09/12/2023
Hi everyone, hope you are all keeping well. As promised, here’s my latest vintage radio repair story. This Marconi model T10 DA is a 1947 table-top sized valve radio in a Bakelite case - quite a clever set for its day, she covers the Long, Medium, and Shortwave bands, with two station presets, which are selected by the waveband switch. The first one covers 200 - 350 meters, whilst the second one covers 330 - 500 meters. When I first had this set working earlier this year, I found the first preset to be tuned to 247m, 1215 K/c’s (Used from March 15th, 1950, until January 20th, 2023.) Also, the second preset appeared to be tuned to BBC Radio Wales’ frequency (340m, 882 K/c’s) – thus giving a clue as to where this set might once have lived.
This particular set I bought for just £2.50 from the vintage Wireless and TV museum in West Dulwich, London, in September 2018 – a real bargin, so many thanks to all concerned! The photos were all taken prior to restoration, the videos in the comments section below show it coming back to life, and subsequently fully working.
I spent several weeks giving the set a full electrical restoration, replacing any high-risk condensers, renewing damaged wiring (this set was made in an era when rubber-insulated wiring was still widely used, so this was all thoroughly checked for it’s condition, and duly replaced where this was found to present a safety risk.) All switches and controls were also cleaned.
Following this it was then time to power the set up, slowly using a variac, so as to introduce the voltage gradually. All went well in that there were no bangs or smoke (thank goodness for that!) but sadly no viable results, no stations no static. A careful tap of the control grid of the Audio Amplifier valve revealed that there was some audio, (Albeit a little on the quite side) so it appeared that there was fault on the R.F. side.
In the first instance, replacing the Frequency changer valve brought some signs of life, with some stations starting to poke through, however the reproduction was still poor. A voltage check revealed that the H.T was a little low. However, what was strange was that it appeared to be getting lower as the set warmed up, with a noticeable decline in performance to match. My first thought was that something could be breaking down, or partially short-circuiting, thus dragging the voltage down. Referring to the circuit diagram, I looked at possible areas where this could be happening, particularly where Condensers are subject to high voltages. (These can become ‘leaky’ and start drawing excess current, which can cause the voltage to fall.) I then set about disconnecting different areas of the set in turn, to see if this brought about any improvement. It didn’t. Same time I turned the set on, the same pattern would follow. HT readings starting off good (or at least near to normal) before declining, a decline that would become more and more rapid, until the set had practically stopped performing. Switching off and allowing it to cool, before switching it back on, would restore the H.T supply, before it inevitably began its rapid decline once again.
But what was causing this, when every possible area of the circuit had been checked? The only remaining clue was that it appeared to be temperature related – i.e. the problem worsened as the set warmed up. Valve trouble? Yes indeed, but not of the kind you might think. My first port of call was the rectifier, I tried a few from stock, all with varying results, (one of them even had a blue hue around the anodes, not a good sign!) bit the underlying problem was still there – very odd. However, I then discovered that if I removed the output valve, the H.T voltages remained at normal levels. A further check revealed positive voltage on the control grid of the valve – always a bad thing as many will know already, but a bit surprising as the decoupling condenser feeding it from the anode of the A.F amplifier had been changed during the initial restoration. So assuming I’d been sold a faulty condenser, I replaced it again, only to find the problem still persisted – voltage on the control grid, which started off low, and gradually increased – as this increased, the H.T voltage would drop, as a result of the valve being over-run and drawing excess current. Knowing that I couldn’t possibly have been sold two faulty condensers, I began to wonder if the fault actually lied within the valve itself – some-sort of internal short perhaps? So I tried a replacement valve, and was rewarded with Good H.T. voltage readings, improved (but still not brilliant) audio, and slight negative voltage readings on the control grid.
Although this improved the set’s performance, the audio was still not brilliant, and injecting an audio signal in to the gram sockets suggested that there was still a problem in the audio amplifier stage. - I was actually getting a better result when I injected the audio into the control grid of the output valve than what I was when I injected it into the control grid of the Audio Amplifier valve – it should be the other way round, so time to investigate the Audio amplifier stage.
Some voltage checks revealed that the anode voltage was lower than it should be, this was traced to the feed resistor having deteriorated and gone higher than it is supposed to be. Replacing this corrected the voltage, but did little to improve the audio, which was still pretty poor, so after a few more checks I decided to try another valve. Doing so I was instantly rewarded with vastly improved audio – I was getting somewhere at last!
So now to the R.F. stage, as the tuning was still not brilliant, with some wavelengths suffering from poor sensitivity. A replacement frequency changer and some minor adjustment of the internal R.F and Intermediate Frequency (I.F.) stages improved this vastly, however.
By this time, the electrical work on the set was all but complete, however I decided to do an additional job, which I have not done before, but will make the set safer. This being to wire-in a 150 milliamp fuse (though in most cases 100 milliamps will suffice – and be even safer) into the H.T. supply line – wired in series between the cathode of the rectifier and the rest of the HT line. The idea being that if an over-load occurs on the H.T. line (such as a condenser shorting or an internal short in a valve) then this will hopefully blow the fuse, rather than over-heating and potentially ruining the mains transformer. (As happened with my Philco model 444 ‘Peoples Set’ back in 2020, when a shorting electrolytic condenser caused the mains transformer to over-heat and go up in smoke – both dangerous, and expensive to repair!) This is a cheap and simple modification, which will hopefully protect the sets most vulnerable (and expensive) part – the mains transformer. Eventually I plan to make this modification to all my sets that contain mains transformers.
Once I had completed this and was satisfied that the set was fully working correctly, I re-assembled it into the Bakelite cabinet, and carried out some minor cosmetic restoration. The white plastic backing for the tuning scale was filthy, so was washed in soapy water (this was detached and floating round in the back of the set when I first acquired it!) and the k***s were also washed the same way. Then, I gave the case and the k***s a ‘polish’ using ‘Three in One’ oil, which I gently rubbed into the bakelite and buffed up with some kitchen roll – unorthodox I know, but it’s surprising how this brings out the shine!
So all in all a pretty nice set of the period with a good mellow but clear tone to it. Of course, I decided to utilise the two preset’s, hence I tuned one to 257 meters, (Foxhall Heath transmitter, broadcasting ‘Smooth Radio’, sadly this was closed just weeks later) and the other one to 463 meters. (Orfordness transmitter, blaring out Radio Caroline.) The Short-wave section is also handy for trawling the late-night airwaves, especially when my favorite Short-wave station, Radio Romania International is on the air.
Finally, the valve-lineup in this set is U10 (rectifier) X61M (Frequency Changer) KTW61M (I.F amplifier) DH63 M (Audio Frequency Amplifier) and KT61 Audio Output. (Equivalent EL33 fitted in this case.)
Hope this has been of interest to everybody, remember I’m always happy to hear your own vintage radio stories and publish them to the page if you wish. Doesn’t have to be a repair, even simple videos or photos are welcome. Otherwise, all the very best and will speak again soon - in the meantime, enjoy the Radio Caroline North programs which are currently coming from the Radio Ship Ross Revenge and can be tuned in on 463m 648 K/c's Medium-wave in the East of England and West of Europe, also 219m, 1368 K/c's via Manx Radio on the Isle of Mann.
Best wishes,
Chris
