It's normal for the PG lights to monetarily flash once right after the power indicator comes one. Sure all the lamps are good?
Run into some Trouble with Two Amps
- Thread starter welcomdmat
- Start date
welcomdmat
Super Member
I deoxit'd the right channel RCA really well, and it is working. I don't know how long it will work
Since one amplifier is doing this and the other is not- and this is the same unit giving trouble (I assume you've swapped preamplifier outputs) I think it's pretty likely the problem lies there.
welcomdmat
Super Member
They seem to be good. It is only that one side that does it -- both left and right, and both for the same amount of time. It does not do it all of the time.
I am probably latching onto everything that I notice.
There seems to be a chance that the right RCA was dirty. Would that cause intermittent playing? I have it set up as stereo with everything playing through the right channel, and it is clear.
That same RCA seems to be a little loose. It internal, red part clicks as I rub it. That is the only one.
welcomdmat
Super Member
No -- not 2 sec. Much shorter. Both lights is what I meant.
I agree that the amplifier seems like it may be a problem, but I have had issues with the right channel out of the single amplifier running separately in stereo mode with sources plugged into it directly.
I could well be more than one thing.
I stood up and lost all sound out of the concerning amplifier set in mono mode with the source running into it directly.
The sound then faded back in after a couple of pops.
I know who worked on it, so I may reach out to them.
I agree that the amplifier seems like it may be a problem, but I have had issues with the right channel out of the single amplifier running separately in stereo mode with sources plugged into it directly.
I could well be more than one thing.
I stood up and lost all sound out of the concerning amplifier set in mono mode with the source running into it directly.
The sound then faded back in after a couple of pops.
I know who worked on it, so I may reach out to them.
Is the other amplifier doing the same thing with the PG lamps?
- So using this amplifier in normal stereo mode (as a single), there is noise on the right channel also?
These amps are really easy to open up. Don't be too afraid about taking a look inside.
I would take them out of mono and test in stereo mode.
A momentary blip from the PG lights, at power up is normal for the 752/754 amps.
PG lights have been flickering upon turn on since 1978.....think through how the circuit is designed to work....until PS stabslizes the output can easily exceed the input waveform shape by 1%.
Stick to the preamp to rule it out....must be disciplined about this. It will take time because just like the watched pot never boils fast enough a intermittant channel will not rear up until it is the least convenient to analyze.
Stick to the preamp to rule it out....must be disciplined about this. It will take time because just like the watched pot never boils fast enough a intermittant channel will not rear up until it is the least convenient to analyze.
^^^^^....adding the points above.
Just strip everything down to the very basics. Source directly connect to an amp in stereo mode....later test the other amp in stereo. Then add the preamp....1 amp in stereo....and later both amps in mono. Go from the most simple setup and slowly build up.
It's easy to get scattered and lose track of what has and has not been tested.
Just strip everything down to the very basics. Source directly connect to an amp in stereo mode....later test the other amp in stereo. Then add the preamp....1 amp in stereo....and later both amps in mono. Go from the most simple setup and slowly build up.
It's easy to get scattered and lose track of what has and has not been tested.
I have 3 Mc amps with PG - well aware of momentary PG lamp illumination on startup.
This sounds different:
So is it 1.9 seconds or...?
Is one amplfier doing something different than the other on startup?
This sounds different:
So is it 1.9 seconds or...?
Is one amplfier doing something different than the other on startup?
Precisely my reason for suggesting to first swap phono (and/or other input leads) to see if the problem channel follows suit; if not, then swap the preamp output leads to the amplifiers. After 40+ years in hi fi I have learned a wrinkle or two in the art of identifying faulty components and/or connections.
I am not at the OP's location so I only have his words to go by.
I am not at the OP's location so I only have his words to go by.
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welcomdmat
Super Member
I will go through again. One amp has consistently worked. Mono and stereo — but it has tended to control the same speaker as I try to keep speaker wire as short as possible.
Direct input, amp 1, both channels can be controlled by gain knobs. Preamp, all variations are variable.
Direct input, amp 2, right/ red channel is intermittent. Plugged into preamp, again intermittent. This is the amp with a 1/4 second flash of Power Guard. I have never seen it in the other amp.
I have moved speaker cables enough to rule out the cables. I have moved speaker cables enough to rule out the speakers.
I have run both amplifiers in stereo attached to both speakers.
I have used more than one source and 3 sets of RCA cables to rule out any connections.
I am left with the preamp and amp 2.
I can sometimes get both channels when I twist the variable input on the preamp. I have cleaned it well.
I have trouble getting the left (non-mono) channel to work on amp 2 when I connect it directly to a source.
I feel that I am chasing a ghost. I don’t want to spend a bunch of money on diagnosis, but I can’t nail down the problem.
I fully understand that there are things I do not know.
Direct input, amp 1, both channels can be controlled by gain knobs. Preamp, all variations are variable.
