Recently, the speedometer of my trusty old Volvo 850 station-wagon started acting up. Initially, it would just drop to zero intermittently at speed, then come back. Hard to notice if you only so much as glance at it every now and then. Over the last two weeks this got worse such that it would only very rarely do anything at all. My best guess for the cause would have been the speed sensor at the vehicle underside, but for mysterious reasons the odometer was still going at the normal pace. According to the instrument schematics, this should not be possible if the sensor were broken – same signal for both. Continue reading
A few weeks ago a friend brought me an old subwoofer that was discarded as broken – a JAMO SUB-660, which is an 600W sub for home cinema with integrated amping. The sub receives pretty good reviews, so I set out to try and fix it. I have worked on quite a few power amps until now, but as fully switched designs like this rarely fail, it is always a challenge when they do.
After quite some time, I am finally starting to check my 4002 signal generator in-depth. The first thing I want to do before really starting this project is to get a good idea of the system layout, hence the “part 0” thing above. I will link from here to the different components as I wriggle through the unit and check them. As there is no service documentation freely available, I will go deeper into critical parts of the circuit along the way.
This will also help me simplify things later when trying to figure out which line went where, if things go wrong.
Clock source (Decade stage)
Contained in the bottom RF block, consists of a styrofoam-encapsulated 10.000(00…) MHz oven-controlled precision oscillator and some clock distribution buffering. This part sources the main TTL clock which is also available on the backside ports as an instrument reference. The picture shows the whole top side of the module block, but the actual OCXO and distributor PCB are on the right.
There are three additional circuits in this module: The 10 MHz TTL buffering and switchover for external references, a 10.7 MHz IF generator (PLL+VCO+Mixer) and another phaselocked VCO for a derived widerange signal (49.3-70.7 MHz according to the marking) which is used to fine-tune the RF synthesis stage.
While working up some extra circuits for the spectrum analyzer, I managed to pick up an old signal generator from eBay.
I heard a lot of positive things about the
German (actually French origin, please look at comments below. Thanks to Rohit for pointing this out!) brand “Schlumberger” before, even though there is no relation to any personal experiences with their equipment. Seems like they also ran some kind of subcompany outfit called “Solartron” or “Enertec” which would today sound more than fishy, what with all those copycat-brands out there. But when an auction came up for a reasonable price I decided to go for it after some short research on the net.
What I got was a Schlumberger 4002 signal generator. It ranges from 0.1 to 2160 MHz with 10-20 Hz tuning accuracy, selectable output amplitude from -138.9 dBm up to +13 dBm in 0.1 dB steps, auto-sweeping and several extras like an OCXO for stability, 20 dB of linear attenuation range without using the step attenuator, an internal modulator and IEC bus remote control. If you looked at the photo closely, you will have noticed that the frequency range is written as “0.1…1000/2160 MHz” on the front panel. The reason for this is the optional doubler module included in this instrument. If the module is installed and detected, the software switches over to extended range without any further changes. Else, 1000 MHz is as far as it goes. More detailed specs will follow as soon as I can decypher the bad scan of a manual page that cropped up on Google. Judging from the inventory labels on the backside, the device must have been used in the manufacturer’s own lab. Unfortunately I have not yet managed to find any service info even though the manuals seem to be sold sometimes, for rather terrible prices. Continue reading
As I already mentioned, the highest priority fix is the input RF attenuator. To get access to this, the control panel must be taken out since the connector is placed at a slightly inconvenient place – the bottom of the motherboard. Unfortunately, right beneath this is the aluminum carrier plate that contains all the RF circuits consisting of semi-rigid coax and clunky metal-jacketed modules. I first unmounted the top, bottom and right side panels, which was an easy job:
- Loosen the single screws at the back of top and bottom covers and pull them off towards the back.
- Remove the two screws holding the carrying handle and pull off the side panel along with it.
- Also remove the top and bottom plastic inlays from the front aluminum frame, these cover up the panel screws.
- Remove all visible screws from the top of the frame that seem to belong to the right panel (should be 3) and also from the bottom (should be 2). Take care not to remove the 2 rightmost screws on the bottom, these hold the front connectors and are best left in.
Now pull the control panel right out.
If it sticks, the points to watch out for are the N-type RF connector and the PCB edge of the sweep time selector. Gentle pulling while wiggling the panel up and down some will bring it out. Remove all connectors from the backside. Don’t worry, the plugs can’t be interchanged. Set the panel on a flat surface, front side down (Fig. 1). Continue reading
Originally I was looking around a well known auction house for a digital Spectrum/Network analyzer from one of the older Tektronix 2715 or HP8566 series to extend my measurement rack to higher frequencies, but they are hard to find for a reasonable price in relation to the risk of buying a device in unknown condition. Still, I wanted one of the older models, because of their excellent design and repairability. Maybe without the exception for some very special, custom parts – but we’ll just hope that those don’t break.
