argon39948 Messages: 2 Registered: December 2013 Location: United States
Junior Member
Working on Kustom amp with PC5129 ver 7? This version uses a 1458 instead of a 739 op amp. Found schematics and layout for the 739 version but nothing for this one. Wondering if anyone can help.
stevem Messages: 4728 Registered: June 2004 Location: NY
Senior Member
If it uses a 1458 chip in stock form then use that type, they can still be had.
I do not have that schematic for that PC number so I can not comment otherwise.
argon39948 Messages: 2 Registered: December 2013 Location: United States
Junior Member
Very noisy with reverb turned up and very low gain. Circuit board had one resistor missing. That resistor is connected to pin 6 of what I think is IC1 (1458). I got some better gain with a 5.6K installed.
Amp also seems to boost low frequencies. Have to turn bass control all the way down.
This board is a single channel preamp with trem and reverb. It uses the later design high impedance input stage with a Mosfet driven tone control stack that is very similar to a Fender tube amp tone control section. I suppose that they went to this circuit because the settings would be familiar to anyone that had used a Fender amp before with its all passive cut controls.
The schematic that I have shows 3 ICs, 2 dual opamps (5558 & 739) and 1 transconductance opamp (3080). The 739 and 3080 are used for the audio stages, and the 5558 is used for the reverb driver and the trem oscillator. The reverb return circuit is a single NPN transistor gain stage.
I don't know what the resistor you added does, so I can't comment there. If the reverb circuit makes noise, check the transistor that connects to the output of the reverb pan.
stevem Messages: 4728 Registered: June 2004 Location: NY
Senior Member
Electrolytic cap C18 may have drifted way high in value making for far too much bass response, in fact the age of all of these caps within the preamp could be the root of some problems.
When you state low gain is this in regards to the reverb level, or the over all gain of the preamp?
stevem Messages: 4728 Registered: June 2004 Location: NY
Senior Member
Can you take a resistance measurement of the input and output side of the reverb as it may have been replaced and not the right one for the circuit, in which case with the pan unplugged and the cable that plugs into the pans output side shorted is the noise still there?