Direct input, amp 2, right/ red channel is intermittent. Plugged into preamp, again intermittent. This is the amp with a 1/4 second flash of Power Guard. I have never seen it in the other amp.
I have moved speaker cables enough to rule out the cables. I have moved speaker cables enough to rule out the speakers.
I have run both amplifiers in stereo attached to both speakers.
I have used more than one source and 3 sets of RCA cables to rule out any connections.
I am left with the preamp and amp 2.
I can sometimes get both channels when I twist the variable input on the preamp. I have cleaned it well.
I have trouble getting the left (non-mono) channel to work on amp 2 when I connect it directly to a source.
I feel that I am chasing a ghost. I don’t want to spend a bunch of money on diagnosis, but I can’t nail down the problem.
I fully understand that there are things I do not know.
If I'm following this correctly.....
Amp #1 in your "full" setup is perfectly fine. The only time there is an issue is when you swap out amp #1 with amp #2. If that is correct amp number #2 is the issue. Remember both channels are needed in mono....so in stereo mode if one channel is cutting out you will have total output lost when in mono.
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After spending some time with @welcomdmat on the phone, I think we've conclusively identified the one MC754 as the culprit and will have to go back to Audio Classics.
It also happens to be the same unit on which the PG lamps illuminate on startup. Apparently the other unit is not exhibiting the same PG function on startup. I cannot say whether this is related to the output loss in one channel (when operated in normal stereo mode). Both amplifiers were originally purchased new by the previous owner at the same time and the serial numbers are around 100 units apart, for whatever that may be worth.
It also happens to be the same unit on which the PG lamps illuminate on startup. Apparently the other unit is not exhibiting the same PG function on startup. I cannot say whether this is related to the output loss in one channel (when operated in normal stereo mode). Both amplifiers were originally purchased new by the previous owner at the same time and the serial numbers are around 100 units apart, for whatever that may be worth.
damacman
Resident Psycho
Hmmm . . . the contact cleaner must be sprayed into the inside of the control. This cannot be accessed from the front panel. Rather, the top must be removed and the rear of the control accessed. In many cases on Mc preamps of this era, the controls are "sealed", making this a job for a qualified technician.
From the information you've provided, I'd suspect a dirty control as well. What others are trying to tell you is that you have to use a process of elimination to determine the component at fault. The simplest "system" you could construct would be one source, your preamp, and a pair of headphones. The problem either will or will not creep up. If it does, you've eliminated both of the amps and now have to determine whether the turntable or preamp is at fault. To do that, you would connect any other source to the preamp and re-evaluate. If on the other hand, the problem does not arise with the preamp / tt/ phones, then you add one power amplifier and the speakers. Reevaluate. And so forth.
welcomdmat
Super Member
I’ve shipped the odd amp to Audio Classics
I spoke to Ryan a number of times, and his team is watching out for it.
I spoke to Ryan a number of times, and his team is watching out for it.
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welcomdmat
Super Member
I do appreciate the systematic nature of the diagnosis process, and I narrowed down the amp as the only constant.
I was able to accomplish a cutting in and out on the one amp with the loudness knob. I was then able to accomplish that with my phones volume control when the phone was directly hooked to the amp (bypassing the preamp).
Because of the hunting for a constant in an inconsistent situation, I do plan on cleaning the knobs (especially the loudness know) soon.
I was able to accomplish a cutting in and out on the one amp with the loudness knob. I was then able to accomplish that with my phones volume control when the phone was directly hooked to the amp (bypassing the preamp).
Because of the hunting for a constant in an inconsistent situation, I do plan on cleaning the knobs (especially the loudness know) soon.
welcomdmat
Super Member
Just as an addition before I get feedback on the amp that I sent to Audio Classics:
A single MC754 powers my system a-plenty. I have really enjoyed this amp though it is noticeably different than the MC2100 I had before (beyond the fact that I had to choose my table carefully under the weight of the 2100).
I have very clear channels with the single amp. I have not had any issues dropping a channel, and I am very happy with that. I have opened up 8 different preamps, and I have never seen one as clean and straight forward as the C712. The last three have been Haflers edited by Musical Concepts.
When setting up the single amp, I turned the rear adjustment up to the full 1.4V on the left and right channels. It sounds great, but I have very little volume control.
A single MC754 powers my system a-plenty. I have really enjoyed this amp though it is noticeably different than the MC2100 I had before (beyond the fact that I had to choose my table carefully under the weight of the 2100).
I have very clear channels with the single amp. I have not had any issues dropping a channel, and I am very happy with that. I have opened up 8 different preamps, and I have never seen one as clean and straight forward as the C712. The last three have been Haflers edited by Musical Concepts.
When setting up the single amp, I turned the rear adjustment up to the full 1.4V on the left and right channels. It sounds great, but I have very little volume control.
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