I have a RD-210 One Hundred (ss preamp, 2 6l6s), that I have owned for almost 20 years. Recently, the amp sounded distorted on low volumes, mostly when you strike the strings hard. I thought perhaps speakers or tubes. Replaced both. No change. Then, the amp started blowing fuses. I replaced about 7 capacitors both tube socket diodes and still nothing. Then I took it to my tech, who just returned it after having replaced "shorted driver transistors". He checked the tubes and adjusted bias. I got the amp hooked back up to the speakers, it plays now without blowing a fuse but still distorts (kinda farty sounding) when you hit the strings harder or play anything other than plucked single notes. Does anyone have any other ideas? I am at a loss of what to do at this point. Thanks!
Sun, 04/13/2014 - 06:42
#1
AndrewLarson
Hi Andrew,
Hi Andrew,
you need to find out where the distortion is introduced in the amp. The first preamp stage that drives the tone stack, and the buffer after it, are IC-1. If IC-1 is the problem, the distortion would be present if you took signal from the 'low' loop jack and fed it to another amp. If the signal is clean there, the distortion is introduced further down the signal chain. Start by checking the signal at the 'high' loop jack - if it's still clean, your distortion is happening around IC-7.
Clear as mud?
Sincerely,
Lars Verholt
Ok. Thanks! I will try this
Ok. Thanks! I will try this today. So, if feeding another amp via the hi loop jack produces a clean signal, then the IC 7 itself is bad, or something in that vicinity? Would this be why the transistor shorted?
Ok, I tried the loop feeds,
Ok, I tried the loop feeds, and the distortion is still present from the hi loop jack, but i can't get any signal out of the lo loop jack when i feed the other amp from there.
-
Ok, next thing to try is to swap IC-1 and IC-4. If the sound is now clean, your have a bad IC-1. Turn off the reverb for this test as IC-4 is the reverb drive/recovery chip.
Sincerely,
Lars Verholt
Check ALL Power Supplies & Semiconductors
If all the Power supplies are where they are supposed to be, then my best guess (assuming you do not have a bad tube) is an IC or diode. Pay close attention do D14 (22V Zener Diode). This sets a grid bias point for the output tubes. Zener diodes tend to fail in the shorted mode. So, those grids will be grounded instead of having 22Vdc on them if this part has failed.
If this diode is shorted or is not providing the proper voltage, the power output can be cut in half and premature distortion is possible. Using a multimeter set for Semiconductor Junction tests, just check ALL the diodes for proper results to ensure something like this is not causing you headaches!
Components in sockets (ICs, Tubes) are the easy part but they are not the only source of problems!
Steve
Parts ID help
So, I am having a hard time identifying IC 1 and IC 4 to swap them out for diagnosing. Here is a pic of my amp. The schematics I have seem to show things in different places or more ICs than are really present (schematics for the 2100-RD GD-1 and GD-2A). Looks like at least one IC has been replaced with a different (interchangeable?). Schematics call for IC1,3,4 as TL072 or LF 353, while IC2,3,6,7 as LM1458. Can someone help point out IC1 and 4 as well as the D14 Steve mentioned? Thanks! Oh, and I have two sets of new tubes (both produce same problem), so I think tubes are not the issue.
210-100
HMM-that's weird. Swore I posted a response here for this a while ago. Anyway, do you know exactly which board you have? I seem to remember from past conversations that the boards ARE marked somewhere. I know the driver board is marked on my 2-10. Let us know exactly which version you have and maybe we can help. Obviously, if you trace the board through from input, the first IC in the chain is IC1. I know we posted a layout for ONE in the past but I seem to remember it was an RD50 so probably not the same. Right off the bat though, those old Mallory silver caps and the ones under the doghouse need to be changed.
PS-if you can send me a better pic to my email in full size, I will try to track down what is what. Mike
Use a GPS to find component locations!
Well, not really. However, with a continuity meter (or the "Beep" setting on most multimeters) you can identify components by starting at locations that you CAN easily find.
For instance, to find D14, look at the schematic. D14 connects to pin 5 of each output tube through 220-ohm resistors. Go to an output tube pin 5, follow the wire back to board then confirm continuity to one end of a 220-ohm resistor nearby. Once you find the resistor, confirm continuity ("0" ohms or "beep") from the other side of that same resistor to the Cathode of a diode in the same area. This will be D14.
