UREI 1176LN rev. F goes into oscillation

Just got an old 1176LN rev F... (for quite cheap)

When trying to calibrate the unit, I discovered that the unit would go into self-oscillation when turning the Output knob to its maximum (VU starts to oscillate and then, goes out the roof when pushing the volume higher). Also, the sound starts clipping when I reach the 18 mark on the Output knob.

Anyone has a clue where to search? Caps, dirty pot?
Thanks!

furn1979 said:

When trying to calibrate the unit, I discovered that the unit would go into self-oscillation when turning the Output knob to its maximum (VU starts to oscillate and then, goes out the roof when pushing the volume higher). Also, the sound starts clipping when I reach the 18 mark on the Output knob.

Click to expand...

Check the power supply voltages to see they are fluctuating or are not within spec (+30v, -10v).

A PS recap would also be a good place to start.

Mark

I'm measuring -10V DC on C23 (the big can) and +31.9V DC on C24 (big can also).

I don't have a scope but I'm not measuring any AC voltage...
???

furn1979 said:

I'm measuring -10V DC on C23 (the big can) and +31.9V DC on C24 (big can also).

Click to expand...

What about when it starts to oscillate?

Does the voltage stay constant?

Mark

i don't remember exactly where, but i think i recall a post by david kulka of Studio Electronics describing something similar to what you're seeing.  his conclusion was that there were long signal traces on the UREI pcb that were acting as loop antennae.  i think he still performs a mod as part of his 1176 service package where he uses a shielded cable to replace those long traces and eliminate the issue.  this info may not apply to your model or your particular problem, but it would be worthwhile to check all the shielded cables, especially around the input and output pots, and make sure that the shields still have a good connection to ground.

ed

Biasrocks said:

What about when it starts to oscillate?

Does the voltage stay constant?

Mark

Click to expand...

Thanks Mark!
First, I wanted to calibrate BIAS. The manual says to set Input to 0 (CCW), Output to full (CW), comp OFF and bias internal pot (R81) to full CCW.

With these settings, the unit goes into oscillation and an AC voltage of 21.2V is read on C23... i still get -10V DC on that can. The positive voltage stays intact at +30V DC, no AC reading there.

Now if I turn the bias pot (R81) to  about half-rotation, the unit stops oscillating.
That tells you anything?

furn1979 said:

Now if I turn the bias pot (R81) to  about half-rotation, the unit stops oscillating.
That tells you anything?

Click to expand...

Your power supply sounds like it's okay, I would do as Ed suggests and check the shielding on existing cables and if that doesn't do it try replacing those traces with shielded cable.

Best not calibrate until you figure out what's causing the problem.You could try replacing all the electrolytics, especially if they are original and retesting.

A scope would definitely help to trace down the problem. If you don't have access to one, I'd suggest getting a tech to look at it. Hell, you've got nothing into it so you should be able to sink a few bucks into it.

Mark

Boy... reviving this old thread makes me realize I spend more than 3 years without addressing this problem with my 1176. :-\

Good news is, I recently got back to it and got myself a scope.

Update with the oscillation:
-The unit will go crazy with a high output setting (starts to oscillate when pot is set at 4-5 o clock)

AND

-IT ONLY HAPPENS with meter settings set at +4 or +8... The unit is fine when meter set to GR or OFF.

Rings a bell anyone?

Thanks!

I'm getting either the same or similar oscillation on my 1176LN rev D.

I cant figured out where it's coming from either but must be in the gain stage or in the meter circuit

the oscillation seems to be coming from the signal line amp?
Ive changed all transistors and no change? also changed C10 1uf and still oscillation?

problem was in the signal line amp, had wrong cap in where it needs 270pf, ive put a 220pf, close enough, all works fine now.