Love working on these hand wired vintage solid state amplifiers. Whilst timely, its a methodical process, almost a meditation, and certainly takes a steady hand and attention to detail.
Only the second Leak I've worked on, having had a Stereo 30 on the bench a few years ago, they really make you appreciate the workmanship that went into these. Now uncommon, with PCBs and ribbon cables standard, hand wiring has not been taught in secondary studies for at least 30 years I've been told. I was fortunate enough to be taught within the industry at a workshop that manufactured products with 50% of the internal having to be hand wired. There is certainly a method if you want the result to be production grade. Its not something that every employee was taught.
Both this and the Stereo 30 I found to have a warm sound and really nice EQ response. The Stereo 30 was manufactured in 1963 and the Delta 30 in 1971. No great a difference of inner layout. Timeless, simplistic outer design, still used to this day. Love these vertical plug in PCBs, great design feature. Both models use the same configuration of output transistors, with the only obvious changes stated in the service manual "Uprating circuitry, heat sinking and fusing has lead to significant performance improvements, particularly under high duty conditions"
This Delta 30 was blessed to arrive on the JZ Services work bench after the owner had taken to everyones favourite NOR Sri Lankan engineers workshop several times over the last few years. The repeat visit was because every 6 -12 months or so, it kept loosing a channel...which was obvious within a few minutes after inspection as he had added an unnecessary main electrolytic capacitor...but clearly did not spot this the second, third and fourth time it came back onto his bench. Having removed the capacitor mount for some unknown reason clearly confused him...then cable tying two capacitors together was the solution.? (one too many). Left to right speakers wired back to front was another elementary error (picture above is the before snapshot).
Removed capacitor, replaced a few more within the same circuit, tidied up the poor workmanship, standard preventative process and biased the amplifier (clear signs of it never having been done before) and returned it to the owner. With no abnormalities observed on main voltage test points, there's no need for this amplifier to be on another workbench for the foreseeable future.