2226b Power supply adjustment

[thxdave]

Was helping a friend with his 2226b and wanted to adjust the power supply. However, I cannot get the voltage below 47VDC using J708 and J715 and adjusting R783. Any ideas where I should look next? TIA, Dave

 

[mbz]

Not familiar with the 2226B.

Adjustment is in the power supply area. Over voltage suggests a failed/short component possibly Q747, maybe Q746.
Suggest diode check both transistors, amp powered off/un plugged. Will look at schematic some more.

 

[mbz]

Probably Q746. Check resistors in the area (R775,7,8) for heat stress.
If everything tests ok then post dc voltage measurements for Q746, Q747 e/c/b to ground/chassis, ie, 6 measurements.
Measure at adjoining resistors rather than at transistor legs, one slip...

 

[thxdave]

Many thanks, mbz. Will go at it again tonight.

 

[thxdave]

BTW, BIG thanks for AK'r Mrki for his creation of the hi-res color schematic in the AK Database for the 2226b. Big, big help for newbies like me.

 

[thxdave]

Here you go:
Q746
B = 43.9
C = 49.2
E = 46.73

Q747
B = 43.9
C = 43.7
E = 0.07

Resistors looked fine to me. No discoloration of the parts or the circuit board.

 

[llwhtt]

The emitter of Q747 should be +15.3 VDC, could be either the Zener, Q748 is shorted or C723 is shorted.

Craig

 

[thxdave]

Thanks Craig. Do they both need to be pulled out to test them accurately?

 

[llwhtt]

Since they are in parallel at least one end of one of them needs to be pulled. And as things go the part you choose to pull is going to be the good one, never fails.

Craig

 

[rBuckner]

Are ya feeling lucky Punk?! Well, are ya???

 

[thxdave]

I gots to know if I can fix this thing. ;-)

 

[thxdave]

The cap measured within spec. The diode only showed .006VDC measuring both directions. Also, the symbol printed on the circuit board shows a standard diode but the schematic shows a Zener.....I think. Still don't know what I'm doing but I'm getting there.

 

[llwhtt]

There are two regulators on that board, the adjustable +35VDC and the non-adjustable 14.7VDC. That Zener is used as a reference for both regulators. Your Zener is shorted so replace that for a start, that may be the only thing wrong. Make sure you install it correctly, the anode (base of the triangle) goes to ground.

Craig

 

[thxdave]

Hi Craig. Do I order a WZ-150 Zener or is that version still made? Thanks for the info and help. I have a small bag of diodes that I inherited from an old tech, but the numbering (if there at all) is very tiny and I'm not sure what I have on hand.

 

[llwhtt]

WZ-150 is a Japanese number, not sure where you'd get one even if they are still available. The W in WZ is 1/2 Watt, BZ would be 1 Watt. A 1N5245B, 15V 1/2W, would be an American part available just about everywhere.

Craig

 

[thxdave]

Craig (or anybody), is there enough info printed on most diodes to figure out what they are? As I mentioned previously, I have a bag of them here but don't know what I have. Or, can I tell using my DMM what I actually have? BTW, I ordered a quantity of new ones from Mouser an hour ago so MANY thanks Craig for the cross-referenced part name.

 

[Osibisa]

Dave, in case you didn't know there are WZ-150's in the 2275 and the BOM lists BZX85B15-TR as a replacement. I'm pretty sure they were used without issue.

 

[thxdave]

I don't remember replacing any in my 2275 rebuild but I might have. Regardless, I looked in my leftover parts and didn't see any. That's why I was asking how to ID the old ones I inherited.

 

[llwhtt]

The part number "should" be on the device, get a magnifying glass if you're old.

Craig

 

[thxdave]

Yeah, I'm getting old......63 and only started wearing glasses this year. Not bad for an old photographer, huh?....smartie pants

Most of the old diodes I have on hand are labeled but a bunch of the glass ones have NO lettering or numbers on them at all. Guess I'd like to know how to measure these so I know what the heck they are.. Now.....let me go find my cane (so I can hit you on the head with it!!)