Peavey TKO-65 hum - Page 1
Forum section: electronics / repair

author: cowasaki
author_rank: Frequent Contributor
timestamp: November 13, 2019, 05:10:51 pm
content: I've taken up fixing amplifiers for fun! Anyway I've managed to fix 24 of 25 so far and I think that we could probably all guess that the one I haven't yet managed to fix is the only one where "my mate has already had a look and says it's knackered"....Anyway I got it partially working before slipping up and shorting out of the transistors in the output section. I've fixed that now with two new power transistors. The issues were many and included dirty pots, 2 knackered poly caps (I've replaced all of them now), a transistor removed by his mate to test and re-inserted the wrong way! plus lots of other rubbish......It now works BUT there is a loud 100hz hum. It is as loud as the input signal! If you switch it off at the isolator the hum goes but the input signal continues till the caps loose their current. The actual signal is nice and clean otherwise. I’ve pushed music through it as well as test tonesI've attached a schematic and I'm sure it's coming from the mains being twice the UK 50Hz.I've replaced the 10nF cap across the bridge rectifier, both 2200uF caps (C52/54) and C53/55. None appeared to be faulty according to my Peak cap tester but grasping at straws now.... All other caps measure well within tolerance and low ESR. All polys have been changed as I mentioned earlier. All the transistors have been tested and having discovered that U5 isn't required to get a sound I've tried it in place of the other op-amps of the same type. U3 is an op-amp I don't have and I'm not sure what I could substitute it with (even if just for testing).All pots have been removed dis-assembled cleaned, de-oxitted, re-assembled and tested and are like new.Input jacks are new high quality onesAny help would be appreciated even if just pointing me at other things I may not have thought of. I am quite new to amps having come from computers so I might have missed something obvious!

author: don.r
author_rank: Frequent Contributor
timestamp: November 13, 2019, 05:39:15 pm
content: Routing of mains and/or input wiring? Could be from capacitive or inductive pickup.

author: cowasaki
author_rank: Frequent Contributor
timestamp: November 13, 2019, 06:53:59 pm
content: Quote from: don.r on November 13, 2019, 05:39:15 pmThe mains is on the other end of the board so it's not that unfortunately.

author: Andy Watson
author_rank: Super Contributor
timestamp: November 13, 2019, 07:09:00 pm
content: QuoteDid you replace like with like? Is there a chance that you have created a ground loop? And/or not grounded something that should be grounded.

author: cowasaki
author_rank: Frequent Contributor
timestamp: November 13, 2019, 07:16:09 pm
content: Quote from: Andy Watson on November 13, 2019, 07:09:00 pmI photographed the original jacks - unless they had been messed up by the previous owner's "friend"This is the original photograph:Basically BLACK is ground whilst red and white are the two inputs. The black goes through the eyelet and on to the one next to it. This can't be seen too well in the image.It is shown in the circuit diagram but I can't tell if it's right or wrong. If you look at the input with the black and white wires going to it you can see that the two pieces of metal going to the hot connection don't make contact so the inputs were bent and corroded.

author: Andy Watson
author_rank: Super Contributor
timestamp: November 13, 2019, 08:13:34 pm
content: The wiring of the black wire does not look like something the factory would have done. However, the provision of three contacts in that white connecctor suggests that the jacks were wired to ground (rather than relying on the chassis.)What do you know of the history of this amp? You say it has been "worked on" before - do you know why? What was the original fault - i.e. the fault that caused it to be worked-on? How much of the original wiring has been changed?

author: cowasaki
author_rank: Frequent Contributor
timestamp: November 13, 2019, 09:04:15 pm
content: Quote from: Andy Watson on November 13, 2019, 08:13:34 pmUnfortunately the person who tried to repair it was an idiot who didn’t know what he was doing. He managed to solder a transistor in backwards and his general level of soldering technique was akin to mine using a 6” nail in a pair of mole grips over a camp fire! The wiring to the inputs looks wrong but I can’t quite work out what Peavey are showing in the circuit diagram. The wiring is shown there but I’m not sure what it’s trying to say!

