Went to retest the collectors of Q1 and Q2.

Q1 was not showing as negative on my dvm. It simply said 63.4

Q2, when testing, arc'd to something and flashed. R18 right next to it started to glow bright red before I could kill the switch.

Pretty disappointed here.

 

You slipped...that happens if you are new. You are learning. No need to be disappointed here.

And that is why I did not advice on putting the Power Tr's. You might have blown them too.

Atleast what got faulty weren't expensive.

You should get negative voltages. No negative means trouble.

Retest the drivers and the resistors as in my previous post about testing the Q1 & Q2 and the associated components.
Once done post back.
You need to look at the diagram. There a points much easier to reach with the board installed. For example, you can access all the transistor terminals by looking which resistors they connect to.

I am thinking you faulted a driver. So remove them, test again the resistors and diodes and everything I asked you to test before.

Good Night.

 

You gonna learn from your mistakes. Sometimes too much probing does more harm than good, as you have discovered.

Don't probe if there is a chance of shorting out something.

If you must take a measurement in restricted area, use jumper hooks. Hook on to the component with the power off. Then turn the power on. Turn off power before removing.

 

Thanks guys.

 

Looks like damage is limited to
Q1a and R17a
Q2a and R18a

May take some time to find.

 

Can you get the exact part no. for Q1 & Q2 ?
Or a replacement ?

 

Cannot get exact part number. Discontinued last millennium.

 

Are R15 & 16 OK. Since R17 & 18 blew with the Q1 & 2, I doubt they will remain intact.
Are u sure others are OK.
Post the no. of the original transistor and the replacement part. need to check if they match.

PS. Please resize the picture before posting. Large files are eating up my bandwidth.

 

R15a and R16a tested OK last night - same as R15b and R16b of the good channel. Checked them 3 or 4 times, thinking the same thing you are.

Schematic calls for Q1 = SJE2762. Number on my actual part is 2SC3298B. Cross references to NTE54.
Schematic calls for Q2 = SJE2763. Number on my actual part is 2SA1306B. Cross references to NTE55.

I'll have to use thermo pad to isolate the new drivers from the stand-up heat sink, due to the new ones are NOT INSULATED.

 

You might not have to if the sink ain't touching anything else and if you are not touching them when powered .

So as before need to know the voltages. Refer to post #58. Check the transistors before soldering.

 

So, post 58 asks for four measurements:

Neg probe on neg speaker:

Pos at front of r17a = +67.9
Pos at back of r17a = +67.3

Pos at front of r18a = -67.5
Pos at back of r18a = -67.1

Also, just realized the new r17 and r18 are supposed to be 22 ohm 5%. However, I actually have installed 22 ohm 2%.

Please advise if I need to purchase new resisters at 5%?

 

2% is better than 5% in the sense that the actual value of the resistance of the 2% resistor is going to be more closely matched to its nominal (or stated) value than the 5% resistor.

What a 2% 22Ω resistor means is that the resistance can be expected to fall within 21.5 and 22.4Ω

whereas a 5% 22Ω resistor is expect to be somewhere from 20.9 to 23.1Ω

Hence you can see the 2% resistor has tighter tolerance.

I would be more concerned with the wattage rating of the resistor. Make sure that the wattage of the replacement resistor is equal or greater than the one being replaced.

 

Tighter tolerance is better. Like Chips said if power is lower than the original you are asking for trouble.
Hmmm biasing seems OK for now.
for the PNP's it is 0.6V, a little higher than I thought.
for the NPN's it is 0.4V, a little lower than I thought.

So let's move on.
Solder Q3 & Q7 only. Leave others out.
Clean the sink and apply fresh heat sink compound. Screw the transistors. Put the board back as it was. Put the screw back to hold every thing tight.
For this test you need to monitor the Voltage across R17 & R18 You will get the above DC voltages.
Keep the Volume at 0, Apply power after connecting the DVM in VDC at the speaker terminal, you will get very low voltage reading (mili volt range, less than 100mV )if everything is OK.
At power up there should not be any voltage spiking and reading should be in mV range.
If this the case keep it powered.
Measure the Voltage across R17 & R18. Note them down. You need to keep it powered to warm it up. 10mins or so will do fine.
Keep the amp where there is no forced cooling like a fan or anything. We need to let the heatsink temp rise up to check for thermal drift in biasing.
after 10 mins check the Voltage across R17 & R18 and speaker out. Note them down and post.

Warning : Never Power up without mounting the Power transistors to the sink with fresh heatsink compound. Failure to do so will result in thermal run way of the power tr's. You will blow them even if everything is OK.

 

According to the wiring diagram - General Note #2, "All resistors 1/4W 5%, unless noted". The resisters that I installed are 1W, 2% so I should be good.

I will stop today at lunch and get some heat sink compound. Is there anything else I should go ahead and get, as well? A 10 ohm resistor? Just trying to save as many trips to the radio store as possible. It's about 25 miles away.

One more question...the power transistor that I had to replace...does it matter what position I install it? I know that it can only go in Q3, Q4, or Q5 slot...but do you have a preference as to where new/old transistors are located? Or simply leave it as Q4 (the one that it replaced) and leave Q3 and Q5 right where they are?

 

I doubt very much that R17 and R18 are ¼W.

 

The resistor should be of same size and type. As I dunno which you had before. I think they are carbon. If carbon types same size should suffice.
As for where to put transistors, it does not matter as long as you do not mix up NPN with PNP. If you did it simply won't produce descent sound.
All the outputs are on Parallel, so where you solder them it's up to you. I said where to put just for reference.
The Discharge Resistor you got is enough for test load so I do not think you need an extra Resistor. Besides it's with speaker that you are gonna have issues with.

 

Can you take sharp well focused photo of R17 and R18 of the good channel?

 

Regarding the test procedure in post number 73

Still no fuse in channel b right?

 

Better to leave the B channel alone until you are done

 

Test per post #73 (Q3a and Q7a installed)
Upon power up, millivolt reading at speaker terminals was -62.0 mV and dropping.
Voltage across R17a = starts at +.53 volts and is climbing
Voltage across R18a = starts at -.53 volts and climbing

After a few minutes, R17a is at +.694 volts and R18a is at -.712 volts

After 10 full minutes:
Voltage across R17a = +.677 volts and pretty steady
Voltage across R18a = -.694 volts and pretty steady
millivolt reading at speaker terminals pretty steady at -0.2 mV (-.0002 V)
main heat sink is mildly warm on channel A side only. Channel B side is still cold.
Q1a and Q2a are equally hot.