1176 low gain..

[line61984]

[quote author="skipwave"]I just fired up my '76 for the first time and listened to some old tracks through it. Holy shit! Astoundingly good sounding, and more gain than I know what to do with (the input pot never went up past 3). I can see why a stepdown transformer on the input would make sense. I'm using the electronically balanced in, no money for transformer right now. A mysterious military transformer made by Nytronics on the output, impedance ratio spec'ed at 135:600 ohms.[/quote]

double checked the bd transistors and had to switch them around. no smoke, no smell, but theres no audio and just a loud constant hum at like -3 db.

 

[skipwave]

That's odd. So they were in backwards, and now that you've got them right way round, no audio is passing?

 

[line61984]

[quote author="skipwave"]That's odd. So they were in backwards, and now that you've got them right way round, no audio is passing?[/quote]

thats right. Q6 andQ7 are in right and by taking a look at the BD 139 and BD140 datasheets it shows which pins are which. If it passed audio before ,when the BD were "Backwards" then they werent backwards right?

 

[raysolinski]

My experience has been that any transistors installed the wrong way are fried...replace them and see if it works...double check all your work before powering on...saves $$ on fried parts!

Ray

 

[line61984]

tried switching back the bds to the way they were before but now theres the loud hum with no audio and a smell of burning. do you think think that the bd transisters placed the wrong way would have burnt out the Q6 and Q7 transistors. the pins for the transistors are all on the table and i put it together based on those pins and it doesnt work. maybe the BD transistors are burnt from all this wrong placement and then being powered. the traces on my board are starting to come off in these places :?

 

[skipwave]

Yeah, just think how the poor transistors try to do a very different (and likely impossible) job when installed backwards. They try valiantly, but burn up with the effort.

They were passing audio, but the distortion could have been from them operating in a crippled condition. The maximum voltage emitter-base (Vebo) is only 5V, whereas the voltage collector-base (Vcbo) is 80V. Vebo was probably exceeded with them in backwards.

I checked the sheets on the BD139, 140 and they agreed with the silkscreen orientation on the mnats pcb.

 

[line61984]

[quote author="skipwave"]Yeah, just think how the poor transistors try to do a very different (and likely impossible) job when installed backwards. They try valiantly, but burn up with the effort.

They were passing audio, but the distortion could have been from them operating in a crippled condition. The maximum voltage emitter-base (Vebo) is only 5V, whereas the voltage collector-base (Vcbo) is 80V. Vebo was probably exceeded with them in backwards.

I checked the sheets on the BD139, 140 and they agreed with the silkscreen orientation on the mnats pcb.[/quote]

so I will replace the BD transistors but should I replace Q6 and Q7 again?

 

[bluezplaya]

I just replaced the BD135 and BD136 and my problem remains...signal is distorted when input is all the way up and no compression :?

 

[skipwave]

[quote author="bluezplaya"]...signal is distorted when input is all the way up and no compression :?[/quote]

Input all the way up? It's normal to distort with the input pot wide open. Especially with gain reduction switched off. Most people are reporting barely turning it a few notches up. PRR has expounded on the limitations of FETs in this application, with their low Vgs, we have to knock the incoming signal down and then build it back up with gain in the output amp. And the 24V rail doesn't allow for much headroom.

Maybe I misunderstood the situation.

 

[bluezplaya]

The signal is clipping badly with the input wide open. I do notice that it doesn't start really clipping until it is almost all the way up. However, there is no compression.

 

[skipwave]

Ah, I see. Either the Gain Reduction Disable switch (first position of the meter switch) is miswired, or your threshold is too high. I'm not sure the latter is possible, so I'd check that switch first.

 

[bluezplaya]

I don't have the meter wired yet because I figured I would check levels to make sure it works first. Does it have to be hooked up?

 

[skipwave]

Yeah, the meter board must be connected. On the schematic connection point 22 is the input to the GR amp.

On mnats page see that 22 is on the meter board:

http://www.geocities.com/m_natsume/1176-wiring-rotary.html

The explanation from that page:

"Point 22 of the circuit goes to the Gain Reduction Control Amplifier. In Jakob's implementation, the signal that feeds the GR Control Amp goes to the meter switch board first. This is because the meter switch incorporates the Gain Reduction Off function which, on the original, was part of the attack control.

Pad 4, Meter BoardThe signal that feeds the meter switch comes from the resistor divider we looked at earlier, which is part of the ratio circuit. Connect a wire to pad 4 on the meter board. Note that the designation "4" doesn't show up anywhere on the 1176 schematic, it's a marker unique to the clone.

When I wired this particular unit, I used ordinary stranded wire for this point. Looking at the schematic, the point between the ratio switches and the Gain Reduction Off switch is also shown unshielded. But it may be beneficial to use shielded wire here, since that is what is used after the switch.

Pad 4, Ratio Board Wire the other end to pad 4 on the ratio board, right next to where the output pot wires go."

 

[line61984]

[quote author="skipwave"][quote author="bluezplaya"]...signal is distorted when input is all the way up and no compression :?[/quote]

Input all the way up? It's normal to distort with the input pot wide open. Especially with gain reduction switched off. Most people are reporting barely turning it a few notches up. PRR has expounded on the limitations of FETs in this application, with their low Vgs, we have to knock the incoming signal down and then build it back up with gain in the output amp. And the 24V rail doesn't allow for much headroom.

Maybe I misunderstood the situation.[/quote]


so if all is wired correctly but the bd transistors are burnt up, what would be the symptoms. I rewired the audio input and output pots and theres a low buzz that clips my daw at a very low level and it is not adjustable with the level on my 1176. Im hoping that itll fire up right when I replace the bd transistors

 

[bluezplaya]

Okay, my meter board is wired properly. Could any of the other transistors be fried?

 

[skipwave]

[quote author="line61984"]theres a low buzz that clips my daw at a very low level and it is not adjustable with the level on my 1176.[/quote]

Low in pitch or low in volume? Well, it must be fairly high level if it clips your ADC.

Definitely replace those output transistors. If they are damaged, you won't get a useful test of your unit without replacing. Do you need some new ones? I have a few extras I can send you.


[quote author="bluezplaya"]Okay, my meter board is wired properly. Could any of the other transistors be fried?[/quote]

So I take it you are now hearing gain reduction, but the signal is distorted? It might be time for signal tracing. Got a scope?

 

[bluezplaya]

No scope, just a DMM.

 

[skipwave]

DIY a signal probe: http://www.recording.org/users/kev/signaltracing.htm

From this post by Mark Burnley, which addresses probing a no-signal 1176: CLICK-Y

 

[StudioSQ]

I am having a low gain problem I just can't seem to get on top of. I built a mats rev j with in and out transformers. All the transistors read correctly when tested against the schematic, and the unit seems to work fine except for that it has much lower gain than my other unit that is a rev d. I would say i am about 20db shy of the other unit. It is hard to tell whether the gain loss is in the preamp or the line amp stage. It also is difficult to get it to give me more than 7db of gain reduction when calibrating. Any ideas?

 

[Dsaudio]

Sorry for my english

I have been doing comparisons between urei 1176 revision C and mntas / gyraf 1176 and the gain are very low compared with originals
UREI, even with 1:2 step output transformer the gain are low, i probed some modifications in the line output amp without success, finally
solved with 1:4 step up output transformer and now seems to works with same urei 1176 gain, with same hissssss but any humm.