Bass guitar amplifier - resistor and possible transistor issue?

Thread Starter

tylerwerrin

Joined Oct 1, 2017
4
Hi guys,

This is my first time posting on this forum - I was wondering if I could get some help in determining what resistor value I need to repair this bass amplifier as well as if the new transistors I purchased are any good.

A little backstory:

It's a Drive CD300B Bass amplifier "all in one" unit. Years ago I had lent it to a friend, and when it was returned to me, it no longer worked. After measuring all the resistor specs, it came down to the transistors (D2059) - there are 2 of them on the head unit. Upon measuring their values, they were out of spec and faulty, which I believe was due to the amp being left on for days on end by my friend.

I purchased some new transistors from china and they finally came. I noticed that the "Y" value printed on the transistors were all different from the ones that were already installed on the board. The new ones had a range of Y values, whereas the board had a Y value of 319.

Anyways, I replaced the transistors, but due to a faulty solder joint by me, i shorted them out resulting in a resistor going up in smoke. Normally i would just replace the resistor and try again... but the resistor's values (usually written in colored bands on the resistor) had burned up. I no longer know what the value is, so I dont know what to use in its place.

I have read that the resistance values of resistors go up after being exposed to extreme heat from a short - the other resistors in the neighborhood of the one that melted down are all over the map..

Can anyone help me in determining what the value is? Could i use one with lower resistance value? is there a generic resistor that can be used?
 

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Ylli

Joined Nov 13, 2015
984
Are you any good at tracing out the circuit and drawing up a partial schematic? I can't find anything online, and without an idea of where that resistor is in the circuit and what it is being used for, your guess is as good as mine.
 

Thread Starter

tylerwerrin

Joined Oct 1, 2017
4
OK, i'll snap some pictures tonight - and yeah, i couldn't find a schematic anywhere - i was hoping at the very least the board would have been a rebranded/cross brand item so it would exist as something generic online but nope!

i removed the burnt resistor, as well as one of the junk transistors.

I can trace stuff along a circuit board, but beyond tracing it visually, i wouldn't know exactly what the circuit would be doing.
 

Thread Starter

tylerwerrin

Joined Oct 1, 2017
4
Here is the backside of the circuit board - i am still working on my trace out - it's very time consuming, especially since this board uses so many different types of resistors (very surprised).

Any ideas?

There are 2 burnt resistors - one which i removed, which i believe acted as a fuse (?) and another which is directly below the left hand transistor (the right hand transistor has a matching resistor. I believe the left hand transistor's resistor burnt out because I had removed the transistor from the board after smoking the other (possibly fuseable) resistor.
 

Attachments

Ylli

Joined Nov 13, 2015
984
Looks like that resistor is in series with the two diodes, D1 and D2. Quick perusal, I think that resistor and two diodes is part of the bias network of the output devices - goes between the two bases to set the quiescent current level.
 

Rawjam

Joined Dec 7, 2017
1
Hi just pick up on this thread, I've just come into possession of one of these amps, it was totally dead also it seems someone had been tinkering with it. However i can confirm that R27 is 56R 5% tolerance and R30 & 31are 220R 5%tolerance, hope this helps.
 
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