All things G1176 - the new "repost" thread.

[fum]

Allright, got a minor problem to hunt down on one of my 1176's. Of course I didn't find the problem until their both in the rack next to each other. One of them isn't working right. Passing audio, but at a far lower level than the other.

Edit:

Ok, my problem is the junction where the collectors of Q6 and Q7 feed the 1n4003 diode, and the base of Q8. It should be roughly 15V, and I've got 23V.

the base of Q9 is getting 22V. I'm huntin down that path to ground at present

All other voltages check out.

Thoughts?

 

[mnats]

Isnt that lorlin a 2 pole switch?

When I was around 10 years old I remember trying to make a tic-tac-toe game using DPDT switches...sort of feel that way looking at this rotary idea.

Yeah, the rotaries are 2 pole. But if you look at the diagram pushing in all the switches equates to what Jakob did - it creates a short between R23 and R26 on one side and R87 and R45 on the other. So like Juergen Haible pointed out on his page it's like pushing in 4 and 20; the other two won't change anything.

So to simplify things a bit...if you think about having the rotary at 4 and 20 at the same time how do you do that? Not possible because in spite of being two pole, the poles are connected at the wrong point to be able to create the required short between the points indicated above.

Bummer.

 

[deanp920]

Fum,

Ok, my problem is the junction where the collectors of Q6 and Q7 feed the 1n4003 diode, and the base of Q8. It should be roughly 15V, and I've got 23V.

Of course check your component values and placement/orientation. Make sure C14 is not shorted and is polarized correctly. Make sure C12 and C29 are not shorted. Then, since they are cheap and readily available, I would replace both 1N4003 diodes. If that still does not fix it, I would replace Q6 and Q7 as a pair, followed by Q8 and Q9 as a pair.

Ugly things may happen if you remove these transistors individually and power up.

Sorry to recommend such a shotgun approach, but that is probably what I would do.

Dean

 

[fum]

thanks for the suggestions, Dean

already tried replacing the diodes, same dealio. Will check the other things.

Replacing parts isn't as drastic as it seems. I always buy extras, expecting I'll either build more later, or will need to replace some potentially.

Regards

ju

 

[fum]

Well, no further. swapped all the components out for new ( transistors, caps, diodes).

Checked resistor values, they all are on.

Went back through checking for shorts ( twice).

It's gotta be there somewhere.

ju

 

[deanp920]

Went back through checking for shorts ( twice).

How about opens; ie., broken traces, cold solder joints? Sometimes your ohmeter is better than your eyes for checking these things.

Dean

 

[deanp920]

Also, I would squirt some signal through (~200Hz) and check AC levels at various points with my DMM, comparing figures with the other unit. Then at least you'll know more precisely where the gain is dropping off.

 

[fum]

Well I found the loss in gain. It's actually the output transformer. Looks like one of the output pins is cold ( I was pretty careful about soldering to it, but perhaps it got too hot and the wire lifted on the inside).

I checked between my two boxes, and both of them have 23V at the base of Q8, and 22V at the base of Q9.

Can someone else check to see if this is what they have? The schematic says it should be on the order of 15V and 14V ( from memory), is the schematic perhaps incorrect?

Have ordered new output transformer, will wait till it arrives to resume work.

Regards

ju

 

[Zee1usa]

Fum,

Was the transformer failure a warrantable failure?

:?:

 

[deanp920]

I checked between my two boxes, and both of them have 23V at the base of Q8, and 22V at the base of Q9.

Can someone else check to see if this is what they have? The schematic says it should be on the order of 15V and 14V ( from memory), is the schematic perhaps incorrect?

The voltages at Q8 and Q9 on my unit match the schematic within one tenth of a volt, as do all the other voltages.

Well I found the loss in gain. It's actually the output transformer. Looks like one of the output pins is cold ( I was pretty careful about soldering to it, but perhaps it got too hot and the wire lifted on the inside).

Did you remove the transformer and verify the fault using the Lundahl datasheet and your ohmeter?

Dean

 

[fum]

Not yet, excellent suggestion. Next step =). I've gone cross-eyed starin at these boards tryin to figure where my volts are comin from.

I've swapped out all the components in the aformentioned list, and when I say checkin for shorts/opens, I'm doin it with my Fluke.

Regards

ju

 

[dale116dot7]

To me the transformer doesn't make sense - at least to cause your voltage shift - it's on the other side of a coupling capacitor.

What normally sets that voltage is going to be the following:

R29 and R30 form a voltage divider setting the bias voltage of Q5.

R32 and R34 form a voltage divider in a feedback network. The rest of the output amp tries to servo the voltage so an input voltage of 5V results in a voltage of about 4.3 at the emitter of Q5 (0.7 volt difference between base and emitter as a rule of thumb). Taking 4.3 volts * R34 / (R34 + R32) means the output should sit at about 13.5 volts.