You can also turn on the amp and measure the voltage present at one (or both) of the output tubes at pin 5 and look for a DC voltage close to 22Vdc. Any voltage value not within 2 or 3 volts of the 22Vdc expected indicates a problem.
The same goes for any component. Find a point on the schematic that is close to what you are looking for (a front panel control or jack is a good place to start as well... something not on the board itself) then track the connections using continuity, component types, component values and color-codes to quickly find your way to an IC pin number. The schematic will indicate which IC that pin belongs to.
Without factory documentation as to board layout, this is usually the only method that can get you there quickly.
Steve
My amp has the GD-2A board,
My amp has the GD-2A board, if that helps!
Layouts
I don't honestly know if anybody ever contacted EB about any old layout sheets laying around in their archives but I DID contact them a couple days ago to see if they would PLEASE let them out if they are there. Don't have much hope but you never know. Crazier things have happened! I mean, ONE surfaced so there has to be more SOMEWHERE. Mike.
Update: Distortion Still There! AHHHHH
Ok, I replaced all the silver bullet caps (those things are pricey!) all 7 8 prong ICs, new tubes and the distortion still happens! It sounds ok when you're picking or single string playing, but when you dig in or strike a bar chord it "farts" out. Obviously, this amp should be clean, clean, clean! Any other thoughts/suggestions? The OT checked out fine by the tech who also replaced two drive chain transistors. ALSO, what is the rotary wheel adjuster on the board near where the old Silver caps were? Bias? I am at my wit's end now. Neither I nor my tech can seem to fix the problem. This is getting pricey! Oh, yeah, I mentioned before, new speakers too, so that is covered. Please help!
rd 100
Me thinks that you need to find a "tech" with a scope! You are going to have to find out WHERE the break up is occurring, pre or finals. You say he replaced the two "drive chain" transistors but are they matched up enough to provide balanced drive? I think those are JE1692's and that pot is(according to the schem) the driver balance. You need to look and see if the front end is driving wild(doubt it) or the drivers are overdriving the finals. Did he replace the drivers with the right transistors or a substitute? Is the signal coming out of the last chain IC(IC7) clean before it goes into the driver? That amp has a GAIN section that might be running all the time too. I AM just guessing here since I have NOT worked on one but you do need to isolate WHERE the breakup is occurring. Throwing parts at it is expensive. The gain section could be causing your trouble.
tech
he does have a scope and owns The Amp Shop in Asheville NC. He fixed the fuse popping problem, only to have it come back to me with the original problem of distorting. I got charged 95 bucks to have it half-way fixed, which is why I am plugging away at other problem areas. Just not sure how to proceed now. I have about 300 into this "repair" already and the problem still exists! So, can this amp even be biased? I have heard both yes and no. Whats the truth? Also, the tubes seems to get way hotter than they should, which may point into the gain section as the problem....no?
Is the distortion still there from the 'Hi' loop jack?
Previously in this thread you noted that the distortion was present out of the 'Hi' loop jack. Is that still the case? If so, the distortion is introduced prior to the driver section. One thing we haven't tried is to disable the clipping circuit (Q3+Q4, D17+D18). If one transistor is shorted, you'll get distortion at the drop of a hat - and not symmetric either.
We'll get you through this.
Cheers,
Lars
Yes, Distortion still there
Yes, Distortion still there from Hi Loop jack. Also noticing an output volume drop, its blown away (volume wise) by my peavey classic 50.
2-10
OK- so you have dist on the front end. While I haven't worked on this particular amp, it's all the same. You obviously have a second amp for a sound source. Lars is right, could be the drive transistors. PROBABLY is but I don't know. Check the transistors for short. NPN = E-B Cont C-B CONT B-E=open B-C open C-E open. If this doesn't bring about a shorted transistor, You MIGHT want to make a widget. I know you have an amp for a sound source so you can use this. Build yourself an audio probe and use this to isolate WHERE the dist starts and stops. this will let you isolate it. Turn it down at first and bring it up until you get a signal when probing circuit. By the way, when I say continuity, it only goes one way! Mike
Did you check the Zener Diode?
On my RD 50 110 a bad Zener diode (1N4748, 22v) caused smoking/fried resistors and awful low volume distortion. Replacing it solved the problem. Terry Loose told me to check it, and not surprisingly, he was correct...
What is the signal like from the treble control?
It would be interesting to know if the signal is clean or not coming out of the treble control. If you pull signal from the wiper of the treble pot (middle terminal) to an external amp to check the signal we'd all be a lot wiser.
