LM394 on my Minelab PCB

[mario]

Hi, I'm trying to change the LM394 chip on my SD2100V2. I have a MAT12 from analog devices. It's suppose to be a real quiet chip compared to the LM394 (Mat 12 has only 6 pins, but according to the LM394 spec sheet, pin 4 and 5 are redundant. I also have a SSM2210 chip in case something goes wrong.

Can some one help me find it on my board. According to the SD2000's schematic, I should have continuity between pin 3 and pin 5 of the LM394. However, when I use my ammeter, I'm getting resistance. The two possible candidate chips are obvious on my photo, based on size of the chip when compared to the SSM2210, which is suppose to be a direct replacement.

Can some one help please me

[gef12]

Mat12

The Mat12 only has 6 pins .. I have it on my 2000.
Insert it in holes 1,2,3 and 8, 7,6 on the pcb dont worry about 4 and 5.
Have not got 2100 schematic so dont know where it goes.. i didnt think it used the dual npn tranny. ... and only uses 2 fets and the AD797's..
Maybe the only 2000 with it on unless anyone else has it

[mario]

Hi, is the mat12 a much more quiet chip than the ssm2210? The audio guys really like that chip. Unfortunately, if you are right, I wont be using my chip. Is there a replacement for the 797s?

[gef12]

Hi, is the mat12 a much more quiet chip than the ssm2210? The audio guys really like that chip. Unfortunately, if you are right, I wont be using my chip. Is there a replacement for the 797s?

Tis really hard to say if it is much more quiet .. unless I swap them in and out .. and I dont think I will be doing that.
You could try the LT1028 as replacement for the 797.. I have 2 in front end of my 2000.
Going for the lowest noise floor as possible..

[mario]

where are the AD797s on the PCBs?

Now I'm back to square 1! lol

[Mechanic]

Hi Mario,

A lme49990 is a good replacement for the ad797 if you can get your hands on one. You need to locate your front end fets, and then find the fet that hooks up to pin 3 on the ad797 Then you have the ad797! If you could post a better photo of your board it may help.

Cheers Mick

[Mechanic]

Lets see if the pic works....

[mario]

Better image, fets numbered

Ok, I got a much higher resolution shot of the pcb, found here . Fets are numbered in red, 8 pin IC's are numbered in blue.

I tried checking continuity between the fets to pin 3 of all the 8 pin chips, and noticed the following: I found continuity between the middle prong of big fet 11, and pin 3 of ICs 1, 2, 5, 7 on the main pcb.

On the daughter pcb , the middle prong of big fet 11 also has continuity between it and pin 3 of another 4 "eight pin" chips.

Interesting enough, the right prong of Fet 11 shows continuity between it , and pin 4 of ICs 3,6,9. In addition, the right prong of Fet 11 shows continuity between it and pin 4 of the unmarked 'eight pin IC' to the right of IC 9.

Big fet 11 doesn't show on my pic because it's hidden by that big capacitor on the top-right most corner of the photo, but it looks like fet 5. Fet 11 is screwed to a blue ground wire, and is also screwed to the case.

Also, that component you inquired about is an enclosed resistor.

[gef12]

Hi Mario,

A lme49990 is a good replacement for the ad797 if you can get your hands on one. You need to locate your front end fets, and then find the fet that hooks up to pin 3 on the ad797 Then you have the ad797! If you could post a better photo of your board it may help.

Cheers Mick

Ooor Mick ... I think Zed said LT1028 was better. ..no matter either I reckon

[mario]

From what I have read, the LT1028 chips are a good replacement for the 797s, but I'm also reading good things about the ADA4898-1. My plan is to install a 8 dim socket to experiment with several chips. I do plan on replacing the NE5534s chips also with LT1028s if I find them.

I just wish i could find a schematic of my board online, that way I know exactly what to replace and not risk frying anything.

If I find them I will replace them and report my findings.

[Mechanic]

Hi Mario,

You can get some lt1028 from Linear technology as samples.

The input path to the preamp will be the same as the 2000 schematic.

Ok had a bit of a look at your pic and put some comments on there as to what I think some of the components may be.
http://i590.photobucket.com/albums/s...025_172418.jpg

See how you go.

Do you have a scope?

Cheers Mick

[mario]

Im getting a scope right now. I need one also for testing my coils.

What max frequency do you recommend? 20MHZ ok? some will do +30MHZ but they are pricey.

I have found some that will hook up to pc, but will only do 50kHZ in range.

what max frequency do you recomend for testing (assuming i will be testing coils also?

[mario]

I took a look at your pic , its the same one as the 1st you posted? i measured its resistance, its about 2.6k

[Mechanic]

My apologies!
http://i590.photobucket.com/albums/s...ck/2100big.jpg
try that one....

A 20mhz scope should be fine, but a nice 100mhz would be better! Make sure it has at least 2 channels. I got mine second hand off ebay for about $300 you can get them cheaper sometimes.

Cheers Mick

[mario]

AD797!

BINGO! I tested your circuit and there's continuity between all components, AND pin 3 of IC 8. Hence we found the elusive AD797!

Thank you!

After more testing, I found the choke to fet 11, and it seems like it's a really important fet. I also found the p fet to fet 11.

Question, what is the purpose to fet 11? And what are those chips: 1,2,5, and 7? Are they 5534s like you suggested? Pin 3 of those chips hooks up to the P fet of fet 11, while fet 11 also hooks up to pin 4 of ICs 3,6,9

It seems to me like fet 11 is an important fet, an older part worth considering upgrading.

Should I also upgrade the p fet/n fet of AD797, if so which part numbers should I replace them with?

Last question, is there only 1 AD797?

thanks in advance