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Monday, 2 April 2018

Hohner Arbor Series Bass Guitar Circuit

The controls on my bass guitar make horrid scratchy noises when you adjust them. I have sprayed them with switch-cleaner, and they go quiet for a few months, but gradually the 'orible noise comes back. So, I think it is time to change them. But what type are they?

Carefully removing the scratch-plate screws and looking at the writing on them gives the value, but not the "law" of the track of the potentiometer. The tone control says A500k, but does "A" mean linear or logarithmic? It is quite easy to measure without desoldering anything because the capacitor is open-circuit at d.c, so it won't affect the resistance-meter reading. I set the pot half way and it measures about 27k between the end of the track and the wiper. Measuring between the ends of the track 470k. So this is clearly a logarithmic part - if it was linear it would measure half the value in the middle of its range. The diameter of the body of the pot is 16mm and it has a splined shaft.

The Volume control has the pickup connected across it, which would affect the resistance reading. This meant desoldering wires from the pot. This one measures about 250k in the middle and 500k from end to end, making it a linear law.

While everything was disconnected, it gave me a chance to measure the d.c. resistance of the pickups - 12 kilo-ohms.

It seems a bit strange to have a linear pot for volume, but the adjustment seems smooth and progressive, so no reason to change it.

The overall circuit looks like this:
The 470nF capacitor is not the original component. Soon after I got the bass I was disappointed by how little effect the tone control had on the sound. From memory, there was a 100nF cap in there - (probably what was fitted to Hohner's "normal" electric guitars). Bass guitars need something bigger. I tried a few different values and settled on this one which goes from very soft and muffled to bright and zingy.

I should have enough info to get some replacement controls now. I see CPC sell some with conductive plastic tracks - they might be a quieter alternative to the old-fashioned carbon track.
Order codes RE06797 and RE06795, and quite cheap too.

Hugh M0WYE

Saturday, 31 March 2018

Lecture on Guitar Pickups

While searching Portsmouth University Library for interesting articles about guitar pickups, I came across a paper entitled "PATENTED ELECTRIC GUITAR PICKUPS AND THE

It referenced a Youtube Video of the Conference Lecture where he delivered the paper, complete with guitar demonstrations of the different sounds produced by the pickups. I thought it was a great demo, and include a link to it here in the hope that others will enjoy it.

Hugh M0WYE

Saturday, 24 February 2018

Duratool D03122 Autoranging Digital Multimeter

Haven't posted in a while as I am currently studying for a degree in Electronic Systems Engineering. It is taking up a lot of my spare time! However, I recently bought a new DVM and I spent a bit more money to buy a true-RMS reading instrument. It should read the r.m.s. value of an alternating signal, regardless of the shape of the waveform. Cheaper models will only read accurately with a pure sine wave.

There are limits to everything, and maximum frequency is one of them. I wondered if the meter would be any good as an a.c. millivoltmeter for making audio measurements. The frequency response was not quoted in either the advert, or the instruction book, so I thought I would measure it for myself.

To do this I used my Levell RC Oscillator, which is a vintage piece of gear, but still works well. Also hooked up the oscilloscope across the terminals to check the output level. I set the Levell Oscillator to 50Hz and adjusted the output until the meter read exactly 1.000 Volts on the a.c. Volts range of the DVM.

The 'scope had a nice sine wave with an amplitude of 5.7 divisions, peak to peak. On the 50mV range, with a x10 probe that works out as 2.85V p-p. Divide by two to get peak, then, divide by square root of 2 to find the rms, and it works out at 1.0076 - that's not bad.

So I tried different frequencies, checking the amplitude was constant on the 'scope. The results are plotted here. I didn't use the "smoothed" curve in Excel, because it puts in an artificial hump at 1kHz which isn't really there. Joining the dots with straight-lines doesn't look as pretty, but is probably more accurate.

 Then I reduced the output to 100mV and did the same test using the mV range on the meter.

On both ranges, the meter has a flat response up to 1kHz and then drops off quite sharply reading almost nothing at 10kHz.

So I conclude that it is good for power frequencies, and even for measuring impedance at 1kHz as part of a bridge or potential divider, but it is not much use for general audio use - I'll stick to using the 'scope.

