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Thrift Store Jamo MPA-101 Repaired – She’s Back Up and Running!
I started working on a Jamo MPA-101 amplifier back in August for a friend and after some early work it sat for a while. He’s re-doing his audio system at home so I spent a few hours to finish troubleshooting while waiting on parts for every other project on my bench right now.
The MPA-101 is a nice compact desk amplifier for a stereo speaker system or a subwoofer. It’s 50W/channel into 4 Ohms or 100 into 8 Ohms bridged mono and has a very quiet cooling fan which is almost totally silent and doesn’t even come on all the time. Great understated styling, too. They’re still in production and you can even buy one on Amazon for about $200. This one was $20 at a thrift store, if I remember the story.
Jamo is a part of the Klipsch group, and these amps are pretty well regarded. They’re daisy-chainable with cascading inputs, so several of these would make a nice independent amplifier system when paired with a digital speaker controller or similar.
I e-mailed Klipsch and they sent me the schematic to help with the repair process. You can download a copy here.
The amp wasn’t coming out of protect or when it was, it was incredibly distorted and with basically no volume control, only loud crashing. It looked like the power supply had suffered a failure at one point, with the resistor being discolored. Some of the capacitors looked pretty suspect so I shotgun’d it and replaced all the capacitors on this board with new ones. The power supply board was solder jumpered to the main amplifier boards at an edge connector, which was a pretty annoying connection method.
Jim KJ7QT wrote me a note talking about his experience with a similar problem on this model:
I pulled the boards out of the amplifier, and carefully examined them with a 10x magnifing glass – a 220MF electrolytic capacitor (labeled C39 on the schematic) showed signs of leakage at its base, and less than 3 Ohms resistance across its plates in circuit – which should have been around 1K Ohm based on the value of resistor R88.Capacitor C39 is part of a sensing circuit that takes 32VAC from the main transformer, rectifies it to a 12VDC reference voltage, which is compared by the amplifier’s protection circuitry. I’m assuming that this circuit is intended to sense an overload on the transformer caused by a short-circuit and shut down the amp – so when the capacitor failed, the voltage dropped, and the amp was shut down.We also replaced resistors R78 (2.2K, 2W) and R85 (2K, 2W) with 5W parts, upgraded R90 (39 Ohm, 1W) to a 2W part, and re-flowed the solder joints on all of the main power transistors, as the back side joints were quite dry, and one had been visibly arcing under load.
I did also get my new Rigol oscilloscope, which really let me see what the amplifier was doing at each internal stage.
I used Audacity to generate a 600 Hz test tone about 25 minutes long and saved it as an MP3, then played it back from the laptop. The garbage waveform it produced and the laptop rendered makes me want to move my HP 200CD precision audio oscillator up the repair queue, it needs its power supply reconditioned and to be calibrated. One probe was attached across a dummy load at the output terminals. The other probe I held on to and used it to probe the amplifier stages from the back forward. The idea was to compare the waveforms and see where the distortion was being generated in the circuit.
It’s almost 600 Hz.
I started probing the input ICs on the preamp stage.
It was handy having the entire schematic visible at the same time, more or less, working right under it.
Output trace with the volume turned about half-way up. Terrible distortion.
Even worse when the amp was being adjusted
The signals phase better when both are connected together. I assume it’s something to do with the triggering; I’m still learning how to use the new oscilloscope since even this functionality was just not possible on my old EICO 460. This new scope has around 60 years worth of improvements built in.
Here I am probing one of the driver transistors on the amplifier board. The distortion has cleared up a bit it seems.
And at an earlier stage. At this point all I’d really done was clean some connections, cycle the volume knob completely a few times, and reseat connectors but it looked like the amplifier stage gain was working properly. I decided to switch to some music.
I’m not entirely sure how to make the scope snapshots a consistent size. The software isn’t the most intuitive. The communication protocol has been pretty well hacked, though, someone might write a replacement UI for the scope. I switched to probing the volume control, since the distortion only came back when it was moved.
