I've not had an opportunity to spend time in my workshop for a couple of weeks, but I did find time today to have another look at the Radionette Kurer valve portable.
With the faulty set tuned to 1000 kHz on the medium wave band I was able to pick up a strong 1455 kHz signal on another set next to it on the bench. This confirmed that the Kurer's Local Oscillator was running. I tried injecting a modulated 455 kHz test signal again, coupling the output from my signal generator into the set via a few turns of wire around the DK96 valve. Instead of the pure tone that I expected, there was a beat note from the speaker. This usually indicates an unstable and oscillating IF stage.
Turning again to the internet, I found several references to similar faults with battery sets in various vintage radio forums. I had already changed the decoupling capacitors and checked screen resistor values, which left possible problems with poor contacts in the valve bases. I gave both mixer and IF amplifier valve sockets a liberal application of contact cleaner spray, and again following suggestions in a forum post, tightened up some of the contacts in the sockets. The advice turned out to be spot on, because on powering it up again I could immediately hear received signals.
It's currently sitting on the bench playing my local AM station on 1332 kHz. The sensitivity isn't great, and i'ts pretty deaf on long wave as well. I'm assuming that this is because the loop aerials aren't connected, so I'll try it again once its back in the case.
It's been an interesting repair job, but I can't honestly say that there is anything about this set that I like. I'll be quite happy to hand it back to my friend.
Keep listening!
Wednesday, February 18, 2015
Thursday, February 05, 2015
Pentagrid Converter
The Radionette Kurer is a step nearer to working but I've run into a problem. Apart from an intermittent volume pot the audio stage is operating, but the receiver is otherwise dead on all four wavebands.
The IF and detector stages are also working, because a modulated 465kHz test signal can be heard OK from the speaker. My next thought was that perhaps the Local Oscillator wasn't running, and this was where I began to struggle a bit.
All of the receivers I have worked on have either had separate oscillator and mixer valves, or have had these two functions combined into a valve with two distinct sections, typically a triode-hexode such as the ECH81. This one uses a single DK96 heptode valve as a self oscillating mixer. Here was the problem; I'd never seen this configuration before and I couldn't understand how it worked.
The answer came from Wikipedia in an article entitled Pentagrid Converter. This explained how the first grid is used for the oscillator, with the second grid acting as an anode. The incoming RF signal from the antenna tuning circuits is applied to grid 4. The remaining grids 3 and 5 are connected together to act as screen grids. Grid 2 allows some of the oscillating electron stream to leak through and couple with the mixing section of the valve.
The Wikipedia article can be found here.
The only circuit diagram I have for this set is a very small picture pasted inside the rear cover. I don't have any component layouts or test voltages either, so given my unfamiliarity with this type of circuit this one it's going to be a bit of a challenge!
I need to spend a few days away from the workshop so regrettably this job will have to be put away for a while, but before I do I will check if the LO is actually working. The easiest way to do this will probably be to use another radio. The LO will be running 455kHz above the receive frequency, and should be receivable on another radio in close proximity.
Keep listening!
The IF and detector stages are also working, because a modulated 465kHz test signal can be heard OK from the speaker. My next thought was that perhaps the Local Oscillator wasn't running, and this was where I began to struggle a bit.
All of the receivers I have worked on have either had separate oscillator and mixer valves, or have had these two functions combined into a valve with two distinct sections, typically a triode-hexode such as the ECH81. This one uses a single DK96 heptode valve as a self oscillating mixer. Here was the problem; I'd never seen this configuration before and I couldn't understand how it worked.
The answer came from Wikipedia in an article entitled Pentagrid Converter. This explained how the first grid is used for the oscillator, with the second grid acting as an anode. The incoming RF signal from the antenna tuning circuits is applied to grid 4. The remaining grids 3 and 5 are connected together to act as screen grids. Grid 2 allows some of the oscillating electron stream to leak through and couple with the mixing section of the valve.
The Wikipedia article can be found here.
The only circuit diagram I have for this set is a very small picture pasted inside the rear cover. I don't have any component layouts or test voltages either, so given my unfamiliarity with this type of circuit this one it's going to be a bit of a challenge!
I need to spend a few days away from the workshop so regrettably this job will have to be put away for a while, but before I do I will check if the LO is actually working. The easiest way to do this will probably be to use another radio. The LO will be running 455kHz above the receive frequency, and should be receivable on another radio in close proximity.
Keep listening!
Radionette Kurer
It looks like my interest in vintage radios has influenced one of my friends, because he turned up at my house recently with an unusual radio that he wants me to restore. I forgot to take the usual 'before' photo, so here it is almost as received, but without its knobs.
 |
The set uses 'D' series battery valves which require 90V for the HT, and 1.5V for the filaments. Batteries for valve radios haven't been made for decades, so the chances of finding a serviceable one today must be very low indeed. Fortunately the mains winding on the transformer seems to be OK, so at least this set should be usable.
It took me a few minutes to work out how to remove the chassis from the cabinet. Eventually I realised that the wooden panel onto which the chassis and transformer are mounted slides out.
The wires on the left side of the chassis are connected to a aerial inside of the cabinet.
After the usual strip, dust and clean the first task will be to replace some of the capacitors. Although there aren't any wax paper capacitors in this set there are several brown 'Hunts' branded ones, which in my experience can't be trusted either and usually get replaced on sight.
