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Author Topic: 1988 Randall RG100ES  (Read 1463 times)

Jazz P Bass

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Re: 1988 Randall RG100ES
« Reply #15 on: December 31, 2019, 02:35:06 PM »
Follow the signal into the power amp.
You do have a scope, correct?
What does the signal look like at the base of Q8?
Q10?
Q11?
Q12?

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Re: 1988 Randall RG100ES
« Reply #16 on: December 31, 2019, 03:55:33 PM »
tops are squared off do to clipping i suppose. keeping the same settings on scope the wave is much smaller on 12 then the 8, 10 , 11 ....11 is the largest. Q8 is irregular pattern

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Re: 1988 Randall RG100ES
« Reply #17 on: December 31, 2019, 03:59:55 PM »
the amp controls all seem to operate as they should.....tone sounds ok and can adjust as normal just no volume. like its not amplifying much at all. Must be some what but very little. I mostly repair tube amps so the SS is kind of newl to me

Loudthud

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Re: 1988 Randall RG100ES
« Reply #18 on: December 31, 2019, 04:32:09 PM »
On the schematic you posted, what is the meaning of the note #7 ? Is that leg of the resistor grounded ? Look at the Bases of Q8 and Q9. The signals should look very similar. That tells us that the amp is basically working, but there is some problem in the feedback network or the amp just isn't getting enough signal to begin with. Is the signal at Q8's Base nearly the same as it is at the Effects Send jack ?

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Re: 1988 Randall RG100ES
« Reply #19 on: December 31, 2019, 04:41:01 PM »
the signal at  base of Q8 is same as the effects send. The base of Q9 is much less signal

Jazz P Bass

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Re: 1988 Randall RG100ES
« Reply #20 on: December 31, 2019, 06:13:29 PM »
What does "Q8 is irregular pattern" this mean?


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Re: 1988 Randall RG100ES
« Reply #21 on: December 31, 2019, 07:54:27 PM »
my scope is acting up so cant really go by that right now....but there is signal there

g1

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Re: 1988 Randall RG100ES
« Reply #22 on: December 31, 2019, 09:18:40 PM »
You need to always specify whether your voltage readings are AC or DC.
Going by the readings given in reply #4, some of the transistors are not running.
Once all the DC voltages are correct, then you can move on to AC signal voltages.
The voltages shown in the ovals on the schematic are DC.  Check all those.
Also post DC voltages for E,B, & C for transistors Q13 thru Q16.  Not rounded to nearest volt but nearest .1Volt.
All DC measurements taken with no signal applied.

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Re: 1988 Randall RG100ES
« Reply #23 on: December 31, 2019, 10:14:55 PM »
E 50.3mv        E 51.2mv        E -42.9v       E -42.9v
C 42.9            C 43.0v           C 0.3mv       C 54mv
B 50.6mv        B 430mv         B -42.9v       B -43v

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Re: 1988 Randall RG100ES
« Reply #24 on: January 01, 2020, 02:56:05 PM »
Ok got my scope and signal gen. Straightened out. Looks like im only getting the tops or pos. Amplitude sine wave from the speaker output. The wave looks correct at the reverb IC 4588

Jazz P Bass

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Re: 1988 Randall RG100ES
« Reply #25 on: January 01, 2020, 03:02:15 PM »
What does the signal look like at the base of Q8?
Q10?
Q11?
Q12?
Take care that the specified 250mv AC signal into Q8 is NOT exceeded.
« Last Edit: January 01, 2020, 03:28:42 PM by Jazz P Bass »

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Re: 1988 Randall RG100ES
« Reply #26 on: January 01, 2020, 04:14:56 PM »
the base of Q8 looks just like the signal at speaker out, tops only. The base of Q10 and Q11 have 42v on them. The base of Q12 has a small fuzzy ball shape sine wave

Jazz P Bass

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Re: 1988 Randall RG100ES
« Reply #27 on: January 01, 2020, 04:47:26 PM »
Not too sure what you are seeing.

Reverb output ic looks good but Q8 base looks bad.
That makes no sense unless Q8 is toast.

Q10 should have +49 Vdc on it.
Q11 should have +1.2 Vdc on it.
Which leads me to believe Q10 is bad.

You have more problems here than I think you can solve on a forum.

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Re: 1988 Randall RG100ES
« Reply #28 on: January 01, 2020, 06:32:17 PM »
i told you wrong....Q11 has 1.4 on base, Q10 has 41.2 on base Q8 has 41.9 as it should, and Q9 has 0.75 as it should ....Q12 has 0.5 on base

Jazz P Bass

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Re: 1988 Randall RG100ES
« Reply #29 on: January 01, 2020, 08:59:11 PM »
Q8 according to the schematic:
B:Not specified (47K from ground, it's an NPN transistor so I would expect minimal Vdc)
C: +38Vdc
E: -1Vdc

Q12 should have a negative voltage on the base: 0.6Vdc

(You do have the schematic don't you?
The expected static voltages are well marked.