This is an interesting repair because it illustrates how to repair a set with no service information, even though some of the component values are incorrect due to previous repair attempts.
The Ferguson 352U is a low-cost three-valve MW/LW set dating from the late 50s. It uses a UCH81 frequency changer, UBF80 IF amplifier, detector diode and (maybe) AGC diode, and a UCL83 triode audio amplifier / pentode output valve. The UCL83 was probably Mullardís smallest mains output valve, capable of delivering 2.5W maximum - although this was adequate for the purpose. These are real RMS watts, not the ludicrously exaggerated PMPO watts we have now! The set originally used a metal rectifier.
The setís owner said that a previous "repairer" had told him that the output transformer was faulty, but a local electronics shop had checked it with a meter and said it was OK. This shop referred the customer to me.
The output transformer was indeed OK, although there were signs of wax dribbling from it which suggested it had been subjected to excessive primary current and had been running rather hot. There were several signs of previous repairs; most notably the metal rectifier had been replaced with two silicon rectifiers in series, attached to a broken section of dropper resistor that had been fixed to a spare hole in the chassis. There was also a new capacitor and a couple of new resistors around the UCL83 valve socket.
The first thing to sort out was the power supply section. The decoupling resistor mounted between the two positive tags of the smoothing electrolytic was a 2W-carbon type that had clearly been badly overheated because all the paint indicating the colour code was burnt away. In the process of trying to bend it to one side so that I could get the meter probes in, it crumbled away. In this set, the HT to the output transformer is taken from the main HT smoothing capacitor, while the HT for the rest of the set is taken from after the decoupling resistor. Based on normal circuit design practice the decoupling resistor would be between 1K0 and 2K0. I fitted a 1K0 5W wirewound, chosen mainly because I happened to have it in stock!
The broken section of dropper resistor had been connected in series with the two rectifier diodes, and measured at 60R. The original dropper, which supplied the valve heaters, was still in good order, apart from the bottom section which was the surge limiter feeding the rectifier and had presumably been replaced by the 60R section.
Turning my attention now to the mess around the UCL83 base, it was clear that the original 270K grid leak resistor had failed, because one lead had been cut close to the body and it had been bypassed with a new 220K resistor. This seemed close enough so I fitted it tidily and removed the old one completely. The grid coupling capacitor had been replaced with a 1970s RS replacement. Since part of the body had been melted with a soldering iron the value could not be read, so I replaced it with a 0.01uF capacitor, which is a usual value for a capacitor in this position.
The replacement cathode bias resistor caught my eye because it was 100R, which seemed far too low for a resistor in this position - particularly when used with such a low powered output valve. Referring to the typical operating conditions for the UCL83 in a valve data book, gives an anode current of 27mA, a screen grid current of 4.4mA and a grid bias voltage of -13V. Therefore, the cathode resistor has to drop 13V at (27mA + 4.4mA), which with a bit of Ohms law equates to 414R. The closest I had was a 390R 1W-carbon resistor, so I fitted that.
Because of all these problems, I decided to test the valves before refitting them. The UCH81 was a bit low on emission and the UBF80 was a bit low on a couple of the inter-electrode leakage readings, but neither were bad enough to worry me. However, the UCL83 failed badly on all the inter-electrode leakage readings, so it was clearly dead. Fortunately, I had a new replacement in stock.
Both dial lamps were missing. Since they are in series in the neutral return, with a thermistor in parallel with the pair, I felt they were likely to have to carry virtually the full current drawn by the set, so I fitted two 12V 2.2W (183mA) lamps.
Time for a first test. I connected the test meter across the HT supply and switched on. The HT immediately rose to around 330V and the dial lamps gave a bright flash then settled down rather dimmer. As the set warmed up the HT started dropping, and seemed to settle at around 150V when the set started showing some signs of life. However, the sound was rather weak and distorted, and the HT kept dropping. Something was clearly wrong. I switched it off again, because I did not want to leave it running in that state for long.
For the next few measurements I connected the meter with the set off, then switched it on just long enough to get a valid reading. Firstly, I connected the meter across the primary of the output transformer, which gave a reading of over 50V. Definitely the output valve drawing too much current - but why? It could not be due to positive volts on the grid, because I had replaced the capacitor and the resistor, and the valve was new. A voltage reading across the grid resistor was zero, so it was not that. What about the cathode? The voltage there was also zero! Well that would explain the excessive anode current - but why?
