| 200B 5 PA head hum/hiss issue [message #28094] |
Tue, 09 March 2021 22:47  |
Drjones1106
Messages: 19
Registered: March 2017
Location: Seattle
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Junior Member |
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Hey all, I'm back again for more info on a project. I have a nice white sparkle tuck and roll model 200 b-5, 4 channel PA head that is having some serious hum/hiss issues. I can't quite pin it down as 120Hz, but that sounds pretty close. I read elsewhere on the site that some hiss in normal in these amps, but this seem excessive. I've tried it on every cab I have, at friends houses, etc. It is there with no input, with input, it is unaffected by any of the preamp controls (vol, bass, treble), and is still there with the reverb tank disconnected. I have already replaced the cord with a 3 prong grounded. Using my crappy oscilloscope I've narrowed down the hiss to where the 50V positive and negative supply hits the output transistors. To me that points to the filter caps. I don't have a multimeter that can measure the 4500uf to see if they are still on value (I just can't bring myself to spring for a fluke). Is there another test I could to do see if they are leaky? I have already ordered some 4700uf 50V radial EL caps to test with, if that fixes it I'll spring for some higher voltage big honkin' can caps, but while I'm waiting for them to ship I thought I would ask if there is anywhere else in the amp I should take a look at. The one weird thing is if I lift the tip of the grounded reverb footswitch jack, the volume of the noise decrease by about half, but there is still noise. Which is very weird, because the noise as seen on the scope isn't in the signal path on that circuit board.
Can anybody point me in the direction of other things to test?
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| Re: 200B 5 PA head hum/hiss issue [message #28099 is a reply to message #28094] |
Thu, 11 March 2021 07:11  |
stevem
Messages: 4774
Registered: June 2004
Location: NY
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Senior Member |
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Ok, let's set up some basic common terms, because I am not sure what you are talking about as the main issue your hearing!
Hiss is a normal result of gain in a semiconductor, but of course this can be excessive if the semiconductor has a problem, or if certain resistors in the circuit have gone south and are making hiss and adding it into the audio stream.
I liken hiss to the sound of a small amount of air leaking passed your tire valve when your filling up your car tire.
This then is not like a hum or a ground buzz noise.
There is also a noise present called noise hash that is made up of the sound of the diodes in the rectifier bridge switching on and off, and this noise will be found through the amps audio stream , be it can be reduced or largely eliminated.
Hum issues can be 60 hz AC voltage related, or 120 hz AC ripple voltage related which can only be made after the diodes.
You seem to think that your issue is on the pc 703 board mounted on the rear wall of the amp.
This board is both the driver board for the output section and has the 8 volt positive and 8 volt negative regulator circuit on it.
One way to fully confirm your finding is to disconnect the blue wire(s) that feed the audio signal into it from the pc803 reverb board.
I would also check the 8 volt + and - from this board because if these two voltages that all the other circuits in the amp run on is not pretty well balanced then there will be a hum in the output of the amp.
In this regulator circuit the negative voltage output tracks the positive, so if the positive is not at the correct voltage, neither will the negative.
There are small size tan electrolytic caps in this circuit that go leaky, of flat out bad.
Also if the four TO3 output transistors are not closely enough matched there will be a DC off set voltage on the speaker output which will make for a hum being heard.
In terms of the stock diode bridge you can up grade it by installing four 01uf 200 volt or better ciramic disc cap across the terminals of the rectifier bridge .
A far better thing to do to get the noise hash out of the audio stream is to get some terminal strips and four modern fast type recovery diodes like UF5404 types and just make a whole new bridge for the amp.
Your strange issue you report with the jack in the reverb circuit I will have to put done thought into and get back to you, but now I think you have a few other things to go fourth and check and report back on.
[Updated on: Thu, 11 March 2021 17:43] Report message to a moderator |
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| Re: 200B 5 PA head hum/hiss issue [message #28121 is a reply to message #28094] |
Wed, 17 March 2021 06:04 |
stevem
Messages: 4774
Registered: June 2004
Location: NY
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Senior Member |
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Hi!
Q801 is part of the reverb pan driver circuit and I think you have said that your testing has been done with the pan not being hooked up.
If indeed this is the case then no issue that may be with any of the circuit on that input side of the pan could be getting into the audio stream.
