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EV Q1212 Fault Repair help required!

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Robert871 View Drop Down
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    Posted: 19 September 2013 at 5:34pm
Hello 

I have an electrovoice Q1212 amplifier that i purchased on ebay. The item was stupidly shipped in a rack case with no support and of course got damaged. The amp works but has an intermittent fault that when you move or flex the casing the item will clip and shut down on one channel. The amp then followed on to fry 1 resistor and 3 transistors on the offending channel. I have replaced these and the fault remains.

Any ideas? I have schematics available. 

Robert 
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Tan-2 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tan-2 Quote  Post ReplyReply Direct Link To This Post Posted: 20 September 2013 at 12:09am
Evening to you... sounds like a cracked pcb so that will need checking under a magnifier/strong light to ensure that there's no fine air line breaks. next when, working on amps that have one channel this can be used to guide in repairing the faulty side ie comparing voltage readings.

Ev being standard transistor circuity and thus it should be straight forwards to get up and running again with out to much parts being changed out.  
start by checking the supply lines to the working channel and note down the offset o/p voltage this should be very low say 20mv to even 100mv. now power off and drain psu then wire up a 100w bulb in line with the primary ac main input now with the working channel un-plugged power up and see if the bulb lights up and goes out or stay on low but, if it stays bright you have a short somewhere and needs more checking over.

regards for now,

 

Logic will get you from A to B. Imagination will take you everywhere.
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Robert871 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Robert871 Quote  Post ReplyReply Direct Link To This Post Posted: 20 September 2013 at 9:21pm
Thank you for your reply,

I have looked everywhere on the PCB and cant find anything. I have found a few components that had become intermittent due to the flexing of the PCB. The amplifier has a service connection that provides a pin out for the BIAS, Rail and temperature measurements. On the faulty channel I am getting O bias voltage and the Rail is higher than the working channel. I also have another of these amplifiers so it is useful for comparing measurements. The bias on the working amp is 3.5mv on each channel. From what i can remember the bias on the working channel of the amp I am repairing was much higher. There appear to be no shorts just the intermittent clipping on channel A and the strange readings. 

What would contribute to O bias and a high voltage rail? The voltage rail should be in the 30's and is in the 40's - 50's on the dodgy channel. I am getting the correct readings from the transformer. I can supply the schematic if at all helpful. 

I really appreciate any input!!

Basically I am already at a loss with this amp and have been compensated lightly by the courier. I have no option to send it to EV and no local service guys who will tackle it. 

Thank you!

Robert  
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Tan-2 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tan-2 Quote  Post ReplyReply Direct Link To This Post Posted: 23 September 2013 at 11:40pm
If unit was to go for repair at ev you might pay more than the amplifier was brought for with service cost.. regarding bias troubleshooting you need to check the bias servo circuit is working thus check the transistor that sets up the o/p stage and its trim pot if the circuit has one common value is 470 ohm to 5k... post the schematic of amplifier so we can see what's going on here.

start by checking voltages around the input small transistors these are called the long tail pairs, one thing how many pre-sets to each channel?   Some amps have a offset adjustment to zero off the o/p voltage to low level.. faulty transistors within the tail pair will cause imbalance and raised voltage at the o/p of the amplifier.. voltage amplification stage ie vas is where the amp boosts the small i/p volts then of to the driver transistors and so to the o/p devices. 
if you place you meter across one pair of the low value resistors theses being .47 ohm or .33 ohm each side straps back to a leg (emitter) of each o/p transistor and adjust the bias pot does the mv across these resistors change? if the servo is working you should get a reading try not to trim to high as your run the risk of over heating the power transistors. If no adjustment can be made then check for open resistors and use the working side to carry out a cold check no power on..


regards for now T. 
Logic will get you from A to B. Imagination will take you everywhere.
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alili View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote alili Quote  Post ReplyReply Direct Link To This Post Posted: 27 April 2018 at 4:45am
can you send me the schematics please coz im trying to repair the same amp , but some components arent marked and i cant really tell the value without the schematics , thanks in advance .

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damo76 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote damo76 Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2019 at 1:40pm
Hi there,

do you still have the schematic for the Q1212?  I have one sitting here I cant find the issue without spending weeks on it, or checking the schematic, any help much appreciated!
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Post Options Post Options   Thanks (1) Thanks(1)   Quote hpavictor Quote  Post ReplyReply Direct Link To This Post Posted: 06 March 2019 at 12:54pm
A nice day to all !



Edited by hpavictor - 06 March 2019 at 12:56pm
audio perfection
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tamixdj Quote  Post ReplyReply Direct Link To This Post Posted: 28 February 2021 at 9:36pm
thank you
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