If anyone else has experience with a Forte Model 3 amp and can chime in on if the low level hum is “normal.” These amps are from a long time ago and sometimes, levels of what is considered tolerable may be different than what we expect to hear today.
It appears to be a single transformer with two rectifiers feeding two PSU cap banks with a shared common ground strap bus. It appears that both amps are fed from same PSU cap bank. This may have an inherent mild ground loop hum vs separate secondaries and separate dual rail PSU’s for “monoblock” operation, that would tend to be less prone to ground loop hum. But I don’t know for sure here with the Forte 3, sometimes a common supply can be made to be quiet.
It appears to be a single transformer with two rectifiers feeding two PSU cap banks with a shared common ground strap bus. It appears that both amps are fed from same PSU cap bank. This may have an inherent mild ground loop hum vs separate secondaries and separate dual rail PSU’s for “monoblock” operation, that would tend to be less prone to ground loop hum. But I don’t know for sure here with the Forte 3, sometimes a common supply can be made to be quiet.
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Finally took the time to do this.
I'd like to let you know beforehand that the hum is a bit more noticeable on the left channel than on the right. It is clearly the same hum, but it's louder on the left than it is on the right.
So, first the ground + phone. This was done with my run of the mill Bose speakers and then with the B&W just to make sure:
- Ground on the right, phone on the left -> Hum on the left the same, hum on the right barely noticeable
- Ground on the left, phone on the right -> Exactly the same
This was done with speakers connected.
Left:
- AC - 0.6 mV
- DC - 15.5 mV
Right:
- AC - 0.8 mV
- DC - 20 mV
Does any of this help?
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You sounded like my school teacher, but you're right. XRK mentioned mV throughout his post so yes, it was measured in mV.
Post edited, sorry.
This is probably the last digit in the range with this particular instrument,
and as such it has some uncertainty.
Also, the audibly annoying harmonics of 100 Hz will not be displayed properly.
With respect to these precautions the values are not abnormally high.
Repair teaching is one of my professions.
and as such it has some uncertainty.
Also, the audibly annoying harmonics of 100 Hz will not be displayed properly.
With respect to these precautions the values are not abnormally high.
Repair teaching is one of my professions.
you know, this amp is that old that changing big caps is matter of logical maintenance
how many oil filters should be changed in car, for same time period?
🙂
maintenance, then repair if needed .......... normal procedure for anything technical
how many oil filters should be changed in car, for same time period?
🙂
maintenance, then repair if needed .......... normal procedure for anything technical
First, ripple on the caps re ground should be checked in ac range.
Never recommend to change working parts unless you are advocate of the cap industry.
Never recommend to change working parts unless you are advocate of the cap industry.
The range of this DMM in particular is 200mV-250V.
True, but I need to think about it. What if, like XRK said, this one is prone to hum due to it's design?
Anyway, I need to find out the values for the caps before I even start to think about it. Will need to go in again.
I'm sorry, but I don't know what you mean. I know how to repair some things and have some of the tools, but I'm a beginner when it comes to power amps.
I explained my reasoning in post #44, based on info given about amp
regarding amp construction (specific buzz?!?) , Nelson knew how to make proper amps long before that one
regarding amp construction (specific buzz?!?) , Nelson knew how to make proper amps long before that one
Re "the range of this DMM in particular is 200mV-250V" - the last digit
I suspect is 0,1 mV so any readings below millivolts have uncertainty.
But your measurements seem to show a normal hum level. This could be
confirmed with a scope capture.
In the same way you can check the ac voltage on the four caps to ground
if it has not be done so far. In this way you can check the caps have equal
value which is a strong indicator of their healthy state.
I suspect is 0,1 mV so any readings below millivolts have uncertainty.
But your measurements seem to show a normal hum level. This could be
confirmed with a scope capture.
In the same way you can check the ac voltage on the four caps to ground
if it has not be done so far. In this way you can check the caps have equal
value which is a strong indicator of their healthy state.
Re post 69 I do not agree to exchange healthy caps, but will not interfere
with a moderator here (if Court Jester means something like an elevated
moderation level).
My prediction is the hum will not change after this recap.
By the way did anybody recommend to tightly fasten all screws associated
with the four caps ? This must be done with power off.
with a moderator here (if Court Jester means something like an elevated
moderation level).
My prediction is the hum will not change after this recap.
By the way did anybody recommend to tightly fasten all screws associated
with the four caps ? This must be done with power off.
Yes, I agree about Nelson (how could I not), was just taking into account what a fellow forum member has said.
But yes, what you say on post #44 makes sense (at least to me, with my limited knowledge).
Check post #43 please and refer to post #11.
I've done that, but could definitely recheck. Will go in again later and do it again. Thank you for all your help so far.
Oh, and the last digit is indeed 0.1.
Well, that's what I got with the same DMM I used to check all the other readings. In mV. No input or speakers connected btw. I may check again if you'd like later, but I'm pretty sure it'll be the same.
There is no evidence the caps are bad, there also is no evidence to suggest there are OK either. Given the age and symptoms, any repair person would probably test that with a scope.
Why argue here?
I expect zero millivolts ripple as indicated in post 43 only if the total
consumption of the amp boards is extremely small.
A question to those "skilled in the art" :
why would anybody want to change caps if they show zero ripple ?
PS to 76 I see need to argue against blind exchange of expensive parts.
consumption of the amp boards is extremely small.
A question to those "skilled in the art" :
why would anybody want to change caps if they show zero ripple ?
PS to 76 I see need to argue against blind exchange of expensive parts.
I have the same amp and it has no hum. PS caps were replaced 4 years ago with higher uF and voltage rating.
it is:
-either find appropriate cap bank, to connect instead of existing ones and the see what's hum level
- or bring amp to tech who is going to do his job; I know how and with what I would check it, but none of that OP is having at disposal
repairing this amp is simple job, price for possibly needed replacement is what is problematic here but ...... taking in account value of amp (even if just market price, not thinking of emotional ties) it's still a bargain, enabling it for 20+ more years of enjoynment
-either find appropriate cap bank, to connect instead of existing ones and the see what's hum level
- or bring amp to tech who is going to do his job; I know how and with what I would check it, but none of that OP is having at disposal
repairing this amp is simple job, price for possibly needed replacement is what is problematic here but ...... taking in account value of amp (even if just market price, not thinking of emotional ties) it's still a bargain, enabling it for 20+ more years of enjoynment
Based on the hum I'd say the amp is not working right. Ripple or no ripple. One of the case where ears maybe better than instruments?
A few mV of 100-120 hz hum is very easy to hear but not so easy to measure with a cheap digital voltmeter.