Archive
October Recap
Recapping the projects I completed in the month of October. I started out with a few goals:
- GE LF-116 Radio Repair
- Sanyo Solid-State Receiver Repair
- EICO 460 Oscilloscope Overhaul and Tune-Up
1942 GE LF-116 Radio Repair Part 5 – Alignment and Reinstallation [Finished!]
Part of a continuing series that is now finished – see the previous installments:
Part 1 – History and First Looks
Part 2 – Tool Prep
Part 3 – Capacitor Can Rebuilding
Part 4 – Capacitors and Socket Replacement
After a brief delay while I waited on parts from my supplier, I’m finally set to finish up the GE LF-116 that I’ve been working on this month. It’s going to be returned to my office in playing shape very soon!
I powered up the radio for the first time with the power supply activated to get a feel for it, and played an MP3 through the line input. It played great – for about five minutes, until it heated up, then a nasty 60Hz hum took over. I’d let the set sit for a few minutes, then power it back on and it was fine for a minute or two before humming again. This was very clearly a thermal short, heater to a cathode. There are four tubes in the radio that are prone to H-K shorts: two 6SG7s and two 6SH7s. One of each tested totally dead, and the others were weak. So was the 7Q7 oscillator. I ended up ordering a new set of NOS tubes for the RF stages, and installed them; the radio played perfectly at this point although not very well aligned.
First, I peaked up the IF transformers with a 455KHz reference signal from my generator:
This step could be done outside of the cabinet, but the RF alignment required putting it back in and attaching the antennas.
With the installation complete, I switch over to RF calibration, first with a 17.8MHz reference signal, then a 1500KHz, finally a 580KHz signal to align the Shortwave and AM bands.
I peak the alignment trimmers with the radio installed in the cabinet using a non-inductive alignment tool:
The trimmer locations were clearly called out on the service documents, so it was easy to find where to adjust even though it’s not the easiest to reach. It aligned up nicely! The dial tracks perfectly now. And speaking of the dial scale, it’s interesting – the tuned station is indicated by a thin line of red light surrounded by shadow.
And that’s it! Back to the office it goes. I’m not planning to do much with the cabinet as it’s not that bad, and it’s going to be in a bit of a high-traffic location so if something happens to the finish I won’t have to worry about it.
1942 GE LF-116 Radio Repair Part 4 – Capacitors and Discriminator/Phase Inverter Socket
Part of a continuing series:
Part 1 – History and First Looks
Part 2 – Tool Prep
Part 3 – Capacitor Can Rebuild
Part 4 – Capacitors and Socket Replacement
Part 5 – Finished!
I’m continuing to work on this 1942 GE radio which I’ve been enjoying in my office for a year, and now it’s time to make it play again. I’ve pulled the chassis out of its cabinet for inspection, made a test jig, and rebuilt the multi-section can capacitor above the chassis due to lack of room underneath for mounting replacements. In this issue, I’m going through the capacitors and cleaning up a few other issues that cropped up along the way.
As we’ve already seen, there’s a fair amount going on down here. For all the empty space under this large chassis, General Electric’s engineers decided for one reason or another to use only about 1/3 of the available space and pack everything into that area as tightly as possible. There may have been some interference concerns, but I suspect a cost-saving measure for one reason or another that’s long lost to history. It does make it a lot of fun to work on, as capacitors are tucked in between the band switch, three layers deep under wiring and resistors, or otherwise made as annoying as possible to access. I’ve been spending a good bit of time with a needle-nose in each hand and that requires a special amount of coordination that makes for slow going.
This radio uses an interesting arrangement. On shortwave or AM bands, the first tube is a tuned RF amplifier helping with distant reception. On the FM band, the first tube is switched into the first stage of a cascade converter system where there’s a two-stage stepdown to the 4.3MHz intermediate frequency – this was done because at the time, tubes didn’t have the bandwidth to perform the conversion in one step without losing quality. It makes for a crowded and more complicated circuit, though, as quite a few coils are switched in and out depending on what’s being requested at the time.
The radio bears evidence of having been serviced many times throughout its life with several different brands of capacitors from varying ages, date codes ranging from 1940 up into the ’60s. There’s the usual poor soldering in a few spots, clipped component leads left on terminals, and general quick re-work but by and large it’s in decent shape and doesn’t appear to have been “hacked on” very much.
I began replacing capacitors one by one, mixing radial or axial styles depending on the location, and came to a resistor that had burned out – just to the right of the red clip I’m using as a marker.
My copy of the schematic wasn’t very readable, but another hobbyist was able to supply me a better scan and I ended up purchasing the complete set of high-resolution media as a result of seeing this sample.
With this wiring snip, the full schematic diagram, and some confirmation from another hobbyist I was able to identify the burned out resistor as R-11, 2.2K Ohms 2W, which supplies B+ voltage through IF transformer T1 to the plate of the 6SG7 converter. Capacitor C-32, a 0.02uF coupling capacitor, was shorted which passed B+ directly to ground and caused it to burn up quickly. This obviously happened at least once in the past, as C-32 was a replacement as was that resistor.
Capacitor replacement followed pretty unexcitingly, assisted by my Hakko for cleaning up terminals. I rebuilt the above-chassis capacitor block in Part 3, linked from above. It’s slow going due to the large amount of brittle, crumbling rubber wire and tight quarters. Many restorers advise either replacing or covering with heat shrink this wiring as if it crumbles, it could short out. Mine is mostly intact, and by taking extra care not to bend – just to push – the wiring around I’ve avoided having any crumbling accidents so will not be doing that time-consuming step unless it turns out later that it’s absolutely necessary.
Very unfortunately, though, a tie point snapped off the socket below the molding while trying to replace one of its connections. These loctal sockets seem more fragile than the octal sockets used in 9 of 11 tubes in the radio. I ordered a set of brand new ceramic loctal sockets from Angela Electronics who boast “Since 1977 we’ve supplied thousands of hard to find items to musicians and tube audio enthusiasts worldwide.” Hard to find no question, as they’re the only site I’ve found that sells loctal sockets – as well as brand new ceramic 5- and 6-pin sockets! On this fact alone, they’ve got all my business for new sockets going forward.
The socket arrived; I carefully remove the wiring from the terminals and drill out the rivets for the socket to replace.
Ceramic sockets are nice and durable. But, naturally, there was another issue: the mounting tabs on the 7K7 socket weren’t spaced evenly with the chassis holes AND unless I wanted to mount with rivets instead of 6-32 screws, I wouldn’t be able to get the tube to seat properly – the mounting was interfering with the base. At this point, my significant dislike for loctal sockets was solidified and I said forget it, grabbing an Octal socket from my parts bin. I mounted the octal socket above the chassis held in by the retaining clip to ensure there’s enough space to seat the tube properly.
A 7K7 tube would go into the loctal socket; for the octal socket, the same tube is labeled 6AQ7GT. I don’t have one of those in stock so ordered from eBay for $2. They’re both double-diode/triode tubes, serving as discriminator and phase inverter for the audio output. And, annoyingly enough, they’re laid out somewhat differently. Whether this was for any particular technical reason or just a rivalry between companies, I don’t know but the socket required re-wiring beyond just hooking the leads back up.
7K7(Pin) 6AQ7GT(Pin) Description Visual 1 8 Heater Grey Resistor/Ground Tie 2 6 Triode Cathode Orange Resistor 3 5 Triode Plate Orange Cloth-Cov. Wire 4 4 Triode Grid Orange-Drop Cap and Res. 5 1 Diode 2 Green Cloth Wire into Can 6 3 Diode 1 Green Rubber Wire into Can 7 2 Diode Cathode Cloth Wire into Can 8 7 Heater Black Rubber Wire to Tubes
With that mapping completed, it was time to re-wire. This required slightly extending some of the wires coming from the discriminator transformer.
This radio is by far the most frustrating to recap of any I’ve worked on yet. It might actually be the most complex one I’ve worked on so far anyway, but the construction – layers upon layers of tight components with sensitive lead dress requirements buried deep inside the radio. Much of the time I was replacing some of the deeper components with a pair of needle-nose pliers in each hand, and I even had to remove the output transformer from its mounts to replace three capacitors located basically under it.
Around the output transformer and push-pull output tubes, there were a handful of 1000V-rated 0.05uF capacitors. I don’t have 1KV metal film capacitors in stock, so I used 1KV-rated ceramic multilayer capacitors with a Z5U temperature coefficient…should be mostly sufficient for the application. Z5U-rated capacitors operate between 10C and 85C with a maximum variation of -56% to +22% capacitance, and honestly the original manufacturing tolerances of paper, foil and wax were probably broader than that in the first place.
A resistor bypassed with a capacitor. The resistor itself is a 1.2K 5% resistor, but it’s actually drifted by about 10%; I’m hoping this won’t be a significant issue but if there’s an issue around the oscillator circuit, this’ll be the first place I revisit. If I’d noticed the tolerance marking before reconnecting, I’d have replaced the resistor outright as well.
