Polytone Baby Taurus - Bad Hum and High Current

[misterc57]

I acquired a Baby Taurus that has a loud hum and high current draw. The hum and current increase as I dial up the variac. This output section uses a MJ802 and MJ4502.

I removed the power amp section to discover some fried components in and around the MJ4502 transistor. The board states "PA 378 B"

I thought I had the proper schematic (see attached). But perhaps not as you will read.

In looking at some of the yellow caps and connections, I think someone has worked on this board in the past.

I have little experience working on solid state output issues and I hope to learn from this experience.

I could use some step by step help in diagnosing the bad components. How do I test the transistors in place? How do I test the power supply feeding this section? Etc.

Is the bridge rectifier missing from this board? I thought it might be on the underside but I only found another 470 uF 35V cap there. There are two 470 uF 35V that appear to be in series. One is top side, one is under side. I think this is C10 and C11, however the capacitance of 470 uF is far off from the schematic stating 4000 uF.

Maybe I have the wrong schematic? Schematic shows a Blue/Red/Blue AC connection feeding the bridge rectifier, but this amp has a Red/Blue/Black wiring group. More on this...

• Red feeds the positive side of the 470uF 35V that is underside.
• Black feeds the negative side of the 470uF 35V that is underside. (which is connected to the positive side 470uF 35V that is top side.
• Blue feeds the negative side of the 470uF 35V that is top side.

Much thanks in advance! MC

Here are some pictures... I have not cleaned the burnt section yet, I will do that next with some alcohol.

[Helmholtz]

No pictures showing.

[Jon Snell]

Something went wrong, no schematics or pictures are shown.

[Jon Snell]

The first thing I would check is the pre set pot. If open circuit, it will draw lots of current.
Here's the circuit to the bits that are important;
You will be able to tell if the polarities are correct before any further damage, if there has already been some.

[misterc57]

Weird that I see the pictures. Perhaps I had too many. I am going to load them again in two posts...

[misterc57]

More pictures...

[Jon Snell]

Well, the last time I saw damage like that it was a fatality.
R1 has gone up which means it will have destroyed the input section completely.
To do that sort of damage requires power on the input or an earthed input with a live amplifier. Either way I would replace the lot for peace of mind, otherwise it will keep bouncing back for repairs.
Is the mains transformer still OK or is it leaky.

[misterc57]

I am repairing this amp for myself, purchased it knowing it had issues.

What do you mean by the mains transformer being leaky?

There are two large molex connectors on the top unit (pre-amp). One has 8 pins, the other 7 pins. I took a picture (see attached) before removing these connectors. After having them off I noticed they were installed wrong, reversed, one on the left should have been on the right. Amp would not turn on the way they were connected. Not sure if this caused the burn out. Once I swapped them the amp would turn on.

When you say replace the lot, are we saying all the semiconductors on this board?

Thank you

[misterc57]

Some pics after cleaning with alcohol. Just noticed a C1583 component (is that the 2N2510?) has a big chip missing on the top edge.

[DrGonz78]

Hard to find those National branded transistors these days as they are obsolete. I think I used Tip31/32 types on a Polytone amp repair. Utsource has a reliable reputation IMHO.

https://mob.utsource.net/itm/p/975320.html

https://mob.utsource.net/itm/p/1832219.html

https://mob.utsource.net/itm/p/11849615.html

P.S. check those.075 ohm wire wound resistors to make sure they are not open.

[52 Bill]

There are two large molex connectors on the top unit (pre-amp). One has 8 pins, the other 7 pins. I took a picture (see attached) before removing these connectors. After having them off I noticed they were installed wrong, reversed, one on the left should have been on the right. Amp would not turn on the way they were connected. Not sure if this caused the burn out. Once I swapped them the amp would turn on.

If the two connectors were mixed up, then the full 120 volts ac were connected to something that it shouldn't have been. The ac runs from the bottom section of the amp to the ac switch and then back down to the power transformer that is usually part of the power amp chassis. Minimally, you will need to test all of the transistors in the power amp.

[misterc57]

If the two connectors were mixed up, then the full 120 volts ac were connected to something that it shouldn't have been. The ac runs from the bottom section of the amp to the ac switch and then back down to the power transformer that is usually part of the power amp chassis. Minimally, you will need to test all of the transistors in the power amp.

Based on the pin out I think the 120V AC would have run down the signal feed to the power amp, hence to R1 which was cooked. I replaced R1. All other resistors tested good but I have not confirmed all resistor readings against the schematic yet. The 4 large cement resistors were good and in spec.

The two large 4700 uF 35V caps are located in the upper chassis, as well as the rectifier diodes.

Here is what I have noticed.

With the speaker disconnected I can turn on the amp and pass a good pre-amp signal to another amp. I have also tested this amp's speaker connected to the other amp, the speaker is good.

I only get a hum and see excessive current rise when the speaker is connected. If the speaker is not connected there is no excessive current. Does this help isolate the problem components?

I need to understand how to test all 6 transistors, in circuit if possible. This is an area I lack experience and need guidance. I tried taking readings on the two output transistors. In diode test mode I see forward voltage in all directions, but I am not sure I am doing this correctly. Do I need to remove the output transistors? Looks like they are screwed on and will lift out without having to unsolder them.

Thank you

[Helmholtz]

With the speaker disconnected I can turn on the amp and pass a good pre-amp signal to another amp. I have also tested this amp's speaker connected to the other amp, the speaker is good.

I only get a hum and see excessive current rise when the speaker is connected. If the speaker is not connected there is no excessive current. Does this help isolate the problem components?

When mains current increases with speaker connected, you probably have DCV at the output. Please verify.
Never connect a speaker when there is DCV. It's easy to destroy a speaker with DCV and the DC output current may damage (even more) transistors/components.
The speaker cone needs to be moving to provide sufficient voice coil cooling.

With SS amp problems always first check for DCV at its output and don't reconnect the speaker before the problem is solved.

[misterc57]

When mains current increases with speaker connected, you probably have DCV at the output. Please verify.
Never connect a speaker when there is DCV. It's easy to destroy a speaker with DCV and the DC output current may damage (even more) transistors/components.
The speaker cone needs to be moving to provide sufficient voice coil cooling.

With SS amp problems always first check for DCV at its output and don't reconnect the speaker before the problem is solved.

Now using a dummy load. Yes, I am seeing significant negative DCV on the output. As I dial up the variac the - DCV increases. Saw - 10 VDC with variac partially dialed up.

[Helmholtz]

Now using a dummy load. Yes, I am seeing significant negative DCV on the output. As I dial up the variac the - DCV increases. Saw - 10 VDC with variac partially dialed up.

Unlike a tube amp, a SS amp doesn't need a load (for stability or OT protection).
Troubleshooting a SS amp can and should be done without a load. You need to find and eliminate the root cause for the DCV offset.