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Old 11th February 2009, 05:49 PM   #21
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Quote:
Originally posted by tmhajw
As I understand it, lifting the earth in your monoblocs will encourage the signal grounds to flow back to your preamp, as they should.
The simplest way to do this is to locate your earth wire on the immediate inside end of your power supply socket, cut it & insert a 47 ohm resistor. This should incidentally present little problem for chassis earthing should any catastrophe occur & it ever go live.
Try it and see. You can easily reverse it....
Thomas
I've done some more reading on the subject of ground loop issues and the possibility of "lifting the ground"...as I understand it, this process would help eliminate grounding problems due to other equipment being used in association with the amp and therefore the noise that might result. Since the amp is completely isolated, as I mentioned above, other than its power source and its connection to the speaker and still creates the noise this seems like a dead end. If I'm wrong, please explain why.

While considering the issue of lifting the ground, I had taken another look at the grounding of the amp in question. I noticed for the first time while examining the chassis ground point at the power cable socket that the paint surface was not scrapped away where the ground strap was bolted down. While shifting the wire gently it snapped off in my fingers...gotta love that built in China quality! At any rate, I disassembled it all, cleaned the paint off the grounding point and repaired everything properly. Then I checked for continuity through the chassis from the socket to all the various grounding points in the amp. A re-test of the amp showed no difference, it still makes the same noise.

I also decided to short the inputs after trying to read more about it. I'm not really sure what it rules out in this situation. I took an old cheap RCA connector, cut the leads and shorted them out, tested for continuity between the center contact and outer ground, plugged it into the amp and powered it up. No difference, the amp still makes the same noise.

In considering the situation further, and trying to absorb more information, I've started to understand that there is a difference between "hum" as described in electronic circuits and hum as I would describe it from a layman's perspective. I am trying to determine now if what I am really hearing is what would be considered "ripple" which would be a 120Hz noise (as Ty_Bower previously mentioned). My judgment is completely subjective on this matter, and lacks any real experience. After listening to the amp a few more times today, I would have to say that if this noise was "hum" (60Hz) that I would imagine its frequency to be very deep sounding, this noise does not have that quality to my ears. I would be more inclined to say it sounds more like "ripple" or a 120Hz noise. That being the case, it would seem that I need help trying to troubleshoot the power supply and/or internal wiring according to what I have read in some of the more simplistic troubleshooting flow charts I have seen.

As a matter of course I would like to check the capacitors to see if there are any obvious signs of a bad capacitor. Can this be done with the capacitors installed in the amps circuit board? or do they need to be removed? I have read conflicting info about this...if someone can point me in the right direction I would appreciate it. I assume I am testing for resistance. All I have is a standard Craftsman digital multimeter on hand. Thanks.
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Old 12th February 2009, 01:04 AM   #22
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Not sure if this was mentioned yet or not:
C4 and C5 look to have quite long leads,and it appears that C4 is right next to the AC power wiring?
I would suggest trimming the leads a bit,and getting them a bit closer to the board,and away from the AC wiring. It could be picking up hum and injecting it right into a tube's grid.
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Old 12th February 2009, 08:21 PM   #23
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Quote:
Originally posted by DigitalJunkie
Not sure if this was mentioned yet or not:
C4 and C5 look to have quite long leads,and it appears that C4 is right next to the AC power wiring?
I would suggest trimming the leads a bit,and getting them a bit closer to the board,and away from the AC wiring. It could be picking up hum and injecting it right into a tube's grid.
Yes...this was mentioned a few times. Whether or not it was an issue, I wasn't sure. I thought the extended leads might also add more resistance to the circuit (more on this later).

At any rate, I was checking the resistance of the capacitors in the amp today (I had read that they should pretty much read infinite and I should see the resistance continue to increase as the caps charged if I had my multimeter connected to them in the correct polarity and they were not leaking) this only seemed to work partly in theory with the caps still mounted on the circuit board, as the longer I tested them the slower the seemed to charge, and they never seemed to charge as fast as some of the new caps I tested that had not been installed.

As I sat scratching my head and trying to think of what to do next, I decided that I might give shortening the leads on the C4 and C5 caps a shot. While I was at it, I used Ty's response to my questions about these earlier as a guide to reinstall them with the correct polarity.

Ahhh...the putrid stench of fried resistor. Don't ask me what happened. I shortened the leads and set the caps back in place having to only reverse the polarity of one of them (C5).

The first time I powered the amp up I didn't know what to make of it. The sound through the speaker was sort of like an oscillating distortion of some sort. Almost sounded like a flag waving in the wind. I shut it down quick and took a look at the board and caps to see if anything obvious looked wrong.

I decided to put C5 back to how it had been installed, rechecked everything visually and tried firing the amp again.

Same thing happened. I listened for a second too long, the second time. R14 on the board is fried. I'm not sure what happened. I thought that maybe some solder I lost track of might have shorted something out, I can't find anything obvious. Right now I'm on my way to Radioshack to see if I can come up with a 390 1/2 watt replacement.

This just gets better and better. Am I even learning anything here? or am I just banging my head against the wall and screwing up any chance of getting this thing fixed? Ugh!
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Old 12th February 2009, 08:30 PM   #24
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Quote:
Originally posted by chromenuts
The sound through the speaker was sort of like an oscillating distortion of some sort. Almost sounded like a flag waving in the wind.
I think the term for what you describe is commonly known as "motorboating".

I can't imagine that anything you would have done with C4 and C5 would have cause this. Something else must be going on...
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Old 12th February 2009, 10:03 PM   #25
chrish is offline chrish  Australia
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Quote:
Originally posted by Ty_Bower


I can't imagine that anything you would have done with C4 and C5 would have cause this. Something else must be going on...
First of all, I must state that I am not an expert and I am just learning...

