SB-101 grid meter help
I'm troubleshooting an SB-101, and would appreciate some suggestions from the forum.
I replaced R305 2.5k 7w that was open, and replaced a few carbon resistors in the driver section that were a little high. Someone previously replaced the electrolytics. B+ voltages are a little high at the test points +320 and -124 instead of +300 and -110, although I don't know if this is a problem.
Someone previously replaced V6 with a 4EW6, so I swapped in a 6CB6, although they are pretty similar except for filament voltage, but I figured it would be better use the originally intended tube.
I went through the initial tests power on tests and receiver alignment steps ok, and the receiver works pretty well. On the transmit tests, I set bias for 50 ma plate current on key up, and on tune I can increase CW volume to full scale on rel power, driver preselect and final tune seem to work as expected, although CW volume is a little touchy when tuning. Plate current and rel power seem to be in expected ranges, however the meter does not deflect in grid position. Here is what I checked so far: same behavior on all bands; the meter works fine in HV, ALC, rel power, and plate current; L903 off the final grids measures about 19 ohms, so it doesn't appear to be open; R916 that the grid meter shunts across is ok, the meter rotary switch is ok, and the final tubes are good. Can someone suggest some likely culprits?
Hi OM John,I suppose the most likely cause is one of the contacts of the rotary switch in the 'Grid Current' position. This may be confirmed by a resistance check across the meter shunt resistor with the rotary switch out of 'Grid Current' position and then in. No change in resistance in 'Grid Current' position (or no kick in the rig meter!) would indicate a contact problem.73Nandu.
Thanks for the suggestion, Nandu.
With the switch in the grid current position, resistance across R916 is about 100 ohms (from the internal shunt resistor inside the meter). With the switch in one of the other positions, resistance across R916 is about 10k.
It will be a couple of days before I can work on the radio. I guess I can measure grid current by placing my Fluke in parallel with the meter or R916. Also, is it possible that there just is no grid current (i.e. can the SB-101 be configured to run in a class AB mode that draws no grid current?). I used to know this stuff, but I'm beginning to think the boat anchor is not the radio but between my ears.
Hi OM John,I think you have the answer! I suppose you are looking for grid current in the SSB mode. If the finals are biased for AB1 mode, then there will be no grid current. Grid current can be read only in Tune / CW modes. In fact, you would need to do so only while adjusting the drive in these modes.73Nandu.
Originally Posted by KJ7B