Tektronix TDS 340A Oscilloscope Repair

Tektronix TDS 340A Oscilloscope after repair and cleaning

Repairing a dead Tektronix TDS 340A Oscilloscope and mak­ing some modifications.

I have been in need of a gen­er­al pur­pose two chan­nel oscil­lo­scope since sell­ing my Tektronix 2430A sev­er­al years ago.
I came across this TDS340A on Ebay with a start­ing bid of $25.00 USD. Description stat­ed “For Parts and/or not work­ing and is sold strict­ly in “AS-IS” con­di­tion”, it also stat­ed in bold print “Does NOT Power ON”. The ship­ping was rea­son­able, and I was the only bidder.

The Tektronix TDS 340A is a dual chan­nel 100 MHz Digital Real-Time™ Oscilloscope with a 8 bit 500 MS/s sam­ple rate. The spec­i­fi­ca­tions state that it sam­ples at five times it’s band­width. The unit I received also includ­ed Option 14, the com­mu­ni­ca­tions option, which includes GPIB and RS-232‑C inter­faces, 9‑pin VGA mon­i­tor out­put, and a Centronics-type par­al­lel print­er port. The dis­play uti­lizes a 6–5/8″ diag­o­nal green mono­chrome CRT monitor.

I noticed in the sell­ers pho­tos that it was miss­ing it’s flop­py dri­ve, which was not a prob­lem for me as I did­n’t plan to be using that func­tion of the scope.
The unit arrived in great shape and was well packed. It had many years of dirt, dust, and grime, along with the usu­al scrapes, stick­ers, and stick­er residue includ­ed. It was sur­pris­ing­ly clean on the inside with a very light coat­ing of fine dust.

The ser­vice man­u­al is down­load­able from Tektronix, and pro­vides basic trou­bleshoot­ing infor­ma­tion, cir­cuit descrip­tions, and parts lists. The trou­bleshoot­ing sec­tion almost imme­di­ate­ly sent me to the pow­er sup­ply sec­tion. There it had me check for 1 to 1.5V on pin 13 of J1 the main pow­er con­nec­tor. I checked it and was read­ing 0 volts.
This is the low volt­age sig­nal from the stand­by pow­er sec­tion that pass­es thru the oscil­lo­scope front pan­el ON/STBY switch to turn on the main pow­er sup­ply sec­tion.
The next step said to replace the Low Voltage Power Supply “LVPS”. It was at this point that the Tektronix man­u­al stopped being use­ful. It includ­ed schemat­ics for the rest of the scope, but noth­ing for the pow­er sup­ply.
The pow­er sup­ply is easy to remove by dis­con­nect­ing 4 cables and two Torx screws. Once out of the unit I dis­con­nect­ed the three Faston con­nec­tors to the pow­er input fil­ter, and 7 Torx screws attach­ing the cir­cuit board to the met­al frame. Upon care­ful­ly remov­ing the board I imme­di­ate­ly dis­charged all the high-volt­age capac­i­tors before proceeding.

Backside of Tektronix TDS 340A LVPS pow­er sup­ply board

I did­n’t see any­thing sus­pi­cious on the front side of the board, but while dis­charg­ing capac­i­tors I noticed an over­heat­ed area on the back side which turned out to be the loca­tion of a ceram­ic pow­er resis­tor. I per­formed some basic checks to ver­i­fy that the DC high volt­age was being gen­er­at­ed in the rec­ti­fi­ca­tion sec­tion, and every­thing was as it should be. Next after dis­charg­ing the capac­i­tors again, I checked the resis­tance of the pow­er resis­tor I had not­ed ear­li­er. It read greater than 20 Meg ohms.
After de-sol­der­ing the resis­tor I noticed that it was sup­posed to be a 3.9 K ohm resis­tor rat­ed at 3 watts. A quick check of my high­er wattage resis­tors yield­ed noth­ing close to 3.9K ohms, so I decid­ed to get creative.

Original resis­tor, tem­po­rary resis­tor, and replaced capacitors

I built up a 2.5 watt 3.9K resis­tor using 6 each 33K, and 4 each 56K resis­tors in par­al­lel. After installing the tem­po­rary resis­tor, reassem­bling the pow­er sup­ply, and plac­ing it back in the unit, I now had 1.3 volts on pin 13 of J1.

Temporary resis­tor installed in Tektronix TDS 340A pow­er sup­ply LVPS
Tektronix TDS 340A LVPS par­tial­ly reassem­bled with tem­po­rary resistor

After press­ing the front pan­el pow­er but­ton the fan imme­di­ate­ly spun up, and the front pan­el LED indi­ca­tors lit-up. Shortly after that I noticed the CRT dis­play with sev­er­al pat­terns that changed dur­ing the start­up self-test. After a few more sec­onds of relay click­ing I was pre­sent­ed with a Tektronix logo and “Power-On self test PASSED”.
It was time to place an order from Mouser Electronics for a prop­er resis­tor. I end­ed up with an Ohmite TWM7J3K9E met­al oxide 3.9K ohm 7W 5% Radial resis­tor.
After the resis­tor arrived and was replaced I also replaced a cou­ple of elec­trolyt­ic capac­i­tors, one of which was very close to the failed resis­tor so may have been sub­ject to high­er than nor­mal heat for a extend­ed peri­od of time. After a quick check of sev­er­al oth­er capac­i­tors and the big bulk rec­ti­fi­ca­tion capac­i­tor I decid­ed to replace them as well.

