Heathkit SB-102 S meter reads low.
Recently purchased a SB-102 in pretty good shape but as most of these oldies needs a little TLC. After cleaning all contacts, pots and tube sockets, radio works fairly well but the S meter even though it works, it reads on the low side, when no station is present the S meter goes well under 0 line and bottoms out. Sensitivity seems ok just a bit on the low side, the actual signal strength does not coincide with the meter's low readings. I switched all the tubes in the IF board, no change, I also checked all the resistors on the IF board and most of them are just slightly over their value but I would think that falls within tolerance?
Any ideas, input appreciated, thanks.
Last edited by N2WJW; 03-10-2014 at 03:08 AM.
If it varies from band to band, the heterodyne oscillator coils may need to be readjusted. But BEFORE you get into that, is the meter zero pot ok? Mine was terribly touchy, some deoxit helped but it could stand to be replaced. There is a service bulletin that suggests replacing R107 (100k) with an 82 K resistor to achieve zeroing. I know you said you checked the resistors, but the manual lists R103, 105, 106, 107, 119 or 115 as possible culprits. Keep in mind that by itself, one resistor gone high may not seem like a big deal, but several going high may well be. It doesn't take much.
The tubes at V3 and V4 can make a difference. Even if they test OK, certain brands can work better than others. They recommended GE tubes.
For low sensitivity, check V10, 6HS6. Good luck!!
Dumb question I know! But, did you adjust the S-Meter zero potentiometer?
The meter adjust is all the way open and still bottoms out below zero. I'm still thinking somewhere in the IF board is where the problem is, most likely a resistor or as Bob says perhaps a combination of all resistors being slightly out of spec. I switched all the tubes in the board and still down, however trying different 6HS6 in V10, I found one that brought up the sensitivity, didn't do much for the s- meter though.
The modification I mentioned above is specifically for meter zero problems. Apparently later units were shipped with a different pot, due to trouble with the original one. The swapping an 82k for the 100k at R107 was supposed to fix this. I would try changing that resistor first.
Come to think of it, everything I have ever had to fix on mine was due to a resistor or a tube.
Sounds to me like either V3 or V4, both 6AU6s, is/are gassy.
Originally Posted by N2WJW
I had this exact same problem in my SB-100, gassy V3 (first IF amp). I found the grid voltage, pin 1, was -1.2VDC when power is first applied but slowly the grid voltage became less negative and after 3 minutes the grid voltage on V3 was +1.5VDC and the S meter pegged below zero and the meter zero pot had no affect on bringing the meter above zero. V4, if gassy, will affect V3's grid voltage since they both share the same AGC and operating bias buss voltage.
I recently restored a SB 102. I found 37 resistors that had gone "way high" in value - more than 50% over value. This is due to age, heat, and documented aging problems with some resistors used by Heath. In my SB 102 A couple of 100K resistors in the metering circuit were "open" which caused some really interesting meter readings. In addition to everything posted above I would also replace all of the 33K B+ resistors with 2 watters. I understand the 1 watt resistors used in the B+ in this rig were operating at or over their ratings.