GR8NET sound line decoupling

Transcription

1 Feb 03, 2016 Rev: Feb 05, 2016 Severity: Critical Eugeny Brychkov, RU GR8NET sound line decoupling Purpose. To ensure safe operation of the GR8NET adapter in terms of its sound output and other adapters sound outputs. Background. Two GR8NET adapters have failed in the field with symptoms when they stop to produce sound through standard MSX computer mixer. Research has shown that not all the MSX computers are having same design of the mixing circuits. 1. Japanese native MSX computers are having strong decoupling circuit, consisting of the capacitor and resistor, thus they allow audio devices with unprotected output and those having no impedance reconciliation circuits to be connected directly to pin 49 SNDIN of the MSX BUS. Examples include: Panasonic A1-WX, A1-ST, Yamaha CX-5 family, Sony HB-F500; 2. MSX computers having weak decoupling circuit consisting of resistor only which limits direct current but does not completely decouple DC-DC circuits. Example is Pioneer X7; 3. MSX computers having no decoupling circuits in them at all. These computers assume that decoupling circuits are implemented inside peripheral devices. Examples are Philips NMS8280 and other Philips MSX computers. Native Philips device example is music module NMS1205. GR8NET has unprotected operational amplifier s output directly connected to the SNDIN input pin of MSXBUS (figure 4), thus in the third case, when there s no decoupling at all, may suffer from excessive currents with another device installed in another slot nearby. Page 1 / 10

2 Panasonic A1-WX Panasonic A1-ST Yamaha CX-5 Sony HB-F500 Figure 1. Examples of strong decoupling circuits Figure 2. Weak decoupling circuit in Pioneer X-7 (no capacitor) Philips NMS8280 PC Philips NMS1205 peripheral Figure 3. No decoupling in MSX machine (left), decoupling in music module Page 2 / 10

3 Figure 4. GR8NET SNDIN output design Solution. In order to protect GR8NET and another devices which may be inserted into the slots of MSX computers with no decoupling, it is required to perform modifications to the GR8NET adapter. Please review following action plan, and, depending on the components you have, feel free to do modifications different, given the following: a. You keep the same strong decoupling circuit; b. After modifications (using point-to-point wiring), all the components are firmly fixed in their places (by their soldered pins or by glue), there re no hanging wires which can be accidentally torn, and there re no possibilities to tear board tracks by pulling air-wires you mounted; c. If you glue components to board, you do not use glue which dissolve board s protective mask (green layer on the board) and thus potentially creating unexpected shortages or capacitances between tracks. You may need the following tools: Sharp assembly knife Capacitor 10µF x 25V not thicker than 5mm Resistor kohm Solder sucking tool Pincers or small pliers Soldering iron with small nozzle Please review the whole set of steps and pictures before you start working on modification. Page 3 / 10

4 GR8NET design note #DN0005 Step 1. Unscrew cartridge casing, look at the marked location. You see pin 6 of operational amplifier with via near it. Step 2. Use soldering iron to hear via (no the pin of the chip) and then use solder sucking tool to empty the via. As via and pin of the chip are very close, solder will also be removed from the pin (not completely, but enough). When performing this operation ensure you do not touch edge connector with the iron. Page 4 / 10

5 GR8NET design note #DN0005 Step 3. Your task is to disconnect via from the pad pin 6 is soldered to. Use sharp knife to cut the pad between via and chip s pin. Do not cut the via, and be careful not to cut anything else around, After you make these two cuts, remove part of the pad between cuts. Then use magnifier to examine space to ensure there re no reminders of the pad between cuts, and there re no shortages to the nearby ground copper. Ideally use multimeter to check for lack of direct contact between via and the pin, between via and ground, and between pin and ground. Step 4. We will have capacitor lying this way. Page 5 / 10

6 GR8NET design note #DN0005 Step 5. You will shape pins of the capacitor this way, so that they would apply to the operational amplifier which as shown in the next step. Note that longer pin is + (positive) pin of the capacitor. Step 6. This is how capacitor should be applied. Its longer + (positive) pin goes at the left side of the chip above it, then above chip s body approximately at the center of the body, and then bends exactly above chip s pin 2 and then down to the pin touching the pin. Capacitor should firmly touch board with its body. Shorter negative pin of capacitor should be hanging at the top. Page 6 / 10

7 Step 7. Simple task you cut resistor s pins and shape them this way so that resistor fit the way shown in the next step. Step 8. Resistor, with its unbent pin, touches top short pin of capacitor (negative pin). Second bent pin of resistor goes into the via. Resistor should not touch anything else with its pins. Ideally resistor should not touch capacitor s bent pin with its body (red arrow), because lacquer coating of the resistor is not reliable isolator, and this coating is subject to breaking away from the resistor s body exposing resistor s metallic body to the shortage with capacitor s pin. You first solder unbent pin with small amount of the solder, fixing resistor in the position. Page 7 / 10

8 GR8NET design note #DN0005 Step 9. Next you cut the bent pin from the rear side so that it minimally protrude from the via, and solder this pin from rear side. Step 10. Finally you ensure that unbent pin is well soldered and whole construction made of resistor and capacitor has no backlash and firmly positioned on the board. Page 8 / 10

9 Step 11. Final check: use multimeter in voltrop mode (indicated by the diode sign on your multimeter mode selection switch) by checking for no shortage between resistor s bent pin soldered into the via and capacitor s bent pin soldered to the pin 2 of the chip. Step 12. Put board into the cartridge casing. If you will have resistor s bent pin too much protruding from the rear, it may touch the casing and you may need to use sharp knife to remove small piece of cartridge s plastic so that cartridge closes properly. Questions and answers Q: Why not just remove metallization of the via involved to break the contact between top and bottom tracks, and connect resistor and capacitor to these tracks? A: It is possible, but there re two issues (a) bottom track, which goes to the pad 49 SNDIN of the edge connector, is fixed by the via and its metallization at its one end, if we remove it and solder air-wire to the track, there s a risk of tearing the air-wire off and potentially tear whole track off the board, including edge connector s pin 49; if it will happen it will be impossible to restore pad 49 thus GR8NET will irreversibly lose its audio capabilities; and (b) added capacitorresistor assembly should connect tracks at different sides of the board, thus Page 9 / 10

10 there should be an air-wire which flips along the edge of the board from one side to another, introducing risk of tear-off of the tracks; passing air-wire through the via is not a good option because this via is located in the place where plastic part of the cartridge casing touches the board, thus you will need to somehow cut the plastic in order to accommodate the wire to have casing closing properly. End of DN0005 GR8NET sound line decoupling. Page 10 / 10