Tektronix 2445B repair step 05 fail 44

October 17th, 2015

This scope failed 3 years ago and the symptom was just no display after power on but some LEDs flicks to a point and stopped at “ADD” LED there!  Recently I take it out and attempt to repair it.

The first suspect is the high voltage board defective and I open it and do a check on the high voltage board switching transistor and everything looks fine.  Also I can get about 65k switching frequency at the switch mode power supply.  After looking around, I see there is a “G Bias” pot on the high voltage board and then I turn it clockwise and see some display DOT on the screen.  The scope wordings of time base and input attention can be seen also.  Thus I reboot it and see a “Step 05 fail 44” error on the screen.  After checking the service manual it point to the DAC on the control board and thus I look at it again.  Oh I find the leaking SMD capacitors with corrosion in its surrounding components.

It can be seen that the components surrounding the three E-cap are with corrosion solder join!  So in totally changed all 4 SMD e-caps on the control board as the other corner also have one more leakage e-cap.

After changed the 4 caps, I put back the board and power on the set but the failure symptom is still same step 05 fail 44!!!  So I probe around the DAC chip and check the Vref 10V at pins 14 and pin 15.  The voltage is really low and close to 0 volt.

So I started to check the components around the DAC U2234 and later find out that the resistor R2228 10k is open circuit.  I replace it with another one smd resistor and after that I power up the set and it works again!

I burn in the set for about 15 mins and then I start to smell something from the scope and of course power off it immediately.  Open up and found the line filter caps (RIFA) brands are all burnt together with two more resistors.

C1018, R1018, C1016, R1010 (parallel with RT1010; 15 ohm not on schematic) were all burnt of open circuits.  Later I also find three more Rifa capacitors on the other PCB (2200pf and 10nF 250v ~ is also crack a bit on the body.  I take them out and all the 2200 pf caps are with very low value of about 100pf!  Thus I order them from element14 and they arrive after 3 days.  For the 0.068u 250V~ cap, I use on hand 0.1uf 250V~ cap to replace them (blue color on the picture).  After put back, everything  works fine.  I have brought this 25 years old scope back to life!

I also note that there are many same scope 2445 or 2445B with similar failure of step 05 fail 44.  I hope this information can help some of the people to repair their scope.

IMG_1128 IMG_1142 IMG_1193 IMG_1195 IMG_1202 IMG_1221IMG_1247 IMG_1251 2445B repair


F5T Pre-amp + FDA-2A AD1955 DAC in one box

May 11th, 2014

This is a pre-amp plus a FDA-2A in one box.  It was built in year 2012.  I use a old Yamaha DA8X box to house eveythings insides.  See the pdf files at bottom for details.


FDA-2A with USB, Optical and AES digital input.  Push button for digital input selection at the front panel.

DAC direct output is available in RCA sockets.

F5T Pre-amp:

Alps volume control from DAC output and note that there is no other analogy input for the preamp.

The pre-amp is based on Nelson Pass F5T power amp design and striped down to only one pair mosfet and lower bias current at about 200mA.  Supplier voltage is at about +/-23V with a small transformer.  Power mosfet is using Toshiba J200 / K1529 pair and casing is acted as the heatsink of power mosfet.

A headphone jack is connected in parallel with the pre-amp RCA sockets.  It can drive most headphones due to sufficient high bias of the power mosfets.


Details refer to this pdf file: F5T-Pre_DAC



Toshiba 2SJ74BL measurement

January 3rd, 2013

Many diyers have been buying some fake 2SJ74 from China sources or even from US source and thus some of them ask me for the measurement curve of this genuine Toshiba part.  Here is the file which you can enlarge to see the curve and estimated Yfs (gain) of the part.  The jfet selected is at about 10.8mA Idss (top curve).  The setting of Tek 577 is highlighted for easy reference but you can zoom in the pdf to see actual setting of equipment.


