MC7300 One Channel On Fritz

[Alexandroz]

w3jn, If you allow me one more question, on the driver board (MC7300 and many other models) in the POWERGUARD circuit, there is always an electrolytic polar capacitor with a capacity of 0.47uf at the input, is it necessary to put exactly the same capacity when recapping? Is it possible to install a more common 1uF electrolytic capacitor in this place?

 

[SaturationPt]

Thanks for the answer. Yes, I wrote it wrong, of course, just "BIAS ADJUST".
The ms7300 also has these points, in fact, ms7300, ms300 and ms500 are close relatives and their bias settings are very similar, here is a fragment of the ms7300 circuit, as you can see, it also has points tp1 and tp2.
What do you mean by "and increase the bias on one channel until the AC current draw *just* starts to increase", when the value on the ammeter becomes non-zero and where should the ammeter be connected??



View attachment 2828369

Are you using the schematic that matches the S/N of your MC7300? There are two schematics for the MC7300.

 

[Alexandroz]

Are you using the schematic that matches the S/N of your MC7300? There are two schematics for the MC7300.

I have not seen anywhere mention of two versions of service manuals for mc7300, do you have both of them or know where you can see the differences? I have an amplifier from the middle of 1994, i.e. last revision before the release of the mc300 model.

 

[w3jn]

Yes I do now see on the manual I have the two test points, not sure how I could have missed them! Sorry, I can't help you with what should be measured there however.

For the earlier amps, when you turn the bias all the way down the quiescent current is at its minimum. When monitoring the AC input current to the amp, you turn one channel up until the current just starts to increase. THen turn the other channel up until the current just starts to increase again. However that procedure is probably not correct for your amp since it has the test points which are across the two branches of the output current limiters.

As far as the capacitor is concerned, I wouldn't change the value as it appears to be part of the time constant of the circuit that drives the PG lights. Are you experiencing an issue with the PG lights? If not, might be better to leave them be, or replace with a film cap. Or it may make no noticeable difference at all.

 

[Alexandroz]

Are you experiencing an issue with the PG lights?

No, I want to make a complete recap so that there are no unexpected problems in the future, almost 30 years have passed.

 

[SaturationPt]

I have not seen anywhere mention of two versions of service manuals for mc7300, do you have both of them or know where you can see the differences? I have an amplifier from the middle of 1994, i.e. last revision before the release of the mc300 model.

My apology.

I was confusing this thread with an MC2255, the MC7300 only has one schematic.