Tektronix F7523A1 MOD WQ

[erichayes]

Hi All,

I didn't want to post until I actually had it on my bench, but I've taken possession of a BFNIB Tek TH and IM distortion analyzer. Burned in by the vendor for 72 hours, it exceeds (which I would expect it to) factory specs in all regards.
I plan to repair and/or calibrate most of my other analyzers and oscillators against the 501. I'll then put a few of them on preBay. That would include the Boonton (which crapped out), an HP 8903E (which also crapped out) another Tek 501 (which also crapped out), an Eico factory wired 902, and a couple of Heath IM-5248s. All will be brought up to factory spec, unless it's cost-prohibitive to do so--in which case I'll supply residual base readings so you can apply appropriate fudge factors.

The other, probably more germane, service I'll be able to provide is evaluation/calibration of analyzers that y'all already have. I don't expect any of you to be chomping at the bit for this service, but it's nice to know that we have a certified standard for distortion and noise that we can rely on.

[Shannon Parks]

Eric, what are the Boonton's problems? Poor performance, errors, or bad power supply? I was hoping to set you up with my Boonton software when I came out. I have three 1120s myself - two working and a parts unit. Maybe I can help?

[erichayes]

Hi Shannon,

As I recall, the problem was false high readings. I remember reading something about a Boonton syndrome wherein the fundamental wasn't being rejected properly--or at all--causing erratic and/or high readings. I just put it on the shelf when it started acting up, and went back to my Tek AA-501 and the Meguro.

The other problem with it is it doesn't have a 400~ HPF. Who the Hell would order an audio analyzer without the HPF option? Building a third order Butterworth isn't that big a deal, but the fact that it has to be built chaps my ass.

If you want to fiddle with it, be my guest. My plate's so damned full right now, I won't be able to look at it until next year, at best.

[Shannon Parks]

The fundamental frequency adjustment is a pretty easy one to fix. As far as the filters, I have the parts to make a couple 400Hz high pass filters, as well as one of the original ones for comparison. I'll glom one together along with a A-weighting filter. Yeah, I used the 400Hz HPF all the time when using the 8903B with my phono stage - very handy. Anyhow, don't toss the Boonton out yet. :)

[1audio]

I created a board layout that can be assembled as either a 400 Hz filter, an "audio" filter (20 Hz to 20 KHz) or an a-weighted filter. Its an expresspcb layout and the boards cost $51 for three samples in 4 days. I got side tracked on the project and I don't know where the details other than the PCB file are. I can post it if anyone is interested.

I have several very low distortion sources and recently got a Shibasoku analyzer that has a .0001% (-120 dB) residual in its "analysis" mode. The Boonton oscillator is good down to .0009-.0007 typically and with opamp tweaks the analyzer can get to about the same. But they do drift. The 30 KHz filter is essential to get this low- the noise becomes dominant otherwise.

[Shannon Parks]

1audio, do you mean these are copies of the Boonton plug-in module PCBs? Cool! I think you could sell them on eBay, but if you wanted to post them, that would be awesome.

[1audio]

I have posted the expresspcb file here: http://pws.prserv.net/demian/Boonton/BEC%20filter%20board.pcb
I can't find the schematic file and there will be a bit of reverse engineering to use it. By assembling it with different parts the 400Hz, the A weighting filter and the "audio" filter can be made from the same assembly. The default board will need to be cut down to fit but the height will be OK even though its larger that the original. It will match the schematics in the Boonton manuals.

I may get to untangling the layouts but not for a few weeks. It will be a spreadsheet listing which values go in which location. The mechanicals are correct, however. If anyone wants to undertake the task let me know and I will scan and post the relavant pages from the manual.

I'm also working on a passive low pass filter to use with digital amps. Once I have some confidence in it I will publish it. Its necessary to remove the switching artifacts so you can measure the audio performance. Its loosely based on the AP specs.

[Shannon Parks]

Many thanks!

[1audio]

Updated file, revised layout and assembly notes
http://pws.prserv.net/demian/Boonton/BEC%20filter%20board.pcb
http://pws.prserv.net/demian/Boonton/BEC%20filter%20board.sch
http://pws.prserv.net/demian/Boonton/BEC%20filter%20board.xls

Enjoy.