It looked like the volume control might have a broken track internally. It worked fine when not being touched, and must have been worked into making a better connection by moving through its travel but was still very badly distorted and didn’t seem to be getting any better at the low end. I was feeling confident enough to attach an actual speaker to it at this point.
I could hear the distortion, but it sounds much better than it did before.
I ordered a http://www.mouser.com/Search/ProductDetail.aspx?R=RV122F-20-15F-A50Kvirtualkey14860000virtualkey313-1240F-50K from Mouser for about $4, and set to replacing.
I removed the control from the mini-board it was mounted on. Here you can see I split the control to see the carbon tracks under it.
There was one minor hold-up where the new control has a different footprint than the old one. Not the end of the world: each control has 3 wires, so I used a section of Ethernet cable and removed the extra pair. It’s about 4 inches long. These new controls unfortunately had the reverse pinout of the previous ones so I had to remove and re-solder the outside connections for each one to make the control work in the correct direction.
I mounted the potentiometer board to the LED board using a common screw and hole. That’s convenient!
I reassembled everything and set to test waveforms with speakers hooked up and my oscilloscope. Yellow is left channel, Blue is right channel. 600Hz synthetic wave software-generated MP3 tone:
Dubstep music:
Alternative Endurance streaming station:
Looks perfect to me. It didn’t sound like there was any excessive hum or buzz in the dead time. The original volume control had an additional grounding lug which the replacement doesn’t have. I’m betting this isn’t a significant issue, but if it is, I can reconnect it fairly easily.
Looks and sounds great. These large 6800uF 50V snap-in capacitors fit within about 1mm of the footprint, it can be kind of hard to find good-fitting parts out of all the possibilities out there.
This was a really fun project where I got to use a bit more in-depth troubleshooting techniques, and the end result sounds as good as you’d expect from something by the Klipsch group. I’m excited to hear it out powering a set of Bose 901 Series 1 vintage speakers.
New Project: Jamo MPA-101 Amplifier Repair
My friend brought me this Jamp MPA-101 amplifier he picked up at a garage sale, complaining in only puts out some rumbling and humming. It looks like it’s suffered a failure in the power supply at some point. I’ll be trying to bring it back to life, as it’s a nice compact desk amplifier. There’s no published service information, so this should be an interesting challenge – the boards are very repairable if I can track down where the faults are.
A resistor in the power supply section, the largest blue one near the center of the photo, appears to have been overheated as it’s visibly discolored. This likely means something has faulted in the power supply, most likely a capacitor. I’ll be taking the board apart to test the capacitors, and go from there.
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.
Huge and Powerful: The 1937 GE F-135 Repair Complete
I’ve finally had the time to finish one of my long-running projects repairing GE’s most powerful radio offering from 1937, the GE F-135. I picked it up from Craigslist back in November but other responsibilities kept me from getting much done on it until the 1st of this year. Finally, after a few months waiting, it’s finished and playing!
The radio came to me complete and in decent shape for the age. It’s missing the glass for the dial, but is otherwise completely intact and the finish isn’t in bad shape despite a few scratches here and there.
This radio is the current king of my collection. The best radio GE sold in 1937, it has a lot of innovative features – early APEX hi-fi reception, dual 6L6 output tubes putting out 20W of audio to a large and rich-sounding 12″ speaker; a total of 13 tubes including a tuned RF stage, dual IF stages, separate oscillator and “station seeking” automatic frequency correction.
Under the chassis it’s in decent shape too. It looks like it has been worked on before a few times – there are some ’40s, a ’50s and a couple of ’90s capacitors installed. There’s a sticker from a Seattle-area Jukebox Repair store on the back which probably explains the more-recent repairs; I looked up the address and they don’t seem to be around anymore.