That being said, unless your testing with a transistor tester that needs 3 of its test leads to be hooked up and is powered by a 9 volt battery then there is no guarantee that the Transistor you have tested is not going leaking in circuit when its powered up, and in turn passing some level of positive power supply voltage into the audio stream.
Also when you state that caps you have tested are right on the money uf rating wise you need a dedicated cap tester to test for leakage, you have not stated that your test out does that, if it does then all is good.
For instance, if C805-806 or especially 807are leaky then power rail voltage would be getting into the audio stream.
All of these caps a just mentioned are coupling caps which separate circuits into separate areas.
If for instance you lift the down stream end of c805 up off the board, then the whole 2 Transistor reverb return amp circuit is out of the picture for being a cause of your problem .
[Updated on: Wed, 17 March 2021 07:37] Report message to a moderator |
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| Re: 200B 5 PA head hum/hiss issue [message #28131 is a reply to message #28121] |
Sun, 21 March 2021 18:09 |
Drjones1106
Messages: 19
Registered: March 2017
Location: Seattle
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Junior Member |
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Thanks Steve, I did all of the reverb circuit testing with the reverb pan hooked up and the controls at max, as per the schematic notes. The testing I've been doing so far has just been with a generic digital multimeter. using the capacitance setting for caps, and using a diode test on each juncture of the transistors to see if any have failed. I just bought a cheap transistor and esr tester on amazon. I'll re-test all the components and see if anything is way out of wack. I know these are not super accurate or reliable testers but I'm hopeful it will give me a ball park. I'm about to go test all the mentioned components in the reverb circuit.
In the meantime I have done a little playing around with the head, just to hear how its performing, ignoring the noise. I can play over it with out too much issue. But I've run into some other concerns. the overall volume of the amp is much lower than I would expect for a 100w head. With the volume at noon and it plugged into my 2 8ohm 1x15 peavey cabs, its only about bedroom volume. My 15w orange head is louder than that with vol at 9 o'clock into the same two speakers. If I turn the volume up past 3 o'clock, it get very distorted. the volume gets quite on the attack, then fades back up as the note decays. It sounds like a transistor distortion pedal that's biased poorly.
I realize this is a PA head, and isn't really meant to be dimed like this, but I still expect more volume than this. Distortion at high volumes is desirable, but this isn't usable distortion. its the same on all 4 channels so I'm not convinced its in any of the preamp boards. Is this an issue of impedance matching? I'm aware of how tube amps react, but I'm not as familiar with solid state.
I've taken voltage readings at all 4 of the RCA 36892 output transistor leads and the voltages are pretty close to the values on the schematic. I don't expect them to be perfect on an amp this old. I'd like to take voltages on the 4 RCA 38736 transistors in heatsinks on the board, but I've got to find a way to do that without shorting anything out. After that I might need to remove them to try and get a reading on this new tester. I'll see if I can find the datasheets for value reference.
sorry this keeps becoming more and more of a rabbit hole every time I post, but I really appreciate the help.
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| Re: 200B 5 PA head hum/hiss issue [message #28133 is a reply to message #28094] |
Mon, 22 March 2021 14:55 |
Drjones1106
Messages: 19
Registered: March 2017
Location: Seattle
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Junior Member |
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Actually, I do have an oscilloscope. Albeit an older, funky CRT. I should able to get measurements out of it, if not super accurate ones. If you'll tell me what parameters to set up/ what I'm looking for I can run some tests. I've got a signal generator too. I'm not super knowledgeable about using it for specific measurements, I mostly use it to find noise, and look at waveform clipping in the guitar pedals I build. But I can always hit up youtube university if I need more info.
regardless any audio testing will have to wait for Thursday. I desoldered Q703 yesterday to test, and one of the legs fell off. So that could have been causing some issues. I'll have a replacement nte108 here then. I also picked up uf5404 diodes for a new rectifier, cause why not.
The Peavey cabs in parallel measure, surprisingly, 4ohms exactly.
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| Re: 200B 5 PA head hum/hiss issue [message #28148 is a reply to message #28094] |
Fri, 26 March 2021 16:45 |
stevem
Messages: 4774
Registered: June 2004
Location: NY
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Senior Member |
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Ok, here's what I have for you, but first some notes!
1) my wall voltage during these test was 117.5.
