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First Look: 1934 Grunow 750 “World Cruiser”
I’m getting ready to sell some of my radio collection to make room for new ones, but until that happens I’ve been under something of a self-imposed moratorium on new purchases. This one came up on eBay a little while ago for a steal so I cheated a bit and had it sent my way.
This is a huge tabletop radio – a “grande” coffee from my local coffee shop is provided for scale. It’s a 7-tube model using the tubes 6D6 6A7 6F7 75 76 42 80 – the AM Broadcast Band and the lowest Shortwave band are double-tuned with a single-stage RF amplifier for extra selectivity; on the two higher shortwave bands the second stage of tuning is disabled to increase sensitivity. With an 8″ speaker, it should sound pretty great. There are a couple of minor scratches on the front that will buff out nicely, and it’s otherwise original – this should be a nice, fairly quick radio to bring back to life.
Shortwave Radio Reception with the RTL-SDR Dongle
I bought and did a quick setup on my RTLSDR dongle using SDR# a few weeks ago, where I used it to listen to FM radio stations around my area and a few public safety frequencies. That’s all well and good, but I’m much more interested in shortwave listening – when the weather is good, I can pick up a fair number of stations on my Hallicrafters receiver and there’s even more out there that I can’t tune in with that old equipment.
The RTLSDR tunes from around 64MHz up through around 1800MHz, but shortwave frequencies are much lower – only up to around 30MHz. Using an RF mixer, it’s possible to shift the signal into the RTL’s tuning range. Portuguese designer CT1FFU developed a mixing upconverter which adds 106.25MHz to the incoming signals, shifting them up into the correct receiving range and filtering out signals about 50MHz to prevent interference. His version comes as a kit which requires surface-mount soldering, but German retailer Wimo offers mostly-assembled versions of the kit which only need the antenna terminals and power connector soldered.
Finding those adapters was a bit challenging – I have a helical antenna which terminates in that alligator clip, feeding into a coax break-out, with an SMA-Coax converter. On the other end is an SMA gender-changer and an SMA to MCX adapter. Ultimately I ordered them from eBay and they work as intended. The USB port provides the +5V power supply for the converter’s operation but otherwise isn’t connected.
Reception is acceptable. With the aid of the SDR software, I can see where signals are more readily, but issues with my antenna setup and local interference are keeping it from performing as well as the Hallicrafters. I can identify human voices on more stations, but it seems there are fewer I can actually listen to with this equipment. I’ll probably try building a tuned loop antenna similar to this one, and see what I can do with better noise rejection and directionality. I might also add a low noise amplifier after whichever better antenna I end up using.
If anyone has a favorite, easy-to-build loop antenna for 10-160M I’d love to hear about it.
July Projects
July is always a pretty busy month – it’s finally summer in the Pacific Northwest and so my attention is divided between my electronics work and spending time in the great outdoors. I have a few things in the pipeline for the month of July which I hope to accomplish.
First, a client has brought me their Grundig 5399U/S radio from 1965 for repair. It has some severe mechanical issues and is currently holding for parts, but it will be featured in a repair article later if they come through.
I’ll also be fixing this Bose 901 Series 1 Active Equalizer, brought to me by a friend whose vintage hi-fi speakers started to experience distortion. It turns out modern capacitors can fail too!
Finally, I hope to fix up my early ~1931 General Electric J-105 antique radio. It’s in beautiful cosmetic shape looking like it just rolled off the showroom floor, but the electrics are predictably dead after 81 years and will need reconditioning.
Build Log and Review: Mable Audio VT-86PP Kit Hi-Fi Amp Project [Chinese Gear]
About a year ago, someone left a comment that really looked like spam on one of my pages advertising a web store selling tube audio gear and parts for guitar and audio amplifiers. Against everything I’ve ever learned about behaving on the Internet, and my better judgement, I ended up actually buying a kit from that vendor Mable Audio located in Shenzhen China. They have several kits available with a few options: 6V6 or EL84 for output, 6SL7 or 12AX7 for the input stage and also a beefier EL34/12AX7 amplifier for about double the price. They have a ton of components and guitar amplifier parts and kits as well.
I selected the 6V6/6SL7 kit because I like the larger tubes, and had a set of 1937-issue 6V6G tubes that look more interesting than the small GT-style tubes it came with.