Also, in my understanding a slightly older system from the top series at the time still outperforms most more expensive, modern, all-digital-hey-we-compensate-all-the-errors devices. That is not to say that digital processing and compensation of systematic errors is bogus, of course! But at the same time, any weak measurement hardware can be made to appear top-class by taking several thousand complete sequences and averaging. Getting it right on a single try is an art for itself, and designing a combination of precise hardware and just the right amount of post-processing is the reason for the price. Or maybe I’m just a sucker for retro tech, with all its edges, heavy metal and shiny parts.
So, I finally got a fair deal on this 8565A unit. I might have wanted to choose its bigger brother 8569B instead, which has a wider external mixer span of up to 115 GHz and a digital control interface, but the LED readout certainly adds a special flavor to the set. The seller had informed me that the device would be uncalibrated and the sweep time selector didn’t work anymore. Usually such estimates contain some tolerance, so I already expected some other things that maybe nobody noticed. Since my original plan was to recalibrate whichever device I got anyway and fix all the problems over time, that would be okay. The fixes will be documented here. Continue reading
These are just some short notes I took while inspecting an aged Sony TA-F220 amp some days ago. I have seen several of these over the last years, pretty decent amp with a nice sound. They all have some regular aging flaws in common, though.
This article is about one of my recent spontaneous projects. A few days ago, I got lucky on an ebay auction and picked up a broken Acer K330 projector. I often look for these kinds of offers because I have spent several years in the audio/video repair business and am pretty confident in my skills when it comes to fault-finding. So I figured, saving some money by restoring a broken device couldn’t hurt.
First some facts and features: The K330 uses a three color LED module, which promises a long lifetime and low energy consumption. A Texas Instruments DLP module handles image generation. In sum this lets me hope for good contrast and strong colors, even if the brightness of 500 lumens doesn’t seem that much. An interesting thing about this DLP chip is that it uses an uncommon diamond pixel grid for size reasons. Diamond in this context means that the pixels do not form a rectangular pattern like in the usual TFT monitors but rather a grid of 45° rotated and slightly squeezed, not completely rectangular tiles. Of course, this means that internal resampling has to occur to map the image from the rectangular domain onto the diagonal domain of the IC.
So, this projector was obviously broken when it arrived – what a surprise. The seller had already informed me that it had suffered from overvoltage of unknown cause. The VGA picture was supposed to be very green-ish, the HDMI input dead in whole and the media player erratic. A slight flickering in the picture was also mentioned. I’ll take you through the repair process for this device from here on. Continue reading
I’ve had this lying in a drawer for some time, it stopped working from one moment to the other. Imagine unlocking your car, driving around some, and then being unable to lock it again. Good thing that mechanical locks are not totally uncommon. I still had one remote in spare though, and together with a bought new one the set was complete once again. To be precise, these are pretty old models (’94-’96) made by Volvo and cost some 40 Euros or thereabouts. I would guess that is pretty much the same for each car manufacturer, as are the inner workings.
All was well until another key broke down – same procedure, different day. Maybe static electricity was to blame, I don’t know. But it made me wonder if it could be fixed – so I cracked it open.
This picture was taken after the repair, which explains the flux residues ;-). The circuit you are seeing can be identified using an EPCOS resonator AppNote linked at the bottom of this article. The resonator (marked in picture) is a S+M R701, manufactured by Siemens Matsushita Components, a joint venture that later brought forth EPCOS. While there are other manufacturers like RFM, these models seem to be a pretty common brand for such keys as far as I have seen. Inside the metallic-ceramic casing is a parallel LC resonance circuit which has been precisely trimmed for a specific resonant frequency. The current types are mostly pin compatible to the old ones.
Just in case: Those resonators cost you a few cents at Farnell, but I would guess Digikey also has them. Use the datasheets to get the correct one for your frequency and pin layout. They are a real pain in the ass to solder by hand! But, to be fair, they were not really designed for it. Use hot air or reflow techniques if you can, or else heat the ground plane from the side, OR remove the resonator by applying a small amount of solder to the case top and heat it up completely. Take care not to rip the copper traces off the pcb, as the glue keeping them there becomes flexible when overheated. To solder on the new one, place it onto the cleaned pads and flow solder underneath the pins from the side, for each pin individually. Do not heat the resonator for too long. I did it like this twice, but as said – pain!
A good place to start is the EPCOS R920 resonator.
As I found out later, the resonator was perfectly working. I swapped it anyways – even swapped the old one back once I found the error – to make sure, but the real culprit was the HF output transistor which I have marked in the lower left.
