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January 19, 2020, 05:09:00 PM

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Author Topic: 1988 Randall RG100ES  (Read 1013 times)

vance

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1988 Randall RG100ES
« on: December 26, 2019, 03:17:30 PM »
Picked up a randall rg100es from local pawn shop. someone had removed all 4 output tansistors. I bought replacements my issue is the board isnt labled at all for components. I installed the transistors and done what testing i could before i powered it up. Voltages seem ok from what i can make from the schematic. Im not getting any volume out the speaker outputs. I can hear it faintly and shows a weak signal on scope. It works through the signal out jack into another amp. One question i have is were the original power transistors 2n6254 insulated between the transistor and heatsink? Any help with getting this amp going would be greatly appreciated.

willpirkle

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Re: 1988 Randall RG100ES
« Reply #1 on: December 26, 2019, 10:11:02 PM »
The case is the collector, so yes it should be insulated if the heat sink is connected to chassis ground, which I’m assuming it is. I’d always use the insulator.

vance

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Re: 1988 Randall RG100ES
« Reply #2 on: December 27, 2019, 07:23:28 PM »
I appreciate the reply...i did realize that and insulated them. Still have very low output out the speaker out . I did notice when hooking up the scope probe when i would attach the ground clip to the neg. of speaker output the volume would actually get a little louder. So thats where im at. The Tip 32 transistors test good. Not sure whats up with the low output at this point.

Enzo

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Re: 1988 Randall RG100ES
« Reply #3 on: December 27, 2019, 09:15:02 PM »
 Connect the scope ground to ground, not the speaker neg post.   Many SS amps have a current sense resistor in the speaker return line.

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Re: 1988 Randall RG100ES
« Reply #4 on: December 29, 2019, 03:58:38 PM »
i actually tried both ways. Im getting signal to the output but its not amplifying like it should. voltages on the Q14, 16, 13, 15 are
E 51mv , B 51mv, C43v
E 52mv, B 438mv, C 44v
E - 44v, B - 44v, C 53mv
E -44v, B - 44v, E 53mv

TIP 32  43v, 1v, 43v
TIP 32 53mv, - 43v, 0v
TIP 31 - 1v, 43v, 447mv

juction of the R48, R49 is - 17.4v

The trim pot dosent change the voltage at all

Jazz P Bass

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Re: 1988 Randall RG100ES
« Reply #5 on: December 29, 2019, 09:42:57 PM »
Where is the schematic?
Can you please post it?
On the last three transistors, what are the designated 'Q' numbers?

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Re: 1988 Randall RG100ES
« Reply #6 on: December 30, 2019, 09:50:05 AM »
Here is a pic of schematic

Jazz P Bass

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Re: 1988 Randall RG100ES
« Reply #7 on: December 30, 2019, 12:25:26 PM »
Other than having the designators wrong on the outputs and the leg assignments wrong on the Negative outputs, I see nothing majorly wrong with the static voltages.

The schematic indicates a 225mv AC signal entering the amplifier.
What value do you read coming in there?

vance

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Re: 1988 Randall RG100ES
« Reply #8 on: December 30, 2019, 02:41:19 PM »
350mv with signal

Jazz P Bass

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Re: 1988 Randall RG100ES
« Reply #9 on: December 30, 2019, 05:24:49 PM »
So you feel the output is 'low'.
What is the maximum Vac out into an 8 ohm load before clipping?

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Re: 1988 Randall RG100ES
« Reply #10 on: December 30, 2019, 06:08:15 PM »
right now im getting 325mv ac out the 8 ohm speaker load. with the master on 10 and gain on 10 i get just enough volume to the speaker to hear it. The two center output transistors are getting slightly warm to touch but outer two are cold

Loudthud

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Re: 1988 Randall RG100ES
« Reply #11 on: December 30, 2019, 07:37:54 PM »
Have you tried bypassing the Effects loop by connecting a patch cord from Effects Send to Effects Return ?

vance

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Re: 1988 Randall RG100ES
« Reply #12 on: December 30, 2019, 08:12:00 PM »
yes

Loudthud

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Re: 1988 Randall RG100ES
« Reply #13 on: December 30, 2019, 08:59:06 PM »
The two center output transistors are getting slightly warm to touch but outer two are cold
This is normal for this circuit.

Link: https://el34world.com/charts/Schematics/files/Randall/Randall_rg100.PNG

There is a 0.18 Ohm 10W resistor (R63) between the low side of the speaker and ground. Have you checked this for continuity ?

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Re: 1988 Randall RG100ES
« Reply #14 on: December 30, 2019, 09:16:56 PM »
yes it checks good