From: Randy Kaeding To: HEATH@LISTSERV.TEMPE.GOV Subject: VTVM Mod I recently saw a few requests for parts to make up or repair the probe for Heathkit VTVMs that contained an AC-Ohms/DC switch right at the probe. Someone came up with a mod back in the early 70s where the 1 megohm resistor that is normally inside the probe can be installed inside the VTVM right on the Function switch. This automatically switches the resistor in and out as necessary when you turn the Function switch. The following steps show you how to make this mod in the IM-11, IM-18, IM-13, and IM28. Note that it is not an "authorized mod", but is easy to do and easily reversible. 1. If you have the manual for your VTVM available, refer to Figure 2 (IM-11 and IM-18) or Figure 7 (IM-13 or IM-28). If you don't have a manual, you can still do the mod if you follow my directions carefully. 2. Find the wire that connect between the "hot" side of the probe jack and the Function switch. Note that the hot side of the jack only has one wire connected to it. Disconnect the end of this wire from the lug of the Function switch (this is switch lug 5). Leave it disconnected for now. 3. There is jumper wire connected from Function switch lug 5 (where you disconnected the wire above) to another lug on the Function switch (this is lug 3). Remove this wire and discard it. 4. Connect a 1 megohm, 1/2-watt resistor between Function switch lugs 5 and 3 (where the jumper wire was). 5. Now connect the free end of the wire coming from the probe jack to Function switch lug 3 (this is not the same lug from which it was disconnected). You may have to lengthen this wire to make it reach lug 3. 6. You can now use any straight-through probe with the VTVM, and do not have to worry about switching the resistor in and out. Good luck, and happy measuring. Randy, K8TMK "One of the Hams at Heathkit" (25+ years)