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KRK VXT6 Repair

February 6, 2018 Leave a comment

Some time ago I picked up a powered studio monitor for about $20. The music store was closing down and the speaker had a fault- the tweeter wasn’t working. These studio monitors of this size range in price from about $200 to $400, so I couldn’t pass up the deal, as I was sure I could repair it – one day.

That day finally arrived. I had purchased a spare tweeter which had sat in the parcel unopened for some time. When I unpacked it, you guessed it, it was the wrong part. Given that it had come from the US, the return postage added way too much to the cost of repair, so I decided to repair the existing tweeter.

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There isn’t much to a permanent magnet tweeter, basically the only electrical component in it is a super-fine winding of copper. This usually fails to due physical stress or sometimes an over-voltage or over-current fault. All that is required is to solder the broken wire- easy right? Well the wire is quite a bit thinner than a human hair and enamelled and wound very tightly into a place that is difficult to get to. You got the picture?

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The good story is that after one repair, a subsequent failure and another repair, the speaker works great

 

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Categories: Uncategorized

Breville 330J Restoration Part 1

January 29, 2018 Leave a comment

I picked up this Australian built radio some time ago. It has sat in a carton in my storage for some time. When restoring Dad’s 1935 Ultimate I wondered if I could get some donor parts from this radio, but when I took a look at the condition of the electronics, I decided to restore it. The electronics are in amazing condition. The radio was produced in the late 1940s, possibly 1948.

27066775_1840903212627293_4071677038937855009_nChassis

I removed the chassis from the wooden case and due to the excellent condition, decided to fire it up without replacing the capacitors. I ran it up on a variac and it came to life, with very low audio. The valve line up according to the markings on the chassis is 5Y3G rectifier, 6J8G Osc/Mixer, 6U3G IF Amp and 6G8G AGC, El3G Audio Amp

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5Y3G rectifier socket.

27332373_1840902999293981_817692224466341981_nIF Amp- Actually says 6U3G now that I see the photo
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6G8G or 6B8G RF Amp and AGC

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EL3G Audio amp

The schematic I have shows a slightly different line up and initially it caused me to be confused. I had confused the audio amp with the mixer. The EL3G that is supposed to be in this set is similar in code to the EK2G on the schematic, but anyway I have now sorted it out.

The EL3G is made of unobtainium so someone had inserted a 6F6G which is much lower gain- this would explain the weak audio. So what I need to do is change the EL3G for a 6V6G and change the bias resistor to reduce current draw. I can test to ensure the control grid voltage is around -12V and all should be good.  Unfortunately audiophiles have pushed the prices of 6V6Gs, for their trendy valve stereos, through the roof…. more soon.

Categories: Vintage Radio

My Dad’s 1935 Ultimate Receiver – Part 2

January 25, 2018 Leave a comment

I returned home to Northern QLD, some 2200 kilometres from where I now live in Sydney, for Christmas 2017. I was working, house sitting and dog sitting, which gave me plenty of time in the evenings and weekends to repair Dad’s radio. Unfortunately some late  received communication meant that I failed to bring my test equipment and tools, so I shipped the radio to Sydney via Greyhound, to be picked up on my return.

Fortunately the radio was unscathed by the bus trip and was promptly placed on my workbench for the investigations to begin. It became immediately clear, that the capacitor I had failed to change had destroyed itself in rather spectacular and gooey fashion and taken out a few other components as it went. An inspection revealed a failed 2A7 mixer and the 57 valve (vacuum tube) had a loose glass envelope. I suspected other damage but could see none.

I replaced the capacitor and ordered replacement valves (tubes). When the tubes arrived I replaced the defective ones, and the rectifier just for kicks. I turned the radio on, the lights glowed, the valves glowed but no sound. hmmmmm. A quick check of voltages found nothing out of the ordinary as far as the mixer and audio amp were concerned.

This made me suspicious that I had an open speaker field coil. A failed coil, either field or in primary, was confirmed by removing the audio amp with the radio on- no click from the speaker. So some further digging around is required…. more in Part 3.

