Archive for April, 2011

hp 1660EP Logic Analyzer – new collection!

Sunday, April 24th, 2011

HP 1660EP Logic analyzer

I recently acquired this machine for a reasonable price in Singapore.  The set did not bootup properly when received due to hard disk failure.  After replacing the hard disk, the set is able to power up, passed all performance verification tests and function normally.  The model of IBM drive I use is Travelstar DJSA-210 6.0GB.  I also tried one Toshiba 6G harddisk but the 1660EP refuse to detect the harddisk. Thus I strongly recomment only IBM brand hard disk should be used.

There are few difficulties in using this old equipment.  Firstly is to replace a old hard disk with about 2.1G to 4.3G size.  So the only place is to look into old computer shops (local junk yards or ebay).  The machine is able to use only 2GB of the hard disk space even if the original size of HD is larger than 2GB (may be OS/firmware limitation).  Secondly, the software from hp website has to be loaded into the hard disk from the floopy drive.  It is not easy to get diskette and copy the download files onto it.  I manage to get a used IBM USB floopy drive for cheap price and NOS box of floopy diskette to complete the job easily.  Thirdly, the ROM update has to be done by floopy drive and it is not possible to boot up the machine once the ROM data is erased.  Thus do not erase the ROM content if you are not sure if the floopy or floopy drive is working perfectly.  Lastly, if the floopy dirve need to be replaced, the source is from some old notebook drive which can be very cheap at scrap cost.  Model is Teac FD-05HF and the same model is used in those TDS744 series digital oscilloscopes.

Some photos below:

After a bit study, I install a X-server cygwin on my PC and then I can link the 1660EP X-window to my computer screen.  It is not that reliable due to some wild character transfer and breaks the connection. 

Audio Precision SYS-2522 Repair

Sunday, April 17th, 2011

I have owned this equipment for few years and recently I find that the Digital input from BNC for channel I and II is not working properly.  The symptoms are summary as below:

1. Channel I level cannot detect and thus refuse to do any measurement.  Channel II can detect signal and do measurment.

2. The 75 ohm and Hi-Z switch is not functioning.  For XLR input, the 110 ohm or Hi-Z is able to be selected and hear relay “click”.

3. When Gen Mon is selected, no measurement for both channels.

After checking the signal path of all digital signals using the generator as the digital source, it seems that the circuit is all working fine.  On the front panel connector pcb, there is a MAX310 multiplexter to select different input connector and signals.  Pin 15, 16 and 17 are address input A0, A1 and A2.  In XLR mode, the voltages are solidly at ground level but at BNC mode, the pin A0 is like floating. 

After spending hours of tracing the path (It is a multi layer pcb and thus the trace is not on top or bottom and I do not have a service manual of this equipment!) from the connectors of the main board, it was found that the address bus A0 to A3 is connected to an IC 74FCT16374AT – Pin 2, 3 and 5.  Since XLR mode is working, thus there is no connection issue for this three pins. Pin 1 of 74FCT16374AT is the Enable pin of these three output address and thus I use a scope probe pin to check on this voltage.  When I touch the pin 1, I hear a relay “click” sound and then the measurement of input digital signal is back to normal.  When I remove the probe, the issue is back again!   After I check connection of pin 1 using multimeter, the pin seems to be not connected to the pad due to dry joint (visually I cannot see any issue!).  So the problem is removed after touching up the pin 1 of this IC with some solder paste.  Here I attached a photo for those people who may have similar problem as my equipment.

FDA-1 PCM1794A DAC for Sale / PCM1794

Friday, April 1st, 2011

Last Update: April 1st, 2011

Change U11 from TA7805 to L4941BV for a more dynamics and details sound.  TA7805 remains a good mellow sound signature.  The kit will only provide TA7805 as standard.

Update: Oct 14th, 2010

There is some minor update on FFT measurement in Rev 4 manual.

FDA-1 PCM1794A DAC Manual Rev 4.1

FDA-1 PCM1794A DAC Manual Rev.5 (Updated Aug 23, 2011)

FDA-1 PCM1794A DAC Manual_r6 (Update on 12th Dec, 2011)

FFT measurement and power supply connection diagrams are added in this Rev 4 Manual.


