Is this right or why would someone alter this. Denon PMA-630

I picked up a Denon PMA-630 Integrated Amplifier for pretty cheap. I was told it had no output, just some static. I got it home and powered it up on a dim bulb tester and it powered up correctly. So next I started to disassemble for cleaning and inspection and I noticed on the amplifier driver board that both the negative connections, next to what I believe are the bias trim pots, have been cut. Is this correct or has it been altered and why? SEE PICS

GENXVINTAGE said:

I picked up a Denon PMA-630 Integrated Amplifier for pretty cheap. I was told it had no output, just some static. I got it home and powered it up on a dim bulb tester and it powered up correctly. So next I started to disassemble for cleaning and inspection and I noticed on the amplifier driver board that both the negative connections, next to what I believe are the bias trim pots, have been cut. Is this correct or has it been altered and why? SEE PICS

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My guess is that they are jumpers that are used during production test. They would be cut as the final procedure. I wouldn't call it a "modification" but a schematic should tell you exactly what they are.

Are both ends connected in circuit below the board? Or are they test points? Maybe one was cut and lifted to make it easier to clip onto?

http://elektrotanya.com/?q=showresult&megnev=denon&megnev2=pma 630&kategoria=&kat2=schematics
service manual

Both connected

I added a picture of the back of the board, they are both connected in circuit.

Wow!

ryuuoh said:

http://elektrotanya.com/?q=showresult&megnev=denon&megnev2=pma 630&kategoria=&kat2=schematics
service manual

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:thmbsp:Wow, I have been searching for about a hour on line to find that, Lol. Thank you very much. ow lets see if I can figure something out!:

Hmmm..Still lost?

:scratch2:This schematic has me stumped. Maybe I'm just not seeing whats right in front of me but I'm still not sure?

is that used for checking bias or dc offset?

aidynphoenix said:

is that used for checking bias or dc offset?

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Well I'm not 100% sure, it could be for the DC/Offset. I don't have a complete manual.

They are for the offset adjustment. The jumpers are the dashed lines on each side of the varable trim pot VR-1 and VR-2. If both jumpers are cut, the range of adjustment will be large, and adjustment will be more difficult. If the good jumper is cut, you will have a finer adjustment. I think that if the wrong jumper is cut, your adjustment may be out of range.

GENXVINTAGE:
I have one of this model amplifier, and the associated service manual. Be very careful what speakers you hook up to this beast. The power amplifier portion is an ultra wide band circuit, is marginally unstable, and tends to self oscillate supersonically. My PMA-630 instantly destroyed the treasured Heil tweeter in one of my ESS AMT10B speaker systems without warning while in use. I loved the way this amplifier sounded, but it is not to be trusted. Consider yourself forewarned.

2chanman said:

GENXVINTAGE:
I have one of this model amplifier, and the associated service manual. Be very careful what speakers you hook up to this beast. The power amplifier portion is an ultra wide band circuit, is marginally unstable, and tends to self oscillate supersonically. My PMA-630 instantly destroyed the treasured Heil tweeter in one of my ESS AMT10B speaker systems without warning while in use. I loved the way this amplifier sounded, but it is not to be trusted. Consider yourself forewarned.

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Ok, great information to have, thank you. Do you have that manual in a pdf file by chance?

Ok, so I check and cleaned everything. Pulled all the plugs and cleaned them and the terminals, checked all the ground points, etc. Put it back together, fired it up with the dim bulb tester and all looked good. I have to check the dc offset and such but got excited to see so I Hooked up a mp3 player and it works great on both channels, but....I noticed another issue. It has 3 relays, the main one when you first fire it up and one each for speaker A and B. They all work but the issue is when I power it off it is still getting juice from some where because I can turn the speaker selector and the relays continue to click for about 20 to 30 seconds after the power is shut down? Thats not normal is it?

GENXVINTAGE:
I haven't fired up my PMA-630 for quite a while (for obvious reasons) and don't remember seeing what you describe. It may be normal and could be explained by the time required for the filter capacitors to discharge. I never had any reason to rotate the speaker switch as I never had more than one set of speakers connected, and that one pair turned out to be one too many.
I obtained the manual (paper) from stereo manuals.com., don't have it on pdf. It really wasn't much help. I followed the instructions for adjusting offset and bias, which MUST be performed "in a swift manner" due to the extreme thermal sensitivity of the DC coupled dual differential input circuitry. I found that rotating the volume control changed the offset at the output on both channels, one especially so. That is the one that vaporized one Heil tweeter. There was no dc being applied to the volume control. This amplifier's frequency response is DC to 100khz +0, -1Db @1watt out. At half power, response is 5Hz to 100khz. This circuit is on the hairy edge of oscillation.
Unless you are looking for an excuse to buy new tweeters frequently, don't connect any speakers you care about to this amplifier.
The heart of this amplifier is a a pair of dual NPN and PNP transistors at the input (inside those metal cans) That must be critically matched. Each channel has this pair. The manual says nothing about what the matching parameters are. They are the 2SA979(F) and 2SC2259(G), which are no longer manufactured. I seriously doubt that Denon would have any of these in appropriately matched sets, nor would anyone else.

So what your saying is its a piss poor design. So even if I get it adjusted properly it could at any time cause a problem? There is no way to fix this issue?

This one has 2SB539C and 2SD287C Transistors in it? Think that Is the same the Pioneer SX-1050 uses.

I was talking about the input transistors on the power amplifier board. I think you are talking about the output transistors on the heatsinks. On the schematic, the input transistors are shown as TR-1 through TR4. They are plastic dual transistors with a common emitter and are inside the metal cans on the amplifier circuit board. The cans must be removed in order to see or remove the transistors.
There may be a later or revised version of this amplifier where the stability was improved with some circuit changes, but I doubt that Denon would have this information because this is a very old design. Perhaps yours is one of the later versions if it seems to be trouble free.
With no output load or input signal, mode switch in "tuner" mode, looking at the output terminals (either channel) with a sensitive DC voltmeter, rotate the volume control and see whether the offset voltage changes to different values at different volume settings. If it does, your amplifier is built like mine and is not to be trusted.
Denon should not have marketed this amplifier as originally designed due to it's dangerously marginal stability.

Yes it does. I have not attemped the procedure for setting it yet but as it sets I'm getting about -63mv on one side and -75mv on the other. Then when rotating the volume they go to around -200mv or a little more?

Should it be showing Negative dc offset like that?

Ok, well after going through the steps (twice now) I keep having the same problem. When I pull the jumpers to set vr1 and vr2, vr1 is well in the negative even when turned all the way up. So from looking at the schematic it maybe the input transistors on that one channel? I guess I will pull the driver board back out and check all the transistors on the left channel.