Hi Chris,
Just think. I did warranty service on those. Most people blocked the vents. We could see the dust void where the owners manual was, or the feet marks from whatever.
Adjusting the fixed resistors and reducing the value of the semi-fixed resistors should do it for you. Just think, Yamaha was all freaky that we get the parts from them so the gains were in the right range. I just bought the proper parts. They wanted to charge me $1.00 for a 1/4 W carbon film resistor. They expected me to charge the customer $1.41 for it.
-Chris
Just think. I did warranty service on those. Most people blocked the vents. We could see the dust void where the owners manual was, or the feet marks from whatever.
Adjusting the fixed resistors and reducing the value of the semi-fixed resistors should do it for you. Just think, Yamaha was all freaky that we get the parts from them so the gains were in the right range. I just bought the proper parts. They wanted to charge me $1.00 for a 1/4 W carbon film resistor. They expected me to charge the customer $1.41 for it.
-Chris
I'm only going to do the bias mod on the Rotel. . . the Yamaha will be a
straight spec fix. . .
I appreciate all the help. I have a different thread about the Yamaha
fix.
Had to waste the day getting a surprise 70th birthday party for my mom together. . . won't get back to work until Monday or Tuesday.
Grrrrr!
No, my mom is worth it. . .need my Yamaha up and running. . .
tunes for the party.
straight spec fix. . .
I appreciate all the help. I have a different thread about the Yamaha
fix.
Had to waste the day getting a surprise 70th birthday party for my mom together. . . won't get back to work until Monday or Tuesday.
Grrrrr!
No, my mom is worth it. . .need my Yamaha up and running. . .
tunes for the party.
Hi Chris,
Sounds fine. Two comments.
1. You have done enough research to warrant adjusting the resistor values. This would be intelligent and what I would do if it were mine.
2. At the very least, clean the metal contacts on the bias controls. Remove them, clean them out and reinstall them. You could always just replace them.
You can feel a slight "bump" when the wiper reaches it's original position if you hold your tool lightly enough as you rotate it. It's a good starting point for the bias setting.
Nice amps though. They look pretty nifty in service too.
-Chris
Sounds fine. Two comments.
1. You have done enough research to warrant adjusting the resistor values. This would be intelligent and what I would do if it were mine.
2. At the very least, clean the metal contacts on the bias controls. Remove them, clean them out and reinstall them. You could always just replace them.
You can feel a slight "bump" when the wiper reaches it's original position if you hold your tool lightly enough as you rotate it. It's a good starting point for the bias setting.
Nice amps though. They look pretty nifty in service too.
-Chris
Hi Chris,
I have seen more than my share in for service, all makes and price points. Professional and consumer. Aside from some pro and semi pro stuff (and Revox), I didn't fix anything if it had a handle on it.
Generalizations must be made. If it's not too bad to service and it sounds worthwhile when I'm done - that is my preference. So by that criteria, Marantz 500's and a standard, good receiver (say Nak TA-2 or a Denon 325). There is a balance of aggravation and sound quality. I dislike things like B&O, Tandberg, Nikko Alpha 440/450 and Fisher new cheap stuff. These are far too difficult to service vs the sound quality. Pioneer SX-9's fall into the YUCK category also. I hate garbage.
Product loses points for a lack of reliability or pointless difficulty to service. I can accept a difficult to service unit if the process is justified. Nakamichi OMS 5/7 falls into that catagory, also the 1000ZXL and most standard transport series 1 cassette decks.
A joy to work on is a complete upgrade on something like a Marantz 2325. Well worthwhile in my book.
-Chris
I have seen more than my share in for service, all makes and price points. Professional and consumer. Aside from some pro and semi pro stuff (and Revox), I didn't fix anything if it had a handle on it.
Generalizations must be made. If it's not too bad to service and it sounds worthwhile when I'm done - that is my preference. So by that criteria, Marantz 500's and a standard, good receiver (say Nak TA-2 or a Denon 325). There is a balance of aggravation and sound quality. I dislike things like B&O, Tandberg, Nikko Alpha 440/450 and Fisher new cheap stuff. These are far too difficult to service vs the sound quality. Pioneer SX-9's fall into the YUCK category also. I hate garbage.
