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Old 28th June 2010, 07:09 PM   #1
Capital is offline Capital  United States
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Default Fuse Blowing Amber Series 70

I picked up an Amber Series 70 with a dead right channel. Both fuses were blown in the right channel and after replacing them I powered up the amp again but they instantly blew. I'm thinking it's the 2N5686 and 2N5684 output devices but before I start throwing parts at it I was curious if anyone had any suggestions.

Any advice would be greatly appreciated.
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Old 28th June 2010, 07:29 PM   #2
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Iīd start by checking the output devices with either a transistor checker or a multimeter set to continuity test (there are plenty of guides on how to test transistors and what readings you should get between pins available online). Itīs probably also a good idea to do a visual inspection and check whether any resistors have scorched or any capacitors or transistors have blown. Also, check the power supply voltages are within a decent range.

A quick google didnīt bring many results for this amp and I checked all of the usual sites I use and could not find a service manual or schematic. If you can find one, it would help out a lot and give something for you to compare with.
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Old 28th June 2010, 09:17 PM   #3
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If they are gone I would be asking why and if the original problem is still there.
After all you dont want to fit new components only for them to blow again.
Were you giving the amp some serious stick when it went ?

I would start off by checking speakers and leads for low resistance or a short.

Semiconductors rarely blow for no reason.

I usually fire my faulty amps up without output transistors in the circuit and feedback the VAS output into the LTP to make sure LTP, VAS and bias are working OK.
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Last edited by nigelwright7557; 28th June 2010 at 09:19 PM.
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Old 29th June 2010, 12:51 AM   #4
Capital is offline Capital  United States
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Thanks for the replies guys. I should have been more clear. The amp would instantly blow fuses in the right channel without speakers or RCA connections. The 2N5686 and 2N5684 output devices pulled out easily and I checked them. The 2N5686 both read the same but the 2N5684 read differently on my multimeter. I decided to swap the 2N5684 to see if powering it up it would then pop the left channel. I was surprised to see that it didn't. Now a logical person would have stopped here and ordered a new part. I being the dufus I am decided, "it's not popping fuses I wonder if I can give it a listen". I plugged everything up and turned it on. It sounded fantastic......for 30 seconds. Bright sparks started flying from the board of the left channel and I quickly unplugged the amp. The LM391N chip was all burned up and the cause of the fireworks.

I'll post pictures later. I know it's beyond my skill level to fix the board, I may run by a shop and see what they think or just keep an eye on the classifieds for another Amber with a bad channel.
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Old 27th August 2012, 03:52 PM   #5
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Default Amber Series 70

Quote:
Originally Posted by Capital View Post
Thanks for the replies guys. I should have been more clear. The amp would instantly blow fuses in the right channel without speakers or RCA connections. The 2N5686 and 2N5684 output devices pulled out easily and I checked them. The 2N5686 both read the same but the 2N5684 read differently on my multimeter. I decided to swap the 2N5684 to see if powering it up it would then pop the left channel. I was surprised to see that it didn't. Now a logical person would have stopped here and ordered a new part. I being the dufus I am decided, "it's not popping fuses I wonder if I can give it a listen". I plugged everything up and turned it on. It sounded fantastic......for 30 seconds. Bright sparks started flying from the board of the left channel and I quickly unplugged the amp. The LM391N chip was all burned up and the cause of the fireworks.

I'll post pictures later. I know it's beyond my skill level to fix the board, I may run by a shop and see what they think or just keep an eye on the classifieds for another Amber with a bad channel.
Hello:


After 3 decades operating a pair of Amber Series 70, I have found that one of the reasons for a LM391N failure is a wrong bias setting, which also will destroy the output devices, and in some cases the drivers also .

The bias must be adjusted to 150mA, as per the manufacturer settings.
After checking that all semiconductors including diodes and resistors are in correct working order, you should start with a low setting (6.2Kohm or so) of the 10Kohm potentiometer that together with the 3.9K resistor allow to adjust the bias properly. You must remove the + fuse and install the ammeter on the fuse holder legs in order to read the 150 mA after turning CW slowly the trimmer screw.
Before any adjusment, you should check the reading on the trimmer without AC applied, if this reading is more than 7Kohm is almost shure that bias setting is high in excess.

My Ambers sound icredible thanks to the use of audio grade components that were not available at its manufacture time, like Non polar caps bypassed with MKTs, and metal-oxide (1%) resistors. I do think that it is unnecessary to modify the original design.
I use both in Dual "half Stereo" configuration which increase the separation and perceptibly the power. Under these conditions and with its huge PS (64.000uF) the Amber outperform modern amplifiers costing a lot of more money.

I hope this can help to someone.
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Old 27th August 2012, 08:21 PM   #6
AVWERK is offline AVWERK  United States
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They made a Lm391n-80 and the lm391n- 100. These are not made any more. If you can locate the -100 use this one instead of the 80.

regards
David
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Old 27th August 2012, 09:07 PM   #7
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Quote:
Originally Posted by AVWERK View Post
They made a Lm391n-80 and the lm391n- 100. These are not made any more. If you can locate the -100 use this one instead of the 80.

regards
David
Hi David:

Efectively most people know that both models has been discontinued by NS, but the N100 anyone can buy to a few distributors yet.

Best wishes.
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Old 18th December 2012, 10:02 AM   #8
oldeng is offline oldeng  United States
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Default Schematic for Amber Series 70

The LM391N-100 data sheet contains an amplifier schematic without any resistor or capacitor values. The transistors used are different also, but the way the transistors are used and connected is the same at the Amber Series 70 that I own.
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Old 18th December 2012, 12:26 PM   #9
jaycee is offline jaycee  United Kingdom
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It would probably be quite trivial to build a replacement for the LM391 frontend IC using discrete parts, if you were desperate.
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Old 18th December 2012, 05:17 PM   #10
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another issue will be to have a more proper VBE multiplier function that the 391 doesnt have and beyond other issues thermal runaway might also be a problem

I recall that someone in a Greek forum added an external VBE multiplier outside the 391 still this will require some digging to find

Kind regards
sakis
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