First thing first after doing the complete set of intake checks on the radio, I gave the cabinet a thorough scrubbing and hit it with Howard Restore-a-Finish and it came out beautifully. The intake checks were uneventful so I didn’t take any photos, but the power transformer, all the IF transformers, oscillator and antenna and RF coils and the speaker transformer and coils were just fine. After applying the Howard’s, it really cleaned up the fading and covered over a couple of small scratches and really brought out the natural shine on the wood.
Then I tested all the tubes and found that most were in good shape (especially the 6L6G tubes installed) there were a few 6J5 and 6K7 tubes that needed replaced. I had these in stock and so it was easy to solve that problem.
I worked on this one under my kitchen’s vent hood as it’s currently too cold to vent soldering fumes outside. The chassis stand is the right width though so that’s perfect.
Every paper and filter capacitor needed replaced, as well as nearly every single one of the resistors which had drifted as much as +100% in value in some cases. Fortunately the coils are all intact or this could’ve been a much messier repair.
I also found a couple of places where the previous repairman who worked on the set may have been dyslexic, as there were a couple places were numbered resistors were reversed – i.e. R23 being in R32′s place and so forth. I imagine that couldn’t have made it work any better, anyway. I tested and replaced going along from the bottom up as needed. These are in-progress shots, so you might see a couple places where leads aren’t trimmed or components aren’t yet soldered. All of those issues were taken care of but might not have made it into the photo series.
Small caps dealt with, it was time to replace the multi-section caps. There is a 4-section can, insulated from the chassis, mounted up top with a set of 2 filter caps and 2 cathode-bypass caps. These all ran to a hole in the chassis where they went above. I snipped the long cross-chassis leads and moved the components close to their intended locations. In this photo, I’ve replaced the 10uF cathode bypass capacitor with its replacement. I like using bipolar caps for the large-value cathode bypasses but that’s just my preference and what I keep in stock (I keep those values around for repairing crossovers in old speakers) but you could use a standard polarized capacitor there.
Here’s the totally-complete underside shot.
There’s still a matter to deal with above the chassis, though. This is an AFC radio which uses a special and complicated transformer heading into the diode which recovers the audio. And it has a small resistor which is reading double it’s value and needs replaced as well, or it won’t align right.
Then I sealed the can back up:
I hooked the speaker and pushbutton assembly up on the bench and gave it a test run – it fired up immediately and started pulling in a few stations even on the Shortwave bands. The dial was off alignment a bit though, so it was time for that.
For the alignment, I pulled up the signal generator and started with an IF alignment before going back to the RF stages. This radio has a special IF arrangement with a procedure, so I aligned the 3rd IF primary, second IF secondary and primary, first IF secondary and primary, then went back and aligned the 3rd IF secondary that feeds into the diode. Aligning that discriminator was a maddening 10 minutes spent trying to nudge the adjustment ever so slightly. My goal was to get 0V between two segments, but it approached that point at an incredibly steep slope. I managed to get it there, though.
The original 0V spec was made with a primitive meter; I’ll take 0.01 on a more sensitive modern instrument. That’s perfect IF alignment. It was definitely worth it though. Now onto the RF, which involved tweaking something like 16 trimmers in a precise order with an RF signal generator at various frequencies.
Finally, it was all set! Time to reassemble.
At this point, the radio plays beautifully and pulls in stations from all over, and I’ve added a line input to let me hook up an audio source. The hassle of the AFC calibration was definitely worth it, it’s nearly like magic to watch it work. With the switch off, the radio tunes sharply and a station comes in over just a few degrees of rotation. With the switch activated, it’s like an entirely different radio – the same station will come in across about a quarter-turn of the knob, 2 divisions in either direction from the center frequency and it will block quieter stations from interfering.
The radio sounds great with a pretty good frequency response and more volume than I know what to do with, too. The relay for the motor is burnt out, though. I missed that on the initial checks so when I went to test the pushbutton function…I got a whole lot of nothing. I’ll make another post here when I do get the motor resolved but for now I’m going to hang this one up and start playing it. This was a very fun and enjoyable project and I have a beautiful radio with a commanding presence to enjoy for many years to come.
