HCA 1200 ii bias and output relays - diyAudio
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Old 8th January 2013, 01:23 PM   #1
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Default HCA 1200 ii bias and output relays

I opened up the amp to try and troubleshoot the output relays which are staticy at low volume and that brought up some other questions. The left channel heat sink was a lot hotter than the right, so I investigated the bias. I ended up adjusting to 10 MV from 8 on the left channel and 9 on the right channel. That doesn't explain the temp difference. Anyone have any idea why that would be? The amp sounds good and balanced to my ear- except for the random static at low volumes till the relays make contact. I also noticed that the output relays are in a tricky spot to work with, and looking online they are discontinued- they are

OMRON G4F1123TUSDC12

Has anyone had any success cleaning them? I could use some old-timer trick

I know they can be redone by sending the amp out, but I am out of work and on a severe budget crunch! Not to mention I enjoy fiddling!

Thanks,
Peter
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Old 13th February 2015, 06:56 PM   #2
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Default Different bias between two inverted sets of output transistors.

So I bought used HCA-1200 MK II and it had one issue. Just after I turn it ON there was no signal in left channel. I tried to shake it slap it and suddenly paw! left channel works so I assumed some loose contact.. Well next time when i started cold - same thing no sound in left channel but this time I let it warm up and voila left channel works by it self. I did several times so loose contact became less probable... I thought maybe bias ?
Also i felt there was some difference between left and right channel so I opened it and measured bias on both channels. Then I tried to adjust it a bit and here's what I have found on left channel.

Since I don't have schematics I was just measuring blindly what is what...
And there are two sets of emitter resistors 2 x 5 each 0.33 Ohm. They are inverted on one set rail is (+) and other set of 5 has negative (-) rail.

As far as I understand bias should be identical on both rails but one rail (one on the back of the amp with resistors Q116-Q120) about 2mv higher. Difference seems to be in % of rail value as if I set bias lower difference will be less and is I'll go up with bias difference between those sets also inferences.
On the right channel is much better - difference is is much less after calibration is about 9,9mV on + rail and 10,1 on - rail.

I also noticed that left channel not working when amp is cold is most probably causes by bad contactor ob the back of the amp - if i slap it directly sound comes back to left channel...


My question is - why there is big difference between both bias sections on left channel ? What could be the reason ? Is there any chance this contact switch on the back (input board) has anything to do with it ?


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Old 14th February 2015, 01:34 AM   #3
nattawa is offline nattawa  Canada
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Did you take voltage readings on all 0.33R resistors?

(9.4*5)/4=11.75, quite close to 11.9, that can indicate one of the output transistors is not working. The voltage difference on Re varying in certain percentage with the bias setting also suggests the same possibility.
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Old 14th February 2015, 10:14 AM   #4
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Good point. I'll check all emitters tonight. I don't know why it didn't came to me...
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Old 15th February 2015, 09:05 AM   #5
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Default Different bias between all transistors on left channel

So as advised I have measured all resistors and bias on all emitters.
Result is a puzzle to me. It's different between all transistors! Differences are huge.
Since they are sourced from common rails and only thing that I can see there are resistors on collectors and emitters that I measured to be ok and of same value. So if resistors are fine does it mean transistors are bad ? But not 100% bad just slightly damaged ? If yes - then which ones ?

Any suggestions ?
Here is a table with measurement results:
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Old 15th February 2015, 02:38 PM   #6
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Quote:
Originally Posted by ptrezza View Post
I opened up the amp to try and troubleshoot the output relays which are staticy at low volume and that brought up some other questions. The left channel heat sink was a lot hotter than the right, so I investigated the bias. I ended up adjusting to 10 MV from 8 on the left channel and 9 on the right channel. That doesn't explain the temp difference. Anyone have any idea why that would be? The amp sounds good and balanced to my ear- except for the random static at low volumes till the relays make contact. I also noticed that the output relays are in a tricky spot to work with, and looking online they are discontinued- they are

OMRON G4F1123TUSDC12

Has anyone had any success cleaning them? I could use some old-timer trick

I know they can be redone by sending the amp out, but I am out of work and on a severe budget crunch! Not to mention I enjoy fiddling!

Thanks,
Peter
I had similar issue with my contacts. when i measured resistance on this relay it showed that when it's closed the resistance is starting from around 50 Ohm on one channel and maybe 15 on another then slowly goes down as contacts are pushing... but it didn't went to 0 it was about one to 2 Ohm on right channel and about 4-5 Ohm on left.

I have removed output board, opened relays covers and bent a little bit contact plates so they don't touch each other but they are not fully open. After that resistance came down to below 1 Ohm on both channels but still not to 0. So i took sandpaper of 600 grade cut thin strip on on amp powered when contacts are closed i cleaned a bit both sides of contacts. After that all was working fine resistance came down do 0 (0,2 Ohm actually but my multimeter cables are 0,1 Ohm each). Now every time relay is closing immediately there is zero resistance on contacts.

Hope this helps.
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Old 22nd February 2015, 10:32 AM   #7
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Default Any ideas?

Any ideas why there so much difference in bias between transistors?
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Old 22nd February 2015, 10:37 AM   #8
Mooly is offline Mooly  United Kingdom
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Those readings look normal. I notice they increase as you work down the list which could be just the normal drift/increase in current as the amp warms. Try monitoring just one value from cold and see how it changes.

It all normal though. (you can't measure resistance with the amp on. Any voltage across the resistor totally confuses the DVM)
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Old 22nd February 2015, 10:55 AM   #9
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All measurements ware made on amp fully warmed up (over an hour). I used 3 multimeters at the same time. All those ware stable when I took a note of them. I even tripe checked them back and forth to be sure they are stable before putting this data in table.

Also I have monitored 3 transistors with most different values from beginning (amp cold). From very beginning they had similar difference until it got hos and stable.

Right channel is almost perfectly ok - all transistors have same value when hot and stable and they rise from cold in thecsame paste.
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Old 22nd February 2015, 11:55 AM   #10
Mooly is offline Mooly  United Kingdom
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There's not really anything you can do over it apart from fit matched devices which I suspect wouldn't gain you anything anyway.

Its just the way it is
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