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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.

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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

The Echophone EC-1 Unboxing

March 21, 2017 3 comments

The Echophone Ec-1 has arrived. I had to carry it in it’s box from my work to home, on the train, no mean feat when also carrying a handbag, but I did get it home in one piece. I should say that it was not as well packed as I would have liked but it seemed to have survived the journey from the US nonetheless.

This radio known as the morale radio was commonly used by GIs to stay in contact with news of home. Like all radios of this era, I bought this one, fulling expecting that it would need extensive work, despite it possessing a sticker stating major modifications (1994). I am not sure what the modifier considered major modifications, I imagined a recap, I hoped not a major departure from manufacturers specifications.

At the point of unboxing, it is unclear what those “major” modifications may have been. Looking at the wiring, there is little evidence of significant change other than attempt to fit a polarised mains plug. The fitting of this seems to be non-standard. Typically when modifying an “all American 5” one would fit a fuse and ensure that the polarised mains chord had the neutral wire connected to the chassis and one would fit safety capacitors where necessary. One would also remove all wax paper capacitors and replace with modern poly units.

Whoever modified this radio, has connected the mains chord in a non standard way, with both wires going to the first valve in the series of five. No fuse is fitted. The mains chord is retained in the case by a knot which has resulted in the insulation being cut by the case- a common if deadly error. Not a single wax paper capacitor has been replaced and a large electrolytic is clearly leaking.

Another project for a cold, rainy day or two.

Categories: Uncategorized

The Saga of the FT101E Rectifier Board

February 10, 2017 Leave a comment

Recently I brought all of my vintage radios out of storage including an FT101E. Anyone who owns one will tell you, that they don’t like to be stored, they need to be used or they stop working. This one had done just that on most bands, so I set to, going through the alignment procedure. Rather than try to remove the mains chord from behind the radio bench, I hose to use a spare on I had. Unfortunately it was, I suspect, incorrectly wired for the FT101E. When connected and turned on, R1 on the rectifier board promptly went “bang.”

I purchased the appropriate 5.6K 2W resistor some weeks ago and today got around to installing it. All went well except that now I have a solid hum from the speaker even with the AF gain at zero. gah!

Categories: HF

Antennas, Apartments and AM Reception

February 5, 2017 1 comment

For some time now I have struggled with the fact that no matter what antenna I use, they couple to the high rise building in which I live and are quite deaf. Today I draped the Lake Eyre special antenna off my balcony as far as I was game. The building management would come down on me if they saw it. It’s better but still quite deaf. This is my Hammarlund HQ170 listening to the ARNSW broadcast on 40 meters AM. Screen Shot 2017-02-05 at 11.09.54 AM.png

Categories: HF

Of Injury, Depression and Productivity

January 28, 2017 1 comment

I had planned for some time to attend Symphony in the Park at Parramatta with my lovely friend, but due to my leg injury from falling down the stairs, I could not attend. This put me in bad mood yesterday. I don’t suffer depression as much as I used to do, in fact I rarely have any symptoms at all nowadays, but occasional mild situational depression still arises. I notice it most not by how I feel, but by the state of my apartment which yesterday got quite messy. My productivity suffers badly at such times. Being one who is always doing something, depression for me is most noticeable by my complete lack of interest in anything, and lack of desire to do anything.

I attempted to turn things around yesterday by going over to the community garden and picking a big bunch of various basil. I also contemplated going to the local ham radio trash and treasure but feel my leg would rather I stayed home. Instead I will come up with some recipes today for the basil. I will clean up my apartment and, of interest to all of the readers of this blog, will begin an antenna project. More about that later

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Categories: Daily News