QUAD303

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17-Mar-2020 11:11 PM

LOWTHERMAN62

LOWTHERMAN62

Posts: 83

Hi all, Just revised a 303 including 4 new mj15003 and mj802. First powered on using 100w DBT. light went down to a barely noticible glimmer but not completely out, so powered up and set bias atc,and all good 67v,33.5v and steady 10mV, all very stable. Noticibly this runs hotter than my other 303. even at idle. Has been running for a few days and sounds great but I am concerned about temperature which I can describe as warm to touch ,at a guess about 25-30 degrees c. My other 303 also revised but original transistors runs much cooler. Any thoughts.

Regards Stewart

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18-Mar-2020 05:20 AM

EJP

EJP

Posts: 1742

Lower the bias to about 5mV. I am finding 10mV just runs away now, don't know how I ever used it as a base.
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18-Mar-2020 11:14 AM

LOWTHERMAN62

LOWTHERMAN62

Posts: 83

Thanks EJP I will give this a try and keep you posted

Thank you, Stewart

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19-Mar-2020 12:07 PM

LOWTHERMAN62

LOWTHERMAN62

Posts: 83

Hi EJP,

 Readjusted bias (rail voltages were 100%) to 6mV and ran some pretty heavy programme music yesterday evening. 303 just warm to the touch. Problem solved.

Cant get my head round the variation on bias suggestions, does it depend on the individual unit . Theoretically I suppose bias should be set on the individual unit using test gear?

Thanks and regards Stewart

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20-Mar-2020 02:23 AM

EJP

EJP

Posts: 1742

It's bizarre. I used to use 10-12mV. Now I can't get above 5.5mV without a runaway, and I get the same THD figures as before. I think it may have to do with MJ15003s versus 78494s.

EJP

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20-Mar-2020 04:55 PM

LOWTHERMAN62

LOWTHERMAN62

Posts: 83

Seems all well at 6mV but still slightly hotter than my other 303 with original transistors. I thought that it may have been the mj15003s maybe being fake but I bought these from cpc farnell as they were a good price at £2.10 plus 20% vat . They all tested good prior to fitting. As you say there must be a difference when using the mj15003 as my other 303 is biased at 10mv and runs cool as a cucumber. Will keep an eye in the next few days and may try even lower. What is the lowest bias we can use here?

 Regards Stewart



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21-Mar-2020 11:41 AM

EJP

EJP

Posts: 1742

The service manual says 5-10mA across pins 4/6, which is 3-6mV across 0.6V. The real minimum in any specific 303 can only be established with a distortion analyzer.

EJP

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