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June 27, 2022, 04:42:19 PM

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Traynor Bassmate 25 Repair Update..

Started by TK, February 06, 2018, 12:41:17 PM

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Hi, Just wanted to post the progress so far and note the findings.. I still have not found what is causing the RS Resistor for the Positive zenor regulator side to over heat,,, But I have found where the problem is..

After cutting pretty much every supply trace on the board including Positive and Negative supply rails as well as the Ground rail all in Multiple places I've worked my way back right to the zenor regulator...

With absolutely no load the Pos rail resistor still over heats.. Very interesting for sure.. I have checked for shorts and there are none,, The voltage is not being loaded down and remains constant at 7.5 volts @ around 65 Ma..

I'm thinking at this point is has to be either a Miscalculated zenor resistor problem or the power transformer is bad? I've checked the AC voltage from the power transformer at the Rectifier input(which I have replaced as well) and the voltage is spot on,,

I'm wondering if there are any other possibilities that I have missed.. There is definitively a problem from the RS resistor, to the Zenor, to ground. I just don't know at this point what it could be,, The Zenors have both been replace along with the 330 ohm resistors,, I'm wondering if the 330 ohm resistor is the wrong value but the positive rail is the only one that heats up , the negative rail is perfect.. This amp must have worked at one point,, This is getting really good. LOL,,, I think the Schematic is attached,,,


Measure the voltage across the 330 ohm, calculate the watttage (E squared divided by R).  Is it less that 2 watts? 
At 65mA and about 21V, that would be 1.3W at the resistor.  Not overheating despite how hot it feels.
What I don't understand is why the negative side does not get hot.
What is the voltage across the 330ohm in the negative supply?


Hi g1,, Thank you for your reply and input,, To answer your questions,, The voltage drop across the 330 ohm resistor is 12. 4 volts.. So the power the resistor is dissipating is .466 Watts,, Hmm very interesting again for sure.. Not sure about the negative rail,, Just checked and getting strange voltage readings from that side now.. Transformers still getting around 21 volts A/c..

Really not sure what's going on,,, voltage drop accross the Negative side 330 ohm resistor is 20 volts.. voltage at the rectifier side of that resistor is 29 volts,,, somethings very wrong here for sure.. I'll keep looking,,


Note that positive and negative side supplies are identical.

The transformer AC is used for both, there is no way for the transformer to affect only one polarity.

It is normal for the 330 ohm resistors to get hot.  It might be the negative side one is not as hot as it should be.

The main power supply voltages should be the same, other than polarity.  29vDC?  SOunds reasonable, but both sides should be that.  If they are different by more than say a volt, find out why, how much ripple is there?   Did you replace the main filter caps?  If so are they both in correctly?


Hi Enzo, g1. Thank you for replying,,

The Voltage imbalance seemed to have been cause buy a cracked pcb trace.. I've repaired that and the voltage is now spot on.

The temperature difference is still there with a slight change. Got to tell you that High temp resistor reading really had me freaked but what I did was turned the amp on and left it on all day... The resuslts where..

All voltages DC and AC are completely stable.. there was still a temp difference in the Zenor regulator resistors, But I think the amp can live with that..

My initial concern and lack of understanding was the Burn marks under the original 1.2 watt 330 ohm resistors and how hot the original bridge rectifier was getting,, Those have been replaced with Higher wattage devices.. 2 watt resistors and a Higher Amperage rating Bridge rectifier..

The resistors still get hot.. I've calculated the Power consumption of bother resistors and it's just under 1/2 watt. The good thing is the even though the resistors are running hot the Bridge rectifier is no longer heating up, and compared to the pos rail resistor it's room temp,,,

I thought that I'd blown up one of the new output transistors which were fairly expensive as they had to be ordered from the UK.. But I took it out and rechecked it and it's perfectly ok...

I'm not sure if someone had been in this thing before to try and repair it but there was quit a bit of trace damage around both output transistors which i will attempt to repair this afternoon,,,,

Thank you to you guys big time for your patience and all the help,,, My understanding of SS discrete Guitar amplifiers like this TBM 25 has increased ten fold.. 


Keep in mind that resistors will dissipate power as heat.  So when you see a higher power resistor, it will get hot when it is dissipating heat.  This is normal.  The browning of the board around hotter components is fairly normal after long term use.
If you install a higher power resistor, it just has a larger surface to distribute the heat over, but it is still the same amount of heat.
If one of them is running hotter, it is probably the one on the positive side.  In an amp there is usually more things running off the positive side than the negative, so sometimes the resistor in the plus line runs a bit hotter than the one on the negative side.


Thanks g1,, I will keep that in mind for sure... I'll install the Higher current transistors and let it run for the day to see what happens.... Thank you for your help..