The kit arrived as a box of miscellaneous parts and the chassis. There was no particular documentation included, but the schematic was sent to me via e-mail. Also, the silver decorative plate (in the center of the photo) is no longer included in these kits despite being pictured, as I was told after I purchased. Not the end of the world, though. The kit included a large power transformer, two output transformers (23% Ultralinear), two huge 470uF 450V filter capacitors, all the needed tubes, signal capacitors, connectors, controls, high quality ceramic sockets and lengths of 600V-rated wire to assemble it all with.
The amplifier itself has decent schematics, on paper anyway, although the output impedance is inaccurate on the datasheet: the datasheet, schematic, and transformer color code documents all specify multiple taps but the provided hardware only supports an 8-ohm speaker. This is acceptable, though, as most home theatre speakers are 8 ohm anyway.
INPUT IMPEDANCE : 100K ohm(RCA)
OUTPUT IMPEDANCE:4ohm – 16 ohm8 Ohm
OUTPUT POWER: 12w x2/(ultralinear)class AB P-P
DAMPING FACTOR : >3
FREQUENCY RESPONCE: 20Hz-20KHz(REF.OUPUT)
TOTAL GAIN: 28dB
INPUT SENSITIVITY: 300mV-600mV
S/N: >89dB (HUM NOISE <3mV
CHANNEL BALANCE: <1dB 20Hz-20KHz (MAX.VOLUME)
CHANNEL SERPRATION:>65dB 20Hz-20KHz
TUBE COMPLEMENT: 6P6P(6V6) X4
6N9P(6SL7)X2
POWER REQUIREMENT: AC220V OR 110V OR 240V + -5% 50~60Hz
Assembly wasn’t terrible, but the build quality of the chassis was marginal at best. The top surface of the chassis bolts to the chassis pan itself to hide mounting rails and dividers, and the holes on the top panel didn’t perfectly line up with the socket locations on the bottom panel. This meant that after mounting the sockets to the bottom where they belong the top cover wouldn’t slide over them because it was interfering by about 1mm on one of the sockets. It took significant pressure to force it on and made a very loud snapping noise when it did so, but the socket and cover were all intact. It’s definitely not coming off again, though. Additionally, the cover over the transformers is supposed to mount using several screws at the corners, but only two screw holes lined up.
The sockets are ceramic and the sockets gold-plated, and the transformers look appropriately sized, though, so it’s on to building past mounting up the parts. This project was a long time in the making, as I mounted the sockets and chassis up on July 12th 2011 and then did no further work until March 2012. I worked my way through the schematic, roughly right to left. This amplifier is a power supply, and two identical amplifier channels. I’d highlight on the schematic as I completed both mirror halves, and this made it easy to keep track of where I was going.
Some photos showing filling in the components:
After assembly, I had a few left-over parts:
A really weird assortment of leftover parts at that. I declined to connect the headphone output (on advice that it was probably not safe for any headphones that I might actually like due to the connection – not unique to this amp, many other circuits have similar issues) which accounts for 2 of those resistors, but there’s still 13 more + a small capacitor that weren’t called for anywhere. And there are two extra panel-mount RCA input jacks. And a ton of extra wire. But for all the extra they did include, there were 4 resistor types (8 resistors total) that were not included, so it was a trip down to Vetco Electronics in Bellevue to pick up replacements. The replacements are the Red- and Tan- bodied resistors. Not quite sure what to make of that.
I elected to replace the LED power indicator with an NE-2A neon bulb instead, so it would be the same color as the glow of the tube heaters.
There is a minor issue with this one, though. There is an anti-arcing capacitor across the switch. I placed the NE-2A between the switched side of the switch and the AC neutral (it runs directly from AC mains through a 150K 1/4W resistor). With the switch turned off, the capacitor allows AC leakage of a few mA to pass to the cold side of the switch. The current leakage is less than the excitation current of the transformer, so the transformer appears “open” to the low signal as it’s entirely eaten because it’s not strong enough to set up a magnetic field. This causes the current through the 150K resistor and neon….so the bulb lights up when the power is off, now. With the power on, the capacitor is out of the circuit and the transformer is an extremely low impedance, so all the current goes through the transformer primary and none into the 150K resistor and the NE-2A bulb. The result? The power switch indicator is backwards. The light comes on when the device is turned off, and goes out when you turn it on. I intend to fix this at some point, but am not entirely sure what the best approach might be. The volume control itself is abysmally low quality, probably the only part in the kit I actually think is just plain “bad”. The return spring is weak and the switch touchy and it binds up easily.
