Looks like tracing it out myself is my only option. Apparently I'm "not authorized" to send email to Bogen. 🙄
The pot I assumed was for bias adjustment. Do you want me to measure the resistance on it?
It's not that I don't feel comfortable poking around in it, I have to start somewhere after all, I'm jut not sure what I'd be looking for.
The pot I assumed was for bias adjustment. Do you want me to measure the resistance on it?
It's not that I don't feel comfortable poking around in it, I have to start somewhere after all, I'm jut not sure what I'd be looking for.
Hi Turbo7MN,
Give Bogen a call. There still provide schematics for PA amps in the owners manual so I don't think this should be a problem for them.
-Chris
Give Bogen a call. There still provide schematics for PA amps in the owners manual so I don't think this should be a problem for them.
-Chris
I think I may have found a couple problems. I was testing a few of the resistors and discovered that what I think is the negative feedback resistor, rated at 15K ohms, appears to be wide open. I'be been checking them just by placing the multimeter leads on each side of the resistor, seems to be working for most of them but it shows that one as being 0 ohms.
Also, that cathode resistor, the one between the tubes, also reads 0 ohms.
Do these sound like logical explanations? I'd like to get this thing fixed so I can start playing through it again.
-Darren
Also, that cathode resistor, the one between the tubes, also reads 0 ohms.
Do these sound like logical explanations? I'd like to get this thing fixed so I can start playing through it again.
-Darren
Okay, I decided to fire it up again. Turns out one of the output tubes is certainly hotter than the other one. They both get pretty warm, but when I touched them I could certainly tell that one was hotter.
So where do I go from here?
So where do I go from here?
Switch the position of the tubes. If the other tube gets hot in that socket then it's a circuit problem. If the same tube get's hot in the other socket then it's a tube problem.
desolder one leg from the resistors that measure bad "in circuit" and remeasure them.. it could be that something else in the circuit is interfearing with your readings.. OR they might be part of the problem.
Owen
Owen
Hi Turbo7MN,
I sent Bogen an e-mail asking for this diagram. I'll ask our local distributor (in Canada) as well on Monday since I know a couple guys there.
-Chris
I sent Bogen an e-mail asking for this diagram. I'll ask our local distributor (in Canada) as well on Monday since I know a couple guys there.
-Chris
Thank you! The diagram will be really helpful. I can trace out the components myself, but the Bogen schematic should have voltage numbers as well, which is really what I'm concerned with right now.
Well, I switched the tubes and fired it back up. To me, they both felt pretty much the same. My method of testing is hardly scientific, basically touch the tubes with two fingers and whichever one stings the worst is hotter, and sometimes one tube felt hotter, and sometimes the other one felt hotter. If I had to say one for sure, it would be the tube that got hot before in the other socket, so I guess that points to more of a tube problem. If one of the tubes was shot, wouldn't there be a fairly large difference though?
Well, I switched the tubes and fired it back up. To me, they both felt pretty much the same. My method of testing is hardly scientific, basically touch the tubes with two fingers and whichever one stings the worst is hotter, and sometimes one tube felt hotter, and sometimes the other one felt hotter. If I had to say one for sure, it would be the tube that got hot before in the other socket, so I guess that points to more of a tube problem. If one of the tubes was shot, wouldn't there be a fairly large difference though?
Hi Turbo7MN,
Don't worry about it until we can get schematics. Testing the tubes may be the only reliable way to find out short of replacing them.
-Chris
Don't worry about it until we can get schematics. Testing the tubes may be the only reliable way to find out short of replacing them.
-Chris
What kind of places would be likely to test tubes? The only place around me that I thought there would even be a chance was the local music store, and they don't so I'm out of luck as far as that goes.
I desoldered one side of the NFB resistor and tested it. Sadly, it checks out fine and I'm back to square one.
Bogen certainly built these things with no intention of ever having them taken apart again. I mangled one side of that resistor connection pretty good, but at least now I know how to deal with them.
I desoldered one side of the NFB resistor and tested it. Sadly, it checks out fine and I'm back to square one.
Bogen certainly built these things with no intention of ever having them taken apart again. I mangled one side of that resistor connection pretty good, but at least now I know how to deal with them.
Darren,
Try contacting someone from the MN Audio Society. There may be someone in your area that can help test your tubes.
rick
Turbo7MN said:
Try contacting someone from the MN Audio Society. There may be someone in your area that can help test your tubes.
rick
I found something on R.G. Keen's site that might be the cause of this:
http://www.geofex.com/ampdbug/lowpower.htm
I think it could be a bad cathode bypass cap. Since the problem happens no matter which input I plug into, I think it could be in stage V2B as shown in the stock M330A schematic. The M330A is about as close as I can get to my M60A for now. The cathode bypass cap would be C18, correct? I should recap it all anyway, so I guess that's going to be the next step from here. Is there any way I can test this thing for sure?
http://www.geofex.com/ampdbug/lowpower.htm
I think it could be a bad cathode bypass cap. Since the problem happens no matter which input I plug into, I think it could be in stage V2B as shown in the stock M330A schematic. The M330A is about as close as I can get to my M60A for now. The cathode bypass cap would be C18, correct? I should recap it all anyway, so I guess that's going to be the next step from here. Is there any way I can test this thing for sure?
Hi Darren,
I got an e-mail return from Bogen. No joy. Sam's may have it though (I bet they do).
I can not open your attachment. None of the php viewers I've tried yet work. Any suggestions for windblows?
-Chris
I got an e-mail return from Bogen. No joy. Sam's may have it though (I bet they do).
I can not open your attachment. None of the php viewers I've tried yet work. Any suggestions for windblows?
-Chris
You mean the one I just linked to? It's just a .zip file containing a word document - there should be no reason you can't open it. It opens fine of my system running Windows XP.
I believe Tom Bavis already pointed out that Sams doesn't have the M60A.
I tested the cap for resistance and got 1.2k ohms, but this was in-circuit and means all but nothing until I can desolder it and measure it properly.
-Darren
I believe Tom Bavis already pointed out that Sams doesn't have the M60A.
I tested the cap for resistance and got 1.2k ohms, but this was in-circuit and means all but nothing until I can desolder it and measure it properly.
-Darren
Hi Darren,
Thanks. My version of XP was convinced it was a *.php file and downloaded it as such.
Wonderful world of windows.
So, changed the name to *.zip and there it is.
Yes, C18 is a bypass cap. You need to use some kind of capacitor tester, r just replace it to test. With an oscilloscope, there should be no signal on pin 9. Just approx. 1 VDC. Same with C24 except 1.2 VDC.
I think the 15K 1W resistors go to the screens, I've never seen anything but an RF choke in series with the plate lead before it hits the output transformer.
-Chris
Thanks. My version of XP was convinced it was a *.php file and downloaded it as such.
Wonderful world of windows.So, changed the name to *.zip and there it is.
Yes, C18 is a bypass cap. You need to use some kind of capacitor tester, r just replace it to test. With an oscilloscope, there should be no signal on pin 9. Just approx. 1 VDC. Same with C24 except 1.2 VDC.
I think the 15K 1W resistors go to the screens, I've never seen anything but an RF choke in series with the plate lead before it hits the output transformer.
-Chris
My M60A doesn't have those 15k resistors. As far as I know, it's identical as far as the preamp section goes, but since it's a 30W amp and uses different output tubes, the similarities stop there.
I think that my "remote" circuits may be wired a little different also.
I think that my "remote" circuits may be wired a little different also.
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