Where to find 6000uF 50V replacement cap cans??

[phatt]

Well, no sound. Other than the somewhat  diminished hum/buzz that I had previously.

I tested the thermistor and it worked very well when heat was applied. So, I don't think this is my issue.

I changed both TIP32C's and the one TIP31C. And both 1N5484's located near the bias trim pot.

Any thoughts??

What should I be checking next??

1N5484 is suspect,, No such diode can be found in my quick search.
The Schematic says 1n914 which are just small signal diodes.
Using the wrong part at that point will likely end in smoke.
don't frig anymore until you build the light bulb limiter.

This is a DC Amp which means, All the Tr's sit at different DC potentials so if something fails it can take out many parts in a flash of magic smoke.
Now I'm not the expert here but here is a brief run down.

Unlike a Valve amp there are no coupling caps between active Tr's, hence the term DC coupled (no Cap to decouple the DC of each section) The limiter will save $$ and lots of wasted time.

The input is similar to the PI input to Valve power amp, used differently.
The idea is to keep the bases of Q8 & Q9 And the Spk output at close to Zero Volts as possible. Q10 is the Voltage Amp. (I think these Tr's see the biggest voltage and I think they tend to run hotter than output at times)
Q11 & Q12 are predrivers for the Darlington pair outputs, formed by Q13 & Q15 and Q14 & Q16. The diode string and trim pot from Q10 is the bias circuit. the thermister usually mounted to heat sink tracks the heat and slightly alters the bias as the amp heats up.
Google differential SState power amp designs or something like that .. Phil.

Ed I just found this; https://www.eeeguide.com/bjt-power-amplifier-with-differential-input-stages/

[RG100ESROX]

Well, no sound. Other than the somewhat  diminished hum/buzz that I had previously.

I tested the thermistor and it worked very well when heat was applied. So, I don't think this is my issue.

I changed both TIP32C's and the one TIP31C. And both 1N5484's located near the bias trim pot.

Any thoughts??

What should I be checking next??

1N5484 is suspect,, No such diode can be found in my quick search.
The Schematic says 1n914 which are just small signal diodes.
Using the wrong part at that point will likely end in smoke.
don't frig anymore until you build the light bulb limiter.

This is a DC Amp which means, All the Tr's sit at different DC potentials so if something fails it can take out many parts in a flash of magic smoke.
Now I'm not the expert here but here is a brief run down.

Unlike a Valve amp there are no coupling caps between active Tr's, hence the term DC coupled (no Cap to decouple the DC of each section) The limiter will save $$ and lots of wasted time.

The input is similar to the PI input to Valve power amp, used differently.
The idea is to keep the bases of Q8 & Q9 And the Spk output at close to Zero Volts as possible. Q10 is the Voltage Amp. (I think these Tr's see the biggest voltage and I think they tend to run hotter than output at times)
Q11 & Q12 are predrivers for the Darlington pair outputs, formed by Q13 & Q15 and Q14 & Q16. The diode string and trim pot from Q10 is the bias circuit. the thermister usually mounted to heat sink tracks the heat and slightly alters the bias as the amp heats up.
Google differential SState power amp designs or something like that .. Phil.

Ed I just found this; https://www.eeeguide.com/bjt-power-amplifier-with-differential-input-stages/

'
Thank you so much for the in-depth detailed response. Always greatly appreciated.

My mistake. I meant 2N5485's. See schematic excerpt below.

I have a light bulb limiter as well as a Variac. So, we're good to go there.

At this point should I consider replacing all of the 5485's on the board?

I tested all of the power transistors with the new ones, and they all tested exactly the same with a diode tester. So, at this point I do not think the power trans were affected.

let me know if I should consider changing the remaining 2N5484's on the board. And any other components that you feel may have been affected.

[RG100ESROX]

I just connected the amp to power through the current limiter/variac, and at 120VAC, there's no sign of a high current draw as the light bulb emits no light whatsoever.

I'm going to take this as a good sign.

As far as the power light and channel LEDs are concerned. They all work as expected.

So, at this point I'm wondering if one or a couple of the preamp transistors need to be replaced...?

I am getting about -20.00 VDC/0.00 VAC on the + of the speaker outputs. Which apparently is not a good thing.

Is this any indication of what the problem could be??

[g1]

The cap was ok.  The soot on it is from some other part nearby that burnt, maybe replaced previously.  Probably that NTE brand power resistor near the cap.
The trace was not broken, it was burnt. That would be where the smoke came from.  What does it connect to, a transistor?  Which one?
Where did you get 2N5484 Fets from?  They are obsolete so there are probably lots of fakes out there.

[RG100ESROX]

I got the 2N5484's from Amazon. Please don't tell me they're no good...? Would I have been better off with some 2N4416's? When you say fakes...what do you mean exactly??

I tried my best, but here's a picture of the rear of the board with the components from the front pasted in their respective locations, so you can see what was attached where in relation to the broken trace (which is circled in red.)

The picture below the top picture is of the front of the board but flipped to match the rear of the board.

The third picture is of the components attached to the trace that broke.

Hope this isn't too confusing.

Let me know what ya think and if you see anything telling.

Thanks in advance.

[phatt]

You need to get priorities in perspective.
forget the preamp the power amp has blown so please focus on that first. Fix one issue at a time otherwise you just make life hard for you as well as those here trying to help you.

If it's all too hard to follow instructions then it's a fair bet one or more Tr's have blown so you could just replace them from Q10 onwards, with equivalent or higher rated. Q8 & Q9 are likely ok as they don't have to pass big current.

