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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.
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.
Clogged De-Soldering Iron of the Day
My Hakko clogged, and the cleaning pick just wasn’t working. It was time for the teardown.
I’m not entirely sure how that shape formed, but it broke off in a clean piece; the remaining tiny plug melted away after the iron heated up. That’s annoying, though.
September Projects
This September, I hope to accomplish a few things:
First, foreground left, is a 1960 Hornyphon Attache 61 radio that I’m repairing for a customer. He brought it over from Germany when he emigrated here in the 1970s, and it’s been in storage ever since. The case is in the background right. This one uses an early copper clad PC board, instead of point to point wiring. These early boards are extremely fragile after 51 years and are known to de-laminate or split traces if you look at them wrong, so my Hakko desoldering tool has come in especially handy so far in allowing me to cleanly remove the components from the board without having to use too much heat or pressure. Without it, I wouldn’t have been willing to accept this for work.
Next up, in the back standing upside down, is a 1953 Hallicrafters 8R40 communication receiver. This middle of the road receiver can receive AM and CW signals from 0.55MHz up thorough 30MHz in 4 bands. It can also tune all of the main amateur radio bands, so I can listen in on stations from around the world with it. I have a 50 foot antenna strung around the corners of my roof, so it should work nicely once I get it sorted. I’m fixing this one for myself, when I get around to it.
Finally, on the far right, is one of a batch of 7 more inoperative LCDs that I recently acquired from Craigslist for free. They have a variety of issues, ranging from cables having come disconnected internally up through a bad resonant transformer on one. This particular one, a 19″ Viewsonic widescreen, has a bad resonant transformer in the switching power supply. I’m debating whether I want to find a replacement, or convert this one to an external power supply and just feed 5V/12V into it. We’ll see. I’ve fixed one already and have five more that I haven’t opened up yet.
This should be a busy month. I’ll be posting a short photo series of the Hornyphon repair as I complete it.
Westinghouse LCD Repair
After seeing the repair I made on the Samsung LCD monitor, a friend gave me a few-years-old Westinghouse LCD/TV that had quit working – it wouldn’t power on anymore. It’s a Westinghouse SK-19H210S, 19″ LCD accepting VGA or HDMI up to 1440×900 resolution (somewhat smaller than true 1080P) and can also tune ATSC and NTSC television signals to receive HDTV over the air.
It’s apparently a very known fact this one has a weak power supply – all over the web. I opened it up and grabbed the power board:
Tucked away all in the back is one capacitor that’s visibly failed, which means it’s likely several are bad or will be soon.
New parts arrived from Mouser.com:
Using my trusty Hakko, I replaced six capacitors. 4 caps in total showed signs of leaking from the bottom as well (discolored board below), 2 seemed okay but I replaced anyway because why not. I’m getting better at using the Hakko and doing this kind of PCB rework in general, the entire process from start to finish only took about 15 minutes this time.
100uF 400V
2200uF 10V
1000uF 10V
1000uF 25V x 2
47uF 50V
Interestingly (or maybe not), these bad caps were the same brand as the bad caps from the Samsung: CapXon. Obviously those have reliability problems, or are just the cheapest they could buy.
Reassembled and powered on. The first power-up would come online but drop off immediately and it was making a hissing noise; it turns out I hadn’t firmly connected the backlight leads. After fixing that, I snapped everything back into place. Consumer electronics these days aren’t made to be opened up, so the case doesn’t quite fit back together the way I’d like it to around the control panel on the side, but it’s not visible unless you look for it fortunately.
Another one fixed! This one was about $12 of parts. Looks like this one goes for around $80 these days, so I’m half-way to getting my money’s worth out of that rework station already.
My next TV repair will be somewhat more ambitious. I got this Samsung HL-P4663W, a 46″ DLP (720p) HDTV for free from Craigslist. It needs a new bulb, and some other rework, and it’ll be worth a few hundred dollars after I get it sorted. I don’t intend to keep this one (as I already have a 46″ Samsung LCD that does full HD resolution) but just to repair and sell most likely.
Adventures in Dumpster Diving: Samsung 225BW LCD Repair
I found a Samsung 225BW LCD sitting on top of my apartment’s dumpster, and figured I’d drag it upstairs. It’s a few year old model but it’s better than the current older Dell LCD that I’ve been using (1680×1050 versus 1440×900). A quick check showed that it would power on, sort-of, but the power light would flicker constantly and there was no backlight.
I popped it open, suspecting a problem in the power supply – and turns out that was right. Several capacitors on the board were showing signs of failure. Capacitors are the main component I replace in the vintage radios but cost-cutting OEMs are often known to use caps that fail after only a few years when new to save a few cents on each part that goes out the door on new things as well. In this case their 330uF and 820uF @ 25V caps had failed and the logic board was no longer getting good power.
Modern electrolytic caps fail by bulging and leaking out the top and/or the bottom, it’s easy to see at a glance. The top two are bulging and leaking; the bottom ones are bulging only which is a bit difficult to make out in the photo.
This project is one of the reasons I bought a Hakko 472D desoldering tool. It’s made for reworking through-hole and point-to-point boards, and works by melting the solder and then applying a strong vacuum through the center of the nozzle sucking it out of the way and cleaning the connection. It wasn’t cheap, but I thought it’d be important to have one of these as I do more types of electronics hobby work. I tested it out on an antique radio and it works perfectly for the annoying old joints.
This board is pretty easy to work on, the components are widely spaced and marked.
Even though it’s not bad, I’m replacing the large main filter as well – just in case. It’s the same brand as the failed ones.
Here I’ve depopulated the bad components from the board and have placed the main filter back in position, with the old one above it for comparison.
The new caps are larger than the old ones – for the same ratings, a larger size capacitor is going to be a bit more durable. For example these 330uF 25V models:
Slid the components through the top, spread the leads to hold them in position while soldering and reattaching:
Bad planning on my part meant I forgot to take a photo of the board post-repair, but it only took about 30 minutes to do the entire thing – most of which was spent figuring out how to adjust the Hakko. And for the power-up:
Success! Back to life. This LCD goes for around $150 online even today and I’ve been meaning to add a second monitor to my desk anyway, so I’m about 1/3 of the way to recovering the cost of that desoldering station after the first project. One down, two to go….This project required 3x330uF 25V capacitors, 2x820uF 25V capacitors and 1x150uF 450V capacitor which came out to $9.83.
I love my chassis stand.
It’s the winter here in Seattle and it gets cold outside occasionally. Cold enough that I can’t vent my soldering fumes out an open window like I normally do most of the year, but I don’t want them in my apartment. Fortunately for me, my chassis stand is the same width as my stovetop and sits neatly under my externally-venting range hood so I can move it over there for ventilation.
The stand can hold the chassis up at any angle and supports a lot of weight. Every time I use it, I’m impressed with the build quality and flexibility. It’s not as nice working in the kitchen as it is on my normal workbench, but it gets the job done.
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