To my eyes, shorting out C4 and C5 has caused problems. C4 and C5 are coupling capacitors that keep the DC voltage out of the signal that is sent to the grid of the pentode section of the valve. Shorting these out means that you are biasing the grid of V2B with +180 volts and the grid of V1B with 45 volts. Looking at the plate curves for the pentode section of the 6BM8, with those kinds of bias voltages, you are way off the curves at 200mA+. 200mA across your bias setting resistors R14 and R15 is going absolutely fry them (as you found out).

Also, using a little Ohm's law, you will see that there is an error in the schematic for the rating of R14 and R15. with about 40mA flowing through the valve, these resistors will be dissipating around .6 to .7 Watts, a 1/2 watt resistor is not going to be good enough here. I would use a 2 watt or better.
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Old 12th February 2009, 10:38 PM   #26
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He shortened the leads - as in "make them not as long." He did not short the leads, or make it a short circuit. At least, I hope he didn't. That would be a bad thing.
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Old 12th February 2009, 10:57 PM   #27
chrish is offline chrish  Australia
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Guess I should put my glasses on

Maybe whoever sold them had problems after the mods they made and tried selling them to make them someone else's problem...

FWIW, I am in my study now, and my amp in here is a copy of this cct. I do get some humm at the speakers, but not much and not audible from 30cm away. I did, however, add more power supply filtering. It was a cheap build to try my hand at a point to point design, so I did not use a choke. I ended up with a CRCRCRC filter, as i had two JJ dual can caps (40+40 and a 100+100).

Chris
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Old 12th February 2009, 11:12 PM   #28
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I'm still extremely suspicious of the power supply caps (C8 and C9). The original hum, now the motorboat symptoms all seem to point towards it. Were C8 and/or C9 ever replaced?
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Old 13th February 2009, 02:33 AM   #29
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Quote:
Originally posted by Ty_Bower
He shortened the leads - as in "make them not as long." He did not short the leads, or make it a short circuit. At least, I hope he didn't. That would be a bad thing.
No I did not create a short circuit in place of C4 and C5.


Quote:
Originally posted by chrish
Guess I should put my glasses on

Maybe whoever sold them had problems after the mods they made and tried selling them to make them someone else's problem...

FWIW, I am in my study now, and my amp in here is a copy of this cct. I do get some humm at the speakers, but not much and not audible from 30cm away. I did, however, add more power supply filtering. It was a cheap build to try my hand at a point to point design, so I did not use a choke. I ended up with a CRCRCRC filter, as i had two JJ dual can caps (40+40 and a 100+100).

Chris
You may be right about the reason for sale...I guess the reason doesn't really matter. That's the risk you take with used equipment. I've had my share of issues with used pieces...nothing like this before though. The thing about the "hum" is that the
other mono-block sounds fine...as you describe your amp, very little noise unless you listen very closely to it, and there is no obvious difference (except the cap I changed out in the noisy one not being up to spec). The bad amp is has a very pronounced and distracting hum which I have speculated (due to inexperience) is more likely considered ripple since it sounds more like a 120Hz noise than a 60Hz noise to me.

Quote:
Originally posted by Ty_Bower
I'm still extremely suspicious of the power supply caps (C8 and C9). The original hum, now the motorboat symptoms all seem to point towards it. Were C8 and/or C9 ever replaced?
I'm now back to square one. I tested R14 before moving forward, and remarkably it still showed the correct resistance. I did manage to get replacements, but decided to leave it for now. I cleaned down the board with some alcohol and a toothbrush so I could see everything more clearly. As I was working on it, I noticed a small blob of solder seemed to wash out from under one of the tube sockets. It was in close proximity to where I had been working on one of the caps and must have migrated off the backside of the iron without my noticing it.

P.S. C9 was the cap that was replaced because it was not up to spec according to the manufacturers schematic.

So now I have the same wonderful humming amp as before...except with shortened leads on the filtering caps. Yippie!


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Old 13th February 2009, 05:10 AM   #30
chrish is offline chrish  Australia
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Once again, I am not an expert, so take my advice for what it is worth...

If there is evidence of any damage to components, replace them, even if they test OK. If the resistors were smoking, they were passing too much current. Try to find out why before re-applying power. R14 and R15 require 2Watt resistors. They are only a few cents each, so use the right rating or you WILL end up with more problems in the future.

1. You have two monoblocks, one works one does not.

2. You have a multimeter.

3. You have a schematic.

4. You have the help of the members of this forum.

5. You should be able to get a free program to turn your PC in to a simple tone generator.

I would suggest printing out two copies of the schematic and mark them clearly "working amp" "humming amp" or similar.

I would then start taking readings with the multimeter, both AC and DC in a systematic manner. Ty suspects power supply, so it might be as good a place as any to start.

Check the diodes of the power supply with the diode test of the multimeter, should be open cct when reverse biased and show correct forward drop when forward biased.

Check the voltage (AC and DC) at C9, then C8, then C2, then C1. Hopefully this will show any problems with power supply and filter caps. If the non-working amp has greater AC, then there is a filtering problem.

Measure at all of the points on the schematic that have a DC voltage reference.

Measure the DC voltage at the junction of R8 and C4 and also the junction of R9 and C5 to see if you have leaking coupling caps (there should be 0 DC voltage at these points).

When taking these measurements with the power on BE EXTREMELY CAREFUL! Clip the negative probe of the multimeter to GND and use the positive probe to take readings. Put your other hand behind you back, in you pocket etc. Do not use it to steady the amp, hold the chassis etc!

Report back with results...

Chris
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