Drawmer 1960 repair

[Aleguitarpro]

I have a 1992 drawmer that works perfectly except compression on channel 1.
I think is the potted black box vca inside because 2 100R resistors that feeds +/-15v to the module are burned.

I have schematics but I don't know if I can post them here.
Is copyrighted?

Anyone has experience on this kind of repair?
I'm designing the black box PCB but I don't know what is T7 (seems a fet).

 

[gyraf]

..yes, the 1960 is a jfet compressor, not a tube comp..

I think I have a schematic of blackbox-part somewhere

Drawmer usually gets mad if we post their schematics here - but perhaps either redraw or just post temporarily in tech libary?

/Jakob E.

 

[Aleguitarpro]

..yes, the 1960 is a jfet compressor, not a tube comp..

I think I have a schematic of blackbox-part somewhere

Drawmer usually gets mad if we post their schematics here - but perhaps either redraw or just post temporarily in tech libary?

/Jakob E.

Ok, I'm going to redraw it and post .
Maybe I can remove it later
I'm only wondering which kind of FET I could use.
It's between two opamps.
I'll post it asap
Thanks

P.s. I wrote them asking a replacement.
They told me they haven't this part and that they can make a replacement but that is going to be more expensive.
After having asked how much they disappeared.
I want to fix it and I'd like to share the black box if someone has one broken... But seems that I can't.
I don't understand their policy at all

 

[Aleguitarpro]

However, the FET seems to be a to-18 (metal case)..
This is a photo of a newer 1960 without black module

 

[Whoops]

What is the blackbox? Is it an encapsulated amplifier?

Post the schematic on the tech docs section or post it in imgur and link it tho there, you are repairing your own equipment and the original company doesn’t even have spare parts...

 

[jensenmann]

I have only good things to say about the Drawmer service dept. I have a shitload of their gear in my dayjob as well as in private use. At the few occasions I had to ask for support they helped me out, even with docs. They are asking not to publish them which I do respect. They went through a lot of shit over the years with several clone companies stealing their designs. With that in mind I can clearly understand why they are careful with giving their documentations away.

I have the schematic of the non-blackbox version which is floating around in the www. It doesn´t contain the Fet type, though. No help from that side, unfortunately. I´d just stick in something midle-of-the road, maybe with the focus on low-noise. Drawmer never used anything super-special in their designs, I´m pretty sure that a 1960 is no exception. But they left out the Fet-type in the docs of all their designs. Maybe that´s the exception, who knows....

Every Fet has tolerances in the range of +/-100% or even more. That´s why there are so many trimmers for GR cal. inside. I bet you can get a 2N5457, BF245 or something similar to work. The circuit around the Fet is differential, hence cancelling distortion artefacts out, anyway. So being secretive about the Fet will most likely be because of a low noise type (which is obsolete now obviously). You could give a 2SK170 or 2N3819 or PN4393 a try.

 

[Whoops]

I have a 1992 drawmer that works perfectly except compression on channel 1.
I think is the potted black box vca inside because 2 100R resistors that feeds +/-15v to the module are burned.

You could try to replace the resistors, measure voltages and if rails are OK change the blackbox from channel 2 to channel 1 and see if that fix channel1 problem.
If it fix it you will be certain the problem is in the blackbox

 

[Michael Tibes]

I did dig up a redrawn schematic of the comp section, uploaded it to tech docs.

hope this helps,

Michael

 

[Aleguitarpro]

I did dig up a redrawn schematic of the comp section, uploaded it to tech docs.

hope this helps,

Michael

Thank you very much.
Downloaded and put side by side to schematics I have and they are identical.

I'll put a socket to test different FET.

You could try to replace the resistors, measure voltages and if rails are OK change the blackbox from channel 2 to channel 1 and see if that fix channel1 problem.
If it fix it you will be certain the problem is in the blackbox

Already done, yes it's the black box.

However from photos on the net of the non-encapsulated VCA FETs are to-18 type

Maybe a 2n4091 or a 2n4391 or ...?

 

[gyraf]

Quote, from I-don't-know-who, 28. feb. 2018:

V4, has a 2N4091 FET in the VCA section

/Jakob E.

 

[Whoops]

what is the circuit that is inside the blackbox? Is it an encapsulated amplifier?

 

[gyraf]

Differential/balanced gain wrapped across two stages consisting of 5532 opamp plus simple ECC82 gain stage, and TL072 debalancing..

https://groupdiy.com/index.php?action=dlattach;topic=44990.0;attach=77100;image

 

[Michael Tibes]

If I got it right it's the white parts in that drawing. The one bit I don't understand is where the CV is coming out of the black box, leading to the GR Meter circuit.

Michael

 

[Michael Tibes]

I wonder if the choice of fet in that circuit is particuarly critical or if the usual 1176-approach should work the same?

Michael

 

[gyraf]

Michael - there is no black box in this schematic, this is what it is..

Attached "blackbox" overview

Was only blackboxed in the first years of production, probably to conceal the fact that it was not really a tube compressor

The GR metering just gets the control signal also, drives meter..



/Jakob E.

 

[Aleguitarpro]

Thanks for infos and schemes.

Attached my version of black box.

I'm going to have about 5pcbs ready to be stuffed in about 15 days.

Then I'm going to test FET to fits the 1960 needs.

 

[Aleguitarpro]

Most challenging part is to fit everything in a 40x40 pcb...

Probably not the best thing to be seen but I haven't so much time to spend in this repair.
If someone has to repair one I'll have 4 spare pcbs.

 

[Ike Zimbel]

...and I have 4 actual Drawmer p/n 6853 VCA boards if anyone's looking. That's complete, fully assembled, factory VCA modules. I also have a ton of other Drawmer parts, including pots, switches, knobs, jacks, transformers etc. You need anything, just ask!

 

[Aleguitarpro]

I just read your post and sent PM