Urei 1178 meter tracking

[Mick Bailey]

I have a 1982 Urei 1178 limiter that I'm having difficulty in calibrating the meter tracking. The meters zero with the limiting bypassed and metering set to GR but the best I can get on both channels is the meters read roughly -13dBu when the output is limiting at -10dBu. I'm otherwise out of range on both meter range adjustment pots. Other than this issue I have it working nicely and I'm wondering if these units aren't too accurate on the metering.

The service notes refer to calibrating the meter tracking at -5dBu and it isn't too bad at this setting (just a shade over-reading on limiting compared to actual) but as the limiting increases towards maximum the meters get further out. All adjustments carried out after 1/2 hour warmup.

Edit; Forgot to mention - I already checked Q-bias.

[nevetslab]

I have a 1982 Urei 1178 limiter that I'm having difficulty in calibrating the meter tracking. The meters zero with the limiting bypassed and metering set to GR but the best I can get on both channels is the meters read roughly -13dBu when the output is limiting at -10dBu. I'm otherwise out of range on both meter range adjustment pots. Other than this issue I have it working nicely and I'm wondering if these units aren't too accurate on the metering.

The service notes refer to calibrating the meter tracking at -5dBu and it isn't too bad at this setting (just a shade over-reading on limiting compared to actual) but as the limiting increases towards maximum the meters get further out. All adjustments carried out after 1/2 hour warmup.

Edit; Forgot to mention - I already checked Q-bias.

I just checked my service notes, recalling having run into this on one of the UREI 1178's I worked on in March 2017. I found my notes, but will get back to you when I have a moment....need to head over to another building to resume loudspeaker maintenance work.

Out of curiosity.....bypassing the aspect of the two meters tracking each other, have your checked the dB linearity of each meter independently? Easy to do if you happen to have a calibrated Step Attenuator. If not, and you have a known-good Analog Meter so you can adjust the oscillator level in 1dB steps manually, and see how each meter responds. If one is tracking way off in that, then there's the challenge of getting the min/max tweaked so it will. Hopefully both meters themselves are still working well enough to be calibrated. More on all this a bit later.

[Mick Bailey]

Are you referring to disconnecting each meter to see how it responds, rather than in-circuit? I do have a reasonable analog meter as well as a Marconi dB meter. I've been taking measurements in-circuit at 1dB intervals by using a DMM and looking up the value on a printed table. though.

[nevetslab]

UREI 1178 Meter Circuit Adjustments

Are you referring to disconnecting each meter to see how it responds, rather than in-circuit? I do have a reasonable analog meter as well as a Marconi dB meter. I've been taking measurements in-circuit at 1dB intervals by using a DMM and looking up the value on a printed table. though.

I was referring to leaving the meters connected in the 1178 circuit. I also have a Marconi dB meter...a 2610, if memory serves. The 1176LN and LL78 meter circuits are similar, depending on the vintage. A lot of differences over the years in the 1176 meter circuits. I have a copy of the 1176L manual which has numerous notes from some seasoned technicians, and has a collection of the different schematic revisions. I've attached that manual to add to your database. I have dealt with them over the years. They are, as you're finding, a bit of a PITA, and following UREI's alignment procedure is a bit tedious, when the results seem to be worse than before. I have had to trim resistors R69 (+4 Trim)/R70 (+8 Trim), selected to indicate 0VU when the output level from the Limiter is +4dBm/1.23V or +8dBm/1.95V. Loading the output xfmr does, of course, come into play there.

I don't recall if I've ever played with the Peak Reading circuit.....I didn't find any reference to that in my service notes. The Gain Reduction Circuit calibration is trickier if there's any power supply issues...the meter is biased from the +18V supply via GR Trim R61 & shunt resistor R62. I recall having to go thru the calibration procedure several times before I had gotten it as good as it could with the analog meter used. I've never had spare meters to swap out.

I gather you have gone thru to verify the Q-Bias before tackling the meter calibration and tracking between the two meters.

I don't currently have either an 1176 nor 1178 on hand for comparative measurements.

UREI-1178 manual.pdf

I wasn't able to add the extensive 1176L manual with all the revisions and tech notes, both it and the 7-zip file were larger than the downloads would allow. So, I've broken this up into sections below:

UREI-1176LN_manual_SN 7652+.pdf
UREI-1176LN_Svc_Man_SN 7652+.pdf
UREI-1176LNmanual_Limiter Tests Pgs 1-8.pdf
JBL Cal Instructions for 1176LN.pdf
UREI-1176 Schematics thru Rev D.pdf
1176_Schematics Rev E-Rev K.pdf
UREI 1176 Peak Limiter Revisions Log.pdf
1176man_PCB A11108_ FET Data Sheet.pdf

[Mick Bailey]

That's a great deal of help and thank you for going to the trouble of posting the sections. I didn't load the outputs and will do another test with them loaded, though I'll find out what the studio is running after the limiter so I can set it up as they would use it. It did cross my mind to change the trim resistors but I thought this shouldn't be necessary and there must be some underlying reason for the discrepancy I'm seeing. However, in the documentation you've posted UREI refers to changing resistor values, so this is expected.

Q-Bias is good. What I didn't do at the time was check the ESR of the electrolytics. I don't know how those Frako caps hold up after 40 years of service and a rising ESR may cumulatively account for tracking errors. There's a guy named Jim Williams who recommends changing them out to Panasonic and installing film bypass caps across them. The unit is temporarily back in the studio for some recording over the weekend and I'll take another look at it when they're done.

[Mick Bailey]

I've done a lot of reading and research on these limiter since my last post and done some more tests and tweaks. Funnily enough in another thread there's a discussion on ESR meters and their uses. Just before Christmas I had another look at the 1178 and did a quick check on ESR and all the caps still read good in this particular unit. Loading the output made a slight difference to the output level, but there was still a discrepancy that I wasn't happy with so I went through the process from start to finish and found that despite setting Q-bias up as accurate as I could initially, it had drifted out by the time I'd made the last of the adjustments. It's super-critical to get this right and affects the whole operation. So I've been making smaller adjustments and gone back to Q-bias a few times to make sure this held up. I found though that the main problem is that when the lid is back on and it's powered up for half an hour the Q-bias and tracking shifts a little, so I went back to it a few times to get it to remain stable after a few hours of running. This particular unit is fixed at 230v operation and our mains is around 248v, so perhaps the tiny mains transformer is getting a little hotter than the design spec. The outputs are now within 0.5dB at maximum limiting, so I'm pretty happy with that and it corresponds with the notes.

I got a text message yesterday from the studio to say it had been in use all day without any problems and the meter tracking is much better.

Next up is a pair of 1176, so some valuable lessons learned and those notes will come in really useful.