This "Recent Repair" was kindly contributed by Tim Pullin.
He identified this set as the Stella ST113U based on the information on my website. However it is a different colour so it may well be either the Philips B2G25U or the Cossor CR1202U, all of which are identical electrically but differ in presentation. Can anyone positively identify it from his description?
I found this set (along with a Murphy A482) in my local free ads paper. Ten pounds later I was away with both sets.
The set is a bit tatty, but has its back and original knobs. Unlike the on the website, mine has a blue background to the tuning scale and the case is best described as "British racing green". The plastic front is in a fetching "nicotine". An area of the case over the output valve has sagged a little. The front trim is also missing, and this together with an absence of labels on the back caused a slight identity crisis. A quick look inside gave me a clue though. The set has no output transformer; a high resistance speaker is used in the output valve's anode circuit. I remembered seeing a similar set on Paul Stenning's web site only days before. After a little searching the mystery was solved - Stella ST113U.
Itching to see what I had bought for my fiver I removed the back. The set is built on a printed circuit with solder in valve holders and has a dropper resistor mounted vertically at the right hand side. Having belonged to a smoker the inside was covered in a thick layer of dust, which had to be cleaned away to see the components on the board! A previous repair was also evident, the output pentode's cathode resistor had been renewed, the new higher rated one soldered underneath the board. Strangely the original was still in position (though it turned out to be o/c).
The usual pre-plugin checks revealed the heater section of the dropper resistor to be o/c. Resistance checks from the tags of the HT capacitor to chassis revealed nothing alarming. Both sections reformed OK when fed from the bench ht power supply via a 100k resistor. I didn't have the drawing for the set but a look at the valve data enabled me to calculate a likely heater chain voltage and dropper resistance. (38v+19v+19v+50v =126v. 240-126= 114v to drop. 114v and 100mA needs 1140 ohms. I used 1k2).
According to the service data, the main dropper is 850 ohms and the additional section for the voltage selector (bypassed when on the 200-220V setting) is 250 ohms So the total resistance should be 1100 ohms. 1K2 is close enough. Paul.
With this soldered across the faulty section the set was plugged in via the lamp limiter. It was time to light the blue touch paper! The set gave only a few crackles when warmed up, & the ht seemed reasonable. The heaters however seemed rather bright, as did the lamp limiter, so I measured the voltage on the dropper. 150v instead of 126v. Hmmmm. Where was all that current going?
I decided to take a few quick measurements. The anode of the UCL82 pentode seemed to have a reasonable voltage on it, as did that of the triode. However the cathode of the pentode section measured 50v(!) which seemed a bit high to say the least! A check on the grid revealed 50v there too! This set uses those pale blue tubular ceramic capacitors, which are usually quite reliable.
As the set used series heater valves I decided to check for a grid/cathode short. On switch off the set gave a quick burst (the grid went negative presumably) of life. The UCL82 was indeed grid/cathode short, (pins 2-3) a value of about 3k-ohms was easily measurable on a multimeter. All other pins were clear.
I wondered if it was possible to "blow the short". A value of 3k-ohms needed a fairly high voltage and reasonable current. I used a transformer with a 50v output (at about 2 amps) and connected the output to a spare valve holder via a amp fuse. After shorting the heater pins the transformer was energised, and a flash was seen inside the valve. The two-amp fuse had also blown! The valve tested OK now and was plugged back into the set.
This time after switch on stations were heard on both MW and LW. Performance is very good for a cheap set. The dropper was now dropping 120v (mains 245v so I'm happy with that!) and HT a bit higher too. There are no DC voltages on the valve grids. Judging by comments on Paul Stenning's website I was lucky that the high impedance speaker had not burned out as a result of the increased anode flowing as a result of the faulty valve. That probably melted the case too!
The moral of the story: if the cathode resistor has burned out WONDER WHY! The set now awaits further cleaning, tidying up of the cathode resistor (which presumably had gone originally as a result of the faulty valve) new mains flex and a heat shield over the o/p valve and dropper (mine is missing). I may make one up out of a bit of ally. The case needs a good clean and I will try to straighten it out by re melting. Anyone got any ideas or tips? Stay tuned!
Text Copyright © 2001 Tim Pullin