Setting 6060 Bias Current

I did look at the sticky but it didn't help me here .

I was trying to use the service manual for the 6060 I downloaded from Hifi Enging. It shows taking the readings for the bias current using R06 / R08 for R channel and R05 / R07 for L side on the F2648 driver board. The driver board in my 6060 is labeled F2647 and I am not sure which resistor pairs to use for the reading for the 15 MV setting
The system seems to be woring OK but I just want to make sure the bias current is correct. I have set the pots to the middle setting in the interm.

Any suggestions on this? :scratch2:

Thanks,
George

Whether the board ID number is F-2648 or F-2647 shouldn't make much difference, perhaps the actual positions of the resistors on the PCB (if mounted there) might be different, that's about all.

If the R numbers you quote above are the roughly 0.5 ohm resistors connected to collectors/emitters of the output transistors, then just measure across the equivalent resistors associated with the other board.

If the issue is identifying exactly which pair of resistors in the new positions on the different PCB, then you will have to use the schematic and your multimeter (set to ohms) with the unit powered off to discover what the 'pairs' are. Beware that when making readings, what looks like a direct connection could be 0.5 ohms - in other words measuring across one of the resistors. However, I have to say that it is very often quite obvious what the pairs are from their positions.

Pretty simple, or have I missed something here?

Thanks for stepping up Hyperion.


Here is a picture of the driver board. Looks like

TR02 and TR04 emitters are in series with R08 and R10 for the right channel .

TR01 and TR03 emitters are in series with R07 and R09 for the left channel.



Based on the manual, I have been taking the readings from the arrowed positions for both channels. Adjusting the pot is giving me .8 MV to 1.5 MV with a full swing of the adjusting pots on both channels. The manual said the value should be set to 15MV + or - 1MV. is the low value I am seeing a problem and should I address it? If so where should I start?

This is the manual adjustment information to show the dependencies and what confused me.



Thanks

I have now looked at the 6060 schematic, and satisfied myself that there should be a F-2647 AND an F-2648 board - that is according to the schematic.

The resistors you are measuring across are on the F-2647 board - and are the emitter resistors for the input stage to the driver board.

Your 6060 DOES have a F-2648 board - it is the one with the relay on it, and the board that the F-2647 plugs into...

The instructions clearly point to R06 & R08 and R05 & R07 (0.33Ω) on F-2648 - soooo... why are you measuring across R08 & R10, and R07 & R09 (10Ω) on F-2647 ???

Sorry to be the bringer of this news, but that's what it looks like to me is causing your wrong readings.

One thing, if you are going to replace stuff for new, p-l-e-a-s-e change the trimmers. :yes:

The emitter resistors are generally 0.33 ohm ceramic resistors adjecent to the power transistors. Just follow the emitter connection on the power transistors and you will find them. The little sub diagram, 3-2 in your post above shows how to connect the voltmeter across the emitter resistors. This is the textbook method of measuring bias current on this kind of amp.

- Pete

Come to think of it, there may be board variations in the 6060. The later production used different boards than what is shown in the manual. Measuring the bias current across the emitter resistors will work regardless of which boards you have.

- Pete

Thanks for the extra information, and clarification Pete. :thmbsp:

Hyperion said:

The resistors you are measuring across are on the F-2647

soooo... why are you measuring across R08 & R10, and R07 & R09 (10Ω) on F-2647 ???

Click to expand...

Because the drawing from the service manual shows TR02,TR04 and TR01, TR03 on the board labeled 2647 and shows resistors in series in what I though was for the measurement. The drawing makes it look like those are the resistors to use by my interpretation.

LBPete said:

The emitter resistors are generally 0.33 ohm ceramic resistors adjecent to the power transistors. Just follow the emitter connection on the power transistors and you will find them. The little sub diagram, 3-2 in your post above shows how to connect the voltmeter across the emitter resistors. This is the textbook method of measuring bias current on this kind of amp.

- Pete

Click to expand...

Thanks Pete,


I can see the ceramic resistors from the emitters but I only see one each side.

Please take another look at the depiction of the F-2648 Board in Fig 3-1 in the second of your posted attachments, is your board NOT like this? - if not please post a picture of that area - centered on the F-2647 socket as in Fig 3-1.

finally got it dialed in.

Hyperion and LBPete
Thanks for the help :thmbsp:

kaplang said:

finally got it dialed in.

Hyperion and LBPete
Thanks for the help :thmbsp:

Click to expand...

Well done! - I am very glad.