There was also another issue: I wired the volume control backwards. “Right” is lower, now, instead of “Left” as is the convention. I evaluated fixing this problem, but while poking around one of the volume control’s pins started coming out of its molding and I didn’t want to risk destroying the control, so that’s a problem that won’t be fixed in this iteration.
I did a quick visual for obvious shorts and powered it up the first time with the 6P6P (Chinese 6V6GT) tubes which were included, as I didn’t want to risk my classic tubes. If there’s going to be a problem it’d be on first power up, for any amplifier from anywhere. The transformers all made a great *THUNK * noise when they energized for the first time, but quickly de-energized again. I poked around some more and found I’d missed a solder connection on the bridge rectifier. I soldered it back on and tried again and everything was fine! First power-up successful, with no re-work needed.
Then again on the test bench with the 6V6G tubes:
Still good! And for some post-production glamour shots:
Now, I’ve moved the amplifier onto my desk and will use it for personal stereo when not using the Surround Sound receiver.
In conclusion: Mable Audio is a reputable supplier. Their product sounds great, and was of acceptable build quality for an entry-level amplifier. The chassis and platform itself will be a great starting point for more modification projects in the future. I rate this kit as a 3.5/5 due to the mechanical issues with the chassis fit and the power switch, but the electronics quality and circuit design seem to be solid. I would recommend this kit to anyone who wants to build an inexpensive tube amp from parts – but, given the complete lack of instructions or documentation beyond a schematic and some color code diagrams, make sure you go slowly and check your work and are somewhat familiar with how to read electrical schematics.
- Pro: Inexpensive for what it is.
- Pro: Good quality components, for the most part.
- Pro: Attractive styling.
- Pro: Fairly straightforward build.
- Con: Terrible quality power switch. Just terrible.
- Con: Mechanical fit on mine wasn’t the best.
- Con: Only 99% of the parts in my kit were the right ones.
For $175 + shipping from China, it’s not a bad deal. Similar kits from more well-known sources, or U.S./European makers, run at least double the price. Once assembled, it’s very attractive and has a lot of potential for future upgrades. This was a fun and rewarding project and a nice change of pace from vintage gear while still keeping the tube connection.
March Projects
February was a pretty slow month for projects, about the only one I managed to accomplish was fixing the Firestone car radio. March should be a bit more productive, though. I have a list of a few projects I’m planning to try and complete.
First up, a pair of Grunow radios that are vastly similar to the pair of 589s that I’ve fixed in the past – a Grunow 566 and a Grunow 588. The 588 even has a Teledial.
I’ll probably also start working on a a Jewel radio that was a Christmas gift:
I also have a kit amplifier left mid-project, I’ve mounted the iron and hardware but none of the connections or components yet. (It’s a stereo amplifier with a 6SN7 dual triode driving a pair of 6V6 in push-pull for each channel, and should sound pretty good when complete.)
Plus other projects that seem interesting along the way. Stay tuned!
Why I’m excited about the Raspberry Pi [$25 Linux PC]
I generally write about radios and older projects here, but I do in fact work on a lot of other projects as well. This one is pretty interesting, a new and intriguing product that stands a very solid chance at becoming a very interesting force in low-end computing is on the verge of making it to market: the Raspberry Pi. It’s a pretty simple concept: a $25 single-board computer with an ARM chip about as powerful as a mid-level cell phone, some RAM, and a few interfaces. Not a very powerful device, but it’s designed to facilitate teaching computer science in schools.
It doesn’t look like much, but it has a network port, HDMI and TV outputs, two USB ports, audio, and a variety of serial GPIO ports. The educational uses of it aren’t very interesting to me…but the possibility of a reasonably capable computer that can be programmed in high-level languages and used to create complex applications on the cheap is incredibly exciting. I’m interested in using these to develop a variety of “appliance” type network applications, passive single-purpose complex devices that could be produced for under $100-200/set including all the other parts.