How it works, in easy words: The signal generated by the microcontroller is a digital pulse signal containing the remote code. This signal is fed into the resonator through a resistor and a capacitor, and from the resonator back through said capacitor to the base of the output transistor. The resistor limits the output impedance of the controller pin to prevent it from clamping the transistor base to a fixed voltage and allow the resonator to overlay an AC component onto the DC voltage. Now, as the resonator…well…resonates at its designed frequency, excited by the impulse coming through the capacitor when the controller pin turns ON, the resonance waveform is coupled into the base of the transistor, which in turn amplifies it and pushes it out onto the antenna. Imagine repeatedly hitting a tuning fork with a small hammer and touching it to stop the vibration in a certain pattern while picking up the generated sound with a microphone to amplify it.
What you get is a high frequency signal during each high-level time of the remote code signal and no signal during each low-level time. This is called OOK, or on-off-keying. It can be seen as an extreme example of amplitude modulation (AM), and the principle is identical to the way TV remotes work – only that RF is used instead of IR light. There is a lot more theory behind this, you can look it up in dedicated books about high frequency circuits and transmitter circuits. As for this circuit, have a look at page 6 of the AppNote linked below. It features a standard circuit that very closely resembles my remote key, plus a detailed function explanation.
Back to the repair – checking the original transistor with a standard multimeter showed proper diode behaviour (meaning it was a bijunction npn or pnp transistor), but this is not always sufficient for a diagnosis of a working device in HF circuits. The output to the antenna (the copper strip along the left edge) appeared near-zero and the pulse signal after the coupling resistor was pretty deformed, too. With some remote keys you can test the RF output by holding a radio scanner set to 433,92 MHz or whatever frequency the key uses next to it and listening for pulsing tones/noise upon a key press. If you don’t get any response, you are either unable to hear the specific modulation (uncommon, as the modulation is pretty slow to ensure good reception) or the transmitter is (still) be broken. In the end, you probably have to find out by trial and error. Owning a GHz scope greatly simplifies the task, of course. I don’t, so I’d advise you to first swap the transistor and then the resonator. R/C/L are pretty uncommon to fail, whereas the transistor is sensitive to static electricity like most semiconductors. What you can measure, though, is the pulse signal from the controller.
Now, to get a suitable replacement, you can either buy a transistor that works for frequencies up to above the operating frequency of your device and has the same pinout – or you can simply go look for a cheap remote controllable wall socket, wireless weather station or the likes. Whatever you choose should use the same frequency, of course, and run on batteries. The probability of finding a suitable transistor inside such a transmitting device is very high. This will be the one nearest the feeding point of the antenna, like in the picture above. You’ll need to confirm the pinout by looking into the circuit or searching for a datasheet by the SMD marking on the case – if present.
I had to try two different transistors, the first one was probably already dead before transplantation (I already wondered why I never used that remote control). A second one worked like a charm! I have noted the designation of the found replacement in the picture above. The last thing to try is the range, but there is not much to gain with only about 3V of operating voltage – ideally, you’ll get whatever range the key had before. If the range is much less, the transistor is most likely not up to the job.
Remember that most car keys “forget” their assigned codes upon battery removal. You will need to reassign the key to your vehicle according to the proper method designated by the manufacturer.
Two days ago, a small package arrived on my doorstep after 15 day’s worth of travel. Straight from China, 5 pc. of the L7251 spindle/VCM driver (remember the bad harddrive?), sold by Shenzhen Drivestar, who have a very friendly and competent customer service. Thanks again! I figured replacing the chip was the best option, looking at the chances of success.
Well what do you know. After soldering in one of these, the little sucker spins again. It is not accessible, yet, but this is definately progress! Even the o’scope shows perfect waveforms at the motor terminals, but listening to the clicking and squeaking noises I’d say something is wrong with the heads.
After investigating a little further, I spotted the central problem: The drive has suffered a triple headcrash, meaning three of its trackheads have touched the disc surface prior to the fault and were literally ripped to shreds – but in the wrong direction! The head assembly was bent and got stuck near to the platter edges, *almost* in the standby rack. Something prevented it from getting there, as it should automatically move and lock there as soon as the disc loses power. My best guess is that the disc stack rotated against its normal rotation direction, which can happen if you carry the drive.
To explain: Normally, the heads would create their own air cushion using the movement of the platters and “float” over the surfaces during operation. While the platters are motionless, that cushion is not present and therefore there is friction between the surface and heads – and any motion of the platters is also directed at the heads. For that reason, the heads have to be parked before the disc slows down too far. Usually, some kind of mechanical tensioner handles that part. It also ensures that the head assembly never moves freely while the drive is stopped.
The resulting braking force during spinup probably caused the controller to fail, and after repairing that part, the disk spun and pushed the head out of the platter spindle.
To sum it up, it is very likely that the surface of the lower platters is severely damaged, and even if not – the only way to undo this is to mount a new head assembly. I will look into this a little further, but for the moment that’s just that until I get around to building myself a small “clean room box” for repairs on discs and displays.