Original 1176LN "F" has low output, no compression. What to look for?

[rob61]

I've got an original Urei 1176LN version "F". It has low output, just to get a "normal" level out of it both the input and output controls need to be almost fully clockwise. There is no compression. And if you press the Meter GR button (to check gain reduction), it pins the meter needle hard to the right. Upon opening the unit, I found the previous owner had put a 1.8K resistor across the output "C" and "+/_". I know about using a 600 Ohm resistor to load the output transformer, but not sure why he got a 1.8K there. I've removed it, and once fixed would replace with a 604r one instead. This couldn't have damaged the output transformer could it?

I was going to recap, but at this point, I'm thinking it's much more than that. Any ideas on how to troubleshoot, and what to look for? I'd appreciate anything I could check here if you can suggest something.

Thanks

 

[Brian Roth]

First thing to check are the power supply rails.  You should have +30 VDC for the main rail, and -10 VDC for the other.  Both rail voltages are measured using ground/chassis for the "common" "minus" connection to your DMM.  I believe there are test points for the rails, but if not, you can use the Zener diodes as connection test points.

Bri

 

[rob61]

Thanks very much Brian. I don't see any test points, and only one Zener diode (CR6)... how would I test for the two rails this way?

 

[pucho812]

here is a schematic in case you do not have one.

http://www.dripelectronics.com/media/kunena/attachments/69/1176REV_F.pdf

There should be 2 zeners cr5 and cr6. You found cr6. cr5 should be right there too.

 

[rob61]

Thanks for the schematic... I can get 29.25v but I only get a few minus mv... can't get the -10v

 

[rob61]

I've noticed some of the legs on the diodes appear blackened... so I'm wondering if something overheated in the PS. Does this look normal?

This is an original "F", and CR5 looks more like a voltage regulator with a heat sink. But that is what is labeled as "CR5" in this "F". Also, I notice some of the other diodes (CR8, CR9, etc) have a leg that seems blackened (charred?).  Again, with the age of this, I'm not sure if it was a PS issue and heat, or just age.

Any ideas of something to try?

 

[gemini86]

CR5 is a zener diode... it will conduct at the right voltage to keep the rail at a constant. It does dissipate heat, therefor the heatsink.

Is that diode getting warm/hot? mease the AC voltage at CR9/10 If nothing, probably a bad transformer winding or connection.

 

[Brian Roth]

CR5 is the Zener for the +30 rail, and CR6 is the Zener for the -10 rail.  Measure directly across CR6 and be sure you are seeing -10 VDC.

As for the black gunk on the leads of CR1 and CR2 in your pix, I think that is just some sort of corrosion.

Here is the definitive (at least to me) 1176 service info, for all Revs:

http://www.jblproservice.com/pdf/Vintage%20JBL-UREI%20Electronics/UREI-1176LNmanual.pdf

Bri

 

[rob61]

The heatsink on CR5 is getting hot.

Measuring AC across CR9 I get 34.45 V
Measuring AC across CR10 I get 34.65V

Measuring DC across CR6 I get -0.9 mV, and if I flip the probs around I get 0.9 mV

Since I can't find -10 VDC should I suspect CR6 is faulty and needs replacing, or is there something else I should check first?

Thanks for all your help!!!

 

[gemini86]

Check voltage before R86 and R85 to see if they are open. If not, disconnect CR6, see what the voltage is then.

 

[rob61]

Ah hah... on R85 I get -15.75V on one side, but a negligible reading of -1.1 mV on the other...

For R86, I get -15.77V and -31.45V

Do I still need to desolder CR6 or is R85 the culprit?

 

[pucho812]

you can test r85 by lifting one leg of it out of circuit and measure it with a dmm. It should be 1K or somewhere close to that depending on what tolerance it is.

 

[Brian Roth]

Going by your test results,  and seeing "nothing" on the output side of R85....possible culprits are:

R85 has gone open circuit.

CR6 is shorted

C24 is shorted

Something down-stream from CR6/C24 is shorted

Bri

 

[gemini86]

first diconnect cr6, since that would be really easy. retest. then go from there.

just thought I'd add a cropped schematic for the relevant portion of the circuit, so I don't have to keep refering to a big PDF...

 

[Brian Roth]

But DO NOT "power up" the unit with the Zener disconnected!

 

[gemini86]

Yes, thanks Brian. Poorly worded that. Remove CR6, test it. Lift one leg of R86 where it meets C24, check voltage there.

 

[rob61]

OK, thanks... will do that next. I'm going to upgrade my basic DMM, and Fluke comes to mind. Any particular model anyone would recommend for working on audio circuits like this?

 

[gemini86]

I've always used a fluke, and I love my 81438. Only thing I wish it had was transistor sockets and inductance measurement functions. Maybe look for a god LCR meter since you're making a purchase.

 

[Brian Roth]

Remove CR6, test it. Lift one leg of R86 where it meets C24, check voltage there.

I would go at the testing a bit differently. 

First of all, without lifting any leads, measure resistance across C24 or CR6 (same difference).  I suspect a short or very low resistance in this case.  If low resistance is measured, lift one end of CR6 and measure it by itself, and lift one end of C24 and measure resistance across it as well.  It is probably smart to also lift one end of R85 to verify the 1000 Ohms resistance, especially if no shorts are detected across the Zener or cap.

Bri

 

[Brian Roth]

I own a Fluke 87V and am happy with it.  One thing that sets it apart from many DMMs is the frequency response on the AC voltage ranges.  Most meters poop out above 400 Hz, which is the frequency used for AC power on many airplanes.  Thus, the "lesser" meters are only accurate from 50 Hz to 400 Hz, making them unusable for measuring the voltage of audio signals outside of that range.

Bri