Salas hotrodded blue DCB1 build - Page 137 - diyAudio
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Old 1st October 2011, 09:39 AM   #1361
AndrewT is offline AndrewT  Scotland
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the spec tells me that the amplifier has supply rails fuses, some form of IV protection and DC detection with shutdown.
That is OK for me.
You decide if it's OK for you.
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regards Andrew T.
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Old 3rd October 2011, 01:48 AM   #1362
cersepn is offline cersepn  United States
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Thank you, Andrew!

I'm doing more testing with cheap speakers.

I realized the right channel of my DCB1 is dead. Not sure why. Both channels were working before, but after i unmounted the pcb from the chassis because i wanted to mount the mosfets to the bottom of the chassis, i connected everything back and voila, no sound on the right channel.

The offset on the right channel is suspiciously low (1 mv vs 3-4 mv on the left?), makes me wonder if it's even working. Voltage across both resistors are in range; voltage across the mains (?) is ~ +/- 9.50.

Not too sure what else to check. Possible that i fried the volume pot while dismantling everything? Is there any way to check? Pretty sure my solder joints are good.

Also, after mounting the mosfets to the bottom of the chassis, i realize the chassis doesn't seem to get that warm at all. I'm using the first level hot rod option. I used normal screws + nuts - is that not sufficient to maintain decent contact?

What parts should be changed to get offset as close to 0 as possible? Help is much appreciated!
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Old 3rd October 2011, 02:15 AM   #1363
Salas is offline Salas  Greece
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Did you use pads for insulating the Mosfets from the chassis? Knock the relay with a screwdriver in case it got stuck.
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Old 3rd October 2011, 02:47 AM   #1364
cersepn is offline cersepn  United States
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Yes, mosfets are insulated with thermal pads. Okay i'll try knocking the relays. Anything else i should try? Thank you, Salas!
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Old 3rd October 2011, 02:49 AM   #1365
Salas is offline Salas  Greece
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A dead fet or two in the audio couple of the silent channel maybe, connections, practical stuff.
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Old 3rd October 2011, 02:20 PM   #1366
cersepn is offline cersepn  United States
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Meaning to say the 2SK170s right? I took out the board, redid all the solder connections. Still the same result. But i realized if i turn up the volume loud enough, the 'silent' channel does have a *tiny* bit of sound. Really, really soft though, almost inaudible (hands can feel mild vibration of cabinet).

If it's the fet, it makes me wonder how they were spoilt in the first place.. since it was working fine before.. hmm. Could it be a potential relay problem? I'm not sure if i'm imagining things but the relay seems to take a longer time to click. Or is that related to a faulty fet, if any?

Since the voltage points measure properly, is it safe to say i didn't fry the mosfets and they're working fine? Or do i need to change them as well?

Will matching of the 2SK170s help with the offset? Since i'm going to change the FETs then i might as well shoot for as low offset as i possibly can...
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Old 3rd October 2011, 02:30 PM   #1367
Salas is offline Salas  Greece
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Some discharge when you were handling the boards maybe, can't say. You need to feed signal and follow it, to see where it stops. If you don't have an oscope, DVM on ACV can detect. Feed 300Hz (test CD, or there are software generators free, google up) at about 1VRMS and see until where it makes it by probing the path, starting from input RCA, finishing at output RCA including interconnect cables. With DVM on Ohm you can check continuity, if relay engages, etc. 100% audio circuit jfet matching helps offset naturally.
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Old 3rd October 2011, 03:02 PM   #1368
Salas is offline Salas  Greece
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Quote:
Originally Posted by cersepn View Post
"Each channel of the A 21 has a high-quality protection relay with gold-plated contacts for long-term reliability. These relays function to protect either the amplifier, the speakers, or both. When the A 21 is first powered on, these relays remain open for three seconds as the positive and negative power supplies stabilize and reach equilibrium ...

Specialized current-sensing transistors are connected to the output stages of the A 21 to constantly monitor the current flow through the output transistors. If the current drawn by this stage exceeds a predetermined safe level due to a load impedance below 1 ohm or a short circuit at the speaker terminals, the output relay will open immediately to prevent any of the output transistors or other parts from failing.

Each channel of the amplifier has a separate fuse for its positive and negative DC voltage rails. These fuses provide backup protection in case the over-current protection does not work in time, or if an internal part
fails. In the event of a part failure, these fuses halt operation to minimize damage to additional parts."

Not sure if it's only marketing fluff... with what the manual said, is it still advisable to use the DCB1 capless with the amp?
I can assume delayed output engagement to load and over-current sensing for automatic load disconnection, but DC output sensing I can't assume from that text. John Curl is posting in our forum all the time, maybe a PM or a post to him would clarify for sure.
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Old 3rd October 2011, 03:47 PM   #1369
cersepn is offline cersepn  United States
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Quote:
Originally Posted by Salas View Post
Some discharge when you were handling the boards maybe, can't say. You need to feed signal and follow it, to see where it stops. If you don't have an oscope, DVM on ACV can detect. Feed 300Hz (test CD, or there are software generators free, google up) at about 1VRMS and see until where it makes it by probing the path, starting from input RCA, finishing at output RCA including interconnect cables. With DVM on Ohm you can check continuity, if relay engages, etc. 100% audio circuit jfet matching helps offset naturally.
Wow, thank you for all that detailed advice, Salas! Pardon me for being daft because i'm new to all this, but if a simple swapping of new parts will solve the problem.. i think i would prefer that, especially since the dcb1 was working fine before.. so i can't imagine something terribly went wrong? At least i hope nothing did, lol.

So ... if i had to swap a part, it would be the 2sk170BLs? Are FETs very sensitive and easily damaged?

i will pm john to see what he says. Thank you once again!
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Old 3rd October 2011, 03:51 PM   #1370
Salas is offline Salas  Greece
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You are asking for a guess. The right way is to test as I described. If its to guess and eliminate by probability start with changing the audio fets in the bad channel but don't be surprised if they aren't the problem in the end.
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