Those base voltages are as follows. I made the measurement in the order as listed beginning with Q305 and ending with Q411.
Q305 1.12 volts
Q306 (-)1.12 volts
Q311 (-)1.7 volts
Q405 1.06 volts
Q406 (-)0.7 volts
Q411 (-)1.35 volts
While or shortly after making the measurement at Q411 something unexpected happened.
I turned away to write down my findings and when I turnned back the DBT was glowing brightly so I shut off the power. After a minute or so, I turned power back on and the DBT was dim. I prodded and wiggled Q411 a bit, and even placed a probe tip on it's base leg again. The DBT began to glow, increasing from from dim to bright so once again I shut power off.
After a time, I turned power back on and DBT did not brighten, it stayed dark.
Q411 is a 2SA678 and I replaced two of those before beginning this thread. One of them was damaged and I replaced it and it's counterpart in the other channel with KSA1015's.
The 2SA678 transistors in this amp are encased in green colour plastic and all had or have blackened, oxidized legs. Because of past encounters with 2SC458 transistors with black, oxidized legs, I was tempted to replace all 2SA678's for that reason but did not do so. Those that remained all tested as OK when out of circuit.
My thought is that Q411 is faulty and my manipulating it activated the flaw. I am inclined to remove what remain of the 2SA678's and replace them with KSA1015's.
I did as suggested and replaced the 1.2 Kohm resistor at R308 with a 2.2 Kohm resistor. I then measured voltage across all four 0.47 ohm resistors in that channel and orbtained the following. I made the measurements on the component side of the board with full wall power. No DBT.
R319 (-)1.5mv R318 (-)1.6mv
R317 (-)1.3mv R316 (-)1.7mv.
With the 2.2 Kohm resistor in place, the scope of bias adjustment became (-)36mv to (-)42mv. I could not achieve positive voltage.
Should I reinstall the 1.2 Kohm resistor at R308 or leave the 2.2 Kohm in place?
Leave the 2.2k in place. Its normal to sometimes have to tweak the bias range following replacement of transistors as old and new manufacturing processes give slightly different characteristics in forward bias voltages.
Don't worry over the polarity, it will be correct whatever you read as the current can't flow 'backwards'.
If you place the negative meter lead on the positive speaker output and measure to R317 and R319 you will read positive values and if you measure to R316 and R318 you will see negative. All normal.
Don't worry over the polarity, it will be correct whatever you read as the current can't flow 'backwards'.
If you place the negative meter lead on the positive speaker output and measure to R317 and R319 you will read positive values and if you measure to R316 and R318 you will see negative. All normal.
I am having terrible luck.
After replacing the 2SA678 transistors at Q311 and Q411 with KSA1015's, the bias on both channels was -29mv left and 29mv right with no DBT. That was with both bias pots at minimum so it was the least amount of bias current obtainable.
To connect my meter probes to the Left and Right Channel test points and (+) speaker terminals I was using a length of wire with alligator clips at each end.
I wanted to measure bias current again after the amp had warmed up so I left it on for 20 minutes or so.
The alligator equppied leads were disconnected from the meter's probes. The amplifier was still on. I clipped on the alligator lead going to speaker (+) to the meter probe but the red alligator lead still clipped on the Left Test Point, slipped from my fingers and touched the chassis.
That produced a spark and now there is next to no measurable bias current on either left or right channel, Maybe 1.5mv and is not adjustible.
Careless - yes. I should have turned the amp off for the few seconds it takes to clip those leads to the probes.
Both channels seem to be affected. Any thought on what is damaged now?
After replacing the 2SA678 transistors at Q311 and Q411 with KSA1015's, the bias on both channels was -29mv left and 29mv right with no DBT. That was with both bias pots at minimum so it was the least amount of bias current obtainable.
To connect my meter probes to the Left and Right Channel test points and (+) speaker terminals I was using a length of wire with alligator clips at each end.
I wanted to measure bias current again after the amp had warmed up so I left it on for 20 minutes or so.
The alligator equppied leads were disconnected from the meter's probes. The amplifier was still on. I clipped on the alligator lead going to speaker (+) to the meter probe but the red alligator lead still clipped on the Left Test Point, slipped from my fingers and touched the chassis.
