Steve's Corner - Common Amp Failures
The two amp systems that were most common in the Rhodes Suitcase models were the 80 Watt (Peterson) version and the 100 Watt (Haigler) version. The 80W system was used during the early- to mid-70's, after which the 100W version replaced it when Delco decided to stop making the germanium power transistors (120725 or 020725). Both systems were very reliable, but each did have their common failure modes. Of the two systems, the 100W version was extremely reliable, with any component failure being very rare.
80 Watt System
The most common point of failure in the preamp is a broken filament in one or both of the light bulbs used in the Vibrato circuit. Remove the chassis cover and you will find these bulbs located inside the aluminum covers that are soldered to the PCB. Turn the preamp upside down and turn the system on. You should see both bulbs turned on, even through the PCB. If not, turn the system off, then remove the covers and unplug the bad bulb(s). Check the filament: it should be broken. These bulbs were standard GE #19 (or just #19) bulbs. They are still common and are available from electronics supply stores. You can also get them from Newark or RadioShack.com (not the stores).
Occasionally the output capacitors (5mfd non-polarized) or a small signal transistor (2N3391A) would also fail.
The most common power amp failure was in the output transistors, especially since they were germanium, which has a higher leakage then silicone. Their failure is usually complete, and it takes out the .5 ohm resistors and some other resistors. The damage can be so bad as to burn the PCB. But all is repairable if the driver transformer is still good (only damaged in extreme cases). The outputs had Fender's part number on them (120725 or 020725): their common number was DTG110B (listed in substitution manuals). They were always matched pairs, and if you don't use a new matched pair (unfortunately, very expensive) there is a risk of an imbalance and a future early failure. Also, it is advisable to replace all of the resistors (6 ea.) in the output section, even if they read within tolerance (especially the 820 2W resistors). .47 ohm, 5W resistors can be used to replace the .5 ohm, 5W versions.
When the amp is repaired and working, check for crossover distortion. If any exists, change the 820 resistors to 680 2W and re-check.
100 Watt System
The most common preamp failure (even then it was rare) was in the 4558 IC's.
The most common power amp failure was related to connections between the PCB and the harnesses. The male connectors that soldered into the PCB had frequent cold solder joints. Re-solder all connectors.
The female pins inside the connector body were crimp fits and sometimes came loose. Gently tug on each of the wires with needle nose pliers and see if any come out. If one does, remove the terminal from the connector body and re-solder the wire into the terminal. Then re-insert it back into the connector body.
More Info
- Chapter 11: Diagrams, Schematics & Pictorials