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May 20, 2024, 04:39:11 AM

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#1
Amplifier Discussion / Re: Randall RG100HT Weak Chann...
Last post by Loudthud - May 19, 2024, 09:29:38 PM
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.
#2
Amplifier Discussion / Re: Randall RG100HT Weak Chann...
Last post by Randy Lahey - May 19, 2024, 06:35:53 PM
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
#3
Amplifier Discussion / Re: Randall RG100HT Weak Chann...
Last post by Randy Lahey - May 19, 2024, 06:17:54 PM
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.
#4
Amplifier Discussion / Re: Peavey Bandit 65 transform...
Last post by Tassieviking - May 19, 2024, 03:24:03 PM
It would be better if you added your location to your personal info, there have been times when someone had one and was happy to send it if the recipient paid the postage.
I remember one time it turned out they lived in the same town so there was no need to send it in the mail.
#5
Amplifier Discussion / Re: Peavey Bandit 65 transform...
Last post by DrGonz78 - May 19, 2024, 02:58:50 PM
I mean from the description of what caused a failure of the amp we have little to go on. What is the status of the main transistors? We have no idea what traces you bridged. With so little information I personally can't say go buy a transformer. With more information then I might have more of an answer.
#6
Amplifier Discussion / Re: Peavey Bandit 65 transform...
Last post by DrGonz78 - May 19, 2024, 12:19:47 AM
What method did you use to make the conclusion that the transformer is shorted?
#7
Amplifier Discussion / Re: Marshall VS100R transistor...
Last post by g1 - May 18, 2024, 11:25:48 PM
T4 collector connects to T6 emitter, so not sure why you are showing different voltages for those pins. (T4 and T6 are BCE pinout)
#8
Amplifier Discussion / Re: Randall RG100HT Weak Chann...
Last post by Loudthud - May 18, 2024, 08:29:25 PM
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 ?
#9
Amplifier Discussion / Re: Peavey Bandit 65 transform...
Last post by g1 - May 18, 2024, 02:25:27 PM
The value of the amp has nothing to do with what parts cost.
If you can get another bandit 65 for 50 bucks, buy it and use it for parts.  You probably won't find an adequate transformer for that price.
Sorry if that sounds harsh, but that is the reality of parts costs.

You need a power transformer that puts out 60V center-tapped (30-0-30) good for at least 120VA.  If it's not the exact peavey unit, you will have to drill and cut to make it physically fit the chassis.
#10
Amplifier Discussion / Re: Randall RG100HT Weak Chann...
Last post by Umlaut - May 18, 2024, 02:24:26 PM
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.