Tektronix PS503A Repair

[All About Jake]

Hi Everyone.  Long Tim EEVBlog reader/watcher.  First time poster.

I was given a TM5006 mainframe many years ago.  It has sat collecting dust for a while, but I'm setting up my electronics bench and I figured that I'd set it up as well.

The mainframe has 3 modules, a PG 501, a DM 502A, and the PS503A in question in the high power slot.

Here's how its behaving:

• The 5v supply puts out 4.998v, so that seems to work
• The right supply puts out between 1.088 and 1.213v depending on the +v setting on the right knob.  The top dual tracking volts knob doesn't affect this output, just the right course/fine knob.  The LED for this supply is lit, and the current limit LED is off.
  
• The left supply puts out 0.732v consistently.  This is unaffected by ether the dual tracking volts knob at the top, or the left course/fine knob.  The left supply led is OFF, and the current limit LED is ON.

Given that these modules are only about $50 on eBay, I'm guessing its not worth investing too much time to repair, but I was wondering if anyone might have some experience with this power supply and could point me in the right direction.  I don't have any extension cables that would be needed to power this module outside of the mainframe probe around.  (The extension cable is more than a whole new PS503A.)

I figured that it wouldn't hurt to ask if there are any obvious things to check or typical component failures.  I don't mind spending a little time on this, but at a certain point, its just not worth too much investment.

Thanks for any pointers.
Jake

[alm]

The 5V output consists basically of an 7805 hanging directly off the DC rail from the mainframe, so there is very little to go wrong there.

Check if changing slots in the mainframe makes a difference. It uses the power transistors in the mainframe (unique to each slot) as pass transistors for the +/- 20 V. Also check any fuses in both the mainframe or the plug-in. Not all plug-ins use all voltage rails from the mainframe, so a rail could be down without all plugins failing to work. Check if someone used the optional feature of making connections between the rear terminals in the mainframe. It could be used to for example wire up a multimeter (like the DM502A) to monitor the output of a power supply, but could easily mess things up if you put things in different slots than designed.

I would also check for any shorted tantalum caps (check if any of the voltage rails measure low resistance, then remove any suspicious tantalum cap and measure). I assume you have the manual (pretty sure there are some decent scans available for free).

Whether repairing a $50 power supply is worth it depends on how you value your time and if you feel a repair would be enjoyable / educational. I do not think it contains many expensive parts, so a repair could easily be something like a µA741 or a cap.

Taking the panels of the mainframe may allow you sufficient access to measure the voltages in circuit. Either that or clip a bunch of wires to the plugin and then insert it and power it up. Jamma Boards used to have a cheap ($20?) kit for an extender, but looks like their website is currently down.

[David Hess]

The PS503A only uses the isolated 25VAC outputs and pass transistors from the mainframe for the tracking outputs so there is not much to check there.  The transistors can be bad of course and a short could have blown a trace on the mainframe's printed circuit board but that is unlikely.

There are two fuses on the back of the PS503A circuit board.  If the negative (left) side fuse was blown, it would let the negative output drift up to +0.7 volts and disable the reference so that could explain all of the symptoms you describe.

It has the usual assortment of solid tantalum capacitors which might be shorted.  In one of my PS503As, they were all replaced with small aluminum electrolytic capacitors.

On my two PS503As, I ended up replaced the big input capacitors, the output capacitors, and one of the bridge rectifiers.

They use those crummy Texas Instruments edge wipe IC sockets which can become intermittent.

[All About Jake]

Thanks for the tips.  Changing slots does not make a difference, other than lighting the 400mA limit when its in a non-high power slot, so that's expected.  Nothing wired up to the back. Both fuses are good.  The output rails seem to measure ~50kohm when off and in the megaohm range when on, so if its a shorted cap, then I guess I'll have to hunt for it a bit more. 

Not sure it makes a difference to troubleshooting, but the left side current limit light is lit even when the supply output switch is in the "off" position.

Extenders on eBay are more than a new module, but if Jamma Boards comes back online, maybe I'll invest the $25 it takes to get their kit.   Or maybe I'll just re-cap and hope for the best.

[David Hess]

Everything is still powered when the output switch is in the off position which pulls the tracking reference down to zero so the outputs should follow.

In some of the mainframes, the covers can be removed to access the plug-in for maintenance without using an extender.

If you replace any of the 741, use the 44 volt versions.  One of mine has 301As instead of 741s.

If a load was applied while an output transistor was open, then the resistor in series with the drive transistor collector will be open.  This might have happened on your negative output.

[All About Jake]

Thanks again. I'll ask a silly question, but that's the only way to learn.  I've heard that these use odd tantalum caps that go bad, but I only really see 1 capacitor that looks like it is tantalum cap, and that's C3 which is part of the 5v regulator circuit.  The rest seem to be radial electrolytic and ceramic caps.  Am I missing something, or are there tantalum caps hiding in radial or ceramic-looking capacitor packages?

