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Old 17th June 2014, 05:58 PM   #21
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One of the folks at Boonton told me that the big filter caps are prone to failure -- not a tweak, just something to be aware of in case the unit gets un-reliable.
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Old 17th June 2014, 06:57 PM   #22
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Very nice performance coming out of the input board. For a -104 dB reading on the display the monitor output looks correct after the 80dB of post-notch gain. It is easy to see what frequency components need to be improved downstream, including fundamental rejection.

For now, I will hold off on accepting your offer for the modified input board. Thank you for offering; I may like to test it later. I need to get some things set up here in my lab first and run some baseline measurements to capture the present state with my equipment. I still have a few equipment gaps to fill.

Did you manage to get AD797s to work in all locations in the instrumentation amp? If the DC performance of the first two op-amps looks good enough, we may be able to eliminate U4 and U7 and bypass the switch with permanent jumpers. These are used for DC LEVEL mode only.

What light bulbs did you use to replace the 10k resistors?
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Old 17th June 2014, 07:06 PM   #23
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The filter caps dry out and the AC ripple increases. There is a low line voltage trip circuit on the power supply board that monitors the unregulated +5V supply to the CPU and when it falls below an adjustable threshold, it pulls the Non-maskable interrupt on the processor halting its operation. This prevents the processor from running amok during brownout conditions and overwriting the battery-backed-up RAM. When the cap begins to fail, the excess ripple may prevent the unit from starting up and this is a classic indicator that the power supply caps need to be replaced.
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Old 17th June 2014, 11:16 PM   #24
1audio is offline 1audio  United States
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Quote:
Originally Posted by MikeLynch View Post
Very nice performance coming out of the input board. For a -104 dB reading on the display the monitor output looks correct after the 80dB of post-notch gain. It is easy to see what frequency components need to be improved downstream, including fundamental rejection.

For now, I will hold off on accepting your offer for the modified input board. Thank you for offering; I may like to test it later. I need to get some things set up here in my lab first and run some baseline measurements to capture the present state with my equipment. I still have a few equipment gaps to fill.

Did you manage to get AD797s to work in all locations in the instrumentation amp? If the DC performance of the first two op-amps looks good enough, we may be able to eliminate U4 and U7 and bypass the switch with permanent jumpers. These are used for DC LEVEL mode only.

What light bulbs did you use to replace the 10k resistors?
The AD797 is tricky to use and it needs some impedances in specific places or its unstable. The LME49710 is way cheaper as well. The AD797 works well for the differential amp. I'm sure the other two amp could be upgraded but its more involved.

I'll need to look up the light bulb. I was looking for a 3W 220V lamp. Nothing small and neat available on this side of the pond. What I wound up with is pretty crude but it works.

Stable FET opamps are available and not expensive so eliminating them is not a huge issue. It seems all the new low noise opamps are quite stable. the LT1115 is actually a selection of the LT1028, also an ultra stable low noise opamp. They did not work as well as the LME on the input buffers.

I may be able to lend you a GPIB interface if that would move things along. I actually have several.

The notch is not real stable. It seems to drift with time, temp and the phase of the moon.

I also have an issue with AC leakage to the chassis. And I noticed the DCR to ground from the two ground posts on the front is different. This makes test setup a moving target at times. All this can get sorted out.

I have added on all of mine an output from the input board output, pin 27, that I use as an input monitor. Its isolated with a resistor. It has proven really useful.
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Old 1st July 2014, 06:56 PM   #25
RNMarsh is offline RNMarsh  United States
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This looks very interesting and the approach is much like the recent upgrades to the HP339A. If the DIY'er can easily get these models, it will be a great help. We all learn from others and this is a great opportunity for me to learn more and this time direct from the designer :-)


Thank you, and Welcome to our corner of the world !
Richard Marsh

Last edited by RNMarsh; 1st July 2014 at 07:02 PM.
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Old 5th July 2014, 05:59 PM   #26
RNMarsh is offline RNMarsh  United States
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I just saw a 1121 for sale at eBay for over 5000 dollars !! .01% lowest range. That seems a little bit on the (very) high side! What is the typical cost of a used one?


THx-RNMarsh
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Old 5th July 2014, 06:09 PM   #27
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Originally Posted by RNMarsh View Post
I just saw a 1121 for sale at eBay for over 5000 dollars !! .01% lowest range. That seems a little bit on the (very) high side! What is the typical cost of a used one?


THx-RNMarsh
The Stanford Research Audio Analyzer (Stanford Research Systems) is in the same pricing vicinity, but close to SOTA!

I sold an 1120 for around $300 a few years back.
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Old 5th July 2014, 06:22 PM   #28
davada is offline davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
I just saw a 1121 for sale at eBay for over 5000 dollars !! .01% lowest range. That seems a little bit on the (very) high side! What is the typical cost of a used one?


THx-RNMarsh

Has it been recently cal'd or some else special?
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Old 5th July 2014, 08:06 PM   #29
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The only special thing it has is being sold to trolls recently. They are ostensibly offering fresh cal upon purchase.

Checking past sales, it appears the more general, non recent caled, price is 4-500 USD

Alan
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Old 5th July 2014, 09:12 PM   #30
1audio is offline 1audio  United States
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I have never payed over $300 for a Boonton and I have 5 at last count. The current new price is quite high and since its qualified for some MIL projects etc. it may be "valid" but not for general purpose. A lot were retired from military service and pretty cheap which makes them attractive. They were part of some involved communications test systems and changing a qualified system can cost way more than paying $5K for the one that is qualified.

More will appear at more reasonable prices. However using the architecture and designing a replacement that would build on the strengths and use newer solutions is a better long term direction.
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