author: Andy Watson
author_rank: Super Contributor
timestamp: November 13, 2019, 10:08:47 pm
content: Quote from: cowasaki on November 13, 2019, 09:04:15 pmSo there's a chance that the amp is not wired correctly. Do you have an actual wiring diagram - i.e. one than shows the wiring between modules? The schematic alone does not show how the manufacturer implemented the connections.Obviously the jacks should be grounded - somewhere! And it looks as if they were intended to be grounded to chassis - so (I'm guessing) either there is an extra ground elswehere that shouldn't be there (creating a ground loop), or there's a ground missing such that the amplifer is floating with respect to the chassis. Is there any evidence of extra or missing grounds around the main electrolytics ? (i.e. where work has previously been done).You say that the amplifier works (?) apart from the hum? - This suggests that the semiconducctors in the signal path are ok.

author: cowasaki
author_rank: Frequent Contributor
timestamp: November 13, 2019, 10:20:24 pm
content: Quote from: Andy Watson on November 13, 2019, 10:08:47 pmI’ve followed the schematic and rewired the input jacks. It work perfectly for five to ten seconds then hum again. I’m sure the jacks are now correct. Just looking at the zenor diodes and C2.

author: cowasaki
author_rank: Frequent Contributor
timestamp: November 13, 2019, 10:32:24 pm
content: I’ve replaced the zenors as I had the right value here and it was a quick job. C2 cap is fine.

author: cowasaki
author_rank: Frequent Contributor
timestamp: November 13, 2019, 10:38:48 pm
content: What doesn’t help is peavey using their own parts numbers. The main power transistors were SJ6392 which is couldn’t find at a reasonable price. I was then told that they are. 2N3055 I’ve got replacement op amps coming tomorrow as they were less then £3 and will go into my stock but there are two transistors 5332 and 5331 that I cannot find. They test as good using my Peak meter but may be out of spec.At this rate I will have replaced everything but the PCB. :-)

author: Kwakerman
author_rank: Regular Contributor
timestamp: November 13, 2019, 10:40:26 pm
content: Have you actually scoped the power supply rails to make sure they are free from ripple? Could be something as simple as a dead diode in the bridge rectifierAnd yes I'm the same kwakerman from Landyzone where we discussed speedo calibration!

author: cowasaki
author_rank: Frequent Contributor
timestamp: November 13, 2019, 11:19:29 pm
content: Quote from: Kwakerman on November 13, 2019, 10:40:26 pmHello.I’ve measured the ripple at 1.2v 100hz on the + line only. Basically there is a 100hz sine wave sat on top of the 30v rail. I checked the diodes and they show as ok. They have a Peavey part number but I’m sure that I can replace them with an alternative as they are only acting as a rectifier.

author: Kwakerman
author_rank: Regular Contributor
timestamp: November 14, 2019, 07:04:38 am
content: MR502s aren't anything too special and seem fairly common.https://www.web-bcs.com/diode/dc/ma/MR502.htmlhttps://www.ebay.co.uk/itm/MR502-Diode-200V-3A-DO-27-Motorola-lot-de-10-/172238616154

author: cowasaki
author_rank: Frequent Contributor
timestamp: November 14, 2019, 08:19:18 am
content: Quote from: Kwakerman on November 14, 2019, 07:04:38 amI ordered some diodes last night as I didn’t have anything up to that spec. Again if it’s not them they are good to have in stock. Not much else it could be though now!

author: cowasaki
author_rank: Frequent Contributor
timestamp: November 16, 2019, 11:48:14 pm
content: Well the diodes turned up and made no difference....So I hooked up my bench power supply at maximum voltage of 31/0/-31 which is less than the correct voltage but gave it a go and got a nice clean sound with the occasional crackle. I then tapped round the circuit with my trusty chop stick finding no issues then found the two zenor diodes that I replaced are hot to the touch. I think that the ones I fitted are not quite up to the job. I will refit the originals tomorrow or new ones of the correct rating.I am learning a lot with this amp which isn't worth what I have spent on bits other than the learning experience. I have created quite a number of test leads to safely connect a speaker cab and signal generator etc etc and am quite happy with the progress.So on to my next question......I have two "Tenma 72-10495" PSUs which are EACH effectively two 31v current and/or voltage limited supplies in the same box. You can run them independently or alternatively lock them together to give 61V between the positive of one side and the negative of the other side. This also means that the negative of the first side is effectively a centre tap giving 31v GND -31VNOW..... Can I run BOTH PSUs in linked mode and connect the negative of the first to the positive of the second to create a fixed centre point and give me the ability to supply 61v GND -61VIs that reasonable or a stupid idea?Most of the solid state amps I am repairing and looking at expect 40v+ on both positive and negative rails. Or should I just run these amps at a lower voltage?