Then with no load and light bulb limiter bring up the voltage and check DC voltages shown on schematic. The bases of Q8 & Q9 as well as the speaker out should be very close to zero volts. (i.e. under 100mVDC)

****Be aware this circuit has No Short Circuit Protection like later designs so even a momentary short at speaker out will likely blow transistors****
 No need to ask me how I found that out 
This is a very simple circuit so a good learning curve for you.

The simple fact that the trace burnt out tells you there has been a critical short causing big current to flow and melt the copper trace.
Take it one step at a time.
Hint listen to G1,, He has the experience to help you.
Phil.

[RG100ESROX]

You need to get priorities in perspective.
forget the preamp the power amp has blown so please focus on that first. Fix one issue at a time otherwise you just make life hard for you as well as those here trying to help you.

If it's all too hard to follow instructions then it's a fair bet one or more Tr's have blown so you could just replace them from Q10 onwards, with equivalent or higher rated. Q8 & Q9 are likely ok as they don't have to pass big current.

Then with no load and light bulb limiter bring up the voltage and check DC voltages shown on schematic. The bases of Q8 & Q9 as well as the speaker out should be very close to zero volts. (i.e. under 100mVDC)

****Be aware this circuit has No Short Circuit Protection like later designs so even a momentary short at speaker out will likely blow transistors****
 No need to ask me how I found that out 
This is a very simple circuit so a good learning curve for you.

The simple fact that the trace burnt out tells you there has been a critical short causing big current to flow and melt the copper trace.
Take it one step at a time.
Hint listen to G1,, He has the experience to help you.
Phil.

Okay, so...I replaced Q8 ~ Q16.

I then slowly cranked the voltage to 120 VAC. The current limiter light bulb did not light in the least.

With reference to chassis ground I got -20.00 VDC/0.00 VAC on the + side of the speakers outputs.

NOTE: None of the power transistors increase in temp after reaching full AC voltage. They remain at room temp.

(I've attached at the bottom the specification sheet for this amp in the event that it may be useful to someone.)


Awaiting further instructions...

[g1]

You should be using a low power incandescent bulb like 40 or 50W.  A big wattage bulb will not light up or offer much protection.  If anything is warming up, you should be at least be seeing a glow.
Otherwise I suspect the lamp limiter may not be wired up right.  Maybe try it with some other household appliance to see if it lights up.

I am looking at your pictures and at the supplied schematic and I can't find any spot that I think correlates to the orange wire in that connector.
This makes me think the schematic is not a good match which really complicates things.

[RG100ESROX]

I replaced the light bulb in the current limiter with a 40 watt bulb (it's all I had), and it lights. At about 50VAC it's starting to glow. Not real bright, but it's glowing. I didn't take the voltage any higher than 50VAC.

Does this photo help any???

(When I built the current limiter. The instructions said to use a very high wattage bulb. like around 250W to 300W. So, that's what I did.)

[g1]

If you get a glow now, that's good and means the bulb limiter is working.  Not sure why some people recommend high wattage bulbs, it defeats the whole purpose.  I think maybe they are wanting to actually use the working unit at higher power levels, which is not necessary.  It should only be used for troubleshooting at idle conditions.

Original untouched photos would be more helpful.  I can't read backwards  and would like to see both sides of the board with no editing and no tape on it.

P.S.  what is saving things from burning up right now is that there is no load connected to the output.  With the big bulb, if a load had been connected there would have been smoke and sorrow again .
Very important for solid-state amps to not connect a load until everything is working right with no load (exceptions are so rare, not worth discussing here).

[RG100ESROX]

Thank you for all of the knowledge transfer. I appreciate it.

Here are the photos you requested.

Awaiting further instructions...

NOTE: Not sure what has changed, but, I just powered the amp on and the lightbulb did not even begin to glow until I got it to 110VAC. Not sure if this is a good sign or not...(?)

PS: The green tape isn't covering anything other than blank/green PCB. If it really all needs to come off, let me know.

[g1]

Sometimes, as the voltage is being brought up with variac, there will be current at various voltages just due to variance in the supply rails as they are coming up.
So, if you are bringing it up at different speed than you did last time, you may not get glow where you did previously, or vice versa.
I guess you are still showing -20VDC at output?
Next step would be to record DC voltages for E,B, and C of transistors in power amp section (Q8 thru Q16).

[RG100ESROX]

Sometimes, as the voltage is being brought up with variac, there will be current at various voltages just due to variance in the supply rails as they are coming up.
So, if you are bringing it up at different speed than you did last time, you may not get glow where you did previously, or vice versa.
I guess you are still showing -20VDC at output?
Next step would be to record DC voltages for E,B, and C of transistors in power amp section (Q8 thru Q16).

Will do. Voltages of EBC Q8~Q16.👍🏻

[RG100ESROX]

Sometimes, as the voltage is being brought up with variac, there will be current at various voltages just due to variance in the supply rails as they are coming up.
So, if you are bringing it up at different speed than you did last time, you may not get glow where you did previously, or vice versa.
I guess you are still showing -20VDC at output?
Next step would be to record DC voltages for E,B, and C of transistors in power amp section (Q8 thru Q16).

Voltages of EBC Q8~Q16.👍🏻

Are these readings with references to ground?

Also, please note. I am out of town until Wednesday evening. So, look for these voltage readings Thursday morning at the earliest.

Thanks again for all of your help.

Much appreciated.

-Jay

[g1]

Yes, DC volts to ground.
Good luck to Utah in the NHL game Tues. (old Jets vs new Jets).