I was asked by a colleague to take a look at some special speakers that needed repair, I’m glad I did. Here is an example of an amazing rarity of engineering and design that you just don’t see every day. Word has it that only 8 sets of these were ever made. The attempt was to make the best sounding speaker, and perhaps they did. The cost of these would have been astounding, especially back in the 70’s when these were made.
Some notable feats of engineering here:
– Built in amplifiers in each speaker unit, 4 per side!
– Active electronic crossover, no passive crossovers
– Large bass reflex cavity for air for an integrated sub-woofer
– Dipole speaker arrangmeent
– Balanced inputs for low noise signals
– Cast aluminum base
– The amplifier design appeared to be unusual to me:
It’s essentially a Class-A and Class-AB combined… it’s a rare and innovative design called a “Class A Dumper.” It seems to have some real advantages over Class-A and Class-AB giving high efficiency and low distortion. Quad and Wharfedale apparently had a relationship do carry this out since the Class A Dumper is patented technology.
This type of amplifier design gets the best of both worlds, clarity of sound and efficiency. Pretty unique!
It’s difficult to see here but here are the guts found the in base, notably the 4 individual amplifier boards under the active crossover. A total of 8 TO-3 transistors.
For anyone interested, here is the scan of the manual.
In the end, the problem was traced to a faulty capacitor on the active crossover board. It had leaked and shorted one side of the power supply causing the “dumper” part of the amplifier to short the other side also. A tiny little electrolytic cap caused all the problems of blowing fuses – typical of vintage electronics.
And yes, they sounded fantastic and lifelike, amazingly wide sound field and full range.
Farmhouse AM / Short Wave Radio
This radio was particularly interesting because it was built to be completely battery operated. It requires three separate battery packs “A”, “B” and “C” configured with tapped voltages of -1.5V, -3V, high-current 1.5V (for the tube filaments), 45V and 90V. Yes, you read it right, 90V DC of battery power! Common at the time, but non-existent today.
The restoration was challenging because of the age and level of degradation the components sustained over time. All tubes needed replacing, both transformers were open-circuit, several tubes were completely shot and of course the numerous capacitors. After that was sorted out, as well as replacing rubber isolation mounts, frequency needle and glass… the question of how to power it in the long term needed to be addressed.
Fortunately, I did not need to re-invent the wheel and found a reference to a simple schematic for an isolated power supply. With some modifications to get the correct voltages, it worked like a charm. Still working after 80 years.
Here is a view of back of the cabinet, ready to go:
The final test on the bench:
All of the replaced components:
Some more detail on the power supply:
This restoration was a bit tricky. Initially, the symptoms were loud humming and distorted sound which usually means old capacitors. After replacing all capacitors, the humming was gone but the distortion issues continued. Next tubes were tested and replaced as needed. Still, after the unit warmed up only a very low volume was achievable and anything above that was completely distorted.
Finally with some encouragement to keep at it, it was traced to an open-circuit resistor on the emitter bias for the pre-amp before the final amplifier stage. Now, this unit is just like new complete with new needles and rejuvenated mechanicals on the turntable. Amazing how nice 1940’s technology can sound – rich bass!
Prior to replacing components, someone had already been at one of the capacitors:
Finally, a new set of tubes, capacitors and a new resistor as well:
So what is re-capping? No, it’s not the normal repeating-yourself-again summary of what was just said – it’s a solution to perhaps the #1 killer of electronics: failed capacitors. “Re-capping” is a term that you’ll hear when a technician replaces all of the old, leaking or failed capacitors in an electronic device.
This was a very common problem in older and vintage electronics, for example a jukebox that I’m working on right now has a dozen or more capacitors. Especially the paper variety from the 50’s and 60’s don’t last very long at all, a few years is a long time for those. Modern capacitors can last a lot longer.
As we can see on the lab bench, this is still a problem in much newer electronics using electrolytic capacitors in the 90’s. I just recently revived a Dell computer that was completely dead, and revived simply by replacing the electrolytic capacitors.
More tales from the workbench to come…