JAMO 660 subwoofer amp

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.

Fig. 1: Jamo SUB-660 power amp module. High-power PSU (right top), standby PSU (right of centre), Class-D amp (above centre) with large output filter choke (centre). Preamp, sound filters and standby detector PCB on the left, line input on the bottom right.

[Read More]

Running a Planar TFEL display off the Raspberry Pi GPIO

Fig. 1: Archlinux logo running on a raspberry-driven Planar/Finlux EL640.400 display.

Thin film electro-luminescent (TFEL) displays represent an interesting, if somewhat anachronistic display technology, now that we have high-contrast LCDs, plasma displays and of course OLEDs. The latter are actually closest to the principle of TFELs, and their primary optical advantage is identical: light-emitting pixels instead of backlighting, for maximum contrast. Where OLEDs use an organic polymer, which can be excited to emit visible light by applying an electric field, a TFEL does the same with an anorganic dielectric material like e.g., gallium arsenide (GaAs). The emitter pixels are sandwiched between two layers of transparent dielectric to insulate them from the transparent electrode grids on the front and back glass cover of the display panel. When a high-frequency current is applied, a current flows through the selected pixel and the emitter material lights up in a beautiful, saturated orange – in my opinion the most interesting aspect about this technology. Remember the old terminals with the amber CRT screen? Close! In contrast to OLEDs, EL displays are also able to tolerate much harsher environmental and mechanical conditions, which makes them ideal for applications in heavy machinery – or living room gadgets, when they are retired. [Read More]

Monitoring and controlling a JUDO i-Soft plus via LAN

Some time ago, a water softening device was installed in my home main water supply line. Such a device contains one or multiple gel capsules that act as ion exchangers, replacing calcium and magnesium in the fresh water supply with sodium. In regions with a rather hard water, this can save you a lot of trouble with maintenance of valves and the lifetime of water-consuming devices like washing machines and dishwashers.

Fig. 1: JUDO i-Soft plus unit with LCD touchscreen user interface.

For regulation, a conductivity sensor determines the hardness of the incoming water. After running through the gel exchanger, the residual hardness is assumed to be around 0.5 °dH which allows the mixing ratio of raw and processed water to be calculated. I won’t go further into details here, the bottom line is: It works like a charm, water is as soft as it needs to be. The device at hand is built by JUDO and is available in several configurations. Basic models contain a two-capsule exchanger for seamless switchover/regeneration cycles and an integrated electronic control unit for automatic regeneration of the gel. A more advanced “i-Soft plus” model is extended by a touchscreen user interface complete with LAN/WLAN network access. This enables monitoring through a specialized iPad application where the interested user can view total water consumption per day, week, month or year as well as change different system parameters. As a nice bonus, the plus unit has an integrated main line valve which is closed automatically whenever user-set time, volume or flow rate limits are exceeded. This already saved my ass once when a pipe became leaky inside a wall. Unfortunately, the exact protocol for communication with the device is not disclosed, which is where this story begins. [Read More]

Schlumberger 4002: Overview

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)

OCXO top compartment. Oscillator on the bottom right, distribution on the top right. 10.7 MHz mixer (10.625 MHz + 65-85 kHz) and filter in top center. Quad VCO and PLL for widerange signal in bottom center and left.

Fig. 1a: OCXO top.

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.


OCXO bottom compartment. Oscillator on top right, 10.625 MHz and 100 kHz ref generator in bottom right corner. 65-85 kHz generator in center and control logic circuits on left.

Fig. 1b: OCXO bottom.

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.


[Read More]

Schlumberger 4002 signal generator

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.

Fig. 1: Schlumberger 4002 signal generator, already opened up.

Fig. 1: Schlumberger 4002 signal generator.
Don’t mind the tearing effect on the LED displays, not visible to the naked eye. I already pulled all the side panels.

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. [Read More]

HP 8565A: Sweep time selector

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.


Fig. 1: Control panel removed from case.

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). [Read More]

HP 8565A Spectrum Analyzer

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.

HP 8565A front

Fig. 1: HP 8565A running, showing a weak signal in the GSM area.

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. [Read More]

New hosting provider

Due to the inability of my previous hosting provider to maintain the integrity of their machines, resulting in enormous and incomprehensible technical problems for at least a year now, I have decided to switch over to another provider.

To complete the chaos, this page will from now on reside under the new TLD

(blog.) muwave.de

For the sake of existing links to this page, I will keep the old domain (mmueh.net) transparently mapped to the new URL for at least one year while this page will already deliver all content in relation to the new domain.

I expect the transition to go smoothly (most of it already has) and hope for better conditions in the future – especially regarding server features and page loading time.

Please don’t hesitate to contact me if anything remains broken. You’ll find my details on the Impressum page, or you can just hit the comments section of this post.

Sony TA-F220

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.


Fig. 1: Top-down view inside the TA-F220.

[Read More]


Another +1 added to my count of curiosities and eastereggs hidden in electronic devices. While pulling apart blown power bricks for cheap ferrite cores, I stumbled across a novel, environmentally aware concept for the line isolation barrier (click pictures for large version).

fish01 fish02

Seems to work, no fried fish was the cause of this failure. Worth a smile 😉

NOTE: The soldering work does not actually look that bad. Part of that goes to the strong lighting for the photo and part to the blown smoothing capacitor spraying its contents everywhere due to overvoltage in a bad three-phase installation (L1/N exchanged).