These capacitors have identical ratings – 0.005uF and 1000V tolerance. What a difference better manufacturing technology makes. The smaller physical size actually caused some mounting issues of its own, though, as the replacement component had shorter leads that required extending with a small piece of jumper wire in a few locations.
Naturally, it was bound to happen that something else would break during this process. I heated this joint with my Hakko to clear some solder, and the lug cleanly separated the instant I did so – it was held on by solder alone, it seems, the underlying metal having broken from thermal stress or a past workman’s abuse sometime during the last 69 years. Fortunately, very very fortunately, this is just a tie point – it’s not connected to the actual switching pads. I soldered a jumper to the rivet for physical stability and replaced the capacitor as normal. If this had been an active switching lug, this could have potentially permanently crippled or even rendered the radio unservicable.
Finally, I reattach the rebuilt can capacitor. I ran the common to the can’s mounting lug, then a jumper from there to a near-by ground tie point and soldered the lug to the chassis, the wire to the lug and the jumper to the lug as well. This ensures a solid ground even in the event the chassis soldering didn’t take very well. I ended up using my Hakko as a soldering iron in this case, as the thermal mass of the tip allowed it to heat the chassis to soldering temperature without cooling; the iron I use for adding solder to most connections is a thin point that cools too quickly when heatsinked.
I did a tube-less power-up to make sure there were no immediate shorts, such as a stray piece of solder, and found that the Beam of Light lamp was burnt out. Fortunately, I have #44 dial lamps on hand:
With the radio’s electrics fully serviced, now I can continue on to the first power-up just as soon as the tube I need arrives. Stay tuned!
1942 GE LF-116 Radio Repair Part 3 – Capacitor Can Rebuilding
Part of a continuing series:
Part 1 – History and First Looks
Part 2 – Tool Prep
Part 3 – Capacitor Can Rebuild
Part 4 – Capacitors and Socket Replacement
Part 5 – Finished!
I’m continuing to work on the GE LF-116 radio, an AM/pre-FM radio manufactured in 1942. We left off where I’d created a jig to hold the radio upside down because it couldn’t be mounted the way I normally do, and now I’ve started to go through and refurbish the components. I quickly ran into a bit of an issue which spawned an entire new post, which would’ve normally just two or three photos in part of a larger article: It’s cramped down there!
In the center just below the “orange drop” capacitor hanging upside down is the base of the multi-section can capacitor. Seen from the top circled in red:
This capacitor houses the first second and third filters, and the output tube’s shared cathode bypass capacitor. There’s just nowhere good under the chassis to mount a terminal strip and new capacitors, so I’m forced to actually go ahead and restuff this can with modern replacements – it’s a time consuming process I’ve mentioned not finding to be a good use of my time in the past, but necessity dictates I do it this time. This is the first can I’ve restuffed, and it didn’t come out quite as well as I’d have liked but it’s passable to anyone but a purist.
I begin by removing the leads from the terminals on the bottom of the can and marking which they came from. The body of the capacitor is the common negative for all four segments and is tied to chassis ground which makes it slightly less messy than if it were an insulated can.
The lugs are marked with a shaped cutout in the phenolic base, and the mapping is indicated on the side of the can.
In this case we have:
- 30uF 450V (C-73A) First Filter
- 15uF 450V (C-73B) Second Filter
- 10uF 450V (C-73C) Third Filter
- 20uF 25V (C-73D) Cathode Bypass
1942 GE LF-116 Radio Repair Part 2 – Tool Prep
Part of a continuing series on restoring this radio:
Part 1 – History and First Looks
Part 2 – Tool Prep
Part 3 – Capacitor Can Rebuild
Part 4 – Capacitors and Socket Replacement
Part 5 – Finished!
I’m continuing to work on my 1942 GE LF-116 radio. In part 1, I showed a few photos of it and talked briefly about the history and showed some photos of the different views of the cabinet and chassis. This radio has protrusions on all sides of the chassis – screws, metal seams, switches, an RCA jack, antenna and speaker terminals, and controls in the front – there’s no viable location to do my normal mounting trick of a C-Clamp and a small piece of plywood to form a stand, suspending the radio from each end. I searched the antique radio forums for a few ideas and came up with a simple one – a thick wood base with holes drilled and dowels inserted. The holes are drilled such that the dowel will land on an empty spot on the top side of the chassis, supporting it without damaging any components while it’s being serviced upside down.
I started with a pair of 2′ x 2′ x 3/4″ plywood sheets as the base, with them glued together it makes a 1.5″ thick mounting platform – should provide plenty of stability. This GE radio’s chassis is quite heavy.
