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Randall RG80ES - help troubleshooting dead red channel?

Started by lossfizzle, December 08, 2012, 01:53:33 PM

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lossfizzle

Howdy, y'all. I brought home a Randall RG80ES head to add to the growing vintage-SS collection. Unfortunately, the big selling point of these amps - the distortion channel - is not fully functioning, and I'd like to get it going.

I am at a very basic level of electronics knowledge - happy to dig into things, avoid getting shocked by filter caps, totally willing resolder a component here and there, but I'm def not great at actual theory or troubleshooting. I do note the red channel on the 80/100ES, as shown on the schem, is a surprisingly simple circuit, which gives me some hope I can fix this without dragging it 90 miles one-way to my tech.

Clean side of my 80ES sounds great. The dirty-channel symptoms: extremely low volume (red ch gain / master on 10/10 = volume of green channel on respective 5/1.5), sounds a bit fizzy and the gain definitely sounds way too low vs. the many Youtube clips I've heard.

The amp has two other symptoms: 1) a slight power-supply hum at idle with both channels' MVs all the way down (which may be normal for this amp, I don't know); 2) the famous pull-treble diode clipping is not working at all. I realize the latter could be a bad switch in the pot but the pots are in pretty good, non-scratchy shape as is, so right now I'm guessing it's a circuit problem corresponding to the existing issues on the red channel.

Sound clip of the clean and dirty channels here for direct comparison (I'm riding the fader to add another 10dB whenever I talk, but the amp output itself is always maintained at the same in-room volume throughout the clip).

Getting ready to tear things apart - based on aural symptoms, failing obvious stuff like broken / cold joints and visibly leaky caps, what should be my next thing to examine?

Thanks y'all.

mensur

Several things,shot guts inside amplifier,replace filter caps in power supply(that's where that hum comes) 2x6800uF.
Single coils are not good for this amp, so try dirty ch with humbuckers.
Do these thing and well take it fro there.

J M Fahey

QuoteI'm def not great at actual theory or troubleshooting.
It's a pity, because that's exactly what you'll need to do. :-X
*Visually* looking at it will give you little clues , if any.
Yet there's some hope, if you roll up your shirtsleeves and start working.
1) lucky you; both channels are practically the same.
The schematic shows voltages to be expected on the green channel; you must measure them first, to check they are within 20% of normal, and then repeat measurements on the red channel, hoping to find about the same.
2)
Quotethe famous pull-treble diode clipping is not working at all.
Well, *maybe* the switch was flaky and an earlier owner soldered it close.
Check it with a multimeter.
QuoteI realize the latter could be a bad switch in the pot but the pots are in pretty good, non-scratchy shape as is, so right now I'm guessing it's a circuit problem corresponding to the existing issues on the red channel.
Not wanting to sound harsh, but don't geuess, measure !!
Or to be more precise: guess first, but then confirm or discard .
Good luck  :tu:

lossfizzle

#3
Thanks guys. I finally had some time to pop the amp open and, as is typically the case, there is nothing visually obvious like broken solder joints / exploding or leaking caps on the red channel. (I'm super impressed by the build quality on this thing and it does *not* appear that a PO has done anything inside.) Taking a few measurements (carefully!) while in operation is next, I guess.

Single coils or no, I should be hearing a lot more gain and certainly a *lot* more volume from the red channel. Had plenty of experience with this guitar / bridge pickup through metal-esque preamps and pedals of the same ilk. I plugged my Hafler T3 into the return and used the same guitar-- the power section of my RG80 is VERY healthy. Enough to get local police attention with the Hafler's master volume set on 2. :)

lossfizzle

Just to rule out the single-coil factor for Mensur's benefit (because I know these amps aren't normally popular with us Tele-playin' types), here's a similar demo using a bridge humbucker. Note that there's basically *no* output whatsoever on the red channel with gain on 5 / master on 2, and that the in-room output level of the red channel on gain 10 / master 10 is actually much quieter than the clean channel on gain 5 / master 2. :)

Posted some pics of the red-channel section of the board and the whole amp here... left them at full res for detail and zooming around.

DrGonz78

#5
I think Juan made a great point that looking at the components of the amp is not gonna fix it in most cases.

If I was to question one two things by your pictures alone it would be by the double potted tone potentiometer and those two IC chips(not sure what those are right of the Tone Clipping pot...) I think they are IC's I have an RG80 combo but it does not have that component there so I have no idea what it is used for... Anyway that thing looks cracked or something...

Another thing thing is the green 1KV 202M signal cap looks like the leg has torn and might be leaking... But that is probably not the problem but still can be replaced later on. However, I know that those ceramic caps when the legs are torn can cause a huge loss to signal overall. Not always the case but something to chew on just by looking at pics.
"A person who never made a mistake never tried anything new." -Albert Einstein

lossfizzle

Thanks for having a look, DrGonz - I don't have the amp in front of me right now but I will look more carefully at those components a little later today. Going from visual memory of yesterday's chassis-opening, I believe those are two block resistors to the right of the clipping diode / pot, and I think the "crack" is just a surface-level scoring of the component that the flash brought out, but I'll look more carefully at it now that I see it.

And sure, I know that just looking at things is rarely likely to produce results but the last few SS amps I've had to fix did in fact have visually obvious issues - axial-lead caps with one end blown clean free of the cap body, power transistors with burn marks and/or holes, etc. Sometimes you get lucky. :)

mensur

Quote from: lossfizzle on December 12, 2012, 09:12:56 AM
Thanks for having a look, DrGonz - I don't have the amp in front of me right now but I will look more carefully at those components a little later today. Going from visual memory of yesterday's chassis-opening, I believe those are two block resistors to the right of the clipping diode / pot, and I think the "crack" is just a surface-level scoring of the component that the flash brought out, but I'll look more carefully at it now that I see it.

