EHT Unit

Wait a minute! Why are you 'fixing' the EHT block? OK, the speaker is old, but check the voltages first. You will need an electrostatic voltmeter - fiendishly expensive - or a high voltage probe such as technicians use to measure the HT circuits in televisions and other CRT devices  - See Parts Section

The use of a high voltage probe has some caveats, but is the only device you are readily likely to lay hands on to measure these voltages at the minute currents present. Click image below to see larger EHT wiring image.


EHT Checks:

To check the specification of the EHT, do the following:

  1. Disconnect the thick red wire and the thin red wire (as a minimum) from the EHT block. This is described more fully in the Disassembly... Section.
  2. The EHT must now be plugged into the mains for this test, and allowed to charge for 5 to 10 minutes. The "low" voltage mains terminals that are now exposed carry up to 610 Volts, with sufficient current to be fatal!
  3. Do check the 610 and 590 Volt tags on the secondary of the mains transformer, making sure that they are within a few % of the stated values. If these voltages are "off" then everything else that depends on them will clearly be amiss also.
  4. Connect the high voltage probe to a standard multimeter (10MW input) as directed by the manufacturer of the probe. Ground/Earth the probe to the same earth point as the Mains Transformer, (clip to the terminal where the THIN black wires connect to the block).
  5. Set your DVM to 'auto ranging' or to a high range. The probe manufacturer will have indicated the scaling factor when connected to a standard 10MW input, (usually 1000:1).
  6. Touch the HT probe to the terminal post on the EHT block where the thick red wire was connected. The reading won't be 6kV +/- 7%, that is between 5 580 Volts and 6 420 Volts!! Why not? The HT probes we generally use in the absence of a true electrostatic voltmeter do not have sufficiently high input resistance, and too much current flows. Consequently the voltage drops and you do not measure 6kV out of a "good" EHT block. A "good" rectifier block with no panels attached will read around 3.9 to 4kV with a HT probe. With good bass panels connected, the reading will rise to 4.3 to 5kV. The resistance of the panels is now in circuit. If you get a reading lower than 3.6kV then the panels are likely leaky, in which case they'll probably hum anyway. Take note of the readings immediately after contact is made, as it will tend to drain the supply very quickly. Remember, these probes do drain significant current (mA) in this context. Another caveat is that a faulty supply can give a high reading due to an open circuit component in the multiplier chain -usually a blown capacitor - no, they are not "good for life" as some authors surmise. { Ron Best of One Thing Audio, Coventry, England reports that he has seen faulty EHT units measure as high as 10.6kV (!) with a true electrostatic voltmeter. Thanks to Ron for the above servicing details also. If you live in England, he is the man to see to get your Quads serviced.}
  7. Similarly, touch the terminal post to which the thin red wire was connected. The reading should be 1.5kV +/- 7%, that is, between 1 395 Volts and 1 605 Volts. Again, your meter, connected to the 1000:1 probe should be showing 1.4 to 1.6 with the probe in circuit.

If these measurements fall within the ranges stated, then there should be no real need to disassemble the EHT block any further, and it can be put into service, as is. If any under-voltage conditions are noted when the panels are connected, then leakage from the panels has to be suspected. This leakage will manifest itself as either visible corona on terminals, or leakage to the frame from the stators themselves. In the case of bad leakage, the panels will hum at about the mains frequency.

If these measurements are under the minimum, then it is time to rebuild the EHT block. The low voltage is likely being caused by the breakdown of the diodes used in the ladder circuit in this block. The capacitors will likely be OK, but you can replace those too, if you like.

If the speaker is really old (50's, 60's, early 70's) then it will very likely have a single epoxy block rectifier fitted. If this is under specification, you can only throw it away, unfortunately. It is possible to copy the circuit on a small piece of breadboard, insulate it with anti-corona lacquer and remount this. Alternatively, you can buy Sheldon Stoke's circuit boards, or make your own, if you're keen enough.


EHT Rebuild:

If the speaker is of later vintage, the EHT block will be composed mainly of a small box made of phenolic plastic "board", such as used in the 'old days' for circuit boards. This box is filled with bees wax, to prevent high voltage leakage to the atmosphere (corona). This type of unit can be completely serviced, as follows:

  1. Detach the EHT block from the mains transformer cage. This requires the removal of two small screws that screw into the block from the mains transformer side.
  2. Place the EHT block in a small metal pan. The pan should be just large enough to hold the block lying on its side, so that when we melt the wax out, we won't have to scrape a thin layer of the stuff out of the pan. It is also easier to re-melt the wax, if you have to, in a small pan.
  3. Melt the wax out with a heat gun. I use a variable 1600 Watt gun, set to about half way. Some folks advocate placing the whole thing in an oven. This has four problems: you cannot see what is going on; you cannot control the heat as easily; you can burn the plastic box easily; and, it takes longer, and costs more. If you are applying too much heat with a heat gun, it is a simple matter to move the gun a little farther away. The big plus is that you can see what is happening, and are less likely to cook (literally) the small circuit board.
  4. Remove the small circuit board with pliers, or tongs!! IT'S HOT!!
  5. Allow the unit to cool down, and examine all the parts. You will see two resistors, 16 diodes, and 8 x 3kV x 0.01m F capacitors.
  6. Re-wire the circuit with new parts. Generally, 2 Watt metal film resistors are more than adequate (330kW and 2.2MW), along with 1N4007 diodes, and capacitors rated for 3kV (or higher) will do fine (Area 51). You will see that no finesse was wasted on some of the originals when it came to soldering either! Now, although I have not experienced failures of paired 1N4007 (1kV) diodes in this application (yet), Ron Best, One Thing Audio, Coventry, England, advises that he has seen numerous failures of this particular part in this application over the years. So, to be really safe, and long-term reliable, replace the diodes with either a 3kV (or higher rating) diode for each pair that is in the EHT standard, or, use pairs of 2kV rated diodes to replace those that are in there. The failure of the 1N4007 is more likely at the “top” of the ladder where the voltages are higher.
  7. Spray the completed board with circuit board lacquer (acrylic) or corona dope - pretty much the same thing - but do read the labels.
  8. Place the circuit board back in the small phenolic box. It is a good idea to check the physical integrity of the box, and re-glue the corners if required.
  9. Re-melt the wax, and some spare bees wax, if you have it. (Hint: Saddle makers, and leather workers keep bees wax).
  10. Pour the wax back into the box quickly, so that the bottom layers have no real chance to set before most of the wax is in. Watch it! A hot wax scald is pretty painful - he said - having experienced a few.

OK, that's really about it. Re-fit the EHT block to the mains transformer cage, unless you have that disassembled for work on the neon lamp, or voltage adjustment circuit, or you want to re-spray the cage metal in a tasteful hammer crackle gray.