Dear Thomas and Frank,
Thank you for your support and advice.
I agree that this unregulated supply design should not cause such huge DC fluctuation. The output fluctuation correlated with the AC lines fluctuation, but it’s only affecting the right channel, it does not to the left one. Further, the AC lines fluctuates only 0.3V at the highest, and it’s only 0.1 in average.
We did check to all op points and also did voltage comparison between each point of each channel. It seems everything is fine, but I will do that again and see what really happen.
Yes Frank, we will draw the circuit diagram on Saturday, I need to drive about 140Km to reach his city. This has been done for about six times when we did some measurement, analysis and part replacements. Once we got the schematic, we will share it with you all, hope we can find the cause of problem and solve it.
Thank you for your support and advice.
I agree that this unregulated supply design should not cause such huge DC fluctuation. The output fluctuation correlated with the AC lines fluctuation, but it’s only affecting the right channel, it does not to the left one. Further, the AC lines fluctuates only 0.3V at the highest, and it’s only 0.1 in average.
We did check to all op points and also did voltage comparison between each point of each channel. It seems everything is fine, but I will do that again and see what really happen.
Yes Frank, we will draw the circuit diagram on Saturday, I need to drive about 140Km to reach his city. This has been done for about six times when we did some measurement, analysis and part replacements. Once we got the schematic, we will share it with you all, hope we can find the cause of problem and solve it.
Hi Salas,
Thanks, let's see what would be our finding. We will, again, check the entire path of the circuitry, from upstream to downstream, point to point, side by side on each channel, any other possibilities.
Meanwhile, If somebody there can help me to find and buy Vishay Dale resistor FP2 Series (FP000222K6F9521: 22.6K, 2W, 1%). Leraning form this experience, I need to secure some parts for spare and repair, just in case. Some I can find in Mouser, but this FP2 series is really dificult.
Thanks, let's see what would be our finding. We will, again, check the entire path of the circuitry, from upstream to downstream, point to point, side by side on each channel, any other possibilities.
Meanwhile, If somebody there can help me to find and buy Vishay Dale resistor FP2 Series (FP000222K6F9521: 22.6K, 2W, 1%). Leraning form this experience, I need to secure some parts for spare and repair, just in case. Some I can find in Mouser, but this FP2 series is really dificult.
Dale FP2 is a discontinued line of resistors, and the value you are looking is even difficult to find up to 1/2W or less. Checked a few mainstream vendors, nada. I would suggest considering another series or vendor. Within Vishay Dale, CPF3 or CCF2/CCF02; or Caddock MS220, MS221 or MS223. All have similar specs (Caddocks have higher V ratings). I have bought small batches of odd value resistors directly from Caddock sales, so that would be my first recommendation in the MS precision film series. Current Sense Resistors, Non-Inductive Designs, High Stability, Surface Mount - Caddock
Couple years ago a friend of mine had the same problem. I had checked his preamp (same model) and the output dc fluctuation was following line fluctuation. Contacted Lamm, but they didn't offer any solution except selling!? new output caps. Changed output caps - problem stayed.
Friend decided to sell the preamp.
Friend decided to sell the preamp.
This is an important clue. It suggests that it is not the caps themselves, but something connected to their mounting or wiring. Is there any wiring to these caps, apart from their PCB pads?
Jan
Hi,
There is a small amount of GNFB, probably to reduce the Zout of the SRPP stage to 250R.
But, is this the Deluxe version or the regular one?
From what I read only the Deluxe version has its output caps bypassed.
TS should also have the technician check if the NFB loop is not somehow disconnected on one channel.
Ciao,
There is a small amount of GNFB, probably to reduce the Zout of the SRPP stage to 250R.
But, is this the Deluxe version or the regular one?
From what I read only the Deluxe version has its output caps bypassed.
TS should also have the technician check if the NFB loop is not somehow disconnected on one channel.
Ciao,
Did you have to physically unmount a PCB or part of the chassis? Could it be that a chassis screw connecting is changed?
It is very strange that one channel is fixed, the other not.
You could change the caps of one channel to the other and vice versa. If the problem remains where it is, it's not the caps.
If the problem moves also, it's the caps. Easy.
Jan
Hi,
The difference is that when you swap them from one channel to the other you immediately know if it is the cap that's faulty. (Personally, I don't think so)
Whereas if you solder in a new one it may well take weeks before it develops a fault.
Did you also change the bypass caps when you first replaced the caps?
Also, I presume the feedback loop is taken from behind the cap to the cathode of the ECC82 (or whatever equivalent NOS tube is there). This is probably just a trace on the PCB but unsoldering the caps may have lifted it perhaps.
It may be a bit far fetched but sometimes it's the stupidest thing that causes the most mysterious problems....
Ciao,
The difference is that when you swap them from one channel to the other you immediately know if it is the cap that's faulty. (Personally, I don't think so)
Whereas if you solder in a new one it may well take weeks before it develops a fault.
Did you also change the bypass caps when you first replaced the caps?
Also, I presume the feedback loop is taken from behind the cap to the cathode of the ECC82 (or whatever equivalent NOS tube is there). This is probably just a trace on the PCB but unsoldering the caps may have lifted it perhaps.
It may be a bit far fetched but sometimes it's the stupidest thing that causes the most mysterious problems....
Ciao,
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