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Traynor Bass Mate 25 Power Supply Over heating.

Started by TK, January 21, 2018, 02:25:50 PM

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TK

Hi,, I am in the last stages of repairing my Bass Mate 25 SS amplifier,, I've replaced the output transistors/drivers etc.. and It works and sounds great,, slight buzz,, and 10 mV DC on the output.. so not too bad I hope,,

The problem is the 330 ohm resistor for the positive side of the power supply, the 2200uf cap and the 7.5 volt zenor are getting RED HOT...When I first started the repair I noticed burn mark just under the resistors and noticed they had used 1/2 watt resistors instead of the 2 watt resistors recommended,, I put in 330 oh 2 watt resistors and the one on the positive side is really getting hot.. I checked it with my laser and It's starting to go up into the 200 degree F area and If i left the amplifier on I'm sure the temperature would keep increasing until the resistor started on fire.. I've checked everything and replaced everything in the power supply with the exception of the Rectifier,,

I thought it may have been the shored output transistors that caused the over heating problem but those are new. I'm stumped,, Any help would certainly be appreciated...



Enzo

Wow, talk about minimal - no cap on the zener side of the resistors.

Do you have 7.5v on each supply?  Did you replace the 2200uf cap.  It sounds like you did.

Is ther a lot of ripple on the positive rail?

What would make that cap get hot?  reverse voltage.  Scope it, is there full cycle AC hitting that cap or only the proper pulsing DC?  If full cycle AC was getting tot eh zener, it would "zene" for positive voltages, but any negative cycles would see the zener as a plain old diode, a diode shorting across the supply.

Please make sure your bridge rectifier is healthy.

TK

Hi J.P. thanks for the Schematic post again,,, Hi Enzo I've replaced the zenors and the 2200uf caps as well as installed 2 330 ohm 2 watt flame proof resistors.. Yes the 7.5 volts both pos and neg are correct,, What I didn't replace was the rectifier,,, I will do that tomorrow and then put it on the scope,, I didn't want to scope it if I couldn't get the heat down some how... I'll post the results tomorrow or the next day for you,, thank you again for your help.... Yes this amp is super minimal,, but does it ever sound amazing,, I only wish I could figure out the power amp topology Traynor used.. It looks like a watered down giant 741 op amp,, LOL,,

Enzo

The power amp is about as classic as it can be.  It is what we call quasi-complementary.  All that means is that both outputs are NPN instead of NPN and PNP.  the difference between the two styles is largely just how Q5 is wired.

This schematic is drawn wrong, the output transistor connections show base and collector reversed.  For example, the base of Q7 connects to Q6 emitter, not to V+.  Rather than just replacing the bridge and hoping it does something nice, why not get out that scope and SEE if it is acting wrong?

Enzo

And we invite you to look at the topics in the sticky section of this very forum and find "Teemuk's Book..."  download that and study it.

TK

Hi Enzo,, Yes I'm in the process of reading Teemu's great book,,, As you suspected and I guessed but had to try anyway,, Installing a new Bridge rectifier did not resolve the over heating problem,, So,, out comes the O-Scope... I've checked for signals at key points on the board and with the exception of a couple of locations that have no signal, what is going in at the input is coming out only larger at the output..So the amplifier is working.

I'm using a square wave generator at this point but I'm going to switch to a sine wave gen and take another look at the signal coming into the two output transistors. I will take DC voltage readings as well.

I can only keep the amp on for short periods of time so it's getting tricky to take readings. Resistors and the positive output at the Bridge rectifier are getting red hot still,, I'm not sure what took out the output transistors at this point and not at all sure if it was the over heating problem that was the original culprit.. but will keep on looking.

I did look at what you pointed out on the Schematic with regard to the drawing being incorrect and sure enough you were right,, the base and emitter of both output transistor were reversed on the drawings for some strange reason. Thank you I wouldn't have caught that for sure...

In an effort to try and prevent damage to the new parts I've installed I'm going to try and Isolate sections of the power transformer and see If I can't get closer to what's causing the problem. Will keep you posted. Thank you again for your help and suggestions.