author: sean0118
author_rank: Frequent Contributor
timestamp: November 17, 2019, 01:13:58 am
content: Quote from: cowasaki on November 16, 2019, 11:48:14 pmYou'd have to check the maximum isolation voltage rating of the Tenma supplies.But probably best to avoid if possible anyway, you'd have 120V present then. It's better to use low voltages when troubleshooting if possible.edit:There's a Youtube video that shows the wiring of the jacks. Looks like the wiring of the black wire etc is stock:Does the hum go away if you unplug the jacks? Also, what capacitors did you replace the 2200uF's with?oh, have you checked the resistance between the different connections to chassis gnd on the PCB? Maybe there is a broken track / solder pad.

author: duak
author_rank: Super Contributor
timestamp: November 17, 2019, 04:44:39 am
content: For a ripple voltage of 1.2 Vp-p on +43V with 2200 uF caps, I calculate a current draw of 264 mA. This is a little higher than I would expect. I don't see any adjustment for idle current though.I wonder if the output stage isn't oscillating. I understand that modern 2N3055s use a different construction (epitaxial?) than the ones from the 60s & 70s that gives them a higher fT, ie. current gain bandwidth, that could make them more prone to oscillation. Seehttps://en.wikipedia.org/wiki/2N3055When I was working with power amps, an oscillation often manifested as a hum because of increased current draw as well as a modulation of the oscillation frequency due to the ripple on the power supply. Are the output transistors hot with no signal?This oscillation is a possibility when using emitter followers with capacitive loads and is more likely with faster transistors. There's usually a small inductor in series with the speaker to isolate it from the transistor at high frequencies. Most of these inductors are a number of turns of magnet wire wrapped around 10 ohm, 1 or 2 watt resistors. The wire is soldered to the resistor leads and the resistor acts as a parallel damper.Also, I think the 2N3055 is marginal here. I've considered them to be safely useable up to 60 V whereas this circuit can easily subject it to at least 86 V and probably more. In an audio amp, the inductance of the speaker can cause high voltages to be generated when the load current is reversed. I suggest something like an MJ15015G.Best o' luck,

author: cowasaki
author_rank: Frequent Contributor
timestamp: November 17, 2019, 10:29:46 pm
content: Ok VERY strange. I’m powering the amp using my bench supply and it is still breaking up and noisy. But when it starts up it sounds ok for a short while sometimes.But if I short out C48 it works !!! Not just for a while either, it sounds fine with no breakup pops bangs or anything. I replaced the cap and it’s back to not working then short it and it works again. However Q10 gets hot and Q11 stays stone cold.

author: Renate
author_rank: Super Contributor
timestamp: November 18, 2019, 02:06:40 pm
content: Yow, don't short C48!You are unbalancing the whole output stage.You are putting a large amount of DC through your speaker.Which is why Q10 is hot.Did you check the DC voltage at the speaker?Did you check the ripple on the +/- 15V?Have you tried just putting your test signal in the "PWR AMP IN" socket?Have you tried removing U3 (is it socketed?) or grounding that "T.P.1"?(That defeats the whole overdrive protection.)Odd things (or errors) in the schematic:The bleeder 2k is shown as across only the +43, for under a Watt dissipation.I would guess it is across the + and - in reality, for 4W or so, more logical for a 5W R.The pilot LED is really off the secondary? It really gets hit with a reverse potential of 43V? A hearty LED.

