Sunday, August 31, 2008

Next patient - Mullard Five-Ten

One of my colleagues has just given me this valve (tube) amplifier built from a kit based on the Mullard Five-Ten design published back in 1954. Though this design is still being built by audio enthusiasts today, it seems this one is the genuine 1954 article. It's far from working condition though, so it's going to need a fair bit of work and a few replacement parts.
As it's likely this will be an extended project with a few stops and starts as I seek components - I don't really like to buy new, because it's expensive, and because new parts often don't look right - I'll be keeping notes in my mullard-five-ten wiki page.

Here are a few more "before" photos. Things to note are that this kit uses the higher power rectifier option of the GZ30. The quality of construction of the chassis is good, so it was certainly built from a kit. The quality of the build is competent, but not great.

From the rear we see the two EL84 output valves, these look as though they've been very, very, hot and a check of the output transformer reveals the primary is open circuit. Between the output valves can be seen the ECC83 phase splitter and behind that the EF86.
There's also an empty hole (far left) that's exactly the right size for a Bulgin 3 pin mains connector.

From the underside we can see that there are some very old electrolytic and paper capacitors. Note that the dates on these are "JUL 53".

A closer view of the hole for the missing Bulgin mains connector. It seems there was once an earth tag here that has been replaced with a picture hook!

Mullard Five-Ten resources

Adding a stereo demultiplexer to the Leak Trough Line 3

When I bought my first Trough Line 3 on Ebay a few weeks ago I did a little googling and found this article on replacing the stereo decoder in the stereo version and this one on an external decoder. It struck me that converting a mono tuner to stereo might be a fun project that could be achieved fairly cheaply with salvaged components. All I needed was a tuner with a stereo decoder, which I had. Fortunately for the Sony, before I ripped it apart an older manually tuned Chinon receiver came up on Ebay locally, so I grabbed it for a fiver. Here it is accepting the multiplex output from the Trough Line.

Being a fairly ancient receiver, mid 70s I guess, the stereo demultiplexer chip was the only IC in the box, so fairly easy to spot, a mix of tracing the wiring, from mono/stereo switch, stereo indicator lamp (N.B. lamp, not LED), voltage checks and poking the pins with the CRT revealed what did what and the fact that it used a single 12V supply. I reckoned the easiest way to get it into the Trough Line would be to cut out what I needed from the circuit board using a hacksaw. So that's what I did! Here it is with a voltage doubler power supply taking 6.3V a.c. from the Trough Line heater supply.

So far, so good. Now to try and fit it in the Trough Line. The obvious thing to do is to put it where the decoder is fitted in the stereo version. Wow, it fits! But it's a tight fit. Looking at the board more closely reveals that the tallest component is the smoothing cap for the regulated power supply. A modern cap of the same value taken from an old PC motherboard is half the size!

That done the installation can continue. I decided that screening probably isn't necessary. Perhaps I'll be proved wrong on this later. The voltage doubler goes underneath. Done!

Sunday, August 24, 2008

BBC FM radio transmitters and broken switches

This weekend brings a new FM aerial and the second Leak Trough Line is brought back to life.

As hoped the second Trough Line arrived, albeit with no valves, in fact no glass at all, the fuse and 'fuse lamp' were both missing. Before replacing the valves I checked continuity of the mains transformer and on/off switch. The transformer was OK but one half of the switch was faulty. So I removed the switch from the circuit. Connected to the mains, with chassis earthed, gave ac on the heater and HT windings. So I put an EZ80 rectifier in the appropriate socket and checked the HT dc. All fine. With the rest of the valves in everything was still good. With aerial and amp connected I now had a working tuner. Unbelievably easy.

Checking the on-line Maplin catalogue for fuse lamps was a disappointment, it seems they've gone the way of variable capacitors and 3mm (1/8 inch) wander plugs. Such things are still available from some sources, e.g. Farnell have Belling Lee wander plugs, not cheap though. Then I noticed that my "donor" tuner, bought for £4.99 on Ebay, had 6 of them. They're 8V, rather than 6.3V, but perfectly usable. The main purpose of the donor was to provide a stereo decoder. I'll describe how I extracted it in a later post.
A few minutes use revealed that not only had the mains switch failed, but so had the two slide switches. Fortunately my Trough Line surfing had already take me to Keith Snook's page on repairing and tuning the Trough Line. I pretty much did exactly as Keith described, though I noticed that there were some fine cracks in the rubber cushion behind the brass strips. Which made me think that the rubber was probably quite a bit harder than when new. As a, probably temporary, remedy I cut a couple of small pieces of self amalgamating rubber tape and super-glued one to each of the cushions. Oh, and to clean the contacts I rubbed the ball from an old PC mouse over them. Now both slide switches are working well, and I've replaced the volume control, and its faulty switch, with a rotary switch.

As for the aerial, I've replaced my cheapo omni-directional loft aerial with a cheapo 3 element directional one. Which way to point it though? Perhaps this is another job for Google Earth - see BBC FM.

More Trough Line links -

Monday, August 18, 2008

Experiments with the Leak Trough Line

Reckoned by some to be one of the finest FM tuners ever, the all valve Leak Trough Line is a very simple radio. Trough Lines can be bought fairly cheaply on Ebay (well for under £50, though some do fetch much more). I've recently picked up a couple for under £20 each.

