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Solid State Amplifiers => Amplifier Discussion => Topic started by: vance on December 26, 2019, 03:17:30 PM

Title: 1988 Randall RG100ES
Post by: vance 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.
Title: Re: 1988 Randall RG100ES
Post by: willpirkle 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.
Title: Re: 1988 Randall RG100ES
Post by: vance 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.
Title: Re: 1988 Randall RG100ES
Post by: Enzo 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.
Title: Re: 1988 Randall RG100ES
Post by: vance 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
Title: Re: 1988 Randall RG100ES
Post by: Jazz P Bass 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?
Title: Re: 1988 Randall RG100ES
Post by: vance on December 30, 2019, 09:50:05 AM
Here is a pic of schematic
Title: Re: 1988 Randall RG100ES
Post by: Jazz P Bass 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?
Title: Re: 1988 Randall RG100ES
Post by: vance on December 30, 2019, 02:41:19 PM
350mv with signal
Title: Re: 1988 Randall RG100ES
Post by: Jazz P Bass 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?
Title: Re: 1988 Randall RG100ES
Post by: vance 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
Title: Re: 1988 Randall RG100ES
Post by: Loudthud 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 ?
Title: Re: 1988 Randall RG100ES
Post by: vance on December 30, 2019, 08:12:00 PM
yes
Title: Re: 1988 Randall RG100ES
Post by: Loudthud on December 30, 2019, 08:59:06 PM
Quote from: vance on December 30, 2019, 06:08:15 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 ?
Title: Re: 1988 Randall RG100ES
Post by: vance on December 30, 2019, 09:16:56 PM
yes it checks good
Title: Re: 1988 Randall RG100ES
Post by: Jazz P Bass 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?
Title: Re: 1988 Randall RG100ES
Post by: vance 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
Title: Re: 1988 Randall RG100ES
Post by: vance 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
Title: Re: 1988 Randall RG100ES
Post by: Loudthud 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 ?
Title: Re: 1988 Randall RG100ES
Post by: vance 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
Title: Re: 1988 Randall RG100ES
Post by: Jazz P Bass on December 31, 2019, 06:13:29 PM
What does "Q8 is irregular pattern" this mean?

Title: Re: 1988 Randall RG100ES
Post by: vance 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
Title: Re: 1988 Randall RG100ES
Post by: g1 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.
Title: Re: 1988 Randall RG100ES
Post by: vance 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
Title: Re: 1988 Randall RG100ES
Post by: vance 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
Title: Re: 1988 Randall RG100ES
Post by: Jazz P Bass 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.
Title: Re: 1988 Randall RG100ES
Post by: vance 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
Title: Re: 1988 Randall RG100ES
Post by: Jazz P Bass 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.
Title: Re: 1988 Randall RG100ES
Post by: vance 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
Title: Re: 1988 Randall RG100ES
Post by: Jazz P Bass 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.
Title: Re: 1988 Randall RG100ES
Post by: vance on January 01, 2020, 10:17:53 PM
when checking them against the schematic they are all close. So if there is 42v at the base of say Q11 where the schematic says 38v is that enough to cause issues ?
Title: Re: 1988 Randall RG100ES
Post by: Loudthud on January 02, 2020, 06:10:10 AM
Have you checked the low value resistors R55, R56, R57 and R58 ?
Title: Re: 1988 Randall RG100ES
Post by: vance on January 02, 2020, 09:53:20 AM
Yes they all test good
Title: Re: 1988 Randall RG100ES
Post by: g1 on January 02, 2020, 01:38:29 PM
Quote from: vance on January 01, 2020, 10:17:53 PM
when checking them against the schematic they are all close. So if there is 42v at the base of say Q11 where the schematic says 38v is that enough to cause issues ?
Maybe. (and I think you meant Q10; too many errors in your posts, please try to be more accurate)
Each transistor needs a difference in the range of approx. 0.5 to 0.7VDC between the base and emitter.
That difference is what tells you whether the transistor is running or not.
So Q10 (PNP) with 42V at the base is only ok if it is roughly .5V less than the voltage at the emitter.
For NPN types, the base needs to be approx. .5V more than the emitter voltage.
Title: Re: 1988 Randall RG100ES
Post by: vance on January 02, 2020, 02:04:18 PM
The base of Q10 is 43.5 v the emitter is 44.0v
Title: Re: 1988 Randall RG100ES
Post by: Jazz P Bass on January 03, 2020, 01:30:01 AM
I am stuck on the squirrely voltages on Q8.
Title: Re: 1988 Randall RG100ES
Post by: vance on January 04, 2020, 01:29:58 PM
I have found a couple 2n5484 transistors bad TR1 and TR5 so far. Not sure what effect that would be having on the power section but need replaced