Note that the part is laser marked as it is new part production in 2008, probably the last production batch in Japan.  For ink mark, those are very old part that I have sold before – 2SJ74V.


You may notice that I have increased the price from 2013 because there are not too many stock left for sales.

J74 measurement

F5T Turbo Power Amplifier kit for sales

November 23rd, 2012

Those who had brought my F4 kits few years ago can purchase this kit or boards for a quick conversion from F4 into F5T because all the holes and mosfet locations will be same as the F4 pcb.  The optional MUR3020W can be obmitted if not use.   If F4 buyer has the original IRF9240/240 power mosfet, it can be re-used.

The To-92D transistor are Toshiba 2SC2705Y/2SA1145Y rated at 800mW and thus it is very safe to use at 60 deg C environment for less than 200mW dissipation.  It would survive safely under a maximum supply voltage of +/-32V.  The Panasonic capacitor is rated at 35V and thus a supply voltage of no more than +/-32V is recommended for this kit.

I build a version using Toshiba Mosfet 2SK1530/2SJ201.  The supply voltage is +/-22V and thus I do not use the cascade transistors (2SC2705Y and 2SA1145Y).  I also change 3 resistors and details refer to the “F5T-BOM-cost” document.  The change on R11/R12 is to increase the thermal stability and R27/R28 is for increasing the supply voltage to jfets to be minimum 10V or above for a supply voltage of 22V.  49.9R to replace 47R do to stock issue.

Sound wise, it is more powerful than the F5 original due to higher current output and more pairs of mosfet is used.  Overall the sound is smoother and warmer than F5. Nelson said this amp sound more relax!

Please look at the F5T-BOM-cost documents for the detail kits offer – Price & BOM options.

Details: F5T-BOM-Cost_2017 (updated transistors prices)

Manual: F5 Turbo Assembly Instruction v3

Power supply ripple comparison CLC vs CRC: It can be seen that an inductor of 0.47mH air-coil has improved the ripple peak to peak level to 20 times smaller.  See attached: F5T ripple

After building another set with Cascade transistors, the noise level with grounded input is about half of that without cascade transistors.  The reason is that the filtering of the transistors at the base is much better and thus the jfets is supplied with a more stable supply with lower noise level.  Here is the measurement comparison.

Notes for F4 up-graders:  For the MUR3020W, I have no pre-tap holes for them and thus I use a L-Bar to fix the 3 power mosfet and MUR3020W as shown.

I have built the third set with 32V supply as suggested by Nelson Pass.  Here is the measurement of distortion vs Power Output at 8.2 ohm load.  With higher supply voltage, the distortion is lower at 0.004% 1W.  The non-cliped output is about 50W.  For this measurement, the bias is about 1.6A to 1.7A  for each channel.  The conclusion is that the distortion is lower at 32V and higher at 22V supply.  With no surprise, this set sound the best after adding the Solen 1mH coil in  the power supply (CLC) filter.

RX-V8 Interface Board from “Amanero USB-I2S module” to FDA-1B or FDA-2A DAC

November 1st, 2012
The interface board is used to connect the Amanero Combo384 (http://www.amanero.com/) USB-I2S module to FDA-1B or FDA2A with TOTAL Isolation of the power supply and ground for both I2S bus and control signals.
With this interface board and Amanero Combo384 (buy separately from Amanero), FDA-1B and FDA-2A will have a input options of USB with 24 bit/ 192k Sample frequency capability. 

Price is as below:

1. PCB only: HK$100 (about US$13).  (BOM and Circuit will be provided after order)

2. Fully Assembled board (please specify FDA-1B or FDA-2A use): HK$400 (about US$50).

3. Shipping cost will be HK$31 and paypal fee add 5% on top. This will be waived if brought with the DAC kit FDA-1B or FDA-2A.


Jitter measurement with FDA-2A DAC and the result is EXCELLENT:

RX-V8 Jitter