I have one and really like it. Built in Montreal in the eighties by Amber Electro-design. Wayne Jones and Ed Meitner did the design. I was able to buy a manual reprint from Wayne a few years back.
They came with a variety of options, do you know what it has? If not, I could translate the model number for you. It is on the rear.
Its great to hear of you guys experiences with this analyzer.
I had noticed that some opamps were bypassed with EL's and others with Tantalum's. So I had surmised that the designers had used the tant's there for a reason. My plans were to replace the tantalums with tantalums. I typically go up one voltage range on a tantalum when I replace it. I do not know if that is really buying me that much but it's typically not that much more expensive to move from a 25V cap to a 35V one. On the EL caps, I have a nearly full bag of 22/50 Panasonic FC's in one of my parts bins somewhere that'll probably get used in place of the 10/40's.
I definitely do not want to do anything to compromise the performance of the stock unit. I primarily mentioned the op-amp changes because I know that many users here on DIY-A have greatly improved the performance of the HP 339A, Shibasoku, Boonton, and other analyzers. I was hoping maybe someone had experimented on the 3501 before.
I went through the calibration on mine yesterday and at certain sweet spots in the freq band I'm getting -105dB with the 400 HP & 30kHz LP filters active. A side benefit of recaling is the meter deflection and drift on the low end of the range is only about .25dB now. Before the calibration I was getting up to 1dB of meter drift in the -100dB range.
I have several units which might be called "rough" condition, seems the more used units presently work the best? I just made up a laptop power supply for the 3501, which only has pins 2&3 for the external input of a power supply, that is for an AC transformer, but the DC switcher from an old HP laptop at 18.5 volts and up to 4.94 amps is fine going through the bridge and Main regulator, as I think this Amber 3501 set really only draws about 12 to 15 Watts, maybe an even smaller wall wart supply would do the job, I may try one at somepoint.
I suspect the careful use of AD797's in 3 or 4 places might improve the set overall, but my later sets (the 3501A's) are doing not so well, into the 70dB and the 80dB ranges, while the 3501 is giving me 92dB without filters, and 97dB on loopback for 80Khz. bandwidth. This is for no adjust or cal, or null or anything. Like I said these sets are rough, and three had smoking tantalum's, while two had blown primary fuses, and burned tantalums.
I also think that the older unit with the switcher supply, and the additional shielding, which also has lots of room inside could be adapted to the newer lithium batts, as the charger is quite sophisticated and maybe 12 Amps from six 18650 lithium batts could run the set for over 12 hours, with almost no increase in weight. There are boards to charge multiple 18650 cells made, just add batts!
I don't know how but I missed this post from a few months back.
I agree with you that a few properly placed 797's would help the 3501's performance. If they weren't $11 each I'd probably have a handful. I'm using the LME49710's which are < 20% the cost. The 49710 will be extinct in the DIP8 package soon though so buy some if you want them, both Digikey and Mouser have them flagged for EOL. I doubt they'll be restocked once the existing stock is exhausted.
I spent a good bit of time and went back through my 3501 & 3501A recently. I have the 3501 at -103.5dB @ 1kHz now in loopback with the 400HP and 30k LP active. I have the 3501A down to -105.5dB @ 1 kHz with the same filters active. The 3501 has a high 3rd harmonic that is coming from the oscillator and I've yet to track the source of it down. If I connect my tweaked 239A oscillator up to the 3501 it measures right at -106dB with the same filters active.
For now I settled on 49710's at U1008, U1030 & U2008. I had 49710's at several locations in the oscillator of both units but the 3rd harmonic came up by about +12dB with them installed so I put the 5534A's back in.
I much prefer the 3501 to the HP.