Connectivity appliances? Home security and automation terminals? New communications devices? Weatherproof terminals to web browse in the shower? The possibilities are endless – this tiny device needing only 3.5W of power to run, it can happily run from a cell phone charger or even a battery.
I’ll be the first in line to pick up a pair of these. Granted, I’m not at all excited about the fact it runs Linux as that means I’ll have to learn a new platform and environment to accomplish much, but I think it might actually be worth it to do so.
If you had a tiny fanless networkable PC, what kind of homebrew projects would you want to see?
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
October Projects
September was a good month for projects. I accomplished all the things I set out to do and more:
- I’ve finished the Hornyphon radio for my customer
- Repaired and began using the Hallicrafters
- Got the broken ViewSonic monitor going again
- and put lighting effects on my coffee table out of spare parts.
YAMR [Yet Another Monitor Repair] ViewSonic VP191b
A little while ago, I picked up a set of seven LCD monitors in various states of not working to work on repairing and maybe eventually reselling. The first one was quite easy – it just needed a wire reconnected internally and works perfectly. I grabbed the second one, a ViewSonic VP191b. It’s nothing hugely special, 19″ with two VGA and a DVI offering resolutions up to 1280×1024, a 16:10 aspect resolution.
Nothing I’d use as a main monitor, but a decent consumer device. And it turns out this one’s a little more complicated to repair than the last few I took care of.
Taking the case off, you can see the high voltage power supply which takes 12V DC and converts it to a thousand or so volts AC to power the backlights; in the center is the logic board and on the right the switching power supply.
The power supply is encased in a plastic insulator to keep it from shorting to the case.
Unfortunately, this one wasn’t in as good of shape as the others. In addition to having a few bad capacitors, it turns out that the resonant transformer is also bad (the yellow square to the right in the photo above.) If this supply lost regulation when a part failed as it was running, it could cause a nasty cascade taking out transistors, transformer windings, anything really and that looks like what happened.
The capacitors used in this model are:
- 470uF 25V
- 1000uF 16V x2
- 470uF 16V
- 120uF 400V
I don’t have a spare resonant transformer, and wasn’t able to locate another one online…maybe the end of the road? Nope! I checked the voltage ratings on some of the logic board components and they were all rated 16V…the rule of thumb for capacitors is you overrate their voltage by sqrt(2), or 1.414 times. These were rated 16V, so I estimated from this the logic board wants a 12V input. That’s handy, and pretty easy to supply.
I need a 12V bench supply for a few other projects I have coming up, so I ordered one from eBay. This one’s inexpensive and considerably bigger than I need, but it’ll be good in the future. The ViewSonic draws ~35W, and the eBay power supply can supply up to 120W. It came without connectors, so I hooked up a line cord socket that I’d scavenged out of a dead Ethernet switch. As always, when you’re working with electricity, take proper safety precautions – don’t touch power things while they’re energized and double-check your connections.
I’ve removed the power supply from the back so it doesn’t get in the way, then depopulated it to save the remaining good components for something else:
I ended up recovering 4 small signal transistors, a bridge rectifier, several misc. resistors and small capacitors, two choke coils, an unidentified standard transformer and an NTC thermistor.
Here’s a shot of the back with the power supply removed, ready for other connections:
For the first trial, I’ll just run jumper wires.
And let’s see…
Looks like it works! My estimate about the voltage proved correct. I mount up a terminal strip just like I do with a radio and wire the new power connection to that so it can be accessed from outside the case later. I’m using a computer power molex as the new connector as that’s what I have on hand, preserving the coloring.
Testing out one more time before reassembly:
Not bad!
It joins the ranks of my other spares I’m not sure what to do with yet:
Mission accomplished. I’ll just get it a power supply of its own, a power brick this time, and it’ll be finished! You can only tell it’s been worked on by the dangling wires hanging out the bottom.
Back from Vacation
I’m back from a short vacation and moving apartments, and ready to tackle new projects. My new location has a lot more available space to set up a work bench which should be online in a few days.
In September, I’ll be offering a few of the radios from my gallery for sale to make room for a new acquisition, a 1934 GE M-125. This was GE’s top of the line offering that year, and has an impressive clock-face dial and a whopping 7 control knobs. It’s going to require a complete electrical reconditioning, but will be an excellent performer once it’s cleaned up.
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