It just keeps blowing...

[budm]

Re: It just keeps blowing...

TR528 is part of the active Bias circuit for the output section, and it is conducting already to maintain bias.
What do you that what that TR528 is for?
I guess you did not read those links I provided.
More:
http://www.eetimes.com/document.asp?doc_id=1272214

[keeney123]

Re: It just keeps blowing...

What do think when audio signal is fed into the amplifier, those transistors will be full conducting or partial conducting (depend on audio level) with the signal, but if one of the device in the chain is stuck on what do you think it is going to happen?
What happen if that TR828 has leakage resistance of lets say 100 Ohms
TR532 (Clooector is connected to + supply) is bias on by R606/608/C542 Bootstraping so you have current flow through R620, E-B junction of TR534 so it is turned and give more current flow through R620 , and through 100 Ohms leakage, and then then through R888 (47) to - supply, at the same time when T534 is on, TR538 will also be on. It does not take much current to burn up that 330 Ohms resistor. At static, the circuit is balanced to give center point output to be at 0V, when thing gets out of balance due to static shorts or leakage then the circuit will get out of balance instead of swinging with the sync with the audio signal.

Would it not charge through C542 readily and when the two zeners and two diodes get to the correct voltage It would conduct lowering the voltage on C542 so it would charge again? And at some time TR528 will conduct?

[budm]

Re: It just keeps blowing...

If one of the semi is bad then it does not matter if he has signal or not, the circuit will be thrown out of balance when you have bad device in the circuit.

[keeney123]

Re: It just keeps blowing...

What do think when audio signal is fed into the amplifier, those transistors will be full conducting or partial conducting (depend on audio level) with the signal, but if one of the device in the chain is stuck on what do you think it is going to happen?
What happen if that TR828 has leakage resistance of lets say 100 Ohms
TR532 (Clooector is connected to + supply) is bias on by R606/608/C542 Bootstraping so you have current flow through R620, E-B junction of TR534 so it is turned and give more current flow through R620 , and through 100 Ohms leakage, and then then through R888 (47) to - supply, at the same time when T534 is on, TR538 will also be on. It does not take much current to burn up that 330 Ohms resistor. At static, the circuit is balanced to give center point output to be at 0V, when thing gets out of balance due to static shorts or leakage then the circuit will get out of balance instead of swinging with the sync with the audio signal.

My understanding is he is not putting a signal in it he is just turning it on and it does this. Thank you for the path I will lock at it closer.

[budm]

Re: It just keeps blowing...

What do think when audio signal is fed into the amplifier, those transistors will be full conducting or partial conducting (depend on audio level) with the signal, but if one of the device in the chain is stuck on what do you think it is going to happen?
What happen if that TR828 has leakage resistance of lets say 100 Ohms
TR532 (Clooector is connected to + supply) is bias on by R606/608/C542 Bootstraping so you have current flow through R620, E-B junction of TR534 so it is turned and give more current flow through R620 , and through 100 Ohms leakage, and then then through R888 (47) to - supply, at the same time when T534 is on, TR538 will also be on. It does not take much current to burn up that 330 Ohms resistor. At static, the circuit is balanced to give center point output to be at 0V, when thing gets out of balance due to static shorts or leakage then the circuit will get out of balance instead of swinging with the sync with the audio signal.

[keeney123]

Re: It just keeps blowing...

If TR828N is stuck fully on or has low resistance between E-C then TR534 and the chain will be turned on hard.
It is very simple amplifier with diff input running on dual supplies.

That is not what I am talking about. In order for the E to C to have leakage current there has to be a path from +57 volts to -57 volts. It will have to go through resistors which will be dropping voltage and lower current in the path between these voltages. There is a 47 ohm swamming resistor on the emitter of the transistor.

[budm]

Re: It just keeps blowing...

If TR828N is stuck fully on or has low resistance between E-C then TR534 and the chain will be turned on hard.
It is very simple amplifier with diff input running on dual supplies.
http://sound.whsites.net/amp_design.htm

[keeney123]

Re: It just keeps blowing...

If you look at the service manual, you will see for example, the BASE of Q534 goes out to the Collector of the driver Transistor TR828 another section as shown in PDF schematic 6/13 PDF page 91, if that transistor has bad leakage it will bias on Q534 on hard and will cause the direct couple circuit have bad chain effect, so you should check that transistor and the other two differential pair Transistors. All the amps has the same setup, the driver Transistors are on another section.

It is hard for me to see could you show the leakage path? Please!

[keeney123]

Re: It just keeps blowing...

