Fintec 904 - Hardware Tuning

I made a few modifications to the hardware to ease my everyday life.

Power Switch

Because the Status pin in Fintec's SCART connector is in permanent active state (+12V), owners of this DVD player have made some harsh tricks to get around the problem. Few bought a mains extension cord with a switch, used composite or s-video outputs instead, or what I did - cut the status wire, because the status signal overdrives everything on my TV set.

I wanted to have automatic changing to AV-channel when I turned the player on. First I thought that I could make that green standby led circuit to control a relay connected to the status pin. Then it came to my mind that I could make the player more ecological by converting its standby button to a real power switch.

The Button Maybe the SCART was designed by a trainee, but luckily there had been at least some thinking at the research labs. Front panel and button board have made to accept also a standard power switch. Front panel has additional switch mounting studs at 20 mm raster and PCB has cut-outs to snap the standby button part off. That silver user visible plastic button has also a standard sized hole for the power switch.

The Assembly I cut the unnecessary parts from the PCB and the silver button. Then I made a visit to my electronic parts retailer to get a new switch. Sadly they had only one heavy duty model at 20 mm raster. 1 inch models (same as in AT computer case) would have been smaller, but I did not want to drill new mounting holes. With protective insulation added it was quite a tight fit. I had to unattach the front panel to get the switch and the mic pre-amp assembled.

Mic Gain Boost

Voice feed through Fintec's mic inputs is quite low even with good, semi-professional microphones. Unmodified mic inputs are suitable for loud trash metal singing and not for smooth and tender Barry Whites like me. So I decided to give the mic inputs a little more input gain.

The mic pre-amp board has just a basic inverting op-amp circuits and a TP5299 Echo processor. Mic signals are summed and amplified at the first op-amp (pre-amp) of RC4558P double op-amp IC. Then the signal is fed via volume potentiometer to the second op-amp (output amp) and also to the echo generator.

The Schematics I traced the schema and it appears that the first stage pre-amp is just like from a schoolbook. A = -(Rfeedback / Rin) Information is provided AS IS. blah blah ... Viewers discretion advised.

The Feedback Resistor Pre-amp amplification factor is determinated by size of the feedback resistor R25 (marked green). It is originally 20 kOhms (A=-20) and after some quick tests I replaced it with 47.5 kOhms precision film resistor. At this doubled amplification the board was picking slight interference when I put it back to its original location in the case. With even more gain someone should also consider reducing noise caused by cheap components (mass resistors) and inadequate shielding. Replace mass resistors (at least R28/R29) with low noise thin film resistors, change and reroute cabling and add some metal shielding around the board. Or even better, get a real mixer with balanced XLR-inputs.

Mic Gain Boost, Part 2

After the resistor swap the useful volume level was now in the last quarter instead of maximum only. But the implementation of the pre-amp raised some questions. I wondered why mic signal went thru fixed gain input amp and then it was attenuated by volume potentiometer. I decided to make that potentiometer adjust the gain of the input op-amp instead.

To be cont'd...or not

Continue to
Part 1: Overview
Part 3: Software Hacking (serial EEPROM, serial port)