2) my dummy load resistor checks out at 4.5 ohms.
3) I had to do these test on my 200B-2 amp ( just 2 channels ) because I forgot that my Brother has my 200B-4 head.
This means that your amp will be drawing a bit more power supply current then mine so your peak power wattage will not be quite what mine was.
2a) my amp still has its original main rail power supply filters from 1969 So who knows maybe your new filters will off set the higher current draw of your amps added two preamp boards and reverb board.
I guess you could unpower two preamp boards if you cared to do you would test out closer to my amp.
At idle I had 39.6 volts on the positive rail and 39.4 on the negative rail.
At idle one 8 volt rail was +8.37 volts, the other rail was - 8.23 volts.
My amps max RMS output voltage was 19.6 for a wattage of 85.3 .
This reading was taken at the very onset of clipping seen on my scope and I found I had better control of that setting point by also adjusting the Treble knob.
At this wattage output my main plus rail was down to 34.1 volts, the negative rail was 34.1 also.
Peak output voltage was 25.16 for a wattage of 140.6 .
At this power level the main plus rail was down to 32.3 and the negative was 32.2.
The regulated power rails where + 8.34 and the other was -8.19.
If your amp test out close to these numbers then your power supply is heathy and we then can look deeper for the source of your issue.
[Updated on: Fri, 26 March 2021 16:48] Report message to a moderator |
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| Re: 200B 5 PA head hum/hiss issue [message #28149 is a reply to message #28148] |
Fri, 26 March 2021 19:01 |
Drjones1106
Messages: 19
Registered: March 2017
Location: Seattle
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Junior Member |
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Ok, Long story short, my amp does not test anywhere close to yours. here are the details.
set up the same test parameters as you: 4ohm across the output, ac volt meter and o'scope on load resistor. DC volt meter for internal checks. Test signal set to 1khz, measured at 0.150v AC on meter.
few other notes. I never fully installed new filter caps, I just mocked them in to see if it got rid of hum. when there was no change, I hooked back up the stock.
I have replaced the stock rectifier with 4 uf5404, and I have replaced a few resistors on the power amp board that tested more than 10% off value. I can give a specific list if necessary. haven't checked all, but the ones I've replaced with 1w 1% metal film resistors.
wall voltage read 120 before the fuse.
load resistor was 4.1ohm
Voltage rail at Idle
+39.4v and -38.9v
8v rail (tested at the red and green wires leaving the power amp board)
+8.06v and -7.93v
the output read -20.8mV dc, 0.01Vac with signal, but volume at 0.
With test signal on, volume increased to highest voltage reading
Output AC reading
1.269v
Rails +39v and -38.6
with volume at full clockwise
Output AC reading
0.29V
Rails
+39.2v and -38.9v
So I'm not sure how to calculate wattage from these numbers, but it is clear that I'm way off. Should I start tracking down some replacements for the output and/or heatsink transistors?
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| Re: 200B 5 PA head hum/hiss issue [message #28153 is a reply to message #28094] |
Sat, 27 March 2021 15:50 |
stevem
Messages: 4774
Registered: June 2004
Location: NY
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Senior Member |
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I just tested mine and with a .960 volt drive signal into the RCA jack I got 8.2 volts into my load resistor, so I would have to say that your 703 output board pretty much looks ok and whatever is wrong with the 803 board is not only messing things up, but loading down the power supply some also,or your load resistor might test out higher then mine does at 4.5 ohms.
[Updated on: Sat, 27 March 2021 15:52] Report message to a moderator |
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| Re: 200B 5 PA head hum/hiss issue [message #28154 is a reply to message #28094] |
Sat, 27 March 2021 22:09 |
Drjones1106
Messages: 19
Registered: March 2017
Location: Seattle
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Junior Member |
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Success!! not total, but some major improvements.
It ended up being q804 that was bad. Once I replaced it with a 2n3904, I got all the volume back. I could hear the test signal across the load resistor with no speaker. I did not end up with super accurate voltage reading because I never properly heat-sinked the resistor, melted some plastic and singed my desk! oops. what I got before I realized I was melting probes was 16.8V at the output. Woohoo!
