I have been searching locally for the 1K 5W resistor but none here. I will have to order it along with the other parts I plan to change eventually so it will be at least a week before I can get it and proceed with the troubleshooting per RB's recommendation.

 

Use your initiative.

1k 5watt?

2 470Ω 2.5 watt in series?

2 2k2 or 1k8 in parallel or 2k2//1k8 etc.

or 4 off 1 watt 4k7 or 5k6 in parallel

or parallel 2 sets of 2 1k 1 watt and connect them in series.

You will be close enough.

 

Use your initiative.

1k 5watt?

2 470Ω 2.5 watt in series?

2 2k2 or 1k8 in parallel or 2k2//1k8 etc.

or 4 off 1 watt 4k7 or 5k6 in parallel

or parallel 2 sets of 2 1k 1 watt and connect them in series.

You will be close enough.

I had thought of all of that however our local (Radio Shack) sucks when it comes to parts anymore. They had nothing that i could have put together to come close to that. Nowadays they are only good for cell phones and batteries............

 

How about a 1k 5watt pot (use the ends not the wiper)?

 

Nope just checked their website. All of their pots are low wattage but that's ok........I have patience to a certain degree.....lol

 

Quick question. C60 is rated at 220 nf but what should the voltage be? And I believe this is a multi-layered ceramic cap?

 

C60 goes from the output rail through a 10ohm resistor to gnd. It will be subjected to a maximum of one full rail voltage, which is probably about 35v (if your rails are +/-35v)?

You could get away with a normal 60v cap but since it is a snubber I would use the same type and size package they used, it might be a higher voltage poly cap etc as they can take more abuse.

Re the 5W 1k resistor, you can leave that out for the moment and just measure the voltage from the amp output to ground (along with the other tests I mentioned).

It is not always necessary to have some load there, I prefer to have a small load like 1k there but you can still read the amp's DC output voltage with no load resistor.

The important thing for now is to diagnose the fault mode; either it pulls the output high to +ve or pulls it low to -ve, or maybe it tries to do both and will blow fuses.

If you can at least narrow it town to one of those 3 fault conditions we can focus on more tests which will suit that condition and lead to a solution.

 

The rails are ±50volts.

This is an 80 watt amp.

 

OK don't jump to conclusions on this because I'm only trying to locate one in the event that I need to replace it after troubleshooting. This is C60 cap. I lifted one side out to be able to take a pic of what was printed on the bottom side. The numbers / letters are in this order:

CM
224
M5E

Been trying to sift thru data sheets but now I am even more confused......lol

 

You can use a 50-100 watt 120 volt lamp for a test load. It happens that the cold resistance ranges from 20 to 10 ohms.
It also acts as a convenient fault indicator.

Also the same wattage range works well as a current limiter in series with the AC supply (to save your fuses).

 

You can use a 50-100 watt 120 volt lamp for a test load. It happens that the cold resistance ranges from 20 to 10 ohms.
It also acts as a convenient fault indicator.

Also the same wattage range works well as a current limiter in series with the AC supply (to save your fuses).

Back in my PC monitor repair days I often used a lightbulb to prevent excessive fault current - but as the filament cold resistance has been mentioned, it does cause a turn on surge that occasionally allows the damage you were hoping to avoid.

An alternative approach is to use a variac - crank it up and see where the smoke comes out.

 

....

An alternative approach is to use a variac - crank it up and see where the smoke comes out.

Smoke is good ???

 

Smoke is good ???

The magic smoke only works if you don't let it out.

Same with valves (tubes) - if you let the vacuum out, they don't work anymore.

 

Guys I have a family crisis on my hands and need to step away from this repair till I can sort this out. This week is a wash so I hope to get back to it next week. Thanks and I will check in once things are back to normal.

 

Well things seem to be a little more in order. I'm planning on looking back on this thread and pick up with the last recommendations for troubleshooting. RB I wanted to ask you on the 10 ohm 1/4 watt resistors you said to use in place of the +ve and -ve, did you mean for me to cut a small section of trace as you had recommended to wolf1419 and solder them there? The reason I'm asking is because the amp only has one fuse unless you are meaning to use the resistors as fuses for the testing purposes. I will be starting this Friday evening (eastern standard time) because my wife will be gone most of the evening and it would be perfectly quiet here! I will put the shorted TR8 and TR10 back in the circuit and begin testing. Unless I'm thinking wrong here. In which case please correct me...............Thanks guys

 

The magic smoke only works if you don't let it out.

Same with valves (tubes) - if you let the vacuum out, they don't work anymore.

So, you probably don't want to let it out.

A series lamp will limit the in-rush current with its cold resistance, (a variac won't) unless it has an amp meter which you must read, and your are very quick with the knob.

The lamp resistance increases if there is a short circuit, which further limits the current, so you save your fuses.

I have a variac, but I think the series lamp method works very well in your (this) particular situation.

 

Do you have some un-shorted output transistors? You won't learn much from putting shorted components back in.

 

Last pair then I will need to order again if they fry.

 

To refresh: Check the other diodes and transistors for shorts first. And seriously, use a series lamp in the mains supply. (75 to 100 watts at 120 volts). If the amp is OK, the lamp will momentarily flash, and then quickly dim to a dull glow. If it remains bright, then trouble still exists.

 

...
RB I wanted to ask you on the 10 ohm 1/4 watt resistors you said to use in place of the +ve and -ve, did you mean for me to cut a small section of trace as you had recommended to wolf1419 and solder them there? The reason I'm asking is because the amp only has one fuse unless you are meaning to use the resistors as fuses for the testing purposes.
...

It is very unusual to have only one fuse on a power amp, normally there are two separate fuses, one for the +ve power rail and one for the -ve power rail.

I normally put a 10 ohm resistor instead of each of those 2 fuses, for initial testing.

The schematics you provided don't show the PSU? The first one you posted was the preamp, and the second one was the power amp.