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24.01.2022 We’ve seen a few vacuum tube amplifiers, ranging from HiFi power and integrated amplifiers to Guitar and bass amps. I cut my teeth on vacuum tubes during my apprenticeship so enjoy working on this equipment. Firstly, here’s an essential little device for setting the bias on output tubes such as 6L6, 6V6, EL34 etc. This one was made using a regular 8 pin socket, and the base from an old tube. The pins were soldered together using short lengths of stiff connecting wire, with pi...n 3 of each part not joined, but instead connected to an insulated wire (600V or higher rated insulation) and each wire terminated with a socket to suit a multimeter probe. A cardboard surround with the inner side covered with Kapton tape was wrapped around the assembly and a quality epoxy (JB Weld) was used to cover the pins. After it set, the surround was easily removed, with a bit of sanding required to finish. A spot of high temp silicone was used around the pin 3 connections. Check this video for the assembly procedure and how to use. The only difference with the one I made was the epoxy. Its much safer to handle than one covered in Gaffa tape. It shouldn’t need to be said that this is a potentially dangerous job. If working with the amplifier powered up, always have insulated boots, and keep one hand behind your back. Receiving a 450+VDC shock from these amps across the chest, or from hand to foot can be lethal. For similar reasons in regards to 240VDC, connecting to the mains via an isolating transformer is strongly recommended. https://www.youtube.com/watch?v=WjhNSbQI_9M

21.01.2022 Replacing a key fob case is normally a simple procedure. It becomes a tad more difficult though if rough use has resulted in the key being pressed too hard over many months. This was a family friend’s fob. Dismantling it fully revealed that one of the two switches had almost completely detached from the circuit board, tearing up 2 tracks with it. Unlike most of these tact switches, the contacts for this one are under the switch, so after repairing the very fine tracks, the s...urface mount rework station was stoked up and the tact switch drawer opened, only to find the old one had an odd profile and I didn’t have the exact replacement. Repairing these switches is always touch and go, as dismantling usually ruins them, and this one was already a bit loose. Sometimes only one set of contacts is used, and if only one set faults, merely turning the switch 180 degrees and resoldering it will fix the problem. However, this one used all four contacts in two switching circuits, so all had to function. Luckily, a few sprays with Electrolube ULS cleaner while working the switch, got it operating again. The circuit board was cleaned, the switch positioned and held in place with a drop of cyanoacrylate, and solder paste and flux applied. To protect the surrounding surface mount components from excess heat, pieces of Kapton tape were placed over these components. This tape is a polyimide film (4,4'-oxydiphenylene-pyromellitimide) that has both electrical and thermal insulating properties across a broad temperature range (up to 400 deg C). In this application it helps prevent close terminals from accidentally being soldered together and others from being de-soldered and coming loose. Normally hitting the switch with the heat gun for 20 secs or so would melt the solder and fix the legs to the board, but in this case two of the terminals didn’t solder properly. After repositioning and two more attempts, it was in place with all 4 legs successfully soldered. A new battery was fitted and the fob snapped nicely into the new case.

20.01.2022 A popular speaker kit available through the 70s and 80s was made by KEF. It included a B139 SP1044 elliptical/oval shaped woofer. The kits were supplied with various enclosure design plans, including one big one. These speakers performed well, with a good bass response and top end. We picked one of the large heavy enclosures up from the customer’s house along with the old Realistic receiver used to power it. The woofer wasn’t working most of the time but occasionally would m...ake an intermittent burst of sound. Removed the woofer, impedance was checked and found to be close to a dead short. No wonder the amplifier showed signs of overheating, with burnt components throughout, and noisy transistors in the output stage. Upon removal of the speaker magnet, the voice coil initially looked ok. Closer inspection revealed the lower part of the coil had completely disintegrated, causing a short in the winding. Check out the reading of 0.063 on the impedance meter. It should read close to 8.0 A replacement woofer was found thanks to help from Toby Croce and Mike McKenzie and duly fitted. The crossover was also removed. While checking component values it was noted one leg from a 5mH inductor connected in series with the woofer was broken around the solder joint, causing intermittent operation. Once repaired, the speaker performed well, demonstrating the great design of this now 35 year old model.

12.01.2022 Some time ago I picked up a Marshall Super Bass 100 head and set about restoring it. This amp also came with a nice Celestion loaded quad box. However the head was in a really bad state, with the chassis being incredibly rusty, so this one was also partially stripped down, corrosion removed, masked up and gal-zinc coated. Upon inspection of the circuit it was quickly realized this amp had once been modified, with some of the front end completely disconnected and gain and filter setting components changed to provide overdrive for a 6 string electric (see photo). I decided to rewire it to original specification as this was a classic bass amp. While it took longer than I thought, it came up like new, especially with a new set of JJ tubes and input sockets.

10.01.2022 As it’s been a long while since my last update, here's a few of the more interesting jobs we’ve done over the past months. A Linn Sondek turntable was picked up for service. The client had immigrated to Australia, bringing his beloved HiFi equipment with him. The LP12 model was fitted an Ekos tonearm with an Akiva moving coil cartridge, one of the best Linn cartridges at the time of its release. After cleaning the platter, replacing the belt, adjusting the tonearm antiskate, ...checking VTA and tone arm balance the turntable was tested with a few good quality pieces of vinyl. Those recorded in the 70s with low bass levels played well, with no audible distortion, but anything with a heavy bass, or even playing a LP single with a higher output level resulted in terrible distortion. Checking the stylus indicated very slight wear but adjusting the tone arm balance made no difference except the stylus was more prone to skipping at lower weight and sticking at higher weights. It was concluded the fault lay in the cartridge, likely either with the coil dampening or positioning, or with the mechanical positioning of the cantilever, perhaps from a knock during transport. Unfortunately, as with many moving coil cartridges, repair is not an option. The design of the tonearm on this model only allows for fitting of 3 screw type cartridges. The customer was happy to go with another Linn cartridge and opted for the Kandid, the current top of the range. After fitting the cartridge, the turntable was fired up and produced a very warm sound with a solid and clear bass response.