author: cowasaki
author_rank: Frequent Contributor
timestamp: November 18, 2019, 06:06:08 pm
content: Quote from: Renate on November 18, 2019, 02:06:40 pmThanks. This is the only amp in 25 I haven’t been able to fix. As it seemed to work for a second when switched on from cold and then a short while when switches off I just tried to discharge all the electrolytics and suddenly it sounded right. I’m not running it like that for long it was just a few seconds. I have my suspicions about Q9 and Q8 but they test ok out of circuit. Q9 was the transistor I accidentally shorted in the first place. Splitting pre and power amp is one of the first things I do. With the bench power supply the pre amp section works perfectly. The issue is post R45.I’ve replaced all the 4558 op amps, Q3 and 4, all the electrolytics. The input jacks are now wired exactly as they should be. All the large resistors and most other ones have been tested. I’ve done a visual check with my amscope. All the pots have been removed, disassembled, cleaned, lubed, tested and reinserted.

author: Renate
author_rank: Super Contributor
timestamp: November 18, 2019, 06:42:26 pm
content: So, what's the DC output of the amp when it's clean and when it's not?Is the "Comp" or "Clip" or whatever LED ever on?Did you verify that 0.54V & 0.51V on R75, R78, R82?

author: duak
author_rank: Super Contributor
timestamp: November 18, 2019, 10:51:20 pm
content: Here's a link to a Peavy parts cross reference:http://matthieu.benoit.free.fr/cross/competitive/Peavey_Transistor_Cross.pdfAccording to the above, the 6392 devices cross to 2N3055A, an improved version of the 2N3055. I still think it's marginal because of its low Vce rating.When you have a moment, give us some circuit voltages, in particular the ones on Q3 & Q4, those called out near Q5, Q8 & Q9 and across R78 & R79.Is anything getting hot?

author: cowasaki
author_rank: Frequent Contributor
timestamp: November 19, 2019, 12:27:05 am
content: Quote from: duak on November 18, 2019, 10:51:20 pmMy 3055s are ones I've had for years but not used and likely not the A model. I've been servicing my Peavey valve amp this evening so not looked at the TKO. If I re-instate the capacitor I mentioned above and use the bench supply I get very poor quality and noisy output. I will measure these locations though. I may get chance tonight but it may be tomorrow.

author: cowasaki
author_rank: Frequent Contributor
timestamp: November 19, 2019, 01:05:45 am
content: Quote from: duak on November 18, 2019, 10:51:20 pmQ10 is mildly warmQ11 is stone cold but eventually becomes warmQ8 is very mildy warm eventually getting warmQ9 is at room tempQ5 is at toom tempAcross R78 = 0.26VAcross R79 = 12-13mVQ3 EBC = 1.41 / 1.03 / 27VQ4 EBC = 1.37 / 1.95 / 30.1VQ5 EBC = 30.6 / 6 to 6.5 av 6.3 / 30Q8 EBC = 30.6 / 30.9 / 7.3 (if physical pin out is same as Q5 ie left to right)Q9 EBC = 3.6 / -30 / 3 to 3.2Q10 EBC = -30.9 / -30.6 / 2.5Q11 EBC = 2.2 to 2.7 / 3.2 to 3.6 / 30.9-V rail = 31+V rail = 30.9615V rail = 15.37-15V rail = -15.24

author: duak
author_rank: Super Contributor
timestamp: November 19, 2019, 05:51:28 am
content: OK, we are now cooking. There is a large DC offset of about 2.5 V on the output. As long as C48 is not shorted it should be less than 100 mV because the DC gain should be X1 from Q3-B to the output. Q3-B & Q4-B should have about -100 mV (or less) on them AND should be within 100 mV of each other. The measurements show 1.03 & 1.95 V respectively so something is wrong. Since Q3-B is lower than its emitter, there was either a measurement error or the output offset voltage applied to Q4-B thru R70 is so overloading the differential input stage that Q3 is being cut off. This explains why it works when C48 is shorted out - it reduces the DC voltage fed back to Q3 & Q4 and allows them to work relatively normally.Since the output offset is positive, something in the circuit is either leaking current from a positive voltage, or something is not accepting current and passing it to a negative voltage.- Q9 has unusual voltages on the B & C pins - is this an error? If not, then something is not right.- Check Q7 & CR22 for shorts. Since Q9 was shorted out, mayhaps Q7 & CR22 were also damaged.- check R72 & R84 for correct values,- C37 could also have developed some leakage but it would require U4A-1 to have a high DC voltage on it.- check Q5 then Q3 and/or Q4 for leakage and current gain (beta),- R66 (22K) could have drifted upwards in value but it's unlikely.Do you have a 'scope to check for oscillation? Often when there's an oscillation it causes a big change in DC values that doesn't make sense. If you don't have a 'scope, use a 100 n cap in series with an AC meter around the circuit to see if there's some AC voltage that shouldn't be there.Most every part in this circuit can affect the DC operation, so if all else fails measure the values of the resistors in situ. If you read a value that's higher, then that resistor is probably bad.Cheers,