When you tried to get sound out of the loop jacks, did you use a t-r-s (stero) jack? The output is on the ring.
Sincerely,
Lars Verholt
Component ID
Ok, so I need help identifying what the Zener Diode is (it looks like there may be two side by side near the old silver caps, and where do the drive transistors live? I can only identify 4 transistors on this board (two smaller ones and the two that "bolt" the board onto the chasis. Thanks!
Look at messages #6 & #9 above.
I described in detail how to find the zener diode with only a meter in Message #9 above. Frankly, if this is beyond your present capabilities you really shouldn't be working inside a tube amp. This can turn deadly and resemble the situations that lead to people being added to the "Darwin" list (posthumously).
The schematic is your only road map and you must be able to interpret the information and logically overlay this on the real components in a meaningful, safe and effective way to get from point A to B. The first clue is that there is only ONE Zener diode. You probably cannot identify it by what it looks like, you must verify what it connects to. The best way to test the zener is also outlined in that message (and again requires a meter at minimum).
Zener
Actually, according to the schem, there ARE two zeners. 1n5353's d15 and 16. right off of c28 and c29.
D14 is the "Loss of headroom" Zener
I misspoke and didn't make myself clear regarding the "one" zener. The original post complained of low headroom. D15 & 16 are low-voltage zeners that only pertain to the power supply for the front-end preamp op-amps and switching circuits. Loss of one of these will result in distortion at almost all input levels or gain settings and lack of proper switching functions. These problems were not stated as the primary symptom.
Loss of headroom is more typically an output stage symptom (if the term is properly being applied here). The only zener in the output stage is D14 and if this zener shorts (as they can do and I have seen myself) you will get an immediate and most noticeable "loss of hearoom" (about half the output power will be lost). Since this one component is a primary candidate for the primary symptom it should be one of the first places that the voltage is checked before "shot-gunning" anything else. Output tubes and power supplies fall into the same "primary" status for this symptom.
Checking ALL VOLTAGES in the amplifier is the first step of troubleshooting (unless there is a blackened hole in a PC board or other "clue"). Much time is wasted by proceeding or conjecturing without first checking all the voltages (power supplies, bias, tube plates, etc.).
Also, when checking semiconductors you are NOT measuring straight static resistance. You MUST set the meter to the semiconductor setting to measure the junction properly or you will probably will get the same reading in both directions because the junction is not being turned on and off when reversing the probes.
zener
the zeners test the same resistance on both sides. guess they must be failing. will pop in a couple new ones. stay tuned. Thanks for your concern, but I am aware of the dangerous voltages in the caps if not discharged. Safety first! No, I am not a tech, but I do enjoy learning by working on my own gear whenever possible, and building pedals etc. Thanks for the help everyone.
Transistors
Don't forget to check the transistors in the drive circuit.
Pin 5 voltage over 22Vdc
"You can also turn on the amp and measure the voltage present at one (or both) of the output tubes at pin 5 and look for a DC voltage close to 22Vdc. Any voltage value not within 2 or 3 volts of the 22Vdc expected indicates a problem."
Does the above apply to an RD 50 112? I have a similar problem with this amp- very low power, have to hit the strings hard to get any sound and the sound I get is nasty distortion. I checked my pin 5 voltage and it's around 32v. The zenner diode attached to pin 5 has no continuity across it, except if I heat it up with a lightbulb. I bought the amp for $30 from someone who said sometimes the power would suddenly decrease by half. It seemed to work fine for me for a couple years until recently, though I didn't play it very much. I'm thinking the zener diode may be the problem. Oh, also it doesn't seem to switch channels now with or without the footswitch.
RD 50 is completely different...
The RD-50 is a different animal. It uses a Zener but it is a 30 Vdc zener. 32Vdc sounds fine for that amp.
I would first check every power supply for proper operation and low hum. I am not saying your Zener isn't an issue but if it always has 32Vdc then it is probably OK.
Steve
RD-50 low power
Ok, thanks for the reply Steve. I'll do that.
Replaced Zener
I replaced the 22v Zener diode and that seems to have definitely helped the distortion issue. However, I remember this amp being much louder without having to crank the Gain knob for more volume though. One other strange issue, is that every since I got this back from my tech who replaced the Drive Transistors, when you power up the amp and take it off Standby, you can strum regularly on the guitar and there is no sound. BUT, if you dig into the strings and hit them hard, the amp suddenly gets jarred to life. Anyone experience this before? Thanks for all the help everyone!