Toasted 400 still toast

Also after looking over the schematics and suspecting Q3,4, and 5 I looked through the parts I have been collecting and found I do have the MPSA42's here...
 
roccus said:
No Joe I did not replace any of those... I never saw a problem till I took the DBT out which then gave full 120V voltage...
I did check the resistors in circuit, all 3 on the left side come in at 132ohm, on the right side Q5 is 132 and Q3 and Q4 come in at 133ohm
Click to expand...
Seems the resistors remain good but the transistors appear iffy. You should replace all 3 with ON Semi MPSA42 devices. Do you have ON Semi originals?
 
Not sure of brand the brand is not marked on them but the bag has Mouser part number..... I'll go over to Mouser and see what the part number comes up as
 
Nope they come back as manufactured by Central.... What else can go wrong??? damn it!
 
I will send you some of the real deal Ray...don't use those parts. I will send you an ON Semi for the MPS651 that you also replaced with a central part.
 
When I order parts for this amp I use the part numbers you provide on the build sheet.... I may have had these around from another poject.... thanks Joe
 
Did some checking out and looks like I did order the Central A42's and 651's. Back when I started this repair when I found a known bad part I woulld order some extra stuff that may be in question as I went through the board. Looks like those 2 parts were not in stock and obsolete so I took what was available. So what happens on this eventually Joe? Will you have to go to another brand, find an ON replacement, or do a whole new redesign for parts that are available??
 
roccus said:
Did some checking out and looks like I did order the Central A42's and 651's. Back when I started this repair when I found a known bad part I woulld order some extra stuff that may be in question as I went through the board. Looks like those 2 parts were not in stock and obsolete so I took what was available. So what happens on this eventually Joe? Will you have to go to another brand, find an ON replacement, or do a whole new redesign for parts that are available??
Click to expand...

Eventually Ray will have to find alternatives and qualify them. Was really sad when ON Semi discontinued TO-92 production, they are the best manufacturer out there. I have a lot on hand, I stock up very heavily when EOL notices are served by the manufacturers in cases like this.

I eventually had to move before on RevD to RevE when ST Micro discontinued production on 2 critical parts on that board. I still have a lot of service stock available for that too.
 
Hmmm I assume you tested the Central parts.... just curious, what did you find you did not like about them?
 
Gepetto said:
Eventually Ray will have to find alternatives and qualify them. Was really sad when ON Semi discontinued TO-92 production, they are the best manufacturer out there. I have a lot on hand, I stock up very heavily when EOL notices are served by the manufacturers in cases like this.

I eventually had to move before on RevD to RevE when ST Micro discontinued production on 2 critical parts on that board. I still have a lot of service stock available for that too.
Click to expand...


And i have quite a bit of stock to back you up too...
 
LOL Lee I hope I will never need another one again... at least not on this amp!
 
WOW.... Just read this thread from start to here !!!! And I have once again learned !!!!! 3 things.... Never try to do a rebuild on one of these amps... That was nothing new !!! Joe has got to be one of the most easy going laid back understanding guys ever.... And... Roccus needs a chill pill !!!
 
Joe... the parts came in today, thanks very much for that. I put them in and everything looked good, 0V on outputs and bias looks good now, so I connected the DCP the points stayed closed as they should and all voltages still look good, then I put D9 and D10 back in and everything still good. I think that is it. I have it sitting here powered up to get good and warm to tweak the bias..... anything else you think I should look at before I connect an input and speakers??
 
Glad you are getting good results Ray.

Did you shotgun all 3 of the MPSA42s on the right channel? Did you measure the balance on the 3-133 ohm resistors now that they have been swapped out?
 
Shotgun??? I replaced all 3 with ones you sent as well as the MPS651..... I am not getting the .27V on Q3,4,5 like you wanted I have Q3 = .2329
Q4 = .2401 and Q5 = .2358... on the resistors R17= 133.28 R18 = 132.70 R19 = 132.78
 
Shotgun is a repair term. Where you just swap things out when they are suspect.

The 0.27V is an approximation Ray, the circuit regulates where it needs to be based on the global feedback loop. The important aspects are that the 3 readings are within 7mV of each other which means all 3 transistors are sharing quite equally and that you output offset is no longer suffering voltage sensitivity. You obviously had walking wounded in there prior.
 
So you think ok to connect my PL3000 and some speakers???
 
Based on your measurements, you are looking good. Get the bias dialed into 0.35V...

BTW, this was your original problem that you just fixed.

Live long and prosper my friend...
 
You must log in or register to reply here.
Back
Top