Mine seems to have 2 socketed 301A's for  U55 and U155.  A socketed 741 for U45 and the rest are soldered 751's.    Also, there seem to be 3 ceramic caps soldered to the back, 2 33pF? (marked 33G) across the balance/compensation pins of the 301A's and another 47pF near U145 that I haven't traced.  These don't seem to be on the schematic.  Also they don't test well with my ESR meter and when I desolder them they don't read with a capacitance meter, so probably should replace them with new ceramics.

Also the values on my board don't seem to match.  A whole host of 4.7uF caps on the schematic are 10uF's on the board. (C34, C38, C138, C20, C120, etc)

Thanks again.

[David Hess]

Thanks again. I'll ask a silly question, but that's the only way to learn.  I've heard that these use odd tantalum caps that go bad, but I only really see 1 capacitor that looks like it is tantalum cap, and that's C3 which is part of the 5v regulator circuit.  The rest seem to be radial electrolytic and ceramic caps.  Am I missing something, or are there tantalum caps hiding in radial or ceramic-looking capacitor packages?

Mine seems to have 2 socketed 301A's for  U55 and U155.  A socketed 741 for U45 and the rest are soldered 751's.    Also, there seem to be 3 ceramic caps soldered to the back, 2 33pF? (marked 33G) across the balance/compensation pins of the 301A's and another 47pF near U145 that I haven't traced.  These don't seem to be on the schematic.  Also they don't test well with my ESR meter and when I desolder them they don't read with a capacitance meter, so probably should replace them with new ceramics.

It seems you also have one of the 301A versions of the PS503A with the solid tantalum capacitors replaced with aluminum electrolytic capacitors and three ceramic capacitors on the back of the board.  There is another circuit change on the front with an added resistor or capacitor.  I have never seen a specific schematic for this version but the standard schematic is good enough.

The 301A operational amplifier requires external compensation so that is what the small ceramic capacitor under each of them is for.  Internally compensated operational amplifiers like the 741 have this built in.  You will need to put them back for proper operation.

Most ESR meters are not going to work on such small values of capacitance and tiny ceramic disk capacitor hardly ever fail.

The only solid tantalum capacitor on mine is at the output of the LM340T5 (practically the same thing as a 7805) regulator.

I have not bothered to track down all of the differences in the versions but they are not significant.  I do not know why replaced two of the 741s with 301As; I found no engineering reason.

Also the values on my board don't seem to match.  A whole host of 4.7uF caps on the schematic are 10uF's on the board. (C34, C38, C138, C20, C120, etc)

An aluminum electrolytic capacitors replacing a solid solid tantalum capacitor needs 2 to 4 times more capacitance for an equivalent ESR especially at higher frequencies.

Except for the big input capacitors and small output capacitors, none of the capacitors will cause failure unless they are shorted which is unlikely with aluminum electrolytic capacitors.  I would start checking voltages before replacing them except for the output capacitors.

For what it is worth, the PS503A is my favorite power supply; the floating dual tracking ratiometric output is incredibly useful and I like the coarse and fine voltage controls.

[All About Jake]

Again, thanks.

My cheap china ESR meter shows 0.37ohms for the output caps.  (50uf, 50v). Not exactly sure what's right for this vintage cap, but they're both in the same range so I don't see one that's obviously different.

Have to see if the sides come off the mainframe, but maybe I can remove the other modules and clip from the sides from inside the open bays.

I put the caps back on the backside for the 301A's.  Will check some voltages and see what I can see.

[All About Jake]

OK fresh eyes this morning, and I think its obvious.  (See attached picture).

There's a blown trace just below the -24v adjustment potentiometer.

It seems like its one of the 25vac input lines into the rectifier for the negative supply.  This is before the rectifier and the fuse is after, so the fuse did not protect.

I can jump a wire from a via on this trace to the rectifier directly.  Probably easier than removing the pot and fixing the trace, and putting the pot back again.

I'm not sure why this particular trace would blow.  Should I just jump and and give it a go?  Or should I expect something else before I put power to the circuit?

[alm]

It seems unlikely to me that the trace would blow spontaneously. I would check for a short downstream of the blown traces (the rectifier seems the most likely suspect), and check if the fuse (F110) was not bridged or replaced with an incorrect value. Should be 1.5A fast-blow according to my manual.

Any other kind of value like a shorted pass transistor or the crowbar circuit somehow activating should blow the fuse (I tested this theory when I had one of the 741s oscillating, tripping the crowbar). I can not see the circuit around Q15/Q115 drawing sufficient current to blow a fairly beefy trace, unless all the resistors were shorted. The Q15/Q115 circuit uses both the positive and negative 33V rail to bias the transistors, so I could easily see why this failure would screw up both channels.

Any signs of other replaced components (like flux residue) that might indicate a previous repair attempt? It is possible somebody already fixed the cause but then found the blown trace.

After establishing that powering it up after the repair, I would power it up, check for visual indications of components heating up, and then be carefully about loading it until you have established that current limiting works correctly. Again, any issue like this should blow the fuse, but that is clearly not what happened previously .

