Heath IG-18 IG-5218

[richiem]

Using my HP 3458 in four-wire ohms on its 1Mohm scale, I got readings on four 90MBs from 336 to 342 ohms -- obviously at *very* low current. These lamps are amazingly sensitive.

[mediatechnology]

I had a chance to go back into my IG-18 to replace the original 500 Ohm stability pot on the Morrey board which was a single-turn open track carbon comp unit. It had always been very drifty. The 500 Ohm 15 turn cermet that replaced it allowed me to get about 10 dB reduction in second and third and much faster settling time due to better adjustment capability. Stability drift seems to have disappeared.

I also compared the physical construction of the lamps. The Eiko 90MB has a slightly overall shorter filament construction than the original Syl 90V which has longer loops. Having said that I don't think there's a closer replacement.

While adjusting the aforementioned trimmer I did disassemble the Eiko from its base and paralleled it to the Sylvania. Combined, the lamps had the expected shorter time constant and amplitude stability seem to converge faster. Unfortunately the overall output was reduced and I eventually removed the Eiko.

It makes me wonder if two lamps in parallel, each having a slightly different thermal mass but similar current voltage/current characteristics might provide a dual time-constant leveler. I've seen his done with CdS cells in compressor/limiters.

[thaumaturge]

Originally Posted by richiem

Using my HP 3458 in four-wire ohms on its 1Mohm scale, I got readings on four 90MBs from 336 to 342 ohms -- obviously at *very* low current. These lamps are amazingly sensitive.

You OWN a HP 3458??? Whoa! I am so envious!!! Only the finest meter on earth. But where to come up with the spare $8500.... (old list)
Doc

[richiem]

@mediatechnology -- I don't think you can get enough spread in thermal mass to make it work well, unless you use an amp (emitter follower?) to drive each one separately so that sufficient current is available to get them going a little. If the driving amps have gain, you'd be able to play with the gain through each branch -- a lot of work, I think, but maybe fruitful....

@thaumaturge -- I ended up with a total of a bit over $3K in mine after replacing some parts and having Agilent cal it. No tchump change, but I just never think very much about the meter anymore -- I just take the readings at face value and move on. Nice.

[thaumaturge]

Originally Posted by richiem

@thaumaturge -- I ended up with a total of a bit over $3K in mine after replacing some parts and having Agilent cal it. No tchump change, but I just never think very much about the meter anymore -- I just take the readings at face value and move on. Nice.

Don't want to threadjack, maybe we could start a 3458 thread... But briefly, I repaired maybe half a dozen of them. Generally replacing broken input mounts on the lame plastic front panel. But one had a real problem with the RMS converter. It always irked me that Agilent would wrap such a lame package around such a superb piece of technology.
Doc

[richiem]

I've had a couple on the bench and have seen a couple more in other places, and have not seen problems with the panel assembly. Mine needed a new display board and a new A5 ROM board (with the new snap-on backup batteries) -- the old board had the Dallas ROMs with real old batteries and they were fading fast. Mine is overdue for a cal, but that won't happen until next year some time.

[thaumaturge]

DO NOT hard bump your terminal cluster. That entire assembly is held to the front panel by one screw anchored into a little plastic disk that shatters. Can be repaired of course, just a PITA. As to your cal... you can get away with that at 4ppm a year DC drift. It's the 100ppm absolute on the AC from 10hz to 10Mhz that I find the most mind boggling.
Doc

[JimT]

I have finally done some work on my IG-5218. I replaced all of the capacitors and measured the distortion with my uncalibrated HP 332A. I measured 0.06% for the unmodified IG-5218. I installed the meter buffer from Williamson's Greening article. I adjusted the bias for lowest distortion using my distortion meter. The distortion is down to 0.015% which is quite close to the lowest my distortion meter can measure.

Jim

[richiem]

I think your IG-5218 is doing quite well - fixing the meter with a buffer is almost the most important mod that you can do easily. You will need a better analyzer to see any further real improvement from anything else you might choose to try.

I would say that #2 in importance is fixing the ground runs, which pretty much entirely eliminates spiking from the square-wave generator.

Good luck with further work.

[JimT]

Thanks

I am hoping to improve it a bit more, but your right my HP 332 is not quite up to the task of measuring this. I was looking for an excuse to put the Active Twin-T notch filter on my list of things to do. I did notice that getting the distortion down was very dependant on the bias setting, so after fixing the grounding scheme and building the Active Twin-T notch filter, I will look more closely at the differential pair.

Jim