Categories: Vintage Radio

My Dad’s 1935 Ultimate Receiver Part 1

January 23, 2018 Leave a comment

Some time back in 2015 I obtained a 1935 Ultimate AU Receiver. I had just recently restored a 1936 Pilot receiver and was keen to do the same for this radio. What I most liked about the radio was that its “birth year” was the same as my Dad’s. Dad’s 80th birthday was forthcoming and I thought that the radio would make the ultimate present. He and my Mum had been loaned a console radio early in their marriage and I know they loved it and were disappointed to see it return to the owner, many years later.

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I set about doing as much restoration as I could before my Dad’s birthday, which was to be soon. Unfortunately it was during the worst part of my life, when my gender disphoria had resulted in serious mental health issues and the loss of all that was dear to me. I rushed to get the radio ready for the birthday celebration while dealing with my own demons. One capacitor proved difficult to obtain- a filter capacitor across the DC output of the rectifier, so I left the original there. I left the radio running for many hours as a test and it worked a treat.

When I presented the radio to my Dad, on the day after his birthday, it immediately failed with a loud pop. I left it with him, with the view to getting back to it “one day.” BTW he loved it.

However my life was about to take a radically different path and it would be some time before I saw the radio and indeed my Dad again…… More in Part 2

Categories: Uncategorized

A Different Project- An Audio Amp

August 26, 2017 2 comments

A friend of mine contacted me to ask if I knew someone who could trouble-shoot her valve driven audio amplifier- of course I said I could. The amplifier is an interesting Chinese brand, a Yaquin MC-100B. It can be used in two different modes Ultra-linear or Triode mode. The amp uses a bunch of valves including 2x 12AX7 a pretty common  amplifier found in everything from guitar amps to radios, used in this case as voltage amplifiers. It also uses 4x 6N8P as drivers and cathode followers and finally 4x KT88 valves in the output stage.

My friend complained that the amp had not worked for nearly 12 months but the filaments on all valves glowed. According to her tests, it had no output at all. She dropped it off and I promised to take a good look at it. Since I was lacking any means to connect any kind of music source to it, I got straight down to visual fault finding- a mistake but only a minor one- it simply cost me some time. I was later to find that when I did connect it to a source it worked, though not perfectly.

I began by removing the bottom plate and visually inspecting all components and PC boards for damage, burns, broken tracks, anything obvious- there was nothing. I then began checking all voltages at the appropriate test points. Beware if you are attempting to do the same. There are some KILLER voltages in here; some over 1000V AC. Beware!

I found all to be within specification. I at this time did not check the bias as I wanted to first confirm the amp had no output at all. I connected my audio signal generator to the audio inputs on the front of the amp (labelled 0.6V). I configured the amp switches appropriately for these inputs and connected some speakers. I immediately heard a pretty good tone, so the amp did have output after all. I confirmed with my friend that she had not just inadvertently had the inputs configured wrong, but she advised she had tested all configurations. hmmmmmm …some testing at her place may be necessary to check that the audio source is not the issue.

Continuing with my testing I noticed that one channel was lower output than the other. I began by checking all bias voltages, finding all of them low but one non-existant. I swapped the suspected KT88 with one of the good ones and the bias voltage returned- indicating a failed tube. I then set about adjusting the bias voltages to 0.55V as specified. Even with one channel low, the amp sounds pretty good.

I instructed my friend to purchase her choice of new KT88 or a matched set of four. This amp is not very sensitive to valve changes, so I would just purchase one. I finished up the repair with fixing the cage over the valves, as it had begun to come apart.

Categories: Uncategorized

I Need to Restore Before I Restore

July 31, 2017 Leave a comment

Today I began preparing a list of parts required to restore my Echopone EC-1, and familiarizing myself with the alignment procedure. I also made a list of parts required to interface the test equipment with the radio.  It was at this point that I realized things would be a whole lot simpler if I used a Vacuum Tube Voltmeter for the RF alignment. While I can use my signal generator and my oscilloscope for the IF alignment, I liked the simplicity of using a VTVM for the RF alignment.

There is only one problem. I have a habit of purchasing vintage test equipment that needs restoring, for example my capacitance tester and other items. So it is with my VTVM which is a Heathkit V-5. Made in 1952, the V-5 like all electronics of that era, uses wax and paper capacitors. These capacitors will almost certainly have failed or will fail if I power the unit. So it is that I added to the list of parts, two capacitors required to restore the V-5. Of course it won;t be as simple as replacing the capacitors- I will have to check all resistors and calibrate the unit as well as make a probe for it. But the beauty is, I will have a working VTVM that like my radios is from an earlier time.