Tektronix TDS 340A pow­er sup­ply and CRT section
TDS 340A lock­ing fan con­nec­tor adap­tor for 4‑pin Noctua fan

Even after a thor­ough clean­ing of the orig­i­nal 120x25mm NMB fan, it was still very noisy, so I decid­ed to replace it with a Noctura NF-F12 PWM 4‑Pin fan I had on hand. I made a small adap­tor to go from the lock­ing two pin con­nec­tor on the pow­er sup­ply to the 4‑pin con­nec­tor for the fan. Wiring the fan yel­low con­duc­tor to +12 volts, and the fan black con­duc­tor to 0 volts/ground. It made a sig­nif­i­cant improve­ment in sound lev­el, but I still would not say it is qui­et though.

Tektronix TDS 340A ana­log and dig­i­tal sec­tion with Option 14 con­nec­tor left
Dallas DS1644-120 Timekeeping RAM mod­ule in TDS 340A

One issue that I am not quite ready to tack­le just yet is the Dallas Semiconductor bat­tery backed SRAM and real-time clock mod­ule. It is used to store cal­i­bra­tion con­stants along with date and time. The device con­tains a small bat­tery that main­tains the stored data inside the mod­ule, and has a lim­it­ed life­time before the non-replace­able bat­tery fails. The mod­ule is obso­lete and no longer avail­able, but there are sev­er­al workarounds for it.

Tektronix TDS 340A flop­py-dri­ve slot 3D print­ed plug
Tektronix TDS 340A com­part­ment slot 3D print­ed panel

As I men­tioned ear­li­er the unit was miss­ing it’s flop­py dri­ve leav­ing a large open­ing in the front pan­el, it was also miss­ing it’s car­ry­ing han­dle leav­ing more open­ings in the side pan­el. So I decid­ed to 3D print some plugs for those openings.

TDS 340A side pan­el han­dle 3D print­ed plug

I 3D print­ed these on my Prusa Mini print­er using PLA fil­a­ment. I also includ­ed a small flat pan­el to restrict air­flow between case sec­tions. The blue col­ored plug for the han­dle open­ing was print­ed using Printed Solid — Jessie PLA Pure Cyan filament.

Without the flop­py dri­ve the full diag­nos­tic test gen­er­ates a Cpu sec­tion FAILED error stat­ing a diagCpu_floppyDrive prob­lem occurred, which I assume means it could­n’t com­mu­ni­cate or access the flop­py since it was miss­ing.
After a 45 minute warmup I ran the Signal Path Compensation SPC util­i­ty from the Cal menu which ran with no issues.
It looks like I can add a work­ing scope to my bench again.

Tektronix TDS 340A oper­at­ing with probe com­pen­sa­tion square wave and 1KHz sine inputs
Tektronix TDS340A Option 14 com­mu­ni­ca­tions board, VGA con­nec­tor, and print­er pow­er supply

I came across a inter­est­ing web­site while research­ing Tektronix pow­er sup­ply issues, by ToughDev on using the RS-232 port to retrieve raw sig­nal data. They also includ­ed a PC inter­face util­i­ty using a .NET appli­ca­tion that allows the user to retrieve the fre­quen­cy mea­sure­ments, wave­form data as well as tak­ing a screen­shot of the oscilloscope.

Tektronix TDS 340A screen­shot using util­i­ty by ToughDev via RS232
Tektronix TDS 340A mon­i­tor view of 1 KHz waveforms

The next project for this scope is to work on the Dallas RAM/RTC mod­ule hope­ful­ly before it com­plete­ly fails. 

STL file for 3D print TektronixHandlePlug.stl

STL file for 3D print FloppyPlug3.stl

6 Replies to “Tektronix TDS 340A Oscilloscope Repair”

  1. I pur­chased two of these on EBay. One works fine but the oth­er had a failed DS1644. I got the upgrade mod­ule from an EBay sell­er and installed it. It works much bet­ter than the one with the stan­dard DS1644 so I’ll be upgrad­ing the work­ing one as well. I had a dif­fi­cult time remov­ing the DS1644 about 3–4 pins would­n’t des­ol­der so I even­tu­al­ly removed it with a Dremel by sections.

    1. @Chris : Can I ask what part you used for the replace­ment of the DS1644 ? And while I’m at it : where did you source the new part from ?
      I’ve had a look around for sim­i­lar chips and ebay had list­ings for a few bucks and mouser had them for ~30$ so I’m won­der­ing if the ebay list­ings just sell fake parts or not …

      1. Hi Albertino and Chris,
        The one I have been look­ing at on Ebay is this one:
        https://www.ebay.com/itm/303821132831?hash=item46bd26941f:g:RJQAAOSwif9f4gjO

        I was able to remove my DS1644 mod­ule with no issues using a Hakko FR-301 de-sol­der­ing gun.
        Before pur­chas­ing the Ebay unit, I am going to try out a new Maxim DS1230AB instead, which does­n’t have the RTC.
        I will update once the new part is pro­grammed and installed.

        Greg (Barbouri)

  2. I’m about to try replac­ing the DS1644 on mine as well. If you have an tips on how to read from the DS1644, they’d be much appre­ci­at­ed! Mine still has the cal­i­bra­tion con­stants and I’m real­ly hop­ing I don’t lose them. I did pick up a TL866II Plus pro­gram­mer to try to read it once I get around to de-sol­der­ing it.

    1. Hi Andrew,
      I read my DS1644 using a TL866II Plus pro­gram­mer and used the DS1230AB device type, which should be iden­ti­cal except for the RTC.
      The address range was 0000 to 7FFF, and the last 7 bytes of the read data had the hour, min­utes, sec­onds, day, date, month, and year.
      One unver­i­fied tip I’ve seen sug­gests let­ting the DS1644 cool down to room tem­per­a­ture after de-sol­der­ing, before read­ing the data.

      Good luck,
      Greg (Barbouri)

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