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Randall RG100HT Weak Channel 1

Started by Randy Lahey, May 16, 2024, 12:20:37 PM

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Randy Lahey

Hi all,
I have a Randall RG100HT amp head that has a really weak/quiet Channel 1 (not Input 1).  Channel 2 sounds great and is loud as I would expect for a ~100W amp.  When using Channel 1, I have to turn the gain and volume up (past 12 noon) to get a decent signal and even then, there's not much definition to the audio and it is somewhat muffled.

I have reviewed the schematic, confirmed component values, checked DC voltages within the Channel 1 circuit, and ran a signal through Channel 1 and scoped it along the audio path.  I have 24v DC along the DC rail and have good AC audio signal up to the gate of Q2.  All components are within spec and I have replaced Q2 with a new 2N5484 transistor.  Attached is the schematic for Channel 1 and I have annotated it (in blue) with measured DC voltages.  The DC voltages for Q1 are within spec but those at Q2 and Q3 seem to support the notion that something isn't right at either Q2 or Q3.

I'm more familiar with tube amps and not SS amps so I may be not recognizing obvious solutions - so I'm putting this problem to the forum.  What recommendations do you have as a solution to the weak audio in Channel 1?https://photos.google.com/photo/AF1QipM9NMuHgN6irIVMYvnkxXqrwgm2pGozITmv5MDl

g1

The link you posted is not working.  Try to attach your picture directly.  When you respond to this post, click the 'preview' button and you will see the option to attach a file.

Randy Lahey


g1

Since you already replaced Q2, next suspects would be C7 or Q3.

Loudthud

Q2 is not conducting any current. Were Voltages measured on the resistors or directly on Q2's pins ? Verify that the Source of Q2 is connected to R9 and that the Drain is connected to R8. Did you install Q2 facing the correct direction ? Did you Ohm-check Q2 before installing ? Did you Ohm-check the bad Q2 ?

Randy Lahey

I measured voltages both at the transistor and at the dropping resistors.  The pinout of the new transistor is different than the original and I made sure it was installed correctly.  I confirmed resistances on the transistors but will report the values here later today.

Umlaut

#6
If R9 is not connected to ground and/or Q2 souce, you will probably get 24V at Q2 drain. also Q3 could be fried and sending its drain voltage back into Q2's drain.

Loudthud

Were the Voltages in post #2 before or after you changed Q2 ?

You didn't say that you Ohm-checked the new or old Q2. Do you know how ? There is no reason for the pin-out to be different if the new part in an actual 2N5484. Is it a Chinese fake ?

Randy Lahey

#8
Attached is the data sheet for the 2N5484 transistors I got from Mouser.  On the flat side of the original transistors, from top to bottom, the pins go Drain, Source, Gate.  On the new 2N5484 transistors, from top to bottom, pins go Gate, Drain, Source.  The original part was actually a TIS-58 but indicated on the schematic that 2N5484 could be used as well.
I've also attached a schematic with DC voltages measured with the gain pot at zero as well as at max.  These were all measured with a 1V AC 500 hz signal injected at the input jack.  I can also provide voltages of the AC signal throughout the circuit if that helps.
I should also mention that Q2 and Q3 have been swapped with new transistors.  I've also replaced C4, R7, C7, R8, and removed C5 and C6 for the meantime.
All voltages have been measured after all the new parts have been installed.  I've also verified conductivity of all the traces in this part of the circuit.

Randy Lahey

Transistor pin resistance measurements:
Original Gate - Source: 550 ohms
Original Gate - Drain: 306 ohms
Original Drain - Source: 300 ohms

New Gate - Source: 11M ohms
New Gate - Drain: 320 ohms
New Drain - Source: 10M ohms

Loudthud

Quote from: Randy Lahey on May 19, 2024, 06:35:53 PMTransistor pin resistance measurements:
Original Gate - Source: 550 ohms
Original Gate - Drain: 306 ohms
Original Drain - Source: 300 ohms

New Gate - Source: 11M ohms
New Gate - Drain: 320 ohms
New Drain - Source: 10M ohms

The Gate should measure like a diode to Drain or Source. If you measure from Drain to Source you should see the ON Resistance of the JFET (a couple of hundred Ohms). If you short the Gate to one of your meter leads while you are measuring resistance from Drain to Source, the resistance will go down slightly on one lead and rise slightly on the other lead. Those readings of 10M or 11M are probably just the resistance of you fingers across the leads.

From your schematics with the Volume pot at min and max it looks like the JFET is installed with Gate and Source reversed. It may pass a little signal but will have a gain less than one.

See my comments in this thread: https://music-electronics-forum.com/forum/amplification/guitar-amps/build-your-amp/debugging-your-build/20115-please-help-me-out-debugging-my-ruby-bassman-amp
about how to Ohm-check a JFET out of circuit. Note that with the vast majority of JFETs, the Drain and Source are interchangeable.

Randy Lahey

Thanks so much Loudthud!  I swapped those JFET leads and we now have audio!  Thanks to everyone who contributed to this thread!

g1

Just for the record, I'm attaching the datasheet for the TIS58.  It's pinout does not match the interfet 2N5484, nor the on-semi or centralab 2N5484.  So that's 3 possible pinouts for the Randall Fet's.   :)