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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

Back to the Drawing Board

February 26, 2017 2 comments

Today I finally got myself organised enough to begin work on another mag loop, in the hope that this one will work. I believe the steel framed high-rise building is to blame for my lack of reception but in a last ditch effort, I decided to make a new multi-turn loop out of some heliax that I had brought back from Queensland. Unfortunately the PVC support I made was not robust enough for the heavier than expected heliax- back to the drawing board

Categories: Daily News, HF, Uncategorized

Rebuilding a Life, Rebuilding a Shack- it is all Drama

January 26, 2017 1 comment

Well I got all of my vintage radios out of storage where they have been for about 2 years and have now got them all set up in my tiny apartment, though no suitable antenna yet.  I also managed to bring along all of my test equipment and various cables and audio mixer etc etc.

The problem is of course, when you leave stuff for so long, especially stuff that was the subject of restoration and many little and large projects, you kind of forget where you left off. I recalled as I began to test equipment with my meagre antenna and dummy load, that not all was 100% when I left it. The Yaesu FT101E was, but now isn’t- that is pretty normal for an FT101 left in storage, so no surprise there. I was disappointing when I connected the mains chord to it and blew resistor 1 on the rectifier board, because that was not my chord but one I had purchased and it was probably wired incorrectly – ouch!

More interesting when I remember that the plan of using the Johnson Viking Ranger II and the Hammarlund HQ170 as a transmit/receive pair had fallen on issues as well- issues I had forgotten. See the HQ170 receiver pulls wildly off frequency when I transmit on the Viking II and sounds very chirpy. I am convinced it is the way this radio uses AVC(automated volume control not AGC) – although switching off the AVC makes no difference. So now I recall that I had a huge project to do with the Hammarlund and had kind of decided to ditch it and get something vintage but more stable. That is out of the question now because I am nowhere near as financial as I was when I had my own business. Wonder what I can do to stabilise it? My fist attempt will be to replace the OB2 valve (tube in the US) with a solid state version.

The Viking Ranger II I was sure had no issues. I remember using it on CW and AM and it sounded great on AM, but then did I not change the value of a capacitor in the audio circuit in order to improve performance and some how stuffed that up? Or was it that an audio amplifier tube was to blame? Either way there is nasty hum and no audio from the microphone on AM today when I went to use it on AM. Since I can’t remember what I did two or more years ago, I will have to go through the service and construction manual and backtrack.

Maybe these radios will be working properly two years from now. You can follow the saga here http://antiqueradios.com/forums/viewtopic.php?f=5&t=314753

(Shack is a term used to describe the location from which amateur radio operators operate)

Categories: Uncategorized

I Love my Vintage Radios

January 21, 2017 Leave a comment

Just a quick post to say I got all my old radios moved to my new home Sydney. They are in my bedroom so now to dress it up so it doesn’t look like a teenage boy’s bedroom 🙂

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I still have to solve the issue of my Hammarlund HQ170 moving off frequency when receiving strong signals.

 

 

Categories: Uncategorized