The smoothing electrolytic is only rated at 150V working. My capacitor re-former won't go this low so this time I will have to bring the supply Volts up slowly using a variac. This trick doesn't work well with sets which have a valve rectifier, but fortunately this one uses a selenium rectifier in the HT supply which will work at low supply voltages.
I'll let you know how I get on.
Keep listening!
Wednesday, February 04, 2015
Visit Counter
When I changed the blog layout in February last year I lost the site visit counter. I've finally figured out how to add the HTML code back into this layout, so as of today there is a Site Meter counter at the bottom of the page.
I've reset the counter to zero today, so it will be interesting to see how many visits this blog is actually getting.
Keep listening!
I've reset the counter to zero today, so it will be interesting to see how many visits this blog is actually getting.
Keep listening!
Monday, February 02, 2015
A bit of HRO history
A couple of people have asked me why I find the HRO receiver interesting enough for me to have two in my collection.
I'm interested in World War Two history, and in particular the code breaking activities of Bletchley Park. The HRO played a very significant role in this work as it was the standard receiver used for intercepting the encrypted German radio traffic.
The HRO has its origins in the mid 1930's. The National Radio Company of Malden, Massachusets had previously produced a receiver for the aviation industry, known as the AGS. The successor to the AGS was intended to meet the needs not only of airlines and commercial users, but also Amateur (Ham) Radio Operators who wanted a very high performance shortwave receiver.
The HRO was brought to the market very quickly, and it's name may reflect this. It is said that staff who worked on the project wrote 'HOR' on their timesheets, meaning 'Hell Of a Rush'. Apparently National thought HOR was inappropriate, and rearranged it as HRO (subsequently said to mean 'Helluva Rush Order'). It went on sale in 1935 at a cost of around $200, which must have been a lot of money at the time!
Very early in WW2, the War Office decided that the receivers in use by British armed forces might not be good enough for the anticipated needs of the war, and a search for a better and commercially available receiver was undertaken. A number of types were evaluated but the National HRO was considered to be most suitable. At first the British Government was not able to buy HROs in quantity (I assume this was when the USA was neutral), so British officers traveled to the USA to buy sets and import them personally. Many thousands of HROs were subsequently sent to Britain under the Lease-Lend arrangement, and there were probably more than 10,000 in use by the end of the war.
HROs were used as a general purpose HF receiver by all British Forces. The 'Y' stations (I believe the name came from Wireless Intercept, or WI) operated by the Army, Royal Navy, RAF and the Radio Security Service all used HROs. They were also issued to civilian Volunteer Interceptors, who used them at home under control of the RSS.
The HRO was very advanced for its time. There were several versions produced over the years but due to its superb performance and ease of operation, even the early ones remained in use by both commercial and military users into the late 1950s, and possibly later. After the war many came onto the surplus market and were eagerly bought by Amateur Radio Operators, and HROs were prized posessions in many radio shacks for several decades. In fact, the first one I ever saw was in the shack of a local Ham who introduced me to the hobby around 1975.
There are a number of good websites where information on HRO receivers can be found. One of the best in my opinion ( I have no connection with it) can be found here
Keep listening!
The HRO has its origins in the mid 1930's. The National Radio Company of Malden, Massachusets had previously produced a receiver for the aviation industry, known as the AGS. The successor to the AGS was intended to meet the needs not only of airlines and commercial users, but also Amateur (Ham) Radio Operators who wanted a very high performance shortwave receiver.
The HRO was brought to the market very quickly, and it's name may reflect this. It is said that staff who worked on the project wrote 'HOR' on their timesheets, meaning 'Hell Of a Rush'. Apparently National thought HOR was inappropriate, and rearranged it as HRO (subsequently said to mean 'Helluva Rush Order'). It went on sale in 1935 at a cost of around $200, which must have been a lot of money at the time!
Very early in WW2, the War Office decided that the receivers in use by British armed forces might not be good enough for the anticipated needs of the war, and a search for a better and commercially available receiver was undertaken. A number of types were evaluated but the National HRO was considered to be most suitable. At first the British Government was not able to buy HROs in quantity (I assume this was when the USA was neutral), so British officers traveled to the USA to buy sets and import them personally. Many thousands of HROs were subsequently sent to Britain under the Lease-Lend arrangement, and there were probably more than 10,000 in use by the end of the war.
HROs were used as a general purpose HF receiver by all British Forces. The 'Y' stations (I believe the name came from Wireless Intercept, or WI) operated by the Army, Royal Navy, RAF and the Radio Security Service all used HROs. They were also issued to civilian Volunteer Interceptors, who used them at home under control of the RSS.
 |
| RSS intercept station at St Erth, Cornwall. |
The HRO was very advanced for its time. There were several versions produced over the years but due to its superb performance and ease of operation, even the early ones remained in use by both commercial and military users into the late 1950s, and possibly later. After the war many came onto the surplus market and were eagerly bought by Amateur Radio Operators, and HROs were prized posessions in many radio shacks for several decades. In fact, the first one I ever saw was in the shack of a local Ham who introduced me to the hobby around 1975.
There are a number of good websites where information on HRO receivers can be found. One of the best in my opinion ( I have no connection with it) can be found here
Keep listening!
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