any one ever repaired this amp? the low voltage section?
I have 6 quiet channels, that are same med reduced output , not distortion just same level output
1 channels is normal level louder than the rest,
visual inspection there is a small chip in the low voltage section of the power supply board that has the marking :
CHN
LM358N
k3Y751
some resistors around this chip, and one leg of this chip have solder joints that look a bit overheated.
Considering buying this chip and replacing. Thoughts?
I have 6 quiet channels, that are same med reduced output , not distortion just same level output
1 channels is normal level louder than the rest,
visual inspection there is a small chip in the low voltage section of the power supply board that has the marking :
CHN
LM358N
k3Y751
some resistors around this chip, and one leg of this chip have solder joints that look a bit overheated.
Considering buying this chip and replacing. Thoughts?
I dont think a factory reset is going to be a fix here. Also this is an amp not a preamp processor so there is no settings.
maybe if you could just help me determine these resistor values, that would help, I am new to this!
it seems they are upside down in the pick, so right to left order, as shown by the arrow I drew
Brown, Orange, black , black red ( cant tell blue from black any help appreciated)
starting with the one at the top in the image:
Calculating the Resistance:
Therefore, the resistor has a value of 130 Ω with a tolerance of ±2%.
Is that correct? does not seem right, in circuit it measure as 18 k oms anyway...
it seems they are upside down in the pick, so right to left order, as shown by the arrow I drew
Brown, Orange, black , black red ( cant tell blue from black any help appreciated)
starting with the one at the top in the image:
- First Digit (Brown): 1EE Power+3en.wikipedia.org+3en.wikipedia.org+3
- Second Digit (Orange): 3
- Third Digit (Black): 0
- Multiplier (Black): ×1
- Tolerance (Red): ±2%
Calculating the Resistance:
- Significant Figures: 130
- Multiplier: ×1newark.com+2en.wikipedia.org+2utmel.com+2
- Resistance Value: 130 × 1 = 130 ohms
- Tolerance: ±2%
Therefore, the resistor has a value of 130 Ω with a tolerance of ±2%.
Is that correct? does not seem right, in circuit it measure as 18 k oms anyway...
Attachments
I think maybe that’s backwards ?
2, 0,0 multiplier 3 or 1000, then tolerance
200 000 ohms
200 kohms
Can someone check it for me ? Thanks !
2, 0,0 multiplier 3 or 1000, then tolerance
200 000 ohms
200 kohms
Can someone check it for me ? Thanks !
If it’s playing it’s not the low voltage section , that is for the DC protection, over current and muting circuit.
Problem is most likely on the amp board.
Post a picture of one of the dual channel PCBs and I will try and help you check some things. Most likely the input capacitor, if there’s no distortion, next would be 1-2 of the TO-92s after the input stage, likely CCS or VAS, but might be VBE.
Problem is most likely on the amp board.
Post a picture of one of the dual channel PCBs and I will try and help you check some things. Most likely the input capacitor, if there’s no distortion, next would be 1-2 of the TO-92s after the input stage, likely CCS or VAS, but might be VBE.
Here are pics, voltages and DMM readings for a working channel. Hopefully the numbers can be made out but I repaired 2 pcbs (4 channels) and it was primarily high DC from the input pair (MPSA56) IIRC, Q4/Q5.
I included a spreadsheet you can print out and make notes, a copy of my notes on same spreadsheet. A list of voltages for all transistors as well.
Also a few pics of the boards I repaired - sorry but can’t remember specifically if this was before the repair or after, when in doubt use my list of voltages. All voltages are on a 60w DBT in the US at 123V mains.
I still have my PVA7 so feel free to ask questions.
I included a spreadsheet you can print out and make notes, a copy of my notes on same spreadsheet. A list of voltages for all transistors as well.
Also a few pics of the boards I repaired - sorry but can’t remember specifically if this was before the repair or after, when in doubt use my list of voltages. All voltages are on a 60w DBT in the US at 123V mains.
I still have my PVA7 so feel free to ask questions.
Attachments
is it possible that if this LM358N was malfunctioning it could lower the output of the individual amp sections? Could it partially mute?
I dont think its very likely that 6 out of 7 amplifier channel boards are all individually damaged in a way that makes them equally reduced in output. Do you?
They sound perfectly fine, there just all quieter than one channel. To me that indicates something that regulates voltage on all the boards at once is malfunctioning.
That coupled with the visible heat signs on the board around this LM358N chip make me think that is the place to start.
Thanks so much for all the info that really helps! Will have a look tomorrow!
Also if the amplifier section was damaged that would effect the sound quality but if the power supply was reduced voltage that would reduce the output without distorting it right? I guess first thing to do is check the voltage around the LM358N and compare with yours , then replace it as I already received a few in the mail, however I want to replace everything involved that cause it to fail . So have to understand the circuit better first to make that list.
Power supply issue would affect all 7 channels, since oyu have 1 working channel - it is highly unlikely to be the PSU or anything related to it.
having reduced sounds, is typically something in the input - if you can measure safely with a DMM --- input a small signal, say 100mV of 1kHz sine wave (youtube will work or phone app) and measure the RCA going in to the amp channel, then the speaker out put. You should see 8-10x the voltage on teh speaker out put (check the working channel for a good baseline).
then follow the AC path with your DMM and you will "see" where the signal drops. point #1 100mV, point #2 100mV, then some future point you measure will show 040mV... bingo, that's your problem. I would start with the polar caps on the left side of the board, IIRC C9/C10.
EDIT: be very careful with the ribbon cable from one board to the board with the VAS and outputs - it is tiny solid wire and it only takes 3-4 bends and you will have oneof those wires break off. Then you may have fixed it, but wonder why protection still kicks in.
having reduced sounds, is typically something in the input - if you can measure safely with a DMM --- input a small signal, say 100mV of 1kHz sine wave (youtube will work or phone app) and measure the RCA going in to the amp channel, then the speaker out put. You should see 8-10x the voltage on teh speaker out put (check the working channel for a good baseline).
then follow the AC path with your DMM and you will "see" where the signal drops. point #1 100mV, point #2 100mV, then some future point you measure will show 040mV... bingo, that's your problem. I would start with the polar caps on the left side of the board, IIRC C9/C10.
EDIT: be very careful with the ribbon cable from one board to the board with the VAS and outputs - it is tiny solid wire and it only takes 3-4 bends and you will have oneof those wires break off. Then you may have fixed it, but wonder why protection still kicks in.