Fry Eggs on a AU-7700
- Thread starter craigsc
- Start date
Well, the voltage reading across R72 or R73 should have been ~10mV if the bias is set correctly. So my conclusion is, however you are checking the bias it is not the correct method.
The 10mV voltage reading is based on 45mA of current being the target - this being the sum of current pulled by 2 pairs of OP transistors AND the driver transistors for each channel. So I have guessed that the driver transistors will pull ~5mA leaving a convenient ~40mA for the 2 pairs of OP transistors = 20mA for each pair.
I x R = V
0.020A x 0.47Ω = 0.0094V or roughly 10mV = the voltage across one emitter resistor for the correct bias current.
I suggest you adjust the bias for 10mV across those emitter resistors - and see how it sounds.
The 10mV voltage reading is based on 45mA of current being the target - this being the sum of current pulled by 2 pairs of OP transistors AND the driver transistors for each channel. So I have guessed that the driver transistors will pull ~5mA leaving a convenient ~40mA for the 2 pairs of OP transistors = 20mA for each pair.
I x R = V
0.020A x 0.47Ω = 0.0094V or roughly 10mV = the voltage across one emitter resistor for the correct bias current.
I suggest you adjust the bias for 10mV across those emitter resistors - and see how it sounds.
Last edited:
craigsc
Active Member
I agree I must be doing the measurement wrong but can't determine what I am doing wrong. I'm pulling F01 and F02 on F-2097 (first and third fuse in a row of 4 behind the two sets of diodes) and then putting my DMM across the fuse holder for each and adjusting VR3 and VR4. It seems like I'm following the SM?
F01 & F02 are not the correct fuses, at least not according to the SM - As far as I can make out there are 2 models one with fuses you can remove and one not. I can only guess that the missing fuses are labelled F01 & F02 even though there already are fuses with those labels for a different part of the amplifier on the schematic.
Last edited:
craigsc
Active Member
Yes, I have the version with the fuses. I'm terrible at getting photos or pdfs onto AK but page 4 of the AU-7700 SM, step 3-1 and 4-1 say to measure output at F-2097 F01 and F-2097 F02. The AU-7700 SM does not have a diagram but the AU-5500 SM has the same adjustment and does have a diagram showing the reading being taken at the fuse holder clips?
Yes, but you can't necessarily use that schematic for the AU-7700. Using the same logic regarding the AU-5500 schematic and bias setting procedure, I can tell you that for the AU-9900 (AU-7700's larger brother), you have to remove wires to set the bias (there are no removable fuses) - push on connectors have been provided to facilitate this, but like some versions of the AU-7700 you DO have to remove wires. I see what you mean about the AU-5500 diagram - both the +ve and -ve rails are protected by fuses - perfect.
I see 5 fuses in total on the AU-7700 schematic (which is in the SM incidentally) - and none of them are the correct ones to use for bias setting (by fuse removal) in the way described in text, for the appropriate model in the bias setting procedure.
Here is a link to the SM here on AK, which includes the schematic http://akdatabase.com/AKview/displayimage.php?album=28&pos=105
I think you have:-
F-701 = 6amp - the mains fuse.
F01 & F02 = 1amp on F-2092 - fuses for the pre-amp supply.
F05 & F07 = 7amp on F-2097 - AC secondary fuses for main PSU.
If your unit is the same as ^^this^^, it is one of the units - described in the bias setting procedure - for which you MUST remove a wire to set the bias for each channel. If yours is one of the units where you CAN remove fuses, I think you should have FOUR fuses on F-2097 - but I think you only have two?
The 'measuring voltage across an emitter resistor' method is a perfectly valid alternative way of setting the bias - and I use it wherever possible, even if the 'inline current meter' procedure is described accurately in the SM.
Last edited:
craigsc
Active Member
I have the F-2097 version with fast acting fuses. There are a total of 6 on F-2097. I can’t determine why the SM procedure isn’t working, but set to 15 mv on the emitter resistors and all seems fine. That’s about 5/8 turn on VR3 and 4. I’m going to stop worrying about it and just enjoy.
OK, but 15mV across one emitter resistor is ~32mA - giving a total of approximately ~69mA for one channel. (remember you have 2 output pairs per channel, so the current through one emitter resistor must be doubled to get the actual figure pulled for both pairs in one channel ).
I think my estimation of what it should be for 45mA, namely 10mV is closer to correct.
).I think my estimation of what it should be for 45mA, namely 10mV is closer to correct.
Last edited:
spark1
Lunatic Member
OK, but 15mV across one emitter resistor is ~32mA - giving a total of approximately ~69mA for one channel. (remember you have 2 output pairs per channel, so the current through one emitter resistor must be doubled to get the actual figure pulled for both pairs in one channel
I think my estimation of what it should be for 45mA, namely 10mV is closer to correct.
Well, you gave it your best effort.
Thanks, I was disappointed not to completely 'crack it', but at least the OP has a workaround, his AU-7700 sounds good, and it isn't 'cooking' anymore.
DEAFASABAT
Well-Known Member
Be very careful with those adjustment pots, at their present advanced age they could disintegrate. Mess with them as little as possible. Good luck!