Project Idea: Verve//Remixed 3′s Cello Boombox
I’ve had this idea for quite a few years but haven’t put much time into it, between everything else I’ve been going on. Once or twice a year for the past five or so years, I’ve posted on Craigslist seeking a broken cello to turn into an art project. Finally, the ad hit and I think I can make it happen.
Verve//Remixed 3 has an incredible album cover. You should buy their music for that reason alone.
I’m hoping to make my own version of the Cello Boombox on the cover…and this will be my starting point, a cello with a broken neck the owner wasn’t interested in repairing. The neck snapped off, the bridge is missing, and there are quite a few cracks (although they aren’t visible unless you look closely) and it needs to be re-glued.
I doubt I’ll do anything with this until next year at least, but now it’s a possibility. I’m just glad I was able to find a broken cello, I wasn’t willing to destroy a functional instrument for this. Not to mention, a working cello costs a lot of money.
There are a few things to consider:
Speaker size and placement. How many drivers will I use, and what types?
Cabinet volume and phase cancelling. If I configure this as a stereo boombox, I’d need to isolate the enclosures internally from each other or the out-of-phase parts of the left and right channels of the audio could cancel or introduce distortion.
Audio source. The Verve Remixed album cover is concept art, not a real product, so the knobs and panel meters and a floppy disk drive aren’t things I could realistically include. Do I mount a small music player in the center – maybe an iPod Touch, or a small Android tablet? Or should I just make this into an artistic speaker without its own audio source?
Power. If this is going to have its own amplifier, how am I going to power it? An outboard power pack? Rechargeable batteries inside?
If anyone has suggestions on those design topics, I’d love to hear them, or from anyone else who has attempted a project like this.
Friday Brain Dump [Random Thoughts]
A recent random assortment of ideas:
It’s easy to get media content from a computer to a television, but it’s difficult to get any kind of external media back into a computer. I have a fairly involved computer setup at home. My workstation is connected to two 20″ monitors and a 46″ LCD. A friend was in town for the weekend, and we were looking for an easy way to make a YouTube video play from his laptop onto my monitor, without actually taking away my control of it. I have two sets of input devices, one at my desk and one near the couch, to control the computer when it’s being used for media. We ended up just switching off who had control of the single mouse cursor between the two input devices. The ideal solution? Either multiple mouser pointers, assigned to the unique input combinations – or a VGA capture interface to make another computer’s input appear in a window. VGA capture devices exist but are absurdly expensive, and there’s no multi-pointer support in the operating system. Most laptops these days have dropped an analog TV output in favor of HDMI or DisplayPort outputs meaning they can’t be connected to an inexpensive TV-capture card.
At about the six month mark, as expected, my phone has started acting up. This has been the case with most every smartphone I’ve owned – a Nokia running S60, an HTC 8525 running Windows Mobile 6.5, a G1, and now my G2 running Android. Reliably at about the six month mark they’ve all started acting up in ways that make me suspect the hardware is failing: lockups and reboots, screen glitches, data disappearing off the memory card randomly and visible dust between the LCD and the glass so deep in the phone the only way I can see it getting in is via osmosis. Hard reset doesn’t fix the problem. Have even high-end smartphones become disposable commodities?
Hallicrafters 8R40 Upgrade: On the back of my bench I have a Hallicrafters 8R40 radio receiver from 1953. It will be a good performing radio when I’m through fixing it up, and I’ll be using it to try and pick up long distance contacts. One thing about far away faint signals is that they can be tough to hear even with the audio turned all the way up, the 8R40 only has a single-ended 6V6 output that maxes out around 4.5W of audio power into a not-that-efficient speaker. From Parts-Express, I bought a few Dayton Audio DTA-2 Class T amplifier modules, based on a Tripath TA-2024 chip. They’ll pump out about 20W of power. I’m planning to use one of these modules inside the back of the Hallicrafters to allow it to drive an external speaker at 20W in a reversible modification. I expect I’ll split the detector’s output and use the module amplifier for the external speaker only; leaving the build-in speaker powered by the tube.
