Samsung DA-E750 repair, I am feeling really dumb.  - Page 1
Forum section: electronics / repair

author: RichardM
author_rank: Frequent Contributor
timestamp: May 10, 2025, 10:52:01 am
content: Hi AllI got one of these old Samsung docks with vacuum tubes for less than the 2 12au7 tubes. Anyway, it had almost no output power, one channel was almost inaudible. I am on the right track replacing some of the smd capacitors and one channel is now back to full power, one channel is dead. I have built and repaired a handful of amplifiers recently and have all the required test gear but I am totally lost as to why I cannot measure the audio output on my scope? I am probing across the speaker terminals (4ohm) and even when I have a nice 1khz sine way playing loud through the speaker, my scope sees almost nothing apart from a 300-400khz small noisy sine wave? The scope is fine and I even tried my analog Tektronix 2247a and see the same, no 1khz sine wave !! What on earth am I doing wrong. I must be losing my mind.Richard

author: Psi
author_rank: Super Contributor
timestamp: May 10, 2025, 10:58:30 am
content: Are you grounding one side of the speaker output through the scope ground clip?I don't know how that amp works, is there a transformer on the speaker outputs?Are the vacuum tubes for preamp/poweramp or both?

author: RichardM
author_rank: Frequent Contributor
timestamp: May 10, 2025, 12:01:22 pm
content: Yes, I am grounding the scope probe to one side of the speaker. That’s what has me so confounded.The tube circuit is a pre-amp as I interpret the schematic.Bit hard to troubleshoot when I can’t seem to trace the input or output signal.

author: benj38
author_rank: Frequent Contributor
timestamp: May 10, 2025, 04:47:01 pm
content: If one of the speaker wires is connected to groundinsidethe amplifier, then you can connect the scope ground to that speaker wire, and the scope probe tip to the other speaker wire. If you reverse the connection, or if none of the speaker wires are grounded inside the amplifier then you are creating a short to ground when you connect the scope probe to the speaker wires. Observe that this is the case also if you have the scope input in AC coupling mode.A single channel measurement with a regular scope is in reference to ground, and the probe ground connection should never be connected to anything but ground. It is not like a DMM where both probe wires are floating allowing you to make differential measurements between any two points in your circuit.To make a differential measurement with a scope between two arbitrary points in your circuit (none of which is ground) you basically have four safe options:Use a battery operated scopespecified as having floating inputs(e.g., Tektronix THS720A).Use a differential probe.Use an isolator (e.g., Tektronix A6902).Use two channels of a regular scope as follows: connect channel 1 probe tip to one side of the speaker, connect channel 2 probe tip to the other side of the speaker, connect both channels' probe grounds to the amplifier ground, and set the scope to display the algebraic difference between the two channels (on the 2274A, press the Ch 2 Invert button, and the ADD vertical mode button).I will not mention unsafe options sometimes employed.

author: RichardM
author_rank: Frequent Contributor
timestamp: May 10, 2025, 06:07:55 pm
content: Thanks for taking the time to reply.I just don’t understand what is so different about this amplifier. I am not probing any differently than I have with all the amps I have repaired or built. I will take a break and come back with fresh eyes.

author: benj38
author_rank: Frequent Contributor
timestamp: May 10, 2025, 06:23:57 pm
content: You still did not tell us if one of the speaker wires is ground or not.If you can hear it you can measure it with a scope, as long as you follow the right probing procedure. A speaker is a much more crude signal measurement instrument than a scope, if your signal is detectable with a speaker, it is detectable with a scope.There is nothing magical this amplifier can do to prevent you from seeing the signal on the scope.

author: RichardM
author_rank: Frequent Contributor
timestamp: May 11, 2025, 04:27:26 am
content: Yes, one speaker terminal is grounded per the schematic a couple of posts back.So it appears I can measure the 1khz sine wave on the scope, lesson learnt. It was a triggering issue and combined with the fact that the output is dwarfed compared to the 300-400khz signal at the speaker also. It’s almost as though the output is modulated ? Or probably more likely there are other issues with the amplifier. My confidence is intact at least 😀

author: David_AVD
author_rank: Super Contributor
timestamp: May 11, 2025, 09:55:44 am
content: The unit uses class D amplifiers, so that would explain what you're seeing. The speaker is also acting as a low pass filter.

author: RichardM
author_rank: Frequent Contributor
timestamp: May 11, 2025, 10:36:54 am
content: Thanks for the explanation. I have never worked on a Class D amplifier, time to do a little reading. I managed to fix the dock, all the 10uf smd capacitors were bad.

author: Psi
author_rank: Super Contributor
timestamp: May 11, 2025, 11:31:45 am
content: Quote from: RichardM on May 11, 2025, 10:36:54 amtantalums or electrolytic?I'm going with low quality tantalums.

author: David_AVD
author_rank: Super Contributor
timestamp: May 11, 2025, 08:08:25 pm
content: I've seen a lot of items lately where the 10uF 4mm SMD electrolytic capacitors have gone low capacitance and high ESR.

author: RichardM
author_rank: Frequent Contributor
timestamp: May 12, 2025, 03:40:06 am
content: Electrolytics

author: Bogdan077
author_rank: Newbie
timestamp: June 23, 2025, 12:38:26 pm
content: HiJust to make sure. I need to know have you changed only the 10uf SMD capacitors, for example C1005 and C1002? Or something else? I am asking because I do not want to miss one and i do not know what capacitors you have changed. I have a samsung DA-E750 with no volume what so ever, and managed to see this post. The only caps SMD electrolytics are C7016, C7019, C8001, C7010, C7008 and C7009. These are the ones u have changed? Thnx.

author: TilleyTech
author_rank: Newbie
timestamp: July 26, 2025, 04:42:37 pm
content: Hi AllI trust you are all well.I stumbled across this thread whilst looking for info across the web for replacement parts for my Samsung DA-E750.So, first off - I do not know much about electrics/electronics - I write software for a living (since the 1990s) for banks and I produce music in my spare time. So I am technical and into audio however unlike you, by reading the comments, I cannot crack open electronics/electrics and fiddle or fix!To me it seems as if the valves have gone in the amp as I still hear sound except it's all warbled/distorted/muffled and the valves are dim now. I tried to get a repair however any repair person I've shared the photos with says they are not standard valves.I thought I'd write here to ask if anyone knows what the part(s) is/are in order to replace please?Many thanks.Mr Leigh Tilleyhttps://tilleytech.com

author: Bogdan077
author_rank: Newbie
timestamp: September 22, 2025, 07:58:41 pm
content: HiSamsung DA-E750 is repaired by dynatrack in this videoLamps are ECC82 vacuum tubes.They are glued to their base. They actually have a LED to luminate them. I have not tried to replace them but I recall someone saying that it used hot air gun to remove and replace the initial lamps. Now, if you are not confident with such repairs I would strongly encourage you not to.