Harman Kardon 330B Series 1/Variant 3 FM Problem

[Defcon99]

Thanks in advance for any help. Restoring 330B Series 1/Variant 3. Everything working except FM has very low signal on both channels (must turn volume up to 100% and then can hear faint FM stations.) Replaced caps on the IF and MPX boards with no change. Resistors and transistors on the IF and MPX boards tested well. If I jumper between IF output to the switch, bypassing the MPX board, I get a strong clear signal across the band (in mono) on both right and left channels. Because of this, I focused on the MPX board. I replaced the entire MPX board with another from same model but no change. Seems like even though bypassing the MPX board plays fine, something is wrong with the IF signal causing problems for both MPX boards. I could not find the exact IF schematic that matches my board. It has 4 large silver cans in the IF strip, a 3K variable resistor and a single c458 transistor near the output to the MPX board. I'm stumped at this point and hoping to get some help.

 

[avionic]

Have you checked the power supply voltages for the FM tuner?

 

[Defcon99]

Both boards (IF, MPX) are getting 13Vdc. Also 13v into the FM tuning front end.

 

[Mumbles]

I'd start looking at that MC 1307 chip if everything else checks out..

 

[Defcon99]

Thanks. Replaced it with another used one. Per specs on the MC1307P IC, DC voltages on the IC pins look good except Pin 2 (19kHz), Pin 6 (stereo lamp), and Pin 14 (19kHz). Pin 2 has 0.3v vs 2.9v. Pin 6 has 0.4v vs 0.9v, Pin 14 has 0v vs 1.7v. The stereo lamp is burned out and I have not replace it yet. I am assuming the MPX circuit still works when the bulb is burned out. I have tested the circuit around Pin 2 but cannot find where anything has failed. There is an input from the IF board connecting between the T301 (Pin 2 out to T301/19kHz out to unknown input from IF out to 1/50v cap to ground) and ground but I cannot find the schematic that matches my board that shows where it originates on the IF board. I replaced the entire MPX board including the IC with no change. Still looking for the problem. The board is grounded by all 4 screws.

 

[Defcon99]

Would anyone have a schematic that matches this specific IF board (four large IF transformers with last transistor being TR107, no TR108.) HFE does not have one. Thanks.

 

[Mumbles]

Would anyone have a schematic that matches this specific IF board

Just one question, what's the serial number of your 330B and is it Japan or Taiwan built? A clear photo of the rear panel showing this info might help.

I do have this other 330B service manual and it seems to cover a version similar to yours. No silk screening and the IF and MPX boards look the same. The only thing is transistor identification starts with Q, not TR.

I got it from https://www.manualslib.com/

 

[Defcon99]

Thank you Mumbles. Serial number is S1820. Series 1 built in Japan. My IF board is missing the 8th transistor (Q208 or TR108). Your photo does look like my board, but the parts list shows Q208? I have that manual but the schematic does not match the photo. I don’t know. I believe my problem may be in the FM detector or muting circuits.

 

[Defcon99]

Looking for a schematic for the IF board in the photo. I found the MPX schematic buried in an old post but cannot find the IF schematic. This IF has 4 large tuning cans and only 7 resistors TR01-TR07. The transistors are 2sc460. Serial Number is S1820. Schematic is missing from all the Service manual versions I can find. Thanks.