Author Topic: Need help to repair Hot Rod Deluxe transistor switching circuit EDIT SOLVED  (Read 1167 times)

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Offline Planobilly

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The issue: The "more drive" circuit is not working.

Three amp techs have looked at the amp without results. Link to schematic

I have replaced many of the components in this circuit.

Test point readings:
Per schematic TP 31 +1.37  +1.07  +1.07
Actual              TP 31 +1.36  +1.09  +1.09

Per schematic  TP 32  +.548  +9.76
Actual               TP 32 +.498  +9.78

Per schematic  TP 33 +16  -13.5  -13.9
Actual              TP 33 +16.37  -10.17  -10.17

Per schematic  TP34 -13.0  -5.46  -4.96
Actual              TP34 -11.88  -12.11 - 12.11

Per schematic   TP 35  -.54  -10.0
Actual               TP 35  -.51  -9.5

Per schematic   TP 36  -16  -16  +16
Actual               Tp 36  -16  -16  -16

I have verified all resistors and zener diodes and changed any that were out of value. I replaced the op amp which was a 4558 on the board from the MFG but shows a 4560 on the schematic. The only difference in the two op amps is the slew rate. I replaced the two JFETs and Q3 and Q4 transistors. The push button switches are working correctly.

It is obvious that the voltages at TP33, pin 7 of the op amp are not correct. TP34 is not correct, and the voltage at TP36 does not change to positive. 

I can provide any other measurements that may help.

Anyone have any ideas of what to do next????



EDIT...sorry about the will need to go to the link

2nd EDIT...the 4558 op amp that was in the amp would not work correctly. I installed a 5532 and that works. Other strange issue...the combo LEDs are not red and yellow but red and green which makes yellow when both are lit and shows red when the green LED is turned off.

« Last Edit: February 19, 2017, 08:07:58 pm by Planobilly »

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