What the Cap!!!!?

[Mooly]

The black lead of your meter goes to ground (chassis) and the red lead measures the voltage on those 22 ohms that go to Q901 and Q902. You should see plus 30 and minus 30 volts or more on each.

 

[bored184]

The black lead of your meter goes to ground (chassis) and the red lead measures the voltage on those 22 ohms that go to Q901 and Q902. You should see plus 30 and minus 30 volts or more on each.

Lets try this again.
Q901
B -22.44V
C -34.14V
E -21.90V

Q902
B 22.52V
C 33.86V
E 21.96V

Q906
B 16.50V
C 21.96V
E 15.90V

Q905
B -16.42V
C -21.90V
E -15.74V

D902 fluctuates between -.8 to almost -2 but does it rather quickly.

 

[Mooly]

Those all look good apart from the diode.

Make sure you are on the correct end of the diode... if so then just change the diode and the cap. Was that the cap that blew up ? If so then the diode is duff and that will be what zapped the cap.

 

[bored184]

Those all look good apart from the diode.

Make sure you are on the correct end of the diode... if so then just change the diode and the cap. Was that the cap that blew up ? If so then the diode is duff and that will be what zapped the cap.

All the electrolytics have been replaced. I just doubled check and on the striped side its fluctuating around -0.06V and the non striped end its -.8V to almost -2V.

 

[bored184]

Its a 1N4003 which of course I don't have, I have some 1N4001s and some UF4007s.

 

[Mooly]

The 4007 is perfect.

 

[bored184]

The 4007 is perfect.

I swapped out the diode and now the pre amp lives! The relay now clicks and I get music out of it. The relay does take awhile before it engages though.

 

[nigelwright7557]

I swapped out the diode and now the pre amp lives! The relay now clicks and I get music out of it. The relay does take awhile before it engages though.

That is the anti thump delay, it gives amp time to settle before being connected to the speaker.

 

[bored184]

That is the anti thump delay, it gives amp time to settle before being connected to the speaker.

That I understand but is there a time span in which it is taking to long to engage? I haven't timed it yet so I don't have a number to go along with my observation.

 

[nigelwright7557]

That I understand but is there a time span in which it is taking to long to engage? I haven't timed it yet so I don't have a number to go along with my observation.

Its usually 3 to 4 seconds before relay clicks.

 

[bored184]

Its usually 3 to 4 seconds before relay clicks.

Cool! If I had to guess it's somewhere between those two numbers so all is well.

 

[Mooly]

All sounds good

 

[bored184]

All sounds good

A big shout out to Mooly, he has been a big help with this project. Thank you!!!

A big thank you to the rest of you as well who helped!

 

[Mooly]

You're very welcome 🙂

So what is next... reducing the rails to allow more modern opamps to be used ?

 

[bored184]

You're very welcome 🙂

So what is next... reducing the rails to allow more modern opamps to be used ?

That's what I'm leaning towards. I've been reading my other thread trying to make sense of it all.

 

[bored184]

1/ Check that the PLUS 22 volt rail is OK from regulator Q902.
2/ Check that the MINUS 22 volt rail is OK from regulator Q901.
3/ Check that the PLUS 15 volt rail is OK from regulator Q906.
4/ Check that the MINUS 15 volt rail is OK from regulator Q905.

5/ Check the auxiliary negative volt rail that powers the front panel LED and also operates the muting circuit (relay 🙂) is present and correct. You should have around MINUS 30 volts on the anode (that's the non striped end) of D902.

Check all those 5 voltages first.

I almost forgot to ask, where did you find these rail voltages? I would assume being able to do so comes a long way in trouble shooting errors.

 

[Mooly]

The emitters of Q901 and Q902 are the plus and minus series pass regulators, Q905 and Q906 are simple series pass devices that reduce the -/+22 volts down to -/+15 volts by means of the resistive divider on their base/s. The latter are not regulated and stabilised.