author: Renate
author_rank: Super Contributor
timestamp: November 19, 2019, 01:34:31 pm
content: Quote from: duak on November 19, 2019, 05:51:28 amNo, the input swings less than +/- 15 and the output is +/- 40. Q5 is the gain stage.I presume that the measurements were taken without any audio input???I presume that the voltages Q10 and Q11 are swapped?I presume that the voltages on Q9 are actually ECB?I suspect C49/R72/R84.What is the voltage at the junction?Isn't the "Comp" LED on? Did you mention that already? It's significant.Why did we have to wait to page two to find out that the output DC voltage wasn't zero?That should be one of the first things to check.If you don't have a scope and you're concerned that it may be oscillating,just put your finger on R81 and see if it's at all warm.

author: Axtman
author_rank: Regular Contributor
timestamp: November 20, 2019, 07:22:50 pm
content: You might want to check out Music Electronics Forum.https://music-electronics-forum.com/I repair guitar amps and find the people on that forum to have a wealth of knowledge and experience. (Not that I am dissing anyone on this forum.)

author: duak
author_rank: Super Contributor
timestamp: November 21, 2019, 07:56:40 pm
content: Quote from: Renate on November 19, 2019, 01:34:31 pmRenate, the gain is established by the ratio of the impedances of the feedback network, primarily 1 + (R70 / (R69 + Xc of C48)). The DC gain is X1 because C48 is an open circuit at DC and the full output voltage is fed back into the inverting input of the first stage. OTOH, the midband (1 KHz) AC gain is X23. When C48 is shorted out, the DC gain is increased to X23. Not a good idea because any drift or offsets in the amplifier will cause a current flow in the speaker. C45 reduces gain by increasing negative feedback at high frequencies, say above 50 to 100 KHz.

author: cowasaki
author_rank: Frequent Contributor
timestamp: November 21, 2019, 11:41:54 pm
content: Sorry everyone. I haven’t gone away I’ve been working silly hours. It’s my weekend off so I will have another look. I MAY have mixed up the measurements as I was rushing and they are not labelled on the board. You have to look at the PCB layout and it was rotated. I will double check them either tomorrow evening or weekend.Do you want the measurement with an input sine wave and the volume at a certain level? Or at zero? Please advise and that is what I will do. I’ve just received replacements for the two transistors Q8 and Q9. Both these passed using my tester but Q9 has a crack in it which I noticed after my last post. But still measures fine when removed from the circuit.

author: duak
author_rank: Super Contributor
timestamp: November 22, 2019, 01:01:51 am
content: re silly hours: Been there, done that & hope I don't have to again - unless I really want to.I can see from your measurements there is a DC offset problem so AC measurements may not help.You say Q9's case is cracked but still tests OK? How are you testing transistors?Renate & I have suggested testing certain parts. May we have you do so?I see that you measure the +V & -V rails at around 31 to 32 V instead of 43 V. Is the schematic you've provided the one for the unit in question? If it's correct maybe the power supply reservoir caps C52 & C54 are bad and there's a high ripple voltage on them.

author: Renate
author_rank: Super Contributor
timestamp: November 22, 2019, 02:31:38 am
content: I think he's bench testing with his +/- 31 supply.@duakErm, you're right, the DC gain is unity.I kind of meant that with the differential pair nailed to the wall we're dealing with the open loop gain.It's best to test it right now without any audio, especially since you're using a meter, which lies as soon as it sees any AC.I know that the DC output is only a couple of volts, but I'd prefer running it into a resistor instead of a speaker.

