Yes for me for sure it would have been impossible. I don't know how you do it.
I measured again the LM394 with clips, without touching it.
Left:
Vbe 674 reverse 814
Right:
Vbe 679 reverse 797
Still acceptable?
My meter is really cheap so I'm not sure how reliable are the measurements.
I measured again the LM394 with clips, without touching it.
Left:
Vbe 674 reverse 814
Right:
Vbe 679 reverse 797
Still acceptable?
My meter is really cheap so I'm not sure how reliable are the measurements.
Checked with my other meter and the results are quite different but the difference in Vbe similar:
right Vbe 605 reverse 680
left Vbe 601 reverse 684
right Vbe 605 reverse 680
left Vbe 601 reverse 684
Hopefully everything will work ok. I don't have any 100k metal film resistor, so can't reassemble it right now.
It seems that the previous owner replaced all transistors but never found the real cause.
Pitty you have to wait for a 100K resistor.
Hans
Pitty you have to wait for a 100K resistor.
Hans
Yes that's what I was thinking, they replaced all the transistors (except the LM394 that was however pulled out for testing I suspect since it was installed slightly tilted), plus most 1n914 diodes, but couldn't solve the problem.
Yes it's a pity to wait for the resistor but with mouser I should get it in a couple of days, at least I will put the original Vishay Dale resistor back in place. Plus I had to order the caps for restoring the ML28.
Yes it's a pity to wait for the resistor but with mouser I should get it in a couple of days, at least I will put the original Vishay Dale resistor back in place. Plus I had to order the caps for restoring the ML28.
I also noticed that the trimpot I removed was set all the way clockwise, as an attempt to reduced the initial DC offset, which I remember since I have the amp has always been 40mV (then 0.2mV after warmup).
So I mounted the resistor and now all voltages are correct. I can set the offset of the inverting input to 0V with the trimpot and the LS output behaves like the other channel, going to under 10mV after a couple of seconds from switch on.
It seems that the amp is fully functional for the first time after probably at least 10 years!
Can't spot any big differences in sound though, but it isn't calibrated yet and still open with wires connecting the power rails etc..
I have the impression that the left channel has a slightly lower volume, but I'm not sure yet. Probably the bias are set very different between the two channels too..
It seems that the amp is fully functional for the first time after probably at least 10 years!
Can't spot any big differences in sound though, but it isn't calibrated yet and still open with wires connecting the power rails etc..
I have the impression that the left channel has a slightly lower volume, but I'm not sure yet. Probably the bias are set very different between the two channels too..
Congratulations, well done.
When offering both channels the same input signal, there should be no differential signal between both LS outputs when connecting a speaker between them.
If not, gain is not equal between both channels.
Hans
When offering both channels the same input signal, there should be no differential signal between both LS outputs when connecting a speaker between them.
If not, gain is not equal between both channels.
Hans
Thank you! Congratulations to you who figured it out!
What do you mean by connecting a speaker between them?
Although I think it was more likely due to improper speaker positioning than the amp itself.
What do you mean by connecting a speaker between them?
Although I think it was more likely due to improper speaker positioning than the amp itself.
I mean connecting one speaker between the two hot outputs of the R and the L LS output.
When both channels get the same input signal, the LS should remain silent.
Hans
When both channels get the same input signal, the LS should remain silent.
Hans
Oh ok, didn't know you could do that. So far, however, after adjusting speaker positioning I'm not noticing significative imbalance between channels. The amp seems to be working and sounding great, thanks for your help! Thanks to you I was able to bring the ML back to its former glory!
Glad you are happy and the old lady is back in good condition from the audio hospital.
Enjoy your music, that’s what it is all about.
Hans
Enjoy your music, that’s what it is all about.
Hans
Hans I have a question. After setting the inverting input offset to 0V I noticed that after connecting my preamp the offset goes to 6mV. Since I only use this preamp, would it be better to set it for 0V with the preamp connected? Or it doesn't matter?
This is completely and absolutely harmless, altough from a pure technical point I wonder how this can happen because as you can see in my simmed model that the input is AC coupled.
Could you measure what DC voltage your preamp produces in the XLR plug ?
Pin 1 is gnd.
Hans
Could you measure what DC voltage your preamp produces in the XLR plug ?
Pin 1 is gnd.
Hans
Sorry I meant the output of the inverting amp goes to 6mV, not the LS output. I guess the + and - output of the preamp are not completely simmetrical