Well if you have read the section on output stages on this web site, you will have seen that I recommend checking the cathode bypass capacitor when the resistor is replaced. Perhaps I should listen to my own advice, because this capacitor was short-circuited! I chopped it out and tried again. Much better - the HT settled at around 280V, with 260V on the UCL83 anode and 14.5V across the cathode resistor. More importantly it worked, and sounded really good. I left it running for a few minutes to make sure it was not going to give me any other nasty surprises, but it seemed happy.
If the UCL83 had been a transistor it would have died instantly from this sort of abuse. Valves are much more robust and can tolerate this sort of short-term abuse and overload. Of course, I am not suggesting deliberately overloading them, but it is comforting to know they will probably survive the odd mistake.
Back to the cathode bypass capacitor. I was undecided whether to fit a replacement or whether to leave it without. I was getting ample volume on all stations that were strong enough to be worth listening to, and the sound quality was very good. To prove a point I tacked a new capacitor in place. Definitely not so good. The sound was too shrill, and if anything, it was too loud. This is understandable because omitting the capacitor gives a bit of negative feedback with reduces the gain and also reduces the distortion. I decided this set sounded much better without the capacitor, so that is how I left it.
I was a bit concerned about the HT voltage though, so decided to check the UCL83 valve data. Limiting values - anode voltage 250V. We have 260V. Oops! Typical operating conditions - anode voltage 200V. That is more like I would expect in this sort of set. Clearly, that 60R resistance in series with the rectifier is too low. Further proof of this was the bright flash from the dial lamps when the set was first switched on and the smoothing capacitor charged. With only 60R in series the switch-on surge could be as much as 4A, which is more than the rating of the rectifiers, and more than the rating of the mains switch in the set.
A quick rummage through the wirewound resistor box and I found a 220R component. That seemed like a good starting point, so I connected it between the 60R resistor and the rectifiers. There was now about 230V on the anode. Better, but still higher than the typical conditions and higher than is usual for a small AC/DC set. A but more searching and I found a 470R wirewound resistor. That was much better, 196V on the anode. Transferring the meter to the cathode resistor gave a reading of 12.8V. I could not ask for much closer than that! I left the 60R section in place, mainly because it provided a convenient place to mount one end of 470R resistor securely. The other end was connected to the original dropper resistor.
With the right surge-limiting resistor, the dial lamps were not flashing as brightly when the set was first switched on. In fact, they were not doing very much at all - the dim glow would not be sufficient to light the scale. I tried a few different types and decided 12V 0.1A lamps gave the best results. They were reasonably bright when the set was running, and the switch-on flash was not too bright either. In normal running, they dropped about 9V each. Since they are in series with the supply to the whole set, I was a bit concerned about the effect on the heater circuit if they should have been a much higher voltage. To confirm that things were OK I connected the meter, on AC voltage range, across the heater of the UCL83. The data book gives a heater voltage of 40V at 100mA, and the meter showed 41.2V. I was happy with that!
After re-assembly, I left the set running for several hours to make sure everything was OK, then called my customer. The set used to belong to his late mother, so he was delighted to have it working again.
Some time later I managed to locate a copy of the circuit diagram for the set, in the 1956-57 volume of "Radio and Television Servicing". Although I cannot make any changes to my repair now, it is interesting to compare the component values I selected with what they should have been.
The most important one is the output valve cathode resistor. I fitted 390R and the circuit diagram shows 330R. With the 390R resistor I fitted, the voltage drop was 12.8V giving a current of 32.8mA (calculated with Ohms law and assuming the resistor was exactly 390R). On the service information the 330R resistor should drop 9.5V, giving 34.7mA. Close enough.
The grid coupling capacitor should be 0.003uF whereas I fitted 0.01uF. Since the grid leak resistor is only 270K, the original would limit the response at lower frequencies to some extent. Maybe this was the intention - it was clearly a cheap set. Certainly with the larger capacitor and the removal of the cathode bypass capacitor the set will now sound better than it did when it was new!
According to the circuit diagram, the resistor in series with the rectifier should be 120R, together with a thermistor whose hot and cold resistances are not shown. However, this was with a metal rectifier, the silicon rectifier diodes fitted in the set now would have a much reduced voltage drop, hence the need for a larger series resistance to keep the HT down to a reasonable figure.
The HT decoupling resistor should have been 1K5, whereas I fitted 1K0. Again, that has close enough. Unfortunately, the diagram does not show the ratings of the dial lamps.
Therefore, although the replacement components I selected were not exactly correct I am happy that they are close enough. Most importantly from a technical point of view is that the UCL83 valve is running within its published ratings, so its life span will not be affected. Most importantly from the owner's point of view is that the set now works, and that the repair did not cost him too much!