This is a collection of notes and information on how to repair these supplies. I happen to have and use a number of these power supplies from various vintages. Some of them were bought in working condition, some of them as-is. Some of them developed issues over the years and i had to repair them.
These supplies are extremely long life. they were originally designed in the late 80's and some models are still in production in 2022 today with Keysight badging !
The original E36xx range consists of 9 supplies.
More details can be found in the family specification datasheet
E3610A / E3611A / E3612A
Principle of operation
Power stage oddities
In a pinch you can put the 2N6055. It is essentially the same part but with a lower breakdown voltage. (Vceo is 60V instead of 80V). This has no impact on the power supply except its ability to hold external overvoltage. It has no impact on the E3612A since that one uses the IRF440 mosfet anyway.
The capacitors in this supply are under a lot of thermal stress. The used series suffers from seal degradation around the pins and over the years the capacitors start leaking and drying out. The leakage can damage the printed circuit board and dissolve the copper traces while a dried out capacitor loses value and the ripple will increase, especially under load.
- It is recommended to replace ALL electrolytic capacitors, EXCEPT the bulk capacitor. The bulk capacitor is a snap-in type and uses a different seal type around the pins that does not degrade.
- Remove C5 and C4. they are disc capacitors and can couple noise into the output
- Since we will change capacitors : change C12 into a 470uf 50v capacitor (was a 330uf 35V)
- Replace R10 with a 21.5 Ohm 1/4W 1% metal film resistor
- Replace C10 with a 6u8 35 Tantalum, for example AVX-Kyocera TAP685K035S
- Install a bleed resistor (R46 in newer schematic) across the output. The value depends on the power supply model. The resistor is installed between TP3 and TP4. The board layout actually has the perfect spot to drop this in place.
- Under certain conditions (capacitive loads) it is possible to drive the supply into oscillation. This can be mitigated by placing a small capacitor in parallel with R6 (200..300pF) to create a frequency roll-off in the gain of the system.
Missing button caps / broken switches
First perform all preventative repair, irrespective of problems, before diving into this section.
These power supplies are very robust and are hard to damage. Apart from the leaky capacitors there is very little that can go wrong and troubleshooting is very easy. Assuming the helper voltages are ok : If one of the control loop misbehaves it is one of the opamps that has failed, or, in case of current : the sense resistor. If both loops failed then the issue could be with Q3 or the power stage.
Cleaning up after leaky Capacitors
What is Electrolyte ?
- Ethylene glycol or Boric acid based : medium to high voltage capacitors up to 85degree C. contain about 20% water.
- Organic electrolytes such as Dimethylformamide (DMF), γ-Butyrolactone (GBL), and Dimethylacetamide (DMA). These are used in wide operating temperature parts up to 150°C. They have good long term properties and low leakage. They contain virtually no water.
- Water based electrolytes. Up to 80% water means it can carry a lot of salt ions to increase conductivity. This gives very low ESR and the ability to handle high ripple currents.
There are other types where a polymer is used but this is beyond what we are dealing with here.
Cleanup on isle capacitors
Component level troubleshooting
If you want to do component level troubleshooting without using the shotgun approach. Below is a walk through of what to check.
When performing measurements pay attention to where you place the negative probe of your meter. These supplies have multiple power domains and what is "ground" for one domain is not necessarily "ground" for another domain.
- Reverse voltage damage : in case a reverse polarity voltage has been applied externally the diode CD3 will most likely be damaged. It can fail open or shorted.
- Verify the power rails. Leaking capacitors can create enough of a load to blow the fuses F1 F2 and F3. These are 1 ampere solder-in Littelfuse parts.
- Measure between TP9 and TP10 to check the 5 volts for the displays. Check F3 if failing.
- Check the Reference supply.
- Measure between TP6 (GND) and TP1 for +12 volts
- Measure between TP6 (GND) and TP5 for +5 volt
- If both are missing : Fuse F2 is suspect.
- If +5volt is incorrect U3 may be broken (LM336BZ-5.0). Pay attention when ordering this part. There is a 2.5 volt version that is much more prevalent than the 5 volt version, especially eBay sellers typically only have the 2.5 volt version.
- Measure between TP6(GND) and TP8 for -12 volt. If missing F1 is suspect.
- Main rail :
- Check between TP4 (GND) and TP2 for the unregulated voltage. The voltage depends on the position of the range switch (S2A). S2 gets a pretty large "hit" when switching from low range to high range as C2 needs to be charged up. I have seen cases where S2 developed burned-in contacts.
- No output voltage
- Turn both voltage and current limit pots somewhere halfway.
- Check R35 and R36. Their value is dependent on model !
- Check that the node R4/R5 is not at, or close to, -12 volt. Measure from TP8(GND) to the R4/R5 node. You should be able to move the voltage up and down using the voltage adjust potentiometer.
- If the voltage is close to 0 volts (relative to TP8) the current error amplifier is most likely damaged.(make sure the current limit pot is somewhere halfway).
- Check the Current sense resistor R2 and both input clamp diodes CR10 and CR11.
- Check Cref. Measure between TP6 and TP7 and verify the current limit voltage can be adjusted with R19. Check potentiometer R19 (Front panel) and its wiring.
- Replace U4 if nothing else works.
- Check the R37 (front panel voltage potentiometer) operates properly.
- Check diode CR6
- Check diodes CR4 and CR5
- Replace U1 if all else fails.
- If the voltage at R4/R5 moves then the transistor Q2 or the power transistors are suspect.
Difficult to identify parts
Power resistors (wire wound)
Power and range switch
Current set switch
The following is a collection of datasheets for key components used in these power supplies. Some of these components are obsolete and the datasheets are sometimes hard to find.The original documentation does not really list the manufacturers of the parts.
As the original parts are going, or have gone, obsolete they need to be replaced with different but compatible parts.
New Part number
Nichicon LGY series 10.000uF
C3, C7, C13
Series 470uF 50V
Nichicon UPJ Series
C9, C15, C19
|Power transistor section is wrong (shows four MOSFET instead of Darlington)