This blog post is not intended to be a full review of the product, and I should probably also say that I'm NOT being rewarded in any way for this post - just want to share the information.
The meter is very versatile. It comes with a thermo-couple temperature probe. It also has a USB data output feature which I haven't tried yet. I also like that it has a big display and a backlight.
It will be a useful addition to the work bench, and hopefully last as long as my last one (30 years+)!

Hugh M0WYE

Friday, 18 August 2017

Replacing the Under Saddle Pickup on an Electroacoustic Guitar

Over the last few months the pickup on my guitar has developed an unpleasant distortion. The A string produces a gritty buzzing sound. At first it was a very occasional, and intermittent problem. I thought I had cured it by putting a shim of paper between the saddle and the pickup under the A string, but the sound came back.

The guitar is a Tanglewood Odyssey cut-away. It is a "bowlback" design with a fibreglass back - I bought the guitar back in 1992, so it has done good service, but replacing the whole guitar is an expensive proposition. So I started reading-up about under-saddle pickups on the internet. The name ARTEC kept coming up, and their pickups looked very similar to what was fitted to my guitar.

Artec website.

I was particularly attracted to the SF-607 model because of this phrase " It reduces buzzing sound caused by curved bottom of saddle or rough surface of pickup cavity on the bridge of guitar. " - I suspect this may be the "buzzing sound" that I have been hearing, or else the pickup has developed a fault. Unfortunately the Solidflex pickups are not at widely available on E-bay as the PP and PG versions, but I found one in Singapore for about £12 and made the purchase.

The SF-607 is physically very similar to the pickup I took out. It is about a millimeter shorter, but the spacing of the piezo blocks is exactly the same. The 2.5mm jack-plug is finished more neatly than the old pick-up with a metal cover and gold plating on the contact surface. In the photo at left, the new pickup is at top and old one below

The pickup strip is blue and branded, the old one is covered with plain black heatshrink sleeve, but the width and thickness I measured to be within 0.1mm of the original. This was very useful because it meant it was almost a "drop-in" replacement. I say almost because the original was a snug fit in the slot, which required a little judicious filing with a needle file to accept the new pickup. I expect I could have forced the new one into the slot, but I wanted to be able to get it out again if I had to !

The 2.5mm plug means that there is no soldering of wires - and the preamp and EQ unit in the guitar seems to work just fine with the new pickup. Here are a few more pictures:
 The pickup goes underneath the white saddle, which is a hard plastic, or bone material set in a slot in the wooden bridge of the guitar.

A hole is drilled at the bottom end to allow the screened cable of the pickup to pass through to the inside of the guitar. The hole is just big enough to allow the 2.5mm jack plug to pass through. This picture shows the old pickup lifted out of the slot, and the shim which I had added to try and fix the distortion problem.

The new pickup just plugs into the preamp unit. Remember to tidy and tape down the wires inside the guitar before putting on the new strings - otherwise they may flop about and rattle when you play it. I used "gaffer tape" (otherwise known as "duct" or "duck" tape).

It is a useful opportunity to clear the dust out of the inside of the guitar - it had got decidedly mucky in there!
Here's another picture of the EQ unit. - the battery holder and front panel is separate, and goes on the outside of the body, the screened metal box is on the inside of the body, and when the screws are tightened it all clamps up tight in the side of the guitar.

I couldn't photograph that operation because I needed at least three hands to hold the EQ unit, front panel, guitar, screw and screwdriver - I didn't have any appendages left with which to operate the camera.

So does it work?
Yes it does, but it is different to the old pickup. The output is lower and there is a bigger variation in output between the individual strings. The A and D strings are louder than the others.

Here is an image of the six strings plucked one after the other and recorded in Audacity, first with the old pickup:

You can see the distortion on the "A" string, as the envelope looks "broken".

And then with the new:

The output is lower, overall, but the A string and D string are strongest. I estimate the top E to be about 6dB down, but the A string seems to be as strong as the old pickup. The good thing is that the output is clean and distortion-free. So I shall leave the new pickup in place for the time being and see how we get on with it. I will have to crank the volume up on the amplifier a bit to compensate for the lack of output.

Hugh M0WYE

Thursday, 22 June 2017

Another Whistle

I've just made a second whistle, this one is longer - in the key of G and, this time, using the black PVC pipe. I made the fipple block a bit longer and cut it at a sharper angle. This makes it a bit easier to play in my opinion.