 

Categories: Uncategorized

Echophone EC-1 Beginnings

July 29, 2017 Leave a comment

If you saw my previous posts Echophone EC-1 and The Echophone EC-1 Unboxing you will know that I put off the restoration of this little gem of a radio until such time as I was able to spend some more time on it. As often happens with these tasks, it never really is about the time, but more about finding the headspace to allocate and finding the motivation.

A great deal of organisation and pre-work goes in to any such restoration, even if the final result is not to have it look exactly like it came out of the factory yesterday. So it is that I find, despite having many other things on the go, including moving house, a very busy career, commitments in music and cycling, I have somehow found the motivation to begin.

For those of you who have never undertaken such a task as restoring old radio equipment, I will be pleased to document the process here and via Youtube. The EC-1 is a very simple radio- a variant of the All American 5 chassis. This design is quite simple and therefore very common in earlier radios, but is is not particularly safe- it requires some modifications to make it safer. Restoring valve radios often means dealing with lethal voltages, so if you undertake a restoration yourself, you do so at your own risk. I am not an qualified technician, just a holder of an Advanced Amateur Operators Certificate of Proficiency (AOCP), the highest licence category for amateur radio operators in Australia. I am certainly not an expert. In fact when it comes to radio repair, I am entirely self-taught. If you wish to see someone far more skilled than me restore the same radio model, you can go to the video below.

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The first steps for me after obtaining the radio and inspecting it are now underway.

  • obtain circuit diagram (done)
  • if possible obtain alignment procedure (done)
  • Make a list of components to replace- all capacitors other than silver mica and possibly some resistors depending on how they test
  • construct several pieces of test circuits such as impedance matching circuits for connecting test equipment to the radio

As I work my way through the process of restoring this radio, I will post updates here.

WARNING STATEMENT

As they are wired from the factory, these radios are deadly dangerous and you shouldn’t even plug them in until they have been made safe.  These radios have what is called a “hot chassis,” and that makes them very dangerous to work on or even to plug in.  Do not attempt to test or repair them until you or a skilled technician have rewired them as I will describe below.  I can not emphasise too strongly how important it is that you follow my advice because over the years, many people have been killed by “hot chassis” radios and many others have received painful shocks.  The fact is, the modifications I shall describe are not only very effective, but are quite easy to do and will cost you less than $10, so there is no excuse for not doing the work.

Why are these radios so dangerous?  Depending on which way their original unpolarized plug is plugged into the wall socket, the chassis of these sets have a 50/50 chance of being “hot” if turned on and a 50/50 chance of being “hot” if simply turned off.  On or off, the chassis will be hot at some point and you will get a bad shock if you simply touch the insides of the radio.  To make matters really deadly, if the grommets that insulate the chassis from the outside case are in bad shape, you will get electrocuted just touching the radio.

How did Hallicrafters and other manufacturers of  “hot chassis” radios get UL approval to manufacture and sell such dangerous radios?  Back when these radios were new, there were no polarized wall sockets and power plugs so radio manufacturers made them safe by insulating the case from the chassis with rubber grommets or by putting the chassis in wooden/plastic cases.  All these radios had Masonite rear covers that prevented probing fingers from touching the chassis and they all had bold warning labels telling you NOT TO TOUCH THE CHASSIS OR CHANGE THE TUBES WITH THE BACK OFF AND THE SET PLUGGED IN. The warning labels said that the set was to be worked on only by a Trained Radio Technician (who knew what a “hot chassis” was).

We can’t trust the old safety devices anymore because the insulating grommets have deteriorated with age and the Masonite backs of most of these old radios are now missing.  Even if they were still there, the warnings that were printed on the backs of these radios are equally useless because The Trained Radio Technician profession has been extinct for decades.  Today we have to be our own “Trained Radio Technician” and the truth is, most of us aren’t “trained” and most of us don’t even know what a “hot chassis” is.  The whole idea of this essay is to give you the training you need to make your radio safe before you try to fix it or use it.

73 de VK2KMI
Kimberly

Categories: Uncategorized