2226b Power supply adjustment

[llwhtt]

I'm right behind you at 62 in April.

Craig

 

[thxdave]

Well, I aged a little more today. The diodes arrived, I swapped out the bad one and.....no change. DBT did the normal glow but same 46VDC as before and R783 makes no change in the voltage output level. I'm open to suggestions. At least the diodes were inexpensive!! ;-) Also, just to verify, I'm testing the voltage at J708(+) and J715(-) as per the manual.

 

[llwhtt]

Is the voltage good on the emitter? Measure the voltage across the Zener, 15VDC? Installed correctly, anode towards ground?

Craig

 

[thxdave]

Thanks Craig. Will check and get back to you.

 

[thxdave]

If by emitter, I'm assuming emitter of Q747 which is now 14.8vDC (up from previous 0.07vDC)..
Voltage of Q748 Zener (to ground) is now 14.83 and orientation appears correct.

 

[thxdave]

BTW, I wanted to thank everyone for their suggestions. I guess this is what I get for trying to show off my "skills" to a friend on his Marantz!
If I have to rebuild this power supply to fix the voltage issue, is there a parts list for this model?

 

[thxdave]

....no more ideas? My friend thinks I'm a total loser. ;-) I told him that this repair is worth exactly what I've charged him....$0.

 

[quaddriver]

I am having the same problems in a 2252 and from studying last night - and not sure if the '26 is the same, but in the amp, there are 2 47uf-50v caps that if they fail, will let rail voltage backfeed along the 35V line. ymmv. the 42v I saw was very very suspiciously close to the actual, modern rail voltage

I have noted that if you disconnect the 35v to test, ALL(nearly all) the current available over a ballast resistor (in my case 220ohm 3w) flows thru the zener - so the stock 1W zener wont be healthy enuf

 

[thxdave]

***cricket*** ***cricket***
I'm going to have to return this thing to my friend unless somebody has some other suggestions.....so......

 

[llwhtt]

Is the voltage at J722 normal now, approx. +15VDC? What are the voltages on the EBC of Q746?

Craig

 

[thxdave]

Will check tonight. Thanks Craig.

 

[bryans12v]

It may be a long shot but it couldn't hurt to pull the trim pot and make sure its tracking accurately. Other than that, with the new zener in, please confirm EBC on both Q746 and Q747 for us. Q747 should be E:15.3v, B:15.9v C:35.6v

Q750 I believe is the regulator for the tuner supply but it couldn't hurt to test this as well since its collector is tied to the others.

 

[llwhtt]

Another Marantz schematic error, Q748/C723 should NOT be connected to the base of Q747. There should be NO dot directly below the base of Q747.

Craig

 

[thxdave]

Is the voltage at J722 normal now, approx. +15VDC? What are the voltages on the EBC of Q746?

Craig

J722 measures 14.2VDC
Q746 - E=46.6
B=44.0
C=49.0

 

[thxdave]

It may be a long shot but it couldn't hurt to pull the trim pot and make sure its tracking accurately. Other than that, with the new zener in, please confirm EBC on both Q746 and Q747 for us. Q747 should be E:15.3v, B:15.9v C:35.6v

Q746 = E - 46.6, B - 44.0, C - 49.0
Q747 = E - 14.8, B - 43.9, C - 44.0

Q750 I believe is the regulator for the tuner supply but it couldn't hurt to test this as well since its collector is tied to the others.

Q750 isn't on the power supply board but is connected to the big heat sink along with the other power output devices. Is that the one you want me to measure?

 

[thxdave]

Another Marantz schematic error, Q748/C723 should NOT be connected to the base of Q747. There should be NO dot directly below the base of Q747.

Craig

Sorry Craig, but that made my brain bleed a bit. Can you explain what you mean by "....should NOT be connected..." and "....NO dot directly below..."?

 

[thxdave]

I pulled R783 to test the trimmer pot out of circuit and it tested fine. Still did not change the voltage values after reinstalling, so.....

 

[llwhtt]

If you look on the schematic there is a dot where the base of Q747 and the line from C723 cross, those are NOT connected but there's a dot showing a connection.

OK the 14.7VDC supply is good as is the reference for the 35VDC supply, the Zener fixed that. The voltage going into the collector of Q746 is good but the voltage on the base is wrong as though Q747 isn't conducting, I'll bet Q747 is kaput.

Craig

 

[thxdave]

I'll put together another Mouser order. Suggestions for other caps or transistors I should replace on this board while I'm in there?

 

[llwhtt]

If the board is difficult to get to or work on I'd replace all of the electrolytics while your're in there. Q746 will be only original semi left so maybe replace that if Mouser has a good cross for it.

Craig