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Samuel Rock 21st December 2008 11:38 PM

About to pull my hair out! Working on ORION 2250
First of all i would like to thank Perry, Your Repair CD has given me lots of info and direction, I have been working on Electronics for a bit of time and have learned every thing based on trial and error... and you have filled in a lot of the blanks.

Well now to fill every one in on the topic of the post,

I have an Orion 2250 on the bench ( I work on mostly Orion )
when it came to me it had all the 2n6491 chips blown on the yellow CH.

First thing I did was start pulling the burnt silicone and then checked the drivers, drivers were bad pre drivers were questionable values were not close across them.

I opted to use some TIP32 and TIP31 as predrivers and drivers taking into mind that the cenu07/57 is EBC and the TIP31/32 is BCE so I just moved the chips over one hole and bent the Emitter up and placed a jumper over to the pad on the board... this was not too much of a problem would have looked a bit better if the person that did the repairs before i got the amp had not smoked the lans to the chips they were just kinda hanging in there before i started. i used hot air to remove them so that i did not overheat what was still good.

So once i got the new drivers and pre drivers installed i dropped in some new ST 2n6491

idle rail voltage into the amp section was 46V +/- or both yellow and orange channels.

low db 30hz tone into a 4ohm dummy on the blown CH
low db 30hz tone into a 4hom driver on the good CH
gives me about 45v rail into the amp section
on the out put of the amp i get about .8VAC to the dummy load and the driver.

I let it play like this for about 1 hr then started to turn up the gain once i get up to about 10VAC on my amp output i smoke a 2n6491

now it will only smoke one at a time. and most of the time it is the one where the amp output lead is solderd to the board.

I took the board back out the heat sink and cleaned the back of the transistors and the heat tape, to ensure no trash was creating a ground to the transister. same thing happend then i put some mica strips under the transisters and once again smoke one more 2n6491.

I am thinking i have one of 2 problems, the first is the 2n6491 does not like the TIP drivers, or the ST 2n6491 chips i have are bunk possable restamp or bootleg

has any one used the TIP as a driver/predriver set?

any one have any input?

Perry Babin 22nd December 2008 06:11 AM

Does it blow the same 6491 every time?

If so, check the emitter and base resistors to confirm that all are within tolerance.

If not, run a 50-100Hz sine wave through the amp. Set the output voltage to ~5v AC (normally you would use ~10v but the amp won't allow that). Have a 4 ohm dummy load or speaker connected to the channel being tested. Measure the DC voltage across the emitter resistors to confirm that all of them have the same voltage across them (within 1-2mv).

Are you using TIP31 A, B, or C?

I generally use either 2N6488/91 or MJE15030/31 for the drivers.

Samuel Rock 22nd December 2008 01:37 PM

Well i took some readings of the pre drivers.

On the orange CH i got 1.5v and 1v on my predrivers
On the yellow CH i got 5.2v and 6v on my predrivers.

The yellow CH was the one that had been burnt.. i just orderd some mpsw42/92 from mouser.. I am going to pull the good predrivers from the orange channel and use them as drivers in the yellow channel.. and use the 42/92 as pre drivers.

man i wish i had purchased more mpsu07/57 when they were cheep.

is it possable to use the 2n6488/91 as drivers and the mpsw42/92 as the predrivers?


SineWaveAu7 22nd December 2008 01:37 PM

The CENU07/57's are 100V, the TIP31C/32C is 100V, however the CENU's seem to operate at a higher frequency. They're more compatible with MPS-U07/57 and BD529/530. :)
- Adam

Samuel Rock 22nd December 2008 01:37 PM

I put the TIP31C/32C in.. it was rated for 100v.. i have some 42/92 comming in. i think i am going to go that route..

this is the first time i have had this problem as i purchased a pile of the mspu07/57 and i am now just running out.


Samuel Rock 22nd December 2008 06:20 PM

I just picked up a 50 pack of mpsu07 in new orleans the been on the shelf sence 1990. Gave 1.10 each.


Perry Babin 22nd December 2008 07:22 PM

In this amp, I'd use the TO-202 parts in the pre-driver location and the MJE15030/31 in the driver location. I've used that combination before in this amp and I know it works. On smaller amps, the 6488/91 and the w42/92 will work. The 6488/91 should work as drivers but they will see more than 80v. The outputs see 90+v and survive even though they're only rated for 80v. I prefer not to use parts beyond their ratings even though they may be able to handle it.

I don't know how you measured the voltage on the pre-drivers so I don't know how to interpret the readings.

If you now have enough of the original parts, use them.

Samuel Rock 23rd December 2008 03:33 PM

I just got an email from ON, the tech said that the MJE253G/243G is the best replacement the have.
The data sheets look wacked out on my blackberry I will have to look at them when I get home.

Perry have you used any of the 253/243? I have some of the MPSU07 now but none of the 57, So if the best bet would be to use the 07/57 for per drivers in the large amps then I would like to save the stock for that
And use the alt part for drivers.

I am with you perry on not pushing parts over the limits, I don't like NTE but they do have the 188/189 but they are rated for 80V. What do you think of uising the nte part as predrivers in the large amps?

To get the voltages I stated for the perdeivers I went from what would be the center pin of the MPSU07/57 to the rca ground.

Well my thumbs are tired of typing I will get back with yall this afternoon


Perry Babin 23rd December 2008 06:28 PM

I've never used the MJE253G. The pin configuration is not the same so it doesn't really help.

The problem I have with NTE parts is that you never really know what you're installing. With the 188/189, there aren't many possible candidates. They are most likely rebadged MPSU07/57. They should work as predrivers.

With 5+v on the base of the predriver, the output should have had full rail voltage. If it didn't have full rail voltage on the outputs, I'd suspect an open resistor on the emitter of the prdriver.

Samuel Rock 24th December 2008 02:13 PM

I deleted this message as it was a dup of one above that got reposted when i hit the refresh button.

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