Fullscreen HTML5Med Res (??MB)HTML4

Serviceman’s Log – continued from p43 leads one to expect uncouth, mislead­ ing behaviour from resistors so I replaced it anyway. Much to my disgust, the fault still remained! As a result, I now decided to remove the entire PCB assembly and examine it for bad solder joints and poor wiring connections etc. This assembly actually consists of three separate PCBs: the main amplifier assembly PCB, a valve base circuit PCB assembly and an output socket PCB assembly. All three are connected by wiring and in particular, the valve base PCB assembly has five heavy-duty flat ribbon cables connecting it to the main PCB assembly. I made a thorough inspection of these assemblies using a magnifying lamp but could find no sign of bad solder joints or poor wiring connections. This was really starting to puzzle me by now, so I reassembled the amplifier, carefully fitted all the Ask SILICON CHIP . . . continued from page 103 vember 2005 headphone amplifier to unity by omitting resistor R2. However, the problem is really that the valve preamp provides too much signal. We assume you are using the February 2004 circuit? If so, you can make the volume control work more progressively by connecting a 100kΩ resistor in series with the 50kΩ potentiometer. A valve headphone amplifier is not really practical. Your approach works well anyway and will have authentic valve sound. WiFi interference to headphone amplifier I assembled an Altronics kit for the Portable Headphone Amplifier (SILICON CHIP, April 2011). It works really well except that during quiet music or in between tracks a motor-boating noise can be heard. I switched off my WiFi modem/router and the noise disappears (the response from the rest of the family is not so quiet!) So I moved the amplifier as far as I could from the modem. This reduced the motor-boating but not completely. 104  Silicon Chip valves, reconnected my audio oscillator and tried the amplifier out once more, this time wiggling the valves as it warmed up. And lo and behold, there was life in the old beast yet! When I wiggled V1, the output appeared and disappeared, depending on which way the valve was pushed. Encouraged by this, I dismantled it again and looked even more closely at the valve base PCB assembly, especially around V1. And while I wasn’t completely sure, it appeared that the valve sockets pins were a little too “open”. As a result, I tightened them on all the valve bases, then reassembled the amplifier yet again and this time it worked perfectly! This exercise was a valuable lesson re-learnt – there are more resistors in any circuit than meets the eye. Junction resistance can be a real problem in electronic circuits, so don’t forget to look at connectors, SC plugs and sockets, and so on. If I hold my hand over the amplifier the noise also stops. My main use is playing music from my desktop PC so I cannot separate the amplifier from the Wifi by more than 2m. Can I shield the unit or add any filter capacitors etc, or would earthing help? (M. D., Paynesville, Vic). • The headphone amplifier already has input filtering so if you cannot keep the unit more than 2m away from the Wifi modem, the only answer is shielding. Ideally, the unit should go in a small metal case but you might like to try using a small piece of copper laminate underneath the PCB. Place it so that the copper side is away from the PCB (to avoid shorts) and connect the copper to the earth of CON1. In fact, we found that simply placing the headphone amplifier case on a sheet of aluminium or copper laminate is quite effective in eliminating this sort of noise. Dog visits to letter box not welcome Have you ever done something to scare dogs away from the front path? I always have dogs coming along and doing their business just outside my letter-box, so I was thinking of some- Advertising Index Altronics.................................. 80-83 Clarke & Severn Electronics.......... 6 Element14...................................... 3 Emona Instruments...................... 12 Hare & Forbes.......................... OBC Front Panel Express....................... 8 High Profile Communications..... 103 Icom Australia.............................. 25 Jaycar .............................. IFC,49-56 KCS Trade Pty Ltd........................ 13 Keith Rippon .............................. 103 KitStop............................................ 8 LD Electronics............................ 103 LEDsales.................................... 103 Master Instruments........................ 7 Microbee Technology..................... 9 Microchip Technology................... 37 Mikroelektronika......................... IBC Ocean Controls............................ 11 Qualieco....................................... 39 Questronix.................................. 103 Radio, TV & Hobbies DVD.......... 101 Rohde & Schwarz.......................... 5 Sesame Electronics................... 103 Silicon Chip Binders..................... 97 Silicon Chip Online Shop............. 64 Silicon Chip PCBs...................... 103 Silicon Chip Subscriptions........... 91 Silvertone Electronics.................. 63 Tronixlabs................................... 103 Wiltronics...................................... 10 Worldwide Elect. Components... 103 thing remote-controlled inside the letter box that would go off when I press a button from inside my place. • We published a Barking Dog Blaster in September 2012 and a remote control for it in October 2012. Altronics sell the blaster (but not the UHF remote control part) as a kit K4500 (www. altronics.com.au). However, this may not work as a deterrent to a casual approach by a dog for a pit stop. We have heard that an electric fence placed around the letter-box post and only activated for a short period during the “event” can quickly deter a dog. However, the risk of electric shock to passers-by is a definite possibility which could have legal consequences. A better approach may be to try a natural dog repellent based on chilli, SC capsicum or ammonia. siliconchip.com.au