Initial impressions of the first buy was that the sensitivity really is very low indeed, but it sounds great. The accepted audiophile solution is a big outdoor aerial. Fair enough, but are there alternatives, e.g. using a decent low noise RF amplifier? This was to be my first experiment with my newly acquired Trough Line 3, but then I saw an opportunity to acquire another very cheaply, albeit with no valves. Rummaging through my boxes of valves revealed that I had spares for everything, including the tuning indicator, except for the ECC84. So what is an ECC84 and did I have a suitable alternative? Looking at the circuit diagram I could see it was the first RF stage, but the circuit diagram was hard to read so I looked at the circuit for the stereo version. Hey, it uses a different valve, the ECC88, these I have.

ECC84, PCC84, ECC88, E88CC, PCC88 - what's the difference? If like me you've got a lot of old valves from old TV receivers, radios, and test equipment you probably don't have any spare ECC84s, but you may well have a few of each of the others. The good news is, they're all usable in early Trough Lines that use the ECC84, though some modification to the wiring of the Trough Line will be needed to use any of the 88s. It seems the ECC88 was an improved replacement for the ECC84, though the base connections aren't exactly the same. It also needs a slightly higher anode current. To check this without modifying the Trough Line I put together a simple adaptor. Here's the circuit.

And here's what it looks like.

And yes, it works! In fact it works very well, bringing in stations that couldn't be received at all with the ECC84. So I'm making this a permanent change to my first Trough Line. Hopefully nobody reading this will imagine I have any special powers or knowledge; all the information required to put together the adaptor can be extracted from the two circuit diagrams, but to be completely sure I checked the valve specs at the Virtual Valve Museum. Here you can also find details of the other valves I mentioned. It's worth noting that they aren't exact equivalents, since ECC84 and PCC84, etc. were designed for slighlty different uses and have different heaters.

Some explanation is possibly required regarding the heaters. The standard heater voltage for mains powered radios from about 1940 (earlier in the USA) was 6.3V. However for AC/DC sets and TVs the heaters were connected in series and so valves were designed to all draw the same current. Valves with a first letter 'E' are for parallel use at 6.3V, whereas those with a first letter
'P' are for serial use at 300mA. What you'll find is that for all these valves the heater voltage and current is within 10%, so for practical purposes they are all usable at 6.3V, and most likely all but the ECC88 are usable at 300mA (I've not tried this). Note this is not generally true for E and P valves, especially not output valves. Other heater variants are also possible, for example a UCC84 would have a 100mA heater.

Before -

And after -

I've seen PCC88 as substitution before. Though these are radio frequency (VHF) amplifier cascode triodes they were used unstead of the last ECC82 in my Ferrograph tape recorder when it was built. I'm not sure why though. What this does show is that the two triodes in the (E/P)CC88 are the same, this doesn't seem to be the case for (E/P)CC84.

Sunday, August 17, 2008

Messing about with capacitors

After a couple of messing about with valves posts I was going to call this messing about with transistors, but mostly it has been capacitors. My wanderings through my electronic junk has now reached the 1960s. Again the challenge is what to keep and what to get rid of. Fortunately I've not kept much from this era, the age of black and white TV, transistor radios, and FM. I've got one item to sell, so on to Ebay to check prices and see what's hot or not. Errrmm, interesting maybe there's more to the 60s that I thought - British hi fi seems to be selling well, but with some bargains too. The temptation was too great so I snapped up a Leak Stereo 30 amplifier and Trough Line 3 tuner. These were described as "in good condition", which I took to mean complete, but not working. I reckon it was worth the (not much) money to have a couple of boxes with MADE IN LONDON ENGLAND on them to mess about with.

A few days later the postman struggled to the door with a large box. I quickly unpacked the box and opened up both cases. Everything seemed present and correct, so off they went to the workshop.

The tuner worked perfectly, sure it needs more signal than I have at present, a better aerial is needed.

As for the amplifier, the left channel worked but not the right. So I clipped a lead across the wipers on the volume control and the right channel came alive. Both power amps were fine, so the problem was with the right pre amp stage. A glance at the circuit diagram shows that there are two routes for the tape amp input one through the pre amp, and one direct to the power amp via the tape monitor switch. However plugging a Sony tuner in and flicking the tape monitor switch gave output on the right but not left! Tracing the signal showed the fault here was with the hi/lo slider switch on the rear panel. Replacing switches that will most likely never be used seemed pointless, so I just soldered all the hi/lo switches to the lo setting. That done I traced the signal through the the right preamp. It vanished at C16R! So something was wrong at T3R. The capacitor didn't look quite right, the plastic cover was clouded, so I pulled it and replaced (25uF, 25V axial). Still no joy. Checking the voltages around T3R showed they were very wrong, emitter at almost 0V. C17R (125uF 10V axial) was also faulty, replaced and now everything is fine!!!! Interesting that C16 had failed open and C17 closed. Odd things electrolytic capacitors. Perhaps I should replace them all, but it seems like a lot of work for little gain.

Here's the tuner. It's mono so I'll have a go at adding a stereo demultiplexer soon. I think I'll try and take one from another tuner (maybe the Sony), rather than build one with new components.