[David Hess]

My cheap china ESR meter shows 0.37ohms for the output caps.  (50uf, 50v). Not exactly sure what's right for this vintage cap, but they're both in the same range so I don't see one that's obviously different.

Except for the input and output capacitors, none of the larger capacitors are needed for proper operation; they are there to filter noise.  0.37 ohms on the output capacitors should be fine.  I think mine were down to half capacitance and they still worked but I changed them anyway.

There's a blown trace just below the -24v adjustment potentiometer.

That is on the negative side.  As alm points out, loss of negative power will screw up both outputs.

It seems like its one of the 25vac input lines into the rectifier for the negative supply.  This is before the rectifier and the fuse is after, so the fuse did not protect.

The surge current would blow the fuse anyway if it was before the input capacitor.

I'm not sure why this particular trace would blow.  Should I just jump and and give it a go?  Or should I expect something else before I put power to the circuit?

You should check the input capacitor and bridge rectifier.  Nothing else could blow the trace without blowing the fuse first.

Any other kind of value like a shorted pass transistor or the crowbar circuit somehow activating should blow the fuse (I tested this theory when I had one of the 741s oscillating, tripping the crowbar).

There is a procedure in the manual for disabling the crowbars.  They can cause problems sometimes although with the current limit circuit operating properly, they should not blow the fuses when activated.

[All About Jake]

Well we have it, a shorted input cap on the negative side.  I desoldered the rectifier and found there still to be continuity across the capacitor.  So I removed the capacitor and found it to be shorted.  ESR meter doesn't help in this case.

I guess I should replace both the input and output caps while I'm in here.

If someone has a recommended replacement, I'd appreciate the recommendation.  For the output caps, I think a TE1307 will work nicely, its about the same size and specs. A few quick searches on Mouser and Digikey, however, don't show anything 4500uf @ 40vdc range that's in stock for the input cap. (Also I'd like them to be 1.365" diameter so they'll fit in the same clips, but I guess that's not strictly necessary.). Also, It'd be nice to find something that in pairs isn't more expensive than the whole power supply. . Perhaps this one?

Also any reason to change the rectifiers while I"m in here? (I've already got one out).  I think this is a good replacement: KBL02-E4/51. (4 amps rather than 2.5, same ~5mm lead spacing)

Thanks again.

[David Hess]

Well we have it, a shorted input cap on the negative side.  I desoldered the rectifier and found there still to be continuity across the capacitor.  So I removed the capacitor and found it to be shorted.  ESR meter doesn't help in this case.

Usually can style aluminum electrolytic capacitors fail open when the aged rubber seal allows water and oxygen in which corrodes the lead connections.

I guess I should replace both the input and output caps while I'm in here.

I would replace all of the input and output capacitors especially if you have to order parts.  The other aluminum electrolytic capacitors are not as critical.  Eventually I will replace them with solid tantalums and maybe make some improvements.

If someone has a recommended replacement, I'd appreciate the recommendation.  For the output caps, I think a TE1307 will work nicely, its about the same size and specs. A few quick searches on Mouser and Digikey, however, don't show anything 4500uf @ 40vdc range that's in stock for the input cap. (Also I'd like them to be 1.365" diameter so they'll fit in the same clips, but I guess that's not strictly necessary.). Also, It'd be nice to find something that in pairs isn't more expensive than the whole power supply. . Perhaps this one?

I used a snap-in style aluminum electrolytic and selected first based on diameter so it would fit in the existing clamps as shown in the photograph.  I used 18 gauge solid wire to extend the leads to the circuit board in a way identical to the original which serves to minimize ESR although it probably does not matter in this application.

The capacitance value does not need to be exact.  In retrospect I should have bought higher voltage capacitors so they would have been longer.  Any of these are suitable.

Also any reason to change the rectifiers while I"m in here? (I've already got one out).  I think this is a good replacement: KBL02-E4/51. (4 amps rather than 2.5, same ~5mm lead spacing)

I would change at least the bridge rectifier on the side that was shorted and if you are going to change one, then you might as well change both.  I just pulled one from a junked ATX power supply.

The photograph shows the size of the original capacitors for comparison purposes.  This is my older PG503A which has all 741s and solid tantalum capacitors instead of aluminum electrolytic capacitors.

[All About Jake]

Thanks for the picture and for everyone who helped in this thread.  I'll order some caps and get to work.  I'm glad I didn't just write this one off.  I'll report back after the repairs are complete.

Now I just need to find a button cap for the DM502A and invest in two more modules to fill out the mainframe.

[All About Jake]

Thanks to your help on this.  With your help, I'm back to a fully functional PS503A.

New input caps, new output caps, new rectifiers, and a jumper wire to complete the circuit around the burned trace.

For reference, in case someone else finds this thread someday, I used the following components:

• Bridge Rectifiers: KBL02-E4/51
• Input Caps: EKMH101VSN472MA50T snap in capacitors, 4700 uF, 100 VDC
• Output Caps: TVA1308-E3, 50 uF, 50 VDC
• 18awg solid wire for the jumpers and new leads for the caps

Pictures Attached.

Thanks again.