The most striking feature of the LGP-30 is the shear number of vacuum tubes mounted on plug-card modules, 34 modules to be exact. These plug into card slots mounted in air plenum assemblies that force filtered air over the boards and tubes and into the envelope of the machine.
Thursday, Jan 5:
The modules (having been transported by hand) were unpacked, grouped, and examined. One tube has gone to air, which isn’t a big deal, except that it’s a number the audiophiles now value as pure gold which has driven up prices. Good thing nobody figured that out and decided to part the machine out instead! I kid…but only a little. With all modules in front of me, I put together a spreadsheet of function, card number and quantity, and then for each type, the number and value of all capacitors. I compared these lists against the original schematics and 99% of them agree. There are a couple caps that disagree with the print (likely running changes) but agree from one card of the same type to the next, and the components look original. Some are marked with 56/57 date codes. Regarding tubes, I’m seeing date codes of 1971 and almost all of each type are the same brand. (Remember, the drum was adjusted and declared operational in ’70 by its tag). If true, this machine may not have seen a lot of use after refurb. Here’s hoping.
Friday, Jan 6:
I put in the order for the replacement capacitors (total of 112 wax and sml electrolytic on the way). Any of the pF value will be left alone for now and replaced on a fix-as-fail basis. An ESR meter is also on the way to deal with the huge electrolytics in the power supply. The Museum in Germany had no trouble with their PS even with all original components. I suspect if I do get the machine operational, only then will I tackle shot-gunning the PS. A plea was also put out to the classiccmp list for a copy of the English version of the Maintenance Manual which would explain some of the machine logic in the detail I’ll need for troubleshooting; as of today, only the German version is online.
With the bulk cap order on hand, I started recapping the 34 modules. The first step was cleaning the edge connectors and all board traces with isopropyl alcohol, paper towels and q-tips. Then came the diode check. I actually found a handful of marginal or open germanium diodes and replaced them with 1N4148’s as I went along. The construction of the boards is quite clever, making use of ‘fork split’ turret pins for most connections. The top side of the pin is split so that components on the top of the board ‘lay’ in the fork and get soldered while components that connect to the same point on the bottom side get their lead bent and inserted into the ‘hollow’ of the turret and filled with solder. I made it a practice to use a desoldering tool during cap removal so that fresh solder would flow into all voids rather than just heat the joint and swap parts. The ‘gotcha’ with these turret pins is that solder can flow out the bottom while heating the top and can bridge to a component lead passing over that via, so inspection is critical after rework.
No major issues were found during rework, however it’s clear the modules are a collection from multiple machines of various ages, judging by board color, component condition and date codes, and other random marks. Each module ‘grip’ has its own dymo label with the machine serial number: 393.