Modify 1176 Rev A to B mnats

[dreamix]

I have a 1176 clone, the mnats but with original O-12 and 5002.

Is there someone who could explain what needs to be done, I tried to read up and seems they changed to a bipolar fet design in the signal preamp, but I cant find a good schematic for Rev B so I can see what the differences are...
I found Rev D ones but they are all in pretty bad resolution.

Thanks in advance Mattias

 

[dreamix]

Damn it was hard to find a rev B schematic

 

[Brian Roth]

Years ago, there was a subsection at JBLProService.com that had a bunch of vintage UREI manuals for download. I didn't look all that hard, but that section seems to be deleted now.

In that section, they had a manual that covered all of the 1176 revisions. I have a copy of that PDF but it is too big to upload here. However, I just found it using the Wayback machine (Archive.org).

Bri

EDIT
I forgot to post the link:

https://web.archive.org/web/2006050.../navigation/Vintage JBL-UREI Electronics.html

 

[dreamix]

Years ago, there was a subsection at JBLProService.com that had a bunch of vintage UREI manuals for download. I didn't look all that hard, but that section seems to be deleted now.

In that section, they had a manual that covered all of the 1176 revisions. I have a copy of that PDF but it is too big to upload here. However, I just found it using the Wayback machine (Archive.org).

Bri

EDIT
I forgot to post the link:

https://web.archive.org/web/2006050.../navigation/Vintage JBL-UREI Electronics.html

Wow, thanks alot, really appreciate it, it will help alot !

 

[Brian Roth]

Don't be mislead by the front section of the 1176 manual in the link I posted above. The first part of the PDF is a scan of the user manual for late production units. Further in, there is a ton of info re. serial numbers, adjustments, and schemos all versions starting with "A".

Bri

 

[dreamix]

I (think) I changed my mnats circuit from rev A to B, there is some problems ofcourse See my voltages in the schematic

Unit is passing audio, Input and output pot is working, metering for +4 and +8 is working, metering for GR is not working, barely moving... Bias does change i.e voltages at Q1 gate and metering moves when adjusting, but no voltage drops are done when measuring output pin 2 and 3,

its strange cause the GR section of the circuit I barely touched, I moved R74 to fit between Q12 and Q13 Collector in the meter amp, and changed R43, R45 value in the GR control amp

When comparing to my rev D original UA unit, my values around Q8 and Q10 are weird, especially emitters

EDIT: voltages now seems fine, input seems abit low but with Input fully CW there is decent amount of GR on the METER only, NOT the audio

https://www.dropbox.com/s/2fv7a3icfqtkv3j/1176 mina ändringar och voltages.jpg?dl=0

 

[dreamix]

So... the unit is now metering GR but is not actually doing any compression to the sound which makes me think there is a problem with or around Q1, Attack knob? provided video If anyone have any ideas :/

https://www.dropbox.com/s/ry9r9u84l60v3v4/20220427_142404.mp4?dl=0

 

[dbonin]

So... the unit is now metering GR but is not actually doing any compression to the sound which makes me think there is a problem with or around Q1, Attack knob? provided video If anyone have any ideas :/

https://www.dropbox.com/s/ry9r9u84l60v3v4/20220427_142404.mp4?dl=0

Echo North's Compression test:
IF YOU CAN DO THIS, YOUR UNIT IS COMPRESSING

Basically do the meter tracking adjustment without adjusting the meter.
Do this ignoring the VU meter on the front.

Input = "24" mid rotation
Output = "24" mid rotation
Attack = full CW
Release = full CW
Compression ratio = 20:1
Meter mode = "GR"

1. Apply a signal (1 KHz, 0db).
2. Set output control for 0db as read on an external meter measuring AC volts across output XLR pin 2 and 3
3. Set attack full CCW (off position). Set input control for +10db as read on an external
meter measuring AC volts across output XLR pin 2 and 3
4. Turn the attack control OFF (CCW) and readjust the output level control for "0" if
necessary.
5. Repeat 3 & 4 until the output drops 10dB (2.44V to 0.775V) whenever the attack control is turned ON.

Again, If you can do this, your unit is compressing.

 

[dreamix]

I had put 560 Ω instead of 560k Ω at R9 it does work now , and sounds really damn nice, but I found the mnats isnt the best board to experiment on so Im printing the original rev B board, get them on monday, exciting

 

[dreamix]

Fullsize one-sided Rev B and D pcbs arrived today, over my expectations really

 

[dreamix]

I see some builds and UA reissues have tant caps in C19 and C20, and others electrolytics, what is the difference here really?
Also for C1 I have a WIMA 1uf 100v to choose from or a Siemens 1uf 250v, will the higher voltage be an issue at all for the siemens?

 

[dreamix]

No? Anyone knows what C19 C20 does

 

[Jonas05]

Hi,
I really appreciate your effort (also on the bla mod)! I messed around with my hairball bluestripe as well. What I really liked: Disconnecting the Feedback winding of the output transformer. But you have to solder in some bridges to keep the rest of the circuit intact.
Have you fixed all your problems? I'm thinking about modding to rev B. Was it a notable difference?