So now that I'm at working volumes again, I've still got serious noise issues. It is both the power amp and the reverb still. I disconnected the input from power amp, and it sounded like 60hz hum. I reattached the reverb, tank and all, disconnected the preamp boards (realized channel 3 was not properly soldered after the last time I did this.) the noise now sounds different, and is louder. If I unground the reverb footswitch, the noise goes back to just 60hz hum, with some artifacts, like quite pops and clicks. So I'll start working through the remaining components on the board and see what I find.
Is there a way to test the reverb pan itself? I still get reverb sounds out of it. so much so if I move anything near it the crash is massive. its very sensitive.
I'm also wondering about the replacement transistor I put in for q703 on the power amp. one of the legs fell off when I removed it for testing. It was a metal can RCA, either 38735, 36735 or 35735. The second digit is very hard to read. The schematic says it should be a pet1075, power amp board 900 has a 38735 in this same spot. I replaced with at NTE108, which I saw recommended elsewhere on this forum as a pet1075 equivalent. I think the hum in the power amp board started after this replacement.
anyway, a huge improvement, Thanks so much!!
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| Re: 200B 5 PA head hum/hiss issue [message #28155 is a reply to message #28094] |
Sun, 28 March 2021 08:11 |
stevem
Messages: 4774
Registered: June 2004
Location: NY
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Senior Member |
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Hi!
First off the only place in the amps signal chain where 60 hz hum can get in is my means of the power transformer magnetically coupling into the output side coil / transducer of the reverb pan.
Any in the amp, circuit wise after the rectifiers can only be 120 hz ripple voltage making for hum issues getting into the audio stream.
There was a factory fix for 803 board noise and feedback problems in amps made before April 4th 1969.
The fix is this.
R836 is changed to 82k.
R837 is changed to 82k.
R830 is changed to 47 ohms.
R848 is changed to 15k.
The reverb pan should test out ohms wise at about 175 on both the input and output side.
If you have the correct reverb pan for the amp then only the output side of the pan should be grounded to the main case of the pan and to the output RCA jack on the pan, and of course on back thru that RCA cable to the 803 board.
The input side of the pan should not be grounded to the main case of the pan.
If the output side of the pan is hooked up and mounted or sitting near the PT and the reverb is turned up you will get a lot of 60 hz heard thru the output.
Pops and clicks in the audio stream sound like the case of a noisy transistor to me!
Q703 is a 38735 which crosses to a NTE108.
Maybe that broken off lead has caused other components around q703 to go bad if the amp was powered up like that.
For continued trouble shooting I would disconnect and unpower all but the one preamp board that seems to work normal!
[Updated on: Sun, 28 March 2021 08:22] Report message to a moderator |
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| Re: 200B 5 PA head hum/hiss issue [message #28163 is a reply to message #28094] |
Thu, 08 April 2021 05:54 |
stevem
Messages: 4774
Registered: June 2004
Location: NY
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Senior Member |
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Well lets first confirm you do not have any blown output Transistors fromt that critical bias diode going open on you, because that's what likely took out the fuse.
By the way when I work on the output stage of the amp I like to lower the rating of the fuse by 25 to 50 % so that it blows faster if I do something wrong or something goes south on me.
You just need to of course remember to replace it when making a full output wattage test.
[Updated on: Thu, 08 April 2021 06:00] Report message to a moderator |
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| Re: 200B 5 PA head hum/hiss issue [message #28171 is a reply to message #28094] |
Fri, 09 April 2021 20:54 |
Drjones1106
Messages: 19
Registered: March 2017
Location: Seattle
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Junior Member |
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A lower wattage fuse is a good call. I should have been plugged into my light bulb current limiter but like a dumbass I was plugged into the wall.
All the transistors test out fine. I took q1-4, q704/5, q708/9 and tested them all on my transistor tester. they all registered as working. though my last question about relative hFE values still stands. I get volume out of the amp. I tried a guitar and its pretty loud. I couldn't do a full vol test, so I set up a load test (same as last time) to get a voltage reading, and got some strange results. It peaks around 10v ac at around 11 o'clock volume, but then drops to around 3v after 12 o'clock. at full clockwise it comes back up to around 7v.
I didn't hook up the oscope, so I'll do that later and get some more qualitative info.
At least it powers up and isn't blowing fuses. still has a wicked noise floor. I'll also be doing more work on that front. I intend to disconnect all but one preamp, and bypass the reverb board, straight to the poweramp to see what kind of noise is there.
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