author: duak
author_rank: Super Contributor
timestamp: November 22, 2019, 05:02:50 am
content: @Renate: I think you're right about the OP using a power supply. The ratio of the two just happens to be close to sqrt(2). At my age I often forget things when I put them aside and pick them up again. I'm pleasantly surprised when I find something that I didn't think I had.

author: cowasaki
author_rank: Frequent Contributor
timestamp: November 22, 2019, 09:01:25 am
content: Yes I am testing using by bench power supply. I have removed all the transistors from the board and tested them with a Peak DCA tester. Whilst in circuit it can’t tell what they are. I’m just on a five minute break at work so had chance to answer. If you want the measurement with the actual transformer and with the bench supply I will happily do that. I will give you all the measurements you need. I have a Keithley 2100, how many decimal places do you want?Anything anybody needs checking just let me know. I’m going to use a permanent marker and mark the board off the diagram to avoid issues and swap out the two transistors. If it isn’t them I’ll put the originals back. I have a decent desoldering station so replacing components is a breeze.Thank you everyone. I am learning plenty. Although I have fixed 25 of 26 only 5 or 6 were complicated.

author: Renate
author_rank: Super Contributor
timestamp: November 22, 2019, 01:18:53 pm
content: When you're working on a circuit with a lot of transistors nailed together in close proximity, it's hard to see where the current is flowing.Usually (but not always) resistors are sort of reliable, they are your best guide to where the current is.You see that Peavey put a bunch of voltage measurements across resistors on the schematic.The main point of the amplifier is to DC balance itself with the correct amount of idle current.Producing audio is only a minor feature.My usual philosophy is to try to leave everything in place and not confuse myself by swapping things.I might jumper in resistors to see if a voltage is "hard" or "soft".Q3 seems to be leaking current (30-27V)/10k = 0.3mA, but that's not enough to be your whole problem.Q5 should be mostly off, but it doesn't seem to be getting any pulldown.Is that Q5 or the lack of the bottom current source (C49/R72/R84) or a pullup through Q6-Q11?If you have Q6-Q11 all out already, you can check the collector voltage on Q5 by itself.It should kind of be able to balance the differential pair (Q3-4) by itself.(Don't remove the transistors for this! Only if they are out right now.)

author: Renate
author_rank: Super Contributor
timestamp: November 25, 2019, 08:14:51 pm
content: Quote from: Renate on November 22, 2019, 01:18:53 pmAh, you were taking all these measurements with an audio signal going, that's not leakage.Quote from: cowasaki on November 22, 2019, 09:01:25 am2 significant digits should be fine.This is an audio amplifier, it's really not that precise.The think is, the measurements that we need are mostly differential, they are all under one volt.It's better to say the voltage across R1234 is 0.72 than that the voltage on one side is 42 and the voltage on the other side is 41.Peavey really did a decent job on the schematic, they tell you what to look for.Q5 is supposed to take 10 mA at idle.R72 is supposed to take 10 mA.So where's the current going?

author: cowasaki
author_rank: Frequent Contributor
timestamp: November 25, 2019, 09:12:40 pm
content: Sorry had a few other repairs to do so not had a chance to get back to this.Will be having a look either tomorrow or Wednesday.

author: cowasaki
author_rank: Frequent Contributor
timestamp: November 26, 2019, 07:58:40 pm
content: Q9 !!!The transistor I shorted originally that blew other components STILL tests as good. I replaced it (it was a 5332) with a MJE253. There is a crack in the case of the 5332 and I think the heat caused issues as it warmed up. There are still a few crackles but the amp itself now works. I'm going to replace the 5331 with an MJE243 as its a matching pair then take a good look over it again and trace it through to find that crackle.Thanks for everyone's input I really do appreciate it.

author: sean0118
author_rank: Frequent Contributor
timestamp: November 27, 2019, 07:26:08 am
content: Good work, thanks for posting the fix.

author: Renate
author_rank: Super Contributor
timestamp: November 27, 2019, 07:38:58 pm
content: Err, you should double check the DC level of the output amplifier to make sure that it's zero.

author: cowasaki
author_rank: Frequent Contributor
timestamp: November 27, 2019, 09:38:38 pm
content: Quote from: Renate on November 27, 2019, 07:38:58 pmI will have a look. Had to stop at that point