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1980’s Randall RG80-112 SC

Started by Luthier Vandros, March 02, 2023, 10:17:16 AM

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Luthier Vandros

Hi, all. Long time lurker/first time caller.

I've got the aforementioned amp on the bench and I'm knee-deep in the doo-doo, as it were. I generally don't take on vintage SS amps, but a good customer asked for help.

The amp arrived with a dead red channel that wasn't passing signal. All voltages check out after replacing a few drifted resistors, so I removed the three transistors and replaced them with a set of 2N5484's from an eBay seller. Obviously there are issues with counterfeit obsolete parts and maybe I fell victim. I then tried a set of BF245's with the same result, except it passed the signal almost inaudibly. I made sure to reorient the legs to respect the pinout. Thankfully, I've been refunded on both occasions, but I'm giving this one last try with a more generic, albeit branded, transistor — 2N3819's. General purpose parts should do the trick, but if these fail I'm at a loss.

I don't have an o-scope, but an audio probe/sig-gen that I use for this type of troubleshooting, but solid state gear isn't in my wheelhouse. From time to time, I'll take these on to help financially strapped customers that can't afford to have their gear go down. Can anyone weigh in with any direction should these 3819's fail to resolve the issue?

Much appreciated!

LV

g1

Because these are Fet transistors, they can be a lot more finicky about being exact replacements, mostly with regard to biasing.
If the BF245's made a difference, it's possible something is off in the biasing and they were closer to getting turned on.
But also possible there is a bad connection somewhere that got jostled during the repair process and the part difference is not the issue.
You have a bunch of fets of the same type in there, so you can test them one at a time using the other channel, and swapping the questionable ones into Q4 position.

please confirm attached schematic is correct and same one you are using:
You cannot view this attachment.

Loudthud

Note the DC Voltages in the ovals on the Green channel. Try to get the same Voltages on the Red channel. Chances are the Drain Voltage is way too low. Increase the resistance of the resistor between the Source and ground, R22 or R26 to try to get the Drain Voltage into the right range.

There is no protection for input JFETs Q1 and Q5. To add protection, connect a SIGNAL diode like a 1N914, 1N4148 or 1N4152 between the Gates of Q1 and Q5 and the -12V source point D. Connect the end with the band (Cathode) to the Gate and the Anode to -12V.

Note: It's a good practice to always plug the guitar cable into the guitar (or effects board) first, then plug the cable into the amp. If you hear that loud BUZZZZ! out of the amp, you're doing it wrong. Turning the amp off or the Volume down won't protect the JFETs you can kill them even when the power is off.

Luthier Vandros

Sorry for the delay; not getting notifications. That is the correct schematic. I really appreciate the comments and once the new silicon is in, I'll try to align voltages between green and red, along with biasing these little jerks.

g1

Quote from: Luthier Vandros on March 04, 2023, 12:31:48 PMonce the new silicon is in, I'll try to align voltages between green and red, along with biasing these little jerks.
On another forum J202 fet was working as a drop-in replacement for the 2N5485/TIS58 with no adjustment required.

Luthier Vandros

#5
Quote from: g1 on March 04, 2023, 06:35:33 PM
Quote from: Luthier Vandros on March 04, 2023, 12:31:48 PMonce the new silicon is in, I'll try to align voltages between green and red, along with biasing these little jerks.
On another forum J202 fet was working as a drop-in replacement for the 2N5485/TIS58 with no adjustment required.

TY, G1. I found a broken trace under the board. The 3819's are passing signal, but cery attenuated compared to the green channel. I imagine I'll have to tweak the drain resistors. Is there a rule of thumb? I have a solid 24v on the source rail (thanks to a 24v zener. More on that below.

Btw the schematic posted must be of a more recent model. My circuit board has several variances, such as no 24v zener off the E supply. Instead that tap is supplying 34v. Both the cap and dropping resistor have been replaced, so as a test, I tacked in a 24v zener. 

Cheers