Troubleshooting Hints and Asking for Repair Help

This is intended to tell electronics novices what information they should provide when asking members of the Ampex Mailing List for troubleshooting and repair help. It was adapted by Howard Sanner from posts by Rick Chinn and Jay McKnight, with further revisions by the original authors.

Giving as much of the information below as possible with your initial post will provide a lot of clues about what is wrong, and save everyone the time and trouble of requesting it.

General Guidelines

• For those used to transistor equipment and now starting on vacuum tube gear, remember that tube circuits have high voltages (up to 500 V on Ampexes) that can--at the least--give you a nasty jolt, and at worst kill you.
• For almost any service work, you need either the manufacturer's manual or an equivalent such as the Sams "PhotoFact." Schematics and manuals for many Ampex models are available on this web site.
• Tell us the make and model of the unit, what you expected to happen, what did happen, and what you measured.
• Tell us what test equipment you have available. It is pointless to suggest a test you don't have the gear to perform.

Measurements to Make

• Measure the DC voltages. If you can, try to localize the trouble to a portion of the circuit. You don't have to measure every node; the critical ones are the power supply voltages, and the emitter, base, and collector voltages of the transistors involved, or the plate, grid, and cathode voltages of the tubes.
• Measure the AC signal voltages. One can often do useful signal-tracing even without having the "normal" values in the schematic. That is, the output voltage of an amplifier is usually greater than the input voltage; and the input voltage of a stage is usually the same as the output voltage of the preceding stage. The exceptions are emitter-followers, where the output comes from the emitter, or cathode-followers, where the output comes from the cathode.

If the schematic is not available on the list's web site, but you have a schematic, please list the voltages called out in the schematic along with those you measured.

Some Brief Hints

• In transistorized equipment, if two or more of transistor's pin voltages are the same, the part is bad.
• For Bipolar transistors (not FETs):
  the emitter should be about 0.5 to 0.7 V different than the base.
  The emitter (arrow) and collector voltages should be substantially different, unless the transistor is used as a switch in which case they may be nearly the same (switch on) or wildly different (switch off).
• For FETs, the lead in the middle with the arrow is the gate. It corresponds to the grid of a vacuum tube. The other two are the source (corresponds to a tube's cathode) and drain (corresponds to a tube's plate). The drain voltage is less than the source voltage. Bipolar power supplies may make this somewhat confusing.
• For tubes, with the usual positive plate supply, the plate is always many volts more positive than the cathode.
• If the cathode is grounded, then the grid is connected to a negative supply via the grid resistor.
• Sometimes (with triodes) the cathode is grounded and the grid resistor returned to ground. In this case, the grid resistor is several megohms.
• If you measure the grid voltage with respect to the cathode, the grid will be negative. Sometimes the grid is direct coupled to the preceeding stage (phase splitters and cathode followers), and the grid will be many volts above ground.

Finally, the DC voltages in vacuum tube equipment are only approximate; a measured voltage that is ±20% of the value in the schematic is usually "within normal limits." Even signal voltages in non-feedback amplifiers may be OK at ± 20%.