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polarity and correctness of all components, and replaced ICs 1 & 2 but still have 1V at these positions. (B. M., Wellington, NZ). • The fact that points which should measure +4.5V have only 1V present suggests that the voltage divider consisting of two 220kΩ resistors includes a resistor with the wrong value or the asso­ciated 10µF capacitor is the wrong way around. Our bet is that one of the resistors is 27kΩ instead of 220kΩ. We assume that your 9V battery is fresh and is delivering 9V. Query on TV sound Can you please tell me if the TV stereo sound format is the same used for FM broadcasting apart from the different deviation and perhaps different pre-emphasis? I am wondering if the same stereo decoder chips can be used. (P. C., Glenhuntly, Vic). • While both FM radio and TV sound are frequency modulation systems, their deviation and de-emphasis are different. However the major difference is in how the stereo channels are encoded. FM radio uses a multiplex method with the modulation switched between left and right channels at a rate of 38kHz. By contrast, stereo TV uses two separate subcarrier frequencies, at 5.5MHz and 5.742MHz above the vision carrier. Hence, the methods of decoding stereo TV are quite differ­ ent from FM multiplex stereo and different IC decoders are re­quired. TV pattern generator programming I have purchased your new Colour Television Pattern Genera­ t or and wonder if you could explain the addressing of the EPROM (IC1) via the four 74HC193 counters. It appears unusual and I wonder if it makes programming easier. The least significant bit of the counter is connected to A1 on the EPROM but the most significant bit goes to A10 instead of A15. Is the software available now from SILICON CHIP? (C. M., Salisbury, SA). • This project used unusual addressing to the EPROM to simpli­fy the PC board design. Since the EPROM would be programmed and run with the same addresses then it is not important which order the address lines are ac- Guitar/PA amplifier wanted I wish to obtain information on the building of a guitar/PA amplifier. I have long been impressed with the tonal reproduction of valve amplifiers. My requirement is an amplifier with a 100W capacity and some (or all) of the features of the Fender amps of the 60s era (reverb, echo, etc). I would most appreciate any advice you can offer me with regard to circuit diagrams, sources of supply of valves, power transformers, audio transformers and speaker enclosure dimensions. (J. S., Isle of Capri, Qld). • We have not published any circuits for valve power amplifi­ ers and nor do we think they are a practical or economic alterna­tive to a well-designed solid state amplifier. For a high perfor­mance guitar/ PA amplifier, we strongly suggest the 175W module presented in the March 1997 issue of SILICON CHIP. You might also consider building the Digital Effects Unit published in the February 1995 issue, to provide effects such as reverberation, echo, vibrato, etc. cessed as long as it is consistent. After all, the address labelling on the EPROM is only arbitrary anyway. The software is available from SILICON CHIP for $10.00, including postage and packing. 4Ω loudspeaker. The only way to get the full power would be to use a DC-DC inverter. We published a suitable project along these lines in the December 1990 issue. When do you change the filter? Notes & Errata I have a water purifier but I never know when to change the filter. Can you design a project to detect when the filter needs changing? (D. S., Miller, NSW). • Unless the conductivity of the filtered water changes over time, as the filter deteriorates, we cannot think of any elec­tronic method which could tell you when to change the filter. Perhaps one of our readers can suggest a method. Amplifier module needs DC-DC inverter I am enquiring about details of the 50W amplifier module described in the March 1994 issue of SILICON CHIP. The kit runs on 25VAC and I was wondering if the circuit can be changed so that the amplifier can be run off a car battery with the same power output. Also, I was wondering how to change a stereo signal to a mono signal. Do you simply join the left and right channels to make one channel? (N. B, Gladesville, NSW). • In order to deliver the full 50W, this module needs a DC supply rail of ±35V or 70V in total. If it was to be run di­rectly from 12V DC, the power output would be less than 4W into a Colour TV Pattern Generator, June & July 1997: the patterns produced by the TV Pattern Generator are slightly off-centre on the TV screen due to a slight displacement in the line sync signal. In most cases, the normal over­ scanning of each line on the TV screen will mask out this small shift. It can be corrected by adding an RC network to delay the line sync by the requisite 1.5µs. This involves adding a 4.7kΩ resistor between the D7 output of IC1 at pin 11 and the sync input of IC10 at pin 16. The pin 16 input of IC10 is bypassed to ground with a 270pF capacitor. The resistor is best placed instead of the link on the PC board above the three 330Ω resistors near IC10. Note that IC10 has an incorrect pin 1 labelling on the PC board. The position shown for pin 1 is actually pin 16. The capacitor can connect from pin 16 to pin 1 of IC10 on the underside of the PC board. Flexible Interface Card For PCs; July, 1997: there are two errors in the Basic listing shown on page 28. Line 90 should read: B$ = RIGHT$(TIME$,2): WHILE RIGHT$(TIME$,2) = B$: WEND ‘wait one second. Line 220 should read: LOCATE 24,20: PRINT “Line”;LIN; ‘print it. Note: do not put full stops at the ends of the lines. SC October 1997  93