These kits are designed to replace the obsolete PCM63 balance DAC kit (aka D1V3 and D1V33).

Please click on the link below to download complete manual for Product Specifications, Circuit Description, Circuit Diagrams, Assembly Instructions, PCB Layout, Interface Specifications, Bill of Materials and FFT measurements: PCM1794A DAC Manual Rev3

Price information refer to Store page:


A. Introduction:

  • These DACs are designed for the serious audiophile. They utilize the best DAC chip -PCM1794A that is capable of producing a super dynamic range of 127dB in stereo mode and 132dB in mono mode!
  • The I/V and analogy sections are custom designed and use 100% discrete components.  The I/V converter is a “Current Steering” topology and the analogy section is a pure ‘Class A’ non negative feedback driver and output buffer stages. With this combination, the sound will be musical, dynamic, accurate and real.
  • In Single End version, the DAC uses one circuit board with one PCM1794A DAC chip working in Stereo Mode. In Full Balance version, the DAC uses two circuit boards (one Master and the second Slave) and two PCM1794A DAC chips, each configured in mono mode (namely Mono L and Mono R) to deliver super dynamic range of 132dB.
  • Users can start with Single End version to see if the performance is up to the expectation of Hi-Fidelity. To protect your investment, a very clear and simple upgrade path is built into the design. Users of the Single End version can easily upgrade to the True Full Balance version anytime by adding a second slave DAC board! The sonic improvement of the Full Balance version brings new meaning to the word ‘High-Fidelity’!
  • These ‘serious audiophile’ DACs are peppered with high quality audiophile grade components that are selected because of their complementary sonic signatures after many exhaustive hours of listening tests.  Audio transistors, Paper-In-Oil capacitors, Silmic II capacitors and Dale resistors are widely used in the DACs.
  • However, the most important feature is that these DACs sounds musical, dynamic, open and warm with an unparalleled resolving power. These are truly remarkable DACs capable of holding themselves against any branded DAC costing many times more!

B. Specifications:

  1. DAC chip: Texas Instruments PCM1794A 24bit/192k best in class DAC
  2. Digital Receiver: Crystal CS8416 24bit/192k digital receiver
  3. I/V and Analogy Filter: Discrete transistor I/V (current steering) operating in pure Class A mode with NO global feedback for the best sonic performance
  4. Digital Input: SPDIF, AES, Toslink and I2S Bus (RCA, XLR female, Optical and 8×2-pin header)
  5.  Bit/Sampling Frequency: Up to 24bit/192k
  6. Analogy Output: Stereo Channels (Single End) @ 1.5Vrms (expandable to Full Balance output by using two boards @ 3Vrms)
  7.  Output Impedance: 150 ohm Single End mode; 300 ohm Full Balance mode
  8. Auto muting for noiseless operation when no audio/digital signal is detected or when a digital error occurs (e.g. AC-3 signal) or when powering up or down
  9. Power supply: AC 17~18V x 2, 0.5A and AC 8~9V x 1, 0.5A; total consumption 7W/board
  10. Display LEDs: PLL Lock, 48k, 96k and Mute (for CS8416 input only)
  11. Distortion: Typically at or less than 0.02% 1kHz at FS, 20kBW Single Ended @ 1.5Vrms.  At full bandwidth typically no more than 0.04%.
  12. Board: 6 x 8 inches double sided 1.6mm gold plated FR4; 6 x M3 supporting holes; max height part 35mm.

C. A lot of Audio grade components are included in the kit:

1.      Elna silmic II capacitors in all analogy power supply

2.      Russian PIO K40Y-9 for Low Pass Filter in IV section

3.      Sanyo OS-Con capacitors in digital supply

4.      Panasonic film capacitors for supply bypass

5.      Vishay Dale resistors in all signal path

6.      Low ESR filter capacitors by NCC or Panasonic

7.      High speed rectifiers BYV27 series

Refer to Bill of Materials for component provided in the kit.

D. Pictures: