Help a newbie with a diagnosis - Page 1
Forum section: electronics / projects

author: gooseEL34
author_rank: Regular Contributor
timestamp: February 27, 2013, 06:14:19 pm
content: Hello folks-I have been tinkering with tube (valve) guitar amps for 15+ years. I can read a schematic, identify parts, and have built several amps from scratch. However, other than checking the common suspects (plate load resistors for out of spec, voltages where they need to be, condition of caps, checking for burnt components, etc), I am not as well versed as to the theory behind frequency response, etc.That being said, feel free to use technical terms in any response you may have.I have just scored a 1970s/1980s solid state guitar amp called a Lab Series L5 made by Moog (you may know them as synth makers).Here is the schematic:http://www.rru.com/~meo/Guitar/Amps/Lab/Images/Lab_Series_L5_L7_L9_L11_scheme.jpegHere is my issue: When I strike a guitar chord, the note does not ring as it is supposed to but rings for a short time, the volume decreases faster than normal (on a working amp), and as the volume trails off, the signal distorts slightly (sounds like clipping).My first plan of attack is to check the voltage rails and make sure I have +15 and -15 solidly.Also, It appears that there are certain test points which have expected voltages on the schematic. I can test those also.Assuming I have good power, what should I next look for?Also, in the middle of the schematic (center left) there is a stack showing a trio of 4558 op amps and it appears that the network is some kind of filter (given R167 Multi-Filter potentiometer).Can anyone tell me what this is doing?To me, it appears that the pre-amp signal comes from the VOLUME pot and gets fed into 6 ICs, which appear to be identical except for the cap and resistor network between pins 1/2 and 6/7. So it looks like to me (a non EE), that the signal is split 6 ways and the ICs are tuned to amplify different frequencies, then they are combined with the "dry signal" (R155/R156) and the pot determines how much of this signal is fed on to the next stage.Also. The power section appears to be 8x RCA 1B04. The amp is rated at 100W (face panel graphic).I have a dummy load of 2x16ohm 50W ohmite resistors. Should that be ok for testing this amp?Would anyone recommend reseating the power transistors with new thermal paste. Or, in the alternative, if the amp is not overheating, leave well enough alone....Thanks guys!Tim (Nashville, TN)

author: Kremmen
author_rank: Super Contributor
timestamp: February 27, 2013, 06:36:39 pm
content: The schematic is complex enough that you really should have a signal generator and scope available. Just striking chords from a guitar will be too uncontrolled.The good thing is that there is a schematic. That lets you check the voltages and amplifiers. Just scoping amp inputs and outputs lets you see right away if there are totally dead ones. Also amplifiactions with varying frequencies are handily checked that way.There seems to be a vintage reverb unit in the lower left. I am not really familiar with that but is seems to be an electromechanical unit.P.S. Leave the power rocks alone until you postively know they need looking into. One thing would be to scope the voltage waveforms over the balancing resistors to spot any deviant or dead ones.Edit: completed broken sentence.

author: SeanB
author_rank: Super Contributor
timestamp: February 27, 2013, 06:57:27 pm
content: Check the reverb unit for fallen off foam and dried out rubbr mountings as well. Often the foam damping will fall off and foul up the internal springs, if this is so clean it off gently and try again.

author: gooseEL34
author_rank: Regular Contributor
timestamp: February 27, 2013, 07:28:17 pm
content: Sorry guys. I meant to also note that I have a HP204C sine wave generator and a Tek dual trace scope (40mHZ) and a decent True RMS vom.If that opens up options, I understand.I suspect I would feed a 1k sine wave into the amp and trace it to see oddities in the waveform. What voltage sine might you suggest?Thanks

author: gooseEL34
author_rank: Regular Contributor
timestamp: February 27, 2013, 07:34:40 pm
content: Quote from: Kremmen on February 27, 2013, 06:36:39 pmOk. Can you tell me what this means? How does one "scope the voltage waveforms OVER the balancing resistors"Are the balancing resistors the .33ohm 2W that are R338 - 341 and R345 - 348?If so, what would I do. Put the scope on one side of those resistors, then the other and see if there is a difference?Thanks (and sorry for the newbie questions). I actually bought myself a college textbook on analyzing electrical circuits. I am through chapter 3 and have an understanding of ohm's law and voltage drop over a resistor, butno idea how a scope interacts with diagnosis (other than to see how a waveform is distorted by a guitar pedal, for example). I can see a sine wave get clipped on top, and understand that in the guitar world clipping = distortion, which may be good.I have a law degree so if you are willing to explain a bit, I am sure I will be able to follow

author: gooseEL34
author_rank: Regular Contributor
timestamp: February 27, 2013, 07:36:04 pm
content: Quote from: SeanB on February 27, 2013, 06:57:27 pmI have noticed on the accutronics tanks built for guitar, there is no foam like on some of the ones used in the hammond organs. But I will check. Thanks!!

author: Kremmen
author_rank: Super Contributor
timestamp: February 27, 2013, 08:22:28 pm
content: Quote from: gooseEL34 on February 27, 2013, 07:34:40 pmSure. Firstly the resistors are indeed those you mention. By checking them, you can see that each of the finals stage transistors does its share of passing current. The way to see that, is exactly by Ohm's law i.e. measuring the voltage drop over the resistor.Now a scope basically displays the voltage difference between the hot and neutral signal input (i.e. the input BNC connector center conductor and housing. While the neutral wire usually is at local ground potential, it does not have to be at any particular potential - it can be anything other that ground as well. BUT N.B. if/when you do this, you must understand that the entire scope body will be at the voltage connected to the input "ground". So be extra careful to avoid ground loop short circuits.In this case there is no direct danger because the amplifier power supply is galvanically isolated from wall outlet ground by the mains transformer. But be extra careful that nothing of the scope touches anything grounded in the amp chassis or elsewhere, or sparks will fly. With these precautions, just connect the scope probe across each resistor in turn and check that similar input signal produces a similar voltage over each resistor. The voltages won't necessarily be absolutely identical in amplitude - the resistors have tolerance. But they should be close. The voltage over the resistor will be relatively low so be prepared to crank up the scope gain.

author: gooseEL34
author_rank: Regular Contributor
timestamp: February 27, 2013, 09:04:14 pm
content: Quote from: Kremmen on February 27, 2013, 08:22:28 pmOk. I think I understand, but I suppose I am confused as to why would I want to look at the voltages with a scope rather than a quality DMM/VOM? My Tek 2213 has a trace that might not be good enough. If I am reading the schematic correctly, the notation near those emitter resistors says >3mV each. If my best resolution on the scope is 2mV per division, will I be able to see that with a trace vs. compare the measurements from a 3 digit VOM?I assume we are talking DC here, correct?Also. I am aware of ground loops. Thanks though. I have worked on Ampeg SVT bass amps, which run 500-600 vDC B+ for the 6550 tubes. I am fully aware that electricity can kill (or maim)Thanks for your patience!!!

author: Kremmen
author_rank: Super Contributor
timestamp: February 27, 2013, 09:45:43 pm
content: Of course you can use the multimeter as well, but that will not give you any idea how the waveform actually looks like. A precise voltage level measurement was not my point here, but to see that all transistors behave more or less equally. Eyeballing the waveform is a reliable way to find out.

author: David_AVD
author_rank: Super Contributor
timestamp: February 27, 2013, 09:59:19 pm
content: Have you isolated whether the issue is before or after the pre-out - amp-in link? Run the pre-out into another amp and see if it's clean. If not, the preamp is suspect. If clean there, the power amp section is bad. Also check and clean the pre-out and amp-in link connectors. Corrosion there can cause symptoms like that.If the pre-out is clean, it could be that the power amp output stage is starved of bias and you're hearing massive crossover distortion. This kinda sounds like what you're describing. R311 looks to the the bias adjustment pot. With no signal there should be a small amount of voltage across each emitter resistor as indicated in the diagram.See how you go with that and report back.

author: gooseEL34
author_rank: Regular Contributor
timestamp: February 28, 2013, 05:20:09 pm
content: Quote from: David_AVD on February 27, 2013, 09:59:19 pmHad not thought to try this. will dx tonight and report back.

author: gooseEL34
author_rank: Regular Contributor
timestamp: March 01, 2013, 04:19:39 pm
content: Ok. Update.I removed the preamp pcb and used DeOxit on all the pots. Problem solved. Seems like a dirty contact.So I have nice guitar signal through both channels, with reverb on channel 2.Now I have the issue that the compressor is not working. Can someone with audio experience look at the circuit and tell me what is a likely suspect? When I switch SW103 (to engage the compressor), theL102 LED lights for an instant and then goes out and rotating R185 (level pot) doesn't seem to engage the compressor. When I got the amp, the compressor worked, and the led would light, indicated that the compressor was working. It did limit the volume, but now it is dead.Also...how does a compressor work?Does it have something to do with adding negative feedback to pin 5 of A111?http://www.rru.com/~meo/Guitar/Amps/Lab/Images/Lab_Series_L5_L7_L9_L11_scheme.jpeg

author: David_AVD
author_rank: Super Contributor
timestamp: March 09, 2013, 11:46:36 pm
content: Maybe check and clean the compressor switch contacts. Are all the pots and switches soldered to the PCB or on wires? If on wires, you didn't snap any off during repair?

author: gooseEL34
author_rank: Regular Contributor
timestamp: March 10, 2013, 03:30:23 am
content: Got this fixed. Topic closed I suppose. Thanks for all the suggestions/help.Tim

author: David_AVD
author_rank: Super Contributor
timestamp: March 10, 2013, 03:35:50 am
content: Excellent news Tim. It's quite common for corrosion / dirty contact to cause many problems. That's one of the things I check for first.