C506 is just filter cap, if you at the other page of the schematic, you will see that the Base is connected directly to the Collector of the driver circuit.

Sorry these dyslectic eyes played tricks on me. I had thought C506 was connected to the B+

[budm]

Re: It just keeps blowing...

what is all short open all about ? i never seen that before

The schematic is the master schematic that has all the parts (so the PCB foot-print of all the parts will show in the PCB CAD program) for making PCB, then you stuff the board with parts for the board assembly (PCBA) with the particular function that you need per BOM of the PCBA, so one blank board can be stuffed hundred ways.

[petehall347]

Re: It just keeps blowing...

what is all short open all about ? i never seen that before

[budm]

Re: It just keeps blowing...

C506 is just filter cap, if you at the other page of the schematic, you will see that the Base is connected directly to the Collector of the driver circuit.

[keeney123]

Re: It just keeps blowing...

If you look at the service manual, you will see for example, the BASE of Q534 goes out to the Collector of the driver Transistor TR828 another section as shown in PDF schematic 6/13 PDF page 91, if that transistor has bad leakage it will bias on Q534 on hard and will cause the direct couple circuit have bad chain effect, so you should check that transistor and the other two differential pair Transistors. All the amps has the same setup, the driver Transistors are on another section.

Question wouldn't C506 stop that from happening with just DC?

[budm]

Re: It just keeps blowing...

If you look at the service manual, you will see for example, the BASE of Q534 goes out to the Collector of the driver Transistor TR828 another section as shown in PDF schematic 6/13 PDF page 91, if that transistor has bad leakage it will bias on Q534 on hard and will cause the direct couple circuit have bad chain effect, so you should check that transistor and the other two differential pair Transistors. All the amps has the same setup, the driver Transistors are on another section.

[keeney123]

Re: It just keeps blowing...

NB = non-burning ( which seems not to be always the case )
R620 - Carbon film 330 ohm 1/4W(NB)
R622,624 - Carbon film 1 ohm 1/4W(NB)


I have not checked any of the resistors with e multimeter... just the transistors. I checked the resistors only visually and cut R620 so that I can close the hood.
When the parts arrive, I will disassemble the boards and test everything.

By the way, could faulty emitter resistors cause this damage?
Now I remember that I got good ohm readings on them and decided not to change them.

So the amount of current needed to fry the 330 1/4 watt resistor would be 27.5 mA at 8.33 volts or any combination of voltage and current that would give 1/4watt. The other 1 ohm resistors must of had less voltage on them and therefore did not get to 1/4 watt.

Also, when you get the new transistors very with ohm meter that they are NPN and PNP.

On the original PNP the base can take up to 4 amps of current.

[madan1]

Re: It just keeps blowing...

NB = non-burning ( which seems not to be always the case )
R620 - Carbon film 330 ohm 1/4W(NB)
R622,624 - Carbon film 1 ohm 1/4W(NB)


I have not checked any of the resistors with e multimeter... just the transistors. I checked the resistors only visually and cut R620 so that I can close the hood.
When the parts arrive, I will disassemble the boards and test everything.

By the way, could faulty emitter resistors cause this damage?
Now I remember that I got good ohm readings on them and decided not to change them.

[keeney123]

Re: It just keeps blowing...

Resistor R620 333 ohms fried. I am not sure what NB means. Yet the two front end transistors did not fry. I am not sure why R 624 or R 622 did not fry unless their wattage is higher then R620. To fry R 620 the current path had to go through TR538 collector/base, R 624, R620, R 622 and TR 536 Base/Collector

[keeney123]

Re: It just keeps blowing...

I would concentrate on were your second picture is. That is the two transistors. To blow both transistors it tells you to much current went through them. They are connected together. One connected to the B+ voltage and one to the B- voltage VR504 as all the VR adjustments need to start fully counter Clockwise. When you measure at TP 504 you then need to bring it up to a very low voltage. If these transistors are fully turned on I only see a 0.22 resistor between them.

[petehall347]

Re: It just keeps blowing...

i have used 1/4 watt resistors in place of the output transistors no problem ... they are used to keep the feedback path intact whilst testing . it also saves on output transistors blowing up .
check dc volts where the emitter resistors join together .it should be zero volts .. few mv is ok .

[Khron]

Re: It just keeps blowing...

http://www.ppinyot.com/dimbulb/dim_bulb_tester.htm

https://antiqueradio.org/dimbulb.htm

http://www.geek-tips.com/2015/11/22/dim-bulb-tester/