[The General]

The HP manual was downloaded from the internet & printed out, with
this in hand the electrical restoration begins. The circuit is basically a balanced push-pull Wien bridge oscillator. The active devices are a pair of 6AU6 pentodes driving a pair of 6CW5/EL86s operating as a cathode follower output stage. Frequency range is 5Hz - 600kHz. Well on inspection the power transformer mounting was slightly bent due to the unit having been dropped on its side at some point in the past. This was soon fixed by putting the chassis on its side & applying some gentle muscle to the transformer. The power switch had been replaced by a pushbutton type, a rummage in the spares box turned up a Bulgin toggle which matched the original style. The rubber power cable was perished where it entered the chassis, this was removed & a new rubber cable the same diameter was installed. I took the opportunity to rewire the transformer primary from 115V to 230V, a
simple job of swapping tagstrip jumpers.The only paper capacitors are a pair of 0.5uF 400V caps. I had some 0.47uF 1200V polypropylenes, a bit overkill on voltage but they were the same size as the originals so I used them. Apart from the close tolerance ones on the range switch, the resistors were all carbon composition. I checked them for drift but all were nicely within spec.
The valves (2 x 6AU6A, 2 x 6CW5/EL86, 1 x 5AR4) all tested full spec on an AVO Mk4. As well as the valves there are two candelabra screw lamps on the chassis. These are connected in series & form the dynamic resistance for the feedback loop. These are listed in the parts list as 250V 10W which I thought may have been a typo; however checking the current/resistance against a 240V 7W lamp I had handy, they are fairly similar (470 ohms cold, 8mA at 12V or 1500 ohms). A resistance check across the HT line read 2.5 Meg so I applied power & checked nothing's amiss. the oscillator started OK, the can filter cap seemed very slightly warm after a few minutes so I kept an eye on it, it seemed OK for a while but then suddenly started to heat up. OK, switch off & let's see about a replacement. In the end I decided to fit two tubular electrolytics under the chassis & leave the can in place for appearance & as a binding post for the new caps' negative leads. With power back on, all was OK & a check of the DC voltages agreed nicely with the manual.
I ran through the alignment procedure, setting up the tuning dial tracking & also the output voltage, which is set by adjusting the negative feedback level. The maximum output is specified as 12V RMS into 600 ohms. Distortion is specified as 0.2% from 20Hz - 200kHz. I measured it at 0.22%. The output level is more than I need, it is possible to adjust the negative feedback to lower both the output level & distortion. I tried this; with the max. output set to 6V into 600 ohms -as low as it would go - I could get a distortion figure of 0.06% at 1kHz, however the output level would take a few seconds to stabilise when changing range. Eventually I set the max. output to 10V, stability wasn't compromised & the distortion level was then a respectable 0.12%.
It runs pretty warm; as well as the two power pentodes & the rectifier there are five 10W wirewounds bolted to the chassis. However HP thoughtfully put all the hot bits on one side of the chassis & the oscillator valves & the frequency determining components on the opposite side. The casing is well ventilated, too.
With the electronics completed, it was time to give the casing a final clean before reassembly. It didn't look too bad but on starting cleaning with some Flash & a sponge, the brown wrinkle paint went about 3 shades lighter as a stream of brown water went down the plughole. It seems HP chose the cabinet colour wisely.....
The last picture shows it reassembled & ready for work.