It's possible there's something else wrong with the output amp, too, possibly a short or open that causes it to almost work. If the output level is quite low then it's possible the amp is broken and you're just getting output from Q5 through R34 and through the output. I would not try troubleshooting the audio portion until you have the DC voltages where they are supposed to be.

There's one other thing that could cause the 23 volt output and also a strange sound - oscillation in the output amp. C12, R36, and C27 set the bandwidth of the amplifier and if something is wrong with any of those it might oscillate in the RF region, and this may cause the output to look more-or-less normal (or maybe not) but the DC offset is strange. Normally if you put your finger on various points on the PC board you'll either make it worse or better - watch that DC voltage.

If you post the various voltages - in particular, every pin (collector, emitter, base) of all of the transistors, then someone here should be able to point you to the problem fairly quickly. A difficult thing with feedback amplifiers is when they quit they tend to go to one power rail or the other, so you can't just troubleshoot it stage-by-stage.

One other thing that might be obvious - did you put in PNP transistors for Q6 and Q7 (BC560's)? Are they backwards - or do you have reversed-package parts?

Hope this helps. I just finished troubleshooting my G1176 and it's working now. I put in a 560k resistor in place of a 560 ohm resistor and it didn't work.

 

[fum]

[quote author="dale116dot7"]To me the transformer doesn't make sense - at least to cause your voltage shift - it's on the other side of a coupling capacitor.

[/quote]

Your completely correct here. What I was saying is that I have two problems. On the one unit, the reason it's output is not the same as the other is because of the transformer problem.

both of them have this voltage shift problem, which is a problem I haven't gotten to the bottom of yet( and haven't had much time for lately, I've been in "keep the drummer happy with his drum tone") mode for several days now.

Just got the replacement traffos, givin it a go again ( if it wasn't bloody 100 degrees here today. Ya, I know, we're heat wimps here in the PNW, but I'm used to it being grey and 50 )

The unit with the good traffo but still the voltage problem outputs at a decent level, just isn't quite right soundwise. Will get to the bottom of this =)

I'll do the diligence and post a complete pin/voltage viewpoint.

Thanks for the help

ju

 

[fum]

Ok, heres the news:

Q5

E 4.48V
B 4.93V
C 28.24V

Q6
E 28.82V
B 28.25V
C 23.86V

Q7
E 29.48V
B 28.82V
C 23.86V

Q8
E 23.29V
B 23.87V
C 30.41V

Q9
E 14.20V
B 22.55V
C 0V

I just made the measurements, I'm gonna go back through what ya said dale, and see if I can make sense of it =)

Regards

ju

 

[dale116dot7]

Q5

E 4.48V
B 4.93V
C 28.24V

This stage looks good.


Q6
E 28.82V
B 28.25V
C 23.86V

This stage almost looks good. The C voltage is off because of something later.

Q7
E 29.48V
B 28.82V
C 23.86V

This also looks good. Same thing as above, since Q7 and Q6 form a darlington pair.

Q8
E 23.29V
B 23.87V
C 30.41V

This basically looks right, but why the voltage shift?

Q9
E 14.20V
B 22.55V
C 0V

This appears to be the culprit. The hint here is that there is no 0.7 volt difference between B and E. Check the pinout of this transistor and verify that it's the right type (PNP) and put in the right way. If it is ok, check out R41 for an open or wrong value.

One additional note:

If that's a misprint and it's actually 24.20 volts on E, than this stage is working and I'd look for a 9.3k resistor in place of the 4.3k resistor, or maybe a 1k resistor in place of the 2k resistor (R32 and R34). Either one would result in a shift like this too.

Good luck!

 

[fum]

Ok, think I've found it. Damn eyes and brain.

The resistors R40 and R41 after Q8 and Q9 are 4.3 ohm, not 4.3K

I kept seeing 4R3 as 4K3.

Thanks Dale

I love those "Big Duh!" moments.

I've been staring at this for hours, sure enough ( didn't have 4.3 ohm in stock, but luckily, did have 8.6 in parallel), swap for the correct values, and watch those voltages stand in line the way they're supposed to.


ju

 

[gyraf]

Good work, Dale and Fum!

:thumb:

Jakob E.

 

[Zee1usa]

Hello..


Wiring up my G1176 and I am not quite sure where wire # 1*is supposed to go? I am using a luhndal input transfo.
It is the hole labeled #1 and it has a *

It appears that people wire things a bit differently by looking at others pictures that have been posted. :?


Thank you

 

[fum]

With the optional traffo input, connect to the 1* ( the * are used if you have the traffo there, like the resistors and caps up in that section).

Regards

ju

 

[Zee1usa]

Thanks FUM,

Question:

So.. pin #1 from the input potentiometer which would normally go to
hole #1 (input gain) goes to hole labeled #1* instead if I follow you correctly. So pin # 1 (input gain) will be left empty? :?


:wink: