MEI LI MAG-450 RF Signal Generator - Page 1
Forum section: electronics / rf-microwave

author: Calambres
author_rank: Frequent Contributor
timestamp: June 22, 2020, 04:44:53 pm
content: I've just received a new MEI LI MAG-450 RF Signal Generator, one of the many clones of the infamous TSG-17. I've bought it to align a Zenith TransOceanic Royal 3000-1 I have inherited.The Sig. Gen. is what you'd expect from its price point: a bit distorted and suffers in the Vpp department as soon as you go up the MHz escalation but I think it is going to serve me well for this application. It seems to modulate OK. I too have a gorgeous late 50's Grundig K2 (Grundig Majestic in the U.S.A.) that's also begging for an alignment.Anyways, I'm writing this here to warn any buyer of this Sig. Gen. and share some of my opinions and modifications:- The ON/OFF switch only breaks the secondary of the transformer! This is very bad practice; you think you've turned off the unit but the power transformer is still on. I've managed to rewire the circuit to make it cut the primary. I had to break a couple of tracks as the ON/OFF switch is of the PCB thru-hole kind. Also a couple of Kapton layers over the new PCB connections for security reasons.- I have also wired a third BNC connector in the front panel with the main pre-"RF LEVEL" output. Sometimes I'll need very low output, for instance to drive the grid of a tube, but such low Vpp is not able to excite my frequency counter. With this second output I can still get a low signal to the D.U.T. as well as a simultaneous powerful signal for the counter.- I read somewhere that the unit was not adjustable. This is not true. As much as it was quite decently adjusted from factory I've managed to adjust it even better with the use of an oscilloscope and a plastic trimmer tool. All bands from A to E have slug adjustable coils. The last F has a thick wire three loop coil which is mostly unadjustable. With a bit of patience I adjusted the output to what it reads in the dial. Band A to E are very close with what the dial reads. The last F band not that much.- Talking about adjustments, there are two trimmers (variable resistors) in the PCB. No idea what they do. I haven't found the schematics for this unit. Anyone know what they're for? Anyone's got the schematics?- I hate the original screwdriver adjustable controls. I have substituted the original posts that separate them from the panel with much shorter ones. This make the shafts protrude enough to install knobs. Much usable now.- The output BNC is coupled with a pair of capacitors in parallel: one 20nF ceramic and one 0.470uF electrolytic. Both of them low voltage. I have substituted them with HV replacements.- The PCB work was not top notch precisely. I had to resolder here and there, remove some solder blobs, and clean flux residues with IPA.- The dial was very hard to move. There's a reduction mechanism with a couple of plastic gears. The main dial shaft is held with two rudimentary plastic bearings. All was dry as a bone. A bit of silicone grease here and there and now it is smoother albeit a bit hard yet.This is the original unit. As soon as I finish all tests and adjustments I'll post here a photo of the modified unit.

author: Calambres
author_rank: Frequent Contributor
timestamp: June 24, 2020, 09:26:40 am
content: Once modified. Now it's easier to work with...

author: Moishekoshstein1948
author_rank: Newbie
timestamp: August 27, 2020, 11:08:25 am
content: Is it possible to calibrate this thing and eliminate the distortion?

author: Calambres
author_rank: Frequent Contributor
timestamp: August 28, 2020, 05:36:01 pm
content: Yes and No.Yes, you can calibrate this thing as I said in my first post. The coils for all bands, except the last, are easily tweakable. Now the dial is much more precise but still too small to be of real use, just a mere approximation. Anyways, I always use it with a frequency meter plugged to the third BNC I hacked.No, as far as I know you cannot eliminate the distortion but for the use I make out of it, AM radio aligning, it is not that important. Besides, the distorsion is not that big for such a low price sig. gen.

author: bfrederi
author_rank: Newbie
timestamp: January 02, 2021, 04:24:24 pm
content: I just purchased a TSG-17 for almost the exact same reason you did: to align a Zenith Transoceanic G500. Unless and until I plan to do more vintage restorations, I can't see investing in a higher quality unit. Were you in fact able to successfully align the IF on your Royal 3000-1 with this thing?Thanks for posting this (and for having the courage to do so!).

author: Calambres
author_rank: Frequent Contributor
timestamp: January 02, 2021, 05:20:35 pm
content: Quote from: bfrederi on January 02, 2021, 04:24:24 pmSure I did, at least the AM side of it. The sig. gen. behaved very well. Fortunately the FM circuit was OK. Read the whole story here:https://antiqueradios.com/forums/viewtopic.php?f=4&t=367655

author: bfrederi
author_rank: Newbie
timestamp: January 02, 2021, 07:41:57 pm
content: Great! Thanks for the link. Luckily FM is not an issue for me. Of course, before I undertake the alignment, there's the minor task of replacing all the capacitors, a handful of resistors and an old school selenium rectifier.

author: i8kpv
author_rank: Newbie
timestamp: February 07, 2021, 04:59:17 pm
content: Ho un MAG450 che ebbi in regalo da un amico perfettamente funzionante con i limiti che conosciamo tutti. Dopo averlo testato l'ho tenuto qualche anno pronto per l'uso tra i miei strumenti....ed ecco che improvvisamente ho scoperto che non si accende più. L'ho aperto , insomma nulla di che , ora sto cercando il guasto , ma esiste uno schema di questo apparato? Grazie

author: Calambres
author_rank: Frequent Contributor
timestamp: February 08, 2021, 08:45:26 am
content: Quote from: i8kpv on February 07, 2021, 04:59:17 pmI have never managed to find a schematic for this signal generator.Anyone?If you want to take a look at the manual here you have it for the TSG-17 which is exactly the same thing:TSG-17 Manual... but you'll find no schematic in it.