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Hitachi HA6800 Problem
Hitachi HA6800 Problem
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Old 21st October 2018, 12:01 AM   #31
Telnet100 is offline Telnet100  New Zealand
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Thank You so much --
Those 2 circuits do look a little similar; Those Capacitors the Author changed went to 10 times the Original Values; from 2 x 22pF in series (11 pF) up to 100 pF. Maybe I will try somthing like this on the Vertical input Pcb with those 2 x 10 pF in series.
Maybe, as suggested, there could be a similar problem in this HA-6800.
I can't see that Changing these can do anything but slow the circuit down.
I'll Post the results --
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Old 21st October 2018, 12:47 AM   #32
roger2 is offline roger2  United States
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FYI...Mooly is an amp guru.

I am merely a hacker taking a pot shot at this. Your description of the oscillation problem, and the fact that it is a Hitachi, reminded me of those videos that I have had bookmarked for a while.

Last edited by roger2; 21st October 2018 at 01:12 AM.
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Old 21st October 2018, 01:55 AM   #33
tvi is offline tvi  Australia
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YIf you hit a dead end you might want to try the procedure on page 14 of AN-1645 LM4702 Driving a MOSFET Output Stage 7.1 Determining Gate Resistor Values

Page 7 "1.6 Parasitic Oscillations" of cordellaudio MOSFET Power Amp.pdf might be interest

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james
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Last edited by tvi; 21st October 2018 at 01:57 AM.
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Old 21st October 2018, 02:14 AM   #34
Telnet100 is offline Telnet100  New Zealand
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Well, I watched that Video on the HA-4700 repair, and decided to do similar on this one, to see what happened. I checked those Capacitors C722 and C714, in both channels, both perfectly okay. Then, I bridged them with 100 pF as suggested.
Problem Solved! -- Rock Stock Stable, even with big changes of supply voltage, unlike before. Next, I will connect the Sig.Gen, and feed in a signal to see what the response looks like, before I refit those Power Fets. I dare say that this mod might affect the Highest Frequency Response, But, I'll see by how much.
I'll be posting the results.
Thanks for All!
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Old 21st October 2018, 02:48 AM   #35
roger2 is offline roger2  United States
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If 100pF stopped the oscillation, then perhaps a smaller value (but larger than original) would as well. As I recall, the technician who made the video only tried one non-original value which, as you noted, was significantly higher than original.


Interesting to read of your test results. And I hope this is a legit method...if it is not then hopefully someone will explain why.
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Old 21st October 2018, 02:52 AM   #36
roger2 is offline roger2  United States
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Wait...722 & 714? I don't believe those caps are serving the same function as the ones that were changed in the HA-4700


In the HA-4700 I think those are miller caps, from base to collector of the VAS, whereas in your amp the similar looking caps are in the NFB loop (from the schematic on pg 2 of this thread).

Anyone who wants to see go to 6:47 (where the author is pointing at the caps in question with a pencil) in the 2nd of the two videos I linked earlier, and repeated below
YouTube

Last edited by roger2; 21st October 2018 at 03:02 AM.
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Old 21st October 2018, 04:13 AM   #37
Telnet100 is offline Telnet100  New Zealand
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Hello, All
Thanks for all the suggestions -- What I did next was to Lower that 100pF Capacitor to 50pF; still Rock Stable. I then Injected a Test Signal into the Aux Input for that channel, and swept the frequency to see the response. It still looks reasonably flat until 40 Khz, until it starts to taper off gradually. It is only about 3dB down at 60Khz, still a perfect sine wave with no sign of instability.
That's correct; those caps are the AC Feedback devices.
Is there any thing someone might suggest I do before refitting those Power Fets?
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Old 21st October 2018, 08:31 AM   #38
Mooly is offline Mooly  United Kingdom
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Hitachi HA6800 Problem
I've no answer as to what has gone on here. Post #1 suggests you acquired the amp minus its output devices and so that still leaves the possibility that some unknown work might have been done in the past.

I guess you have to just fit the FET's back and see how it all behaves.
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