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Old 25th April 2009, 01:40 AM   #1
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Angry Repairing Cello Encore 1M Line Pre

My Encore 1M line preamp got problems, the LEDs go dim and the outputs were cut, do anyone has idea on what's happening. I am the first time to handle such an expensive gear and please help.
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Old 25th April 2009, 02:43 PM   #2
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Default Encore

First, check if the power supply is working properly. You will need a voltmeter to make a few measurements.

The Encore has it's voltage regulators on the main preamplifier board.
Take off the top. The input to the regulators comes from the power connector. Follow the red and yellow wires from the connector to where they plug into the main board near the two large black heat sinks. Then look for three "bussbars" running the length of the main board from left to right about a third of the way back from the front. The middle bussbar is "common" and the other two are plus or minus (not sure which) regulated voltage. Measure from the common to the red wire and you should get around +40 Volts DC (unregulated). Measure from the common to the yellow wire and you should get -40VDC. Important thing here is that both voltages should be the same. If one is lower then the filter caoacitors in the power supply could be going. Then measure from the common to one busbar and you should get +25.0 volts, from common to the other should get a reading of -25.0 volts.

If the readings are OK, I suspect the preamp is OK and the problem is in mute relay circuit which, in the power supply chassis on what is called the VCB-2 board. This is a simple delay circuit which after 30 to 45 seconds, turns on the LED and "unshorts" the output via relays in the preamp. When operating properly, this circuit works at about -25VDC which you can read from one side of 33 ohm R4 and chassis. The operating voltage to the relays should be around -23VDC (other side of R33 and chassis). The voltage across C1, the timing capacitor should "rise to" 21VDC.

If you can't follow what I'm saying, get someone else to do the work.

Charles
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Old 26th April 2009, 12:21 AM   #3
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Many thanks Charles,

I spent a day yesterday to follow the circuit for both power supply and main amp. and have a same view as yours. I found that even the output relays have been cutted, the regulated bus is still +/-25vDC.

Previously I worried that there is a fault detection signal feeding from main amp to switch off the relays but the green and white cables (relay control and LED) goes no where elses in the main amp. board other then the relays and LEDs. So I think the fault should be from the delay circuit.

Thank you for your data for the delay circuit, my reading is -6V (output terminal for the relay circuit) when not connecting the preamp. and 13v for the 33ohm 5w. It should be something wrong, should it be the 100uf 50v cap.?

BTW, could you tell me the function for each amp. modules?

Thank again Charles.
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Old 26th April 2009, 01:42 PM   #4
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Thanks again, the problem has been fixed.

However, after this case, I am planning to replace all E-caps including 6x 8200uf Sprague, 2 x 470uf Sprague and the 150uf ic.

BTW, can anyone tell what's the grade for ic caps, are they audio grade e-caps or ordinary e-caps? Since in HK, it is very difficult to buy the original e-caps now.
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Old 26th April 2009, 03:23 PM   #5
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Rachel,

The minus voltage to the delay circuit is derived from the main power transformer, through R1, R2 (100 ohm 5 watt) and then through he two 1N4004 diodes (cathodes connected together), then filtered by C14 , the 100uf cap. Maybe one of the 100 ohm/1N4004 paths has opened up causing the filter cap to see only half-wave rectification which could cause the filtered voltage to drop significantly. Q2 the TIP22 transistor is initially turned fully on, "shorting out" the LED and the relays, it turns off after the delay period.

The circuit board layout is as follows: looking from the front, the left-hand quarter of the board is the left channel balanced input section. It is a "textbook" instrumentation amp (balanced to unbalanced) configuration, with the two VF-3 modules the +/- gain stages with the OTA-7 module acting as the differential amp. The next quarter-section in is the right hand channel input section. The output section is a portion of the front of the board ( the two OTA-7's combined with the left hand rear section (to the right of the power supply). One OTA-7 provides gain, in conjunction with the balance control and the two VF-3's convert the unbalanced signal "back to" balanced for the outputs.

Your last message just popped up - glad you fixed the problem - what was it? I don't believe you need "audio grade" caps in these positions.

Charles
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Old 27th April 2009, 02:06 AM   #6
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Sorry in my previous post did not report and share the cause of fault. The fault was caused by the failure of the filtering caps Sprague 100uf 50v immediately after the rectifiers.

Because Sprague caps are hard to be sourced in HK, I initially replace with a Rubycon for temp fix.

I am now planning to renew all e-caps in the Encore, including all the Sprague silver caps and blue caps, and 2 caps which bear the brand name as "ic" which I don't know is a high grade caps or not. One of the concerned cap is the timming cap. and the other is located at main amp PCB regulator section (the 150uf 50v brown one). Presummably, the timming cap can be anything but the filtering caps in main PCB?

You seem very familiar with the design of Encore, thing that you should be an user of it, or even involved in its design team. If Tom was not killed in 2007, I suspect you were him, just kidding.

Anyway thanks so much to giving me valuable information timely.
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Old 27th April 2009, 02:19 AM   #7
PH104 is offline PH104  United States
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IC stands for Illinois Capacitor. There probably isn't anything special about them but you can check their website to get the specs:
www.illinoiscapacitor.com
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Old 27th April 2009, 02:20 AM   #8
tvi is offline tvi  Australia
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The caps labelled "ic" were from Illinois Capacitor, they are a high quality industrial capacitor.

Hope this helps

Regards
James
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Old 28th April 2009, 06:20 AM   #9
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Thank you all of you.

I have contacted "ic" HK branch and trying to source the caps from them. The problem goes to Sprague caps.

Do anyone know that where can I still found the Sprague caps (8200uf 50v, 470uf 50v and 100uf 50v), or can you advise which repalcement caps having similar sonic characteristics?

BTW, I saw some special caps in the amp. modules, say OTA-7v, the caps are 2.2uf 50v with black color, and some (in VF-3 and OTA-5 and main board are in yellow colour. Can you tell me which type of cap they are and from which company, can I still find them?

Many thanks again in advance.
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Old 28th April 2009, 01:18 PM   #10
tvi is offline tvi  Australia
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Sprague are now part of Nippon Chemicon, also called United-Chemicon .

Regards
James
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