square_10kHz.jpgRight_Square_20kHz.jpgRight_Sine_400Hz.jpg
See next post for Left.
 
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Left: Square Wave 400Hz, 800Hz, 1Khz, 2.5Khz, 5Khz, 10Khz, 20Khz and a Sine wave 100Hz and 10kHz
1_Left_Square_400Hz.jpg2_Left_Square_800Hz.jpg3_Left_Square_1kHz.jpg4_Left_Square_2-5kHz.jpg5_Left_Square_5kHz.jpg6_Left_Square_10kHz.jpg7_Left_Square_20kHz.jpg8_Left_Sine_100Hz.jpg9_Left_Sine_10kHz.jpg
1_Left_Square_400Hz.jpg2_Left_Square_800Hz.jpg3_Left_Square_1kHz.jpg4_Left_Square_2-5kHz.jpg5_Left_Square_5kHz.jpg6_Left_Square_10kHz.jpg7_Left_Square_20kHz.jpg8_Left_Sine_100Hz.jpg9_Left_Sine_10kHz.jpg
 
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whoaru99 said:
Not that it really should matter, but why such low output test voltages?
Click to expand...
I was running the input via phone through RCA to XLR adapters. I had the volume on phone almost maxed out. The gain was set at 12. Didn't want to max out gain. If I had actual preamp with XLR with higher output voltage it most likely would give have given higher amp output. I didn't want to use my WiiM Ultra for the tests.
 
terracode said:
I was running the input via phone through RCA to XLR adapters. I had the volume on phone almost maxed out. The gain was set at 12. Didn't want to max out gain. If I had actual preamp with XLR with higher output voltage it most likely would give have given higher output. I didn't want to use my WiiM Ultra for the tests.
Click to expand...

I suggest getting another source that's different from a phone to confirm it's not just some wierdness from the phone and adaptation.
 
whoaru99 said:
I suggest getting a better source. Who knows what the phone is doing.
Click to expand...
I was using a tone generator app on the phone using usb-c to 3.5mm adapter. Maybe I will try the 10khz and 20 kHz with my laptop instead of phone. I found a good tone generator app for Windows. If it was a phone issue, one would think the left and right reading at 10 kHz and 20 kHz would be the same.
 
terracode said:
I was using a tone generator app on the phone using usb-c to 3.5mm adapter. Maybe I will try the 10khz and 20 kHz with my laptop instead of phone. I found a good tone generator app for Windows.
Click to expand...
I suggest not bothering with a wide array of signals adding complexity right now.

1kHz is perfectly fine for both sine and square wave until there is confidence in the readings. If you can't get a "square" 1kHz and a "sine" 1kHz there is no point to do any of the others right now.
 
whoaru99 said:
I suggest not bothering with a wide array of signals adding complexity right now.

1kHz is perfectly fine for both sine and square wave until there is confidence in the readings.
Click to expand...
What are your thoughts on the 1kHz readings? Do you feel something is amiss and if so what could the culprit be? Caps or transistors?
 
Where are you connecting the scope probe? At the resistors or right at the speaker out?

Should be at the speaker outs from what I have read .
 
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