Product loses points for a lack of reliability or pointless difficulty to service. I can accept a difficult to service unit if the process is justified. Nakamichi OMS 5/7 falls into that catagory, also the 1000ZXL and most standard transport series 1 cassette decks.
A joy to work on is a complete upgrade on something like a Marantz 2325. Well worthwhile in my book.
-Chris
Replaced Bias resistor with 3.58kohm (was 2.7kohm) and new
500 ohm trim instead of 2.2kohm. . .finer control. . .
Now I'm changing the value for negative feedback resistor.
Original value is 10kohms. . . now it is 13.7kohms. . .going to listen. . .
The DC offset went down in both channels with these changes!
DC offset (R) Premod: 24mV
DC offset (R) Postmod: 10mV
DC offset (L) Premod: 34mV
DC offset (L) Postmod: 18mV
Do you think the global NFB change or the bias change is causing this?
500 ohm trim instead of 2.2kohm. . .finer control. . .
Now I'm changing the value for negative feedback resistor.
Original value is 10kohms. . . now it is 13.7kohms. . .going to listen. . .
The DC offset went down in both channels with these changes!
DC offset (R) Premod: 24mV
DC offset (R) Postmod: 10mV
DC offset (L) Premod: 34mV
DC offset (L) Postmod: 18mV
Do you think the global NFB change or the bias change is causing this?
The DC offset change was a byproduct of changing the NFB resistor
to a higher value. I do want to change the LTP transistors some
day soon. . . Bias set to 7.1mV (32mA).
I changed the NFB resistor from 10kohms to 13.58kohms. . . a 36%
increase in resistance or a +2.66dB increase. . .that should produce
less NFB into the LTP.
With an 8 ohm power resistor on the output I measured the output
voltage and the input voltage with my DVM. . . might not be reading
correctly at the high frequencies (really need an oscilloscope).
White Noise Gain: 36dB (6.95 Vout : 110 mVin)
100Hz Gain: 29dB (8.44 Vout : 295 mVin)
1001Hz Gain: 29dB (7.95 Vout : 270 mVin)
9999Hz Gain: 40dB (8.35 Vout : 78 mVin)
19999Hz Gain: 60dB (8.25 Vout : 8 mVin)
to a higher value. I do want to change the LTP transistors some
day soon. . . Bias set to 7.1mV (32mA).
I changed the NFB resistor from 10kohms to 13.58kohms. . . a 36%
increase in resistance or a +2.66dB increase. . .that should produce
less NFB into the LTP.
With an 8 ohm power resistor on the output I measured the output
voltage and the input voltage with my DVM. . . might not be reading
correctly at the high frequencies (really need an oscilloscope).
White Noise Gain: 36dB (6.95 Vout : 110 mVin)
100Hz Gain: 29dB (8.44 Vout : 295 mVin)
1001Hz Gain: 29dB (7.95 Vout : 270 mVin)
9999Hz Gain: 40dB (8.35 Vout : 78 mVin)
19999Hz Gain: 60dB (8.25 Vout : 8 mVin)
Hi Chris,
Your amplifier's DC offset should be corrected in the differential pair area. Once the input pair are matched, you can inject or extract some current from one base to bring the DC offset within 5 mV. I may drift around a little.
I would not be messing around with the feedback network though. You should really put it back to where it was.
-Chris
Your amplifier's DC offset should be corrected in the differential pair area. Once the input pair are matched, you can inject or extract some current from one base to bring the DC offset within 5 mV. I may drift around a little.
I would not be messing around with the feedback network though. You should really put it back to where it was.
-Chris
Hi Chris,
Your amplifiers stability margin depends on compensation and gain (set by feedback). A certain compensation for a given gain and vice versa. Tampering with feedback will affect stability. That’s why Anatech warns you.
By the way, full feedback will give you gain=1.
OT/ I did PM you in another thread since you offered the service manual for this amp, maybe my mail got lost?
Your amplifiers stability margin depends on compensation and gain (set by feedback). A certain compensation for a given gain and vice versa. Tampering with feedback will affect stability. That’s why Anatech warns you.
By the way, full feedback will give you gain=1.
OT/ I did PM you in another thread since you offered the service manual for this amp, maybe my mail got lost?
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.