And sure, I know that just looking at things is rarely likely to produce results but the last few SS amps I've had to fix did in fact have visually obvious issues - axial-lead caps with one end blown clean free of the cap body, power transistors with burn marks and/or holes, etc. Sometimes you get lucky. :)
Quote from: DrGonz78 on December 12, 2012, 06:18:49 AM
I think Juan made a great point that looking at the components of the amp is not gonna fix it in most cases.

If I was to question one two things by your pictures alone it would be by the double potted tone potentiometer and those two IC chips(not sure what those are right of the Tone Clipping pot...) I think they are IC's I have an RG80 combo but it does not have that component there so I have no idea what it is used for... Anyway that thing looks cracked or something...

Another thing thing is the green 1KV 202M signal cap looks like the leg has torn and might be leaking... But that is probably not the problem but still can be replaced later on. However, I know that those ceramic caps when the legs are torn can cause a huge loss to signal overall. Not always the case but something to chew on just by looking at pics.
Those two *IC* are 220nF tonestack caps, I guess that one is broken so it need to be replaced along the 2.2nF treble cap if it's leg is cut offed.If that's the case, signal only goes throughout mid pot, and energy is extremely reduced.Again from the pics, that's the only problem.Here's one trick, route the signal form master pot.middle leg to the amp input, and let's see is the tonestack the problem.

lossfizzle

Quote from: mensur on December 12, 2012, 02:18:00 PM
Those two *IC* are 220nF tonestack caps, I guess that one is broken so it need to be replaced along the 2.2nF treble cap if it's leg is cut offed.If that's the case, signal only goes throughout mid pot, and energy is extremely reduced.Again from the pics, that's the only problem.Here's one trick, route the signal form master pot.middle leg to the amp input, and let's see is the tonestack the problem.

Thanks Mensur. I can't read the top of the components in question because the print is so fine / light, and I have not personally seen caps like this (knowingly), but from a few context clues elsewhere in the amp I imagine you are probably right about them being caps. The cap on the left is *not* broken, there is a light scoring line that goes across the top of the body as shown and continues on to the top of the adjacent cap. It could have been inflicted by a soldering iron chisel tip at the time of mfr and is incredibly fine - definitely does not penetrate the plastic body of the component(s).

The two green caps pictured are also solid - no leg breaks on either side. I have not yet had a chance to pull the board completely out of the chassis to look for obvious things like broken joints on that side, but I have gently poked around the components in the red channel and nothing seems obviously loose yet.

lossfizzle

...Mensur, I just saw your latest edit to your post (about routing things from the middle leg of the red channel master pot to, presumably, the power amp input?). Can you be more specific about how I would achieve that?

mensur

Quote from: lossfizzle on December 12, 2012, 02:36:48 PM
...Mensur, I just saw your latest edit to your post (about routing things from the middle leg of the red channel master pot to, presumably, the power amp input?). Can you be more specific about how I would achieve that?
Can you read the schematics?

J M Fahey

The tone stack is common to both channels, so it's not a suspect.
Please read voltages on both channels (which are practically twins) and post results.

DrGonz78

#12
<Mensur...> I think he meant to solder or clip a wire to the master (red channel) middle leg that then led straight to the hot (tip) part of the input jack as to force the full signal into input on that channel. Well that would be interesting to know if that works.

<J M Fahey> As Juan is saying to look at the Green channel and the voltages that are sought out on that channel and then compare and contrast any discrepancies since they are almost identical. If you look at the Green channel there are voltages posted in the schematic. However, on the Red channel of the schematic there is not much to show for voltage so you have to cross reference the circuit. These circuits are almost the same or are identical as Juan said... So, look closely and cross reference any voltages in a way to find a bad component or loss of voltage.

What I meant about about a break in a leg on the Green cap is in reference to the green part on the leg being cracked off. I am not saying the whole leg is cracked off the circuit... just to note. I have seen these types of tone caps w/ cracked insulation that lead to total loss of effect signal in (for example) a Boss DD-3. That one ceramic cap killed the entire delay effect from working.
"A person who never made a mistake never tried anything new." -Albert Einstein

lossfizzle

Yeah, I see what you mean now, DrGonz - there *is* a slight fracturing of the left-leg insulator on the green cap at the "back" of the red channel. I would be surprised if this would alter the performance of the cap but it sounds like you have had different experiences...

Awkward n00b question / moment of truth: I am happy to try and read voltages as rep'd on the green channel and I can figure out how they are "mapped" to the red side since the two circuits really are quite similar. However I'm not used to having to read such values on live circuits, esp. AC-based. (From the schem, I do get that the red and green channels are receiving 24VDC on the marked "E" leg of the power supply.)

To get these readings, I'm guessing I want red DMM lead on the point / leg shown, black DMM lead clipped to chassis ground somewhere. If this is not the case then please fill me in before I proceed and/or produce sparks.

DrGonz78

#14
If you are at all unsure or worried about taking live voltage readings then keep researching all you can about safety precautions. Like the one's that say take off all your rings on your fingers before testing voltages.  :tu:

How you described the testing is correct: Ground is typically your chassis and it is a good idea to just clip it in place. At that point, powered off, I like to set DMM for a continuity test to make sure I have grounded through to the circuit board. Clip black probe to chassis and test for continuity. For example, test continuity from chassis to something you know is ground on the board. Then yes start to record voltage readings in each different place on the board. If you are unsure about something ask before testing but it is really straightforward for the most part. As you do it more you get more comfortable but just never get too comfortable and always be on your guard.
"A person who never made a mistake never tried anything new." -Albert Einstein