I then clamped it to the edge of my coffee table and went to get lunch while it set.
The dowels are cut to size, 16″ each. I used a mix of 3/4″ oak (stiffer) and 5/8″ depending on where the dowel would touch the radio – if there were tight clearances. Faintly visible under the dowels are the guide marks for where to drill for this particular chassis.
I randomly drilled extra holes after the original marked ones were done just on expectation of using this with other chassis. I used an oversized base for the same reason – I might work on some bigger ones eventually. It came out okay:
And all mounted up:
Now the real work can begin. It’s not perfect, but it’ll do – the 5/8″ dowels and dowel holes fit together like I want them to – slightly snugly – but either the 3/4″ dowels or my 3/4″ bits are slightly off-sized as they fit only loosely. With 8 segments supporting it (including a few in between components where it couldn’t slip out) and the c-clamp arrangement in the back fixing its horizontal position, I’m satisfied it’s sturdy enough for light repair work as I’m doing. This won’t be a permanent solution, though. Steve Strong from the OKC antique radio club makes articulated chassis holders that mount to the existing chassis bolts and rotate 360° to provide perfect control. I’ll be picking one of those up in a few weeks, but sadly after I’m done with this.
Steve, e-mail linked above, is selling these for $57.50 + shipping and they’re custom made to order.
1942 GE LF-116 Radio Repair Part 1 – History and First Looks
Part of a continuing series:
Part 1 – History and First Looks
Part 2 – Tool Prep
Part 3 – Capacitor Can Rebuild
Part 4 – Capacitors and Socket Replacement
Part 5 – Finished!
I’ve had a GE LF-116 radio in my office for a while and now that I’ve started to clear my long backlog of projects it’s finally made it up to my bench. I’ve displayed it in my office holding up a monitor for a while but it’s made it onto my list of October projects.
It doesn’t look like much. Most radios after 1940 until this style died out seem to all look about the same to me. It has interesting enough features and is fairly rare, though, so it was worth preserving – AM Broadcast and Shortwave reception with a tuned RF amplifier, and early FM and a high-end audio output. Relatively few radios were manufactured with the original pre-war FM band, 42-50MHz and they tended to be high end models, this one no exception. With 11 tubes, including two 6V6GT tubes in push-pull output, a tuned RF amplifier on AM and high-fidelity system on FM.
The radio was designed at a time when television had just been invented and wasn’t widely adopted and before the U.S. became heavily involved in WW2 and domestic radio production was halted, and as I mentioned, at a time when FM radio was 42-50MHz instead of the current 88-108MHz. There was an exceptionally nasty series of court battles and “lobbyist activity” which ultimately resulted in the band being moved in June 1945 – and the first year major domestic mass production of radios returned was in 1946. A few niche-market or higher-end radios contained both bands for a very short time – like the Zenith 7H820 – but most stations on the old band quickly went bankrupt. and it was eliminated. As noted in the article, there were some physical constraints – that frequency range happens to have particularly bad interference characteristics and has been mostly abandoned today, so it’s not entirely lobbyists.
It’s important because they designed the most of the radio’s circuitry to be fairly broadly resonant – in this case, this radio is known to work up to around 99MHz of the modern FM band on strong stations by taking advantage of the 2nd harmonic of the local oscillator. There are some slightly different audio parameters between the two sets of broadcasts but the older circuitry is so broadly tuned it makes it work.
The preset buttons are labeled with local Seattle stations, although they’ve since changed frequencies over the years.
There’s a 12″ speaker driven by around 5W of power. The radio itself has 11 tubes, one of the more complicated ones I’ve tackled so far. The chassis also looks the least friendly, of course – but it’s in excellent physical condition at first glance, which will make it easier. It’s just very cramped underneath. With an 11 tube line-up of 6SG7 6SG7 7Q7 6SG7 6SH7 6SH7 6SQ7 7K7 6V6GT 6V6GT 5U4G there’s going to be lot of circuitry.
It’s also difficult to mount anything to the side of it for suspending the chassis above the work surface because of protruding screws and a sheet metal seam. I’ll have to build a new jig from an idea I read – a thick board drilled in a grid for dowels cut to length to suspend the chassis above a work surface by holding it up by the middle. It’ll cost around $20 in materials at Home Deopt, I imagine. That’ll be for the next segment. In the mean time, I’m fairly worried about this power transformer. Combined with the fact this radio came with every tube except the 5U4 rectifier, I suspect the previous owner may have damaged it by trying to plug it in – or else it just failed this way in its past life and was put into storage.
