Audio research corp D125 what fet to buy

[blindboybenton]

Hello all

Had this one in a while ago. Brought a load of Fets from the usa paid an arm and a leg for them + tax

Anyway fitted them and still having issues, Has been else where before me
Such a pain to work on, with the chassis half covering the board and with tight wires no abbilty to really get in there
so i removed the board and spent time make extension wire so i can run the amp with the board out flat
Audio research corp D125SchPL.pdf Page 2-3
I think i got the area of the fault .. But i want to try some Jfets to see if it fixes it. If i does then i will buy the right ones . But Audio research remove the marking and paint on lines of colour. so i have no idea what they have used apart from looking at the map

i would like to know in your option what you would put in

Q3 and Q4

Q9-10-23-24-25-26

these have a different pin out D-G-S

many thanks lm looking forward to your answers

[nickb]

Assuming the ones you bought were the exact type it would be big help to know what there were as a starting point.

[blindboybenton]

The ones I brought were from audio research Corp they were the correct ones. I didn't buy any spares and I think I need a few more jfets.
Bbb

[nickb]

Are they marked with the ARC numbers or the usual numbers?

[blindboybenton]

All gets have there marking rubbed off and replaced with colour coding. I forgot to mention this in the first post.. so I have no idea what they are. In the manual they are listed with a part no or gives you a colour codes
Bbb

[J M Fahey]

I guess they are also scratched and painted.

ARC might have silkscreened new numbers or just paint them black and somehow attach a label (or a couple colour paint dots).

[blindboybenton]

they have sanded the marking of and painted over them. Looked very closely cant make out any markings

BBB

[nickb]

they have sanded the marking of and painted over them. Looked very closely cant make out any markings

BBB

Gotta love 'em...

I don't think they are too critical. Source followers and /or heavy feedback. OTOH I don't know why they use the string of FETS as diodes - seems wasteful. I need to understand why they are doing that there.

What is the exact problem you are having?

[blindboybenton]

When the amp came too me it had a hiss on right channel and left channel was lower output. The Tubes were all new. The was got at by another repairer local to him and has been been fixed or right since.
I ordered 10 fets from ARC and fitted 8 Q7-8 and Q32-33. since replacing these the amp has got worse now the fault is in the pi

Ive double checked and checked with ARC that i had the right fets. checked soldering etc
So i decided to make it easier on myself to work on to run extension wires so the board can put on the table and work on it easier

Now starting at the input checking both channels and working up to the PI... the main problem is at the present that somebody has changed two Jfets Q3 on both channels for a couple of stock one's not in the workshop at present can't remember what type
I want to change both Q3 on both channels. I feel confident that one of them is down

so the info i need at present is if this was your repair, without waiting 10 days +to get them from ARC what would you change them for

I understand that now ive got another fault due to the fets i changed and the Q3-4 wasn't touched.

BBB

[nickb]

Q1, Q3 & Q4 are quite critical as the DC conditions need to be correct. The resistor in the source is quite low in value and that is what makes it critical. Q1 also needs a higish gm to make the gain of the inverter unity.

I've been looking at what RS and Farnell have that might work but it's very time consuming and it's getting close to time for my Sunday pm nap

[Jazz P Bass]

ZZZZZZZZZZ.

[nickb]

I'll type quietly so as not to wake Jazz up...

...Looking at RS & Farnell I've come up empty. For Q1, Q3 & Q4 you need Vds(max) = 55V at least. There is just nothing. I've also come to the conclusion that ARC must have done a binning selection on these to that they will meet the required DC conditions. JFETS off the production line have too wide a spread and even then I could not find anything readily available that came close. I'm throwing in the towel at this point.

Have you been able to measure the DC conditions around these three to verify that the earlier substitutions were satisfactory. Also, how come these FET keep failing? They do fail but it's very unusual for so many to fail in such a short time.

[blindboybenton]

Many thanks for trying I think I will get them from arc. I will get the ones that the previous engineer swapped out and a some of the others in the same circuit. He had used j111 the fault is most defently in this area. Many thanks again for trying. Hope you had a good nap bbb

[blindboybenton]

Hi Nick

I got the parts fitted from ARC. Now the right channel is working perfectly. Left now is better but the i have a voltage being dragged down to 0v at R27. found S/c Q12 pulled it out and voltage is back up. s/c between Drain and Gain both directions .. I'm going to get a few mosfets from RS and see. what happens.. Fed up with this amp now.. it will go on my list of amps not to touch