If you didn't read my last blog, these are "penny whistles" made to the design of Guido Gonzato.

The black pipe makes a perfectly serviceable instrument, but it is much harder to see the pencil marks, and, conversely it shows the scuff-marks from sandpaper much more clearly! A strip of masking tape along the tube was useful to mark out the positions for the holes.
Here's a picture of the two whistles together.
73 Hugh M0WYE

Saturday, 17 June 2017

Low-Tech Whistle

I've got one of those cheap "Generation" tin whistles in the key of "D". It's quite fun to play, but it is quite high and squeaky and when I try to play with other instruments, it is horribly sharp. I like the sound of the lower pitch whistles, such as you hear played on tracks by Capercailie and on the Transatlantic Sessions television programs, but, although not very expensive, still looking at upwards of £50.

But I found an interesting website by Guido Gonzato here: Guide to making low-tech whistles

Now I have found designs for whistles before, but I have had difficulty getting the same materials as were used in the original. If you substitute a different type of tubing it has an affect on the length and hole position. But I found 2 meter lengths of this 20mm diameter PVC pipe in B&Q and Wickes for 99p and 18mm diameter hardwood dowel which can be made to be a snug fit inside by sanding off about half a mm.

So today I made one, and I am very pleased with the result (photo left). I won't go into detail about how I made it because Guido's instructions (in the above link) are very detailed and I followed them very closely.

I made the Alto B-flat instrument, and I found that I needed to leave the tube about 1mm longer than was specified in the diagram.

In fact, because my tube started out quite long, I checked his overall lengths at F#, G, G#, and A and found they all needed to be 1mm longer than specified - that probably comes down to the type of tube, he says "not all PVC pipe is created equal".

So I wanted to upload a sound-sample for you, but I don't see a way of doing that on Blogger, so it will have to be in the form of a video ...
Sorry I'm playing it a bit like a recorder, it's just to give an idea of what it sounds like! Guido has some much better sound samples on his website.

I'm intending to make some more of these, in different keys, and perhaps even longer and more mellow :-)

Anyway, if you like making simple musical instruments, I encourage you to have a go at making one of Guido's low-tech whistles.
Hugh M0WYE

Saturday, 10 June 2017

Lidl's Vice

I've got a variety of bull-dog clips, clothes pegs and "extra hands" to hold small items when soldering. I did have a small table-top vice, but my son "borrowed it" and it has obviously proved very useful to him ... haven't seen it since :-)

When I saw that Lidl's were offering a table-top vice for eight quid, I thought I would give it a try.
Lidl's Table Top Vice Offer.

Some of their tools are very good value for money, occasionally I have wished that they were better quality!
This product seems ok ... but there are some important things to be aware of. One is quite obvious when you pick up the box in store - it isn't very heavy! It is not made of steel, it is made of an aluminium alloy (clearly stated in the instructions) and this means the casting is not going to be very strong. Don't expect to use it like an expensive Record vice for hammering and bending thick metal, it will break.

The aluminium will likely not be as hard-wearing as steel would be.

The main tommy-bar, used for tightening the jaws of the vice is quite nice, with a detent at each end, so that the bar clicks into place at each end of its travel. However I found that all the screw threads felt rough and "graunchy" - not smooth at all. Although the instructions make no mention of lubrication, the first thing I did was to apply grease to all the threads and the grooves where the moving jaw of the vice slides. I also greased the ball and socket and now it feels much nicer to use.
hopefully that will extent the life of the threads a bit too. Time will tell.

The vice is supplied with soft, plastic covers for the jaws, which could be useful for holding some items, but will obviously be useless for soldering, because they will melt! Underneath the plastic covers are smooth metal jaws, one of which has two grooves at right-angles. The grooves are for holding round objects.

Supplied with the vice is a clamp for holding tubes of between 38 and 43mm diameter. This seems rather a limited range to me, and it is complicated to fit the tube attachment. To do this you have to remove a the jaws, which requires the use of the supplied hexagon wrench. However, the fact that the jaws can be removed does allow for the user to create their own attachments.

I'm thinking that this will be very useful for holding plugs, sockets and circuit broards instead of chasing them round the bench with the soldering iron.

Hugh M0WYE