New Project Idea: Oddball Tube Hi-Fi Amplifier
I’m looking for a new summer project, and think I’m going to branch off from repairing radios to building some of my own gear. I’m interested in making a tube hi-fi stereo amplifier to go with my old speakers – and it’s something I can build. Audiophiles, guitar enthusiasts, and other people into tube audio have driven the price of most of the good audio tubes higher than I’m interested in paying, though.
The 6V6 audio tube is known and loved by enthusiasts everywhere, and came in a variety of other versions with different ratings, different glass, different power supply requirements but pretty much all produce the same output. I’m interested in making a miniature-tube amplifier, so I’ll be starting with the 6AQ5A tube – but these are pretty expensive. An alternately powered version, the 5AQ5 goes for only about $2/each so that looks like a winner.
I’ll be basing my amplifier on the Silvertone 69A, a dual-channel push-pull stereo amplifier manufcatured in 1960. This amplifier used a 5U4 rectifier tube to supply the high voltage. By replacing the tube rectifier (right, above) with a pair of 1N4007 silicon diodes (left, above) I’ll free up the 5V power supply. A set of four 5AQ5s draws 2.4A of current; a 5U4 draws 3A of current so there’s more power to spare and a guarantee of being able to find cheap hi-fi tubes for years to come without a lot of competition.
The schematic – with some bias and output mods – not showing the power supply or output tube modifications – that I’m basing the design is here (click for larger version!):
This should be a good sounding little amp if I get around to actually completing construction. I have most of the iron [transformers] in stock already, it’s just a question of buying some hardware and spending the time required to wire it. We’ll see.
Burned Up Aiphone
This is what happens when you connect the high level output of an audio amplifier, to an input designed to be hooked up to an MP3 player or other line level audio source. This Aiphone PA amplifier was incorrectly installed by a technician and it made this 1/2W resistor dissipate about 40W. It held up…briefly…but quickly turned into a charred mess. It was somehow managing to pass a terribly distorted signal but didn’t damage any other components. After replacing the resistor on this pictured unit, and another identical one damaged the same way, it was back in service.
It’s not always overly aggressive cost engineering that makes things fry. Sometimes it is actually operator error.
Don’t Trust Everything You Read (On Old Radio Schematics)
Electrical drafting is primarily done on a computer today, with software such as EAGLE or KiCAD. This wasn’t the case back when tube radios ruled the airwaves, though – schematics were drawn up by engineering draftsmen by hand. And as with any process with a human element, they didn’t always get it right.
I’m working on a 1934 Philco 66. It came to me in excellent original condition with little evidence of having been service, and throughout the process, I’d been relying on the schematics to guide me in the right direction. Unfortunately, along with a laundry list of other issues, my reliance on the schematic to be “the truth” led me around in circles longer than I needed to be to resolve a power supply problem.
Below is a schematic snippet of the power supply and audio sections of the 1934 Philco 66, with the RF chain to the left of the #75 Detector/1st Amplifier tube hidden for simplicity’s sake.
In green, I’ve highlighted the path B+ (high voltage) is supposed to flow from the rectifier cathode to the plate of the first audio amplifier. It’s a very straightforward path…if the draftsman had indicated that tube was supposed to be connected to the power supply. In red, I’ve indicated a missing connection symbol. Without it, there was no power being supplied to the first tube in the audio amplifier stage and the audio signal was being killed at that point before it could make it to the final output amplifier. Using an alligator clip, I restored that connection to test, and the radio sprang to life making noise on the next power-up.
The second filter capacitor should have been connected to both B+ and to the plate path for the #75 tube, rather than just the plate path. (Incidentally, the two capacitors are both at the same potential, so under the correct connection scheme could have been replaced with a single capacitor of a larger value.)
It’s not done yet, but I’m inclined to believe the final wiring issue has been corrected, and it’s on to performance.
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