Top view of modified tuner - click to enlarge.
I purchased this unit used Sept. 2006 on Audiogon, and made the following modifications:
- All electrolytic caps were replaced, and power supply values increased where suggested by FMtunerinfo.com (see below for references)
- Audio circuit resistors replaced with 1% metal film Resistas/PKPs.
- IEC added and rear panel fuseholder added (but not snubber circuit)
- F-connector added, and stock balun disconnected
- IC6 replaced with OPA2604 in socket
- Passive EQ mod done (see FMTunerInfo link below)
- Ammons negative regulator added (and C82 changed to 470/35 per Bill's suggestion). This addresses a relative weakness in this unit - no negative power supply regulator
- Power supply diodes replaced with 11DQ10 schottkys
- New RCA connectors added to rear panel, directly wired after output coupling caps C49 & C50. Note that I’ve left the coupling caps in for now, but DC offset is almost nil, so I’ll probably remove or bypass them later.
I then sent the tuner to Bill Ammons to re-work the IF filters, add his IF filter board, and align the unit. He also cleaned the RF front end and changed a cap for improved AM performance.
Another Interior shot, click to enlarge.
Audio output circuit, showing OPA2604 IC. Blackgate output coupling caps at right; passive EQ caps (MIT Multicaps) standing up at the left. Click to enlarge.
IF Filters and Ammons IF Filter Adder PCB. Click to enlarge.
Power supply, showing Ammons Negative Regulator PCB at top. Click to enlarge.
FMTunerInfo.com is a wealth of tuner information, especially for the KT-7500:
DIY Mods for the KT-7500, including power supply changes, audio circuit improvements and the passive EQ mod. This is the most comprehensive set of mods to the KT-7500, and the first place to start if you plan to modify this tuner.
KT-7500 Service Manual (PDF file)
KT-7500 Schematic (PDF file)
KT-7500 PCB layout (PDF file)
General info on the KT-7500 (scroll down to the KT-7500)
Bill Ammons Filter-Adder PCB (PFD file)