That produced a spark and now there is next to no measurable bias current on either left or right channel, Maybe 1.5mv and is not adjustible.
Careless - yes. I should have turned the amp off for the few seconds it takes to clip those leads to the probes.
Both channels seem to be affected. Any thought on what is damaged now?
I must be cursed.
This morning I intended to do the voltage checks you suggested, beginning with rail voltages.
I want to get measurements at the filter caps but both (~) & (+) were zero volts.
I checked the fuses and both were blown. At the moment, that is a mystery because both fuses were fine yesterday after the accidental spark occurred.
All I’ve done is replace those 1.2k ohm resistors with 2.2k ohm resistors.
I replaced both fuses and they both blew before or just when the variac reached full power.
I am back to not knowing what to do next.
This morning I intended to do the voltage checks you suggested, beginning with rail voltages.
I want to get measurements at the filter caps but both (~) & (+) were zero volts.
I checked the fuses and both were blown. At the moment, that is a mystery because both fuses were fine yesterday after the accidental spark occurred.
All I’ve done is replace those 1.2k ohm resistors with 2.2k ohm resistors.
I replaced both fuses and they both blew before or just when the variac reached full power.
I am back to not knowing what to do next.
I looked at the circuit for rail fuses and couldn't just see any but with you mentioning two fuses I do see some on the voltage selector area so I guess those are the ones you mean?
Is there a fuse in your mains plug?
If you pull the mains plug out and measure between the two pins on the plug (live and neutral) on ohms on your meter it should read quite low, maybe not much more than 10 ohms. Remember to turn the amp mains switch on to check this to give continuity.
Is there a fuse in your mains plug?
If you pull the mains plug out and measure between the two pins on the plug (live and neutral) on ohms on your meter it should read quite low, maybe not much more than 10 ohms. Remember to turn the amp mains switch on to check this to give continuity.
Back to square one 
Disconnect the rectifier bridge from transformer and verify (using variac and DBT as visual load indicator) that fuses don't blow.
If ok then connect then rectifier bridge to transformer and disconnect the amplifier stages - fuses blow or not?
If not then put 100R 5/10W resistors between rectifier bridge and amplifier rails and measure the current in these (voltage drop).
Disconnect the rectifier bridge from transformer and verify (using variac and DBT as visual load indicator) that fuses don't blow.
If ok then connect then rectifier bridge to transformer and disconnect the amplifier stages - fuses blow or not?
If not then put 100R 5/10W resistors between rectifier bridge and amplifier rails and measure the current in these (voltage drop).
Yes, those are the fuses I was referring to 1.6A
If by "a fuse in your mains plug", you mean the plug that goes into a wall recepticle? No, there is no fuse inside. It is a the standard non-polarized two blade plug for North American voltage.
I measured resistance on those blades with good fuses in place and the amp's power switch on. The reading I saw was 5.4 ohms.
I can see where to disconnect the bridge rectifier from the power supply - two red wires.
I do not see where or what to disconnect so as to disable amplifier stages.
Also, I am not clear on the description "100R 5/10W". Does that mean 100 ohm, 5%, 10W? If so, I have some wire wounds that meet that description.
Sorry to be unclear - I meant 5 or 10W. I would use a variac and not use full power - in case of a strong short they may overheat and need to be placed in water.
A photo would be of help but there should be two wires delivering+35V and -35V from rectifier board to amplifier board.
Sounds fine
I would just power it all up now and with your meter on AC VOLTS measure the voltage between ground and the two red secondary wires coming of the transformer. You should see around 24 volts AC for both.
Sounds fine
I would just power it all up now and with your meter on AC VOLTS measure the voltage between ground and the two red secondary wires coming of the transformer. You should see around 24 volts AC for both.
If I did that, I expect those fuses will blow agaiin.
Photos as requested, such as they are.
There are a number of solder joins where there were once wire wrapped posts. I was not reposible for those. I had the two largish red wires (now marked with black and silver Sharpie ink) disconnected a short time ago to disconnect the btidge rectifier and those too were formerly wire wrapped then been soldered and are resolderd now by myself.
First pic is the trace side of whee the rectifier sits.
2nd pic is the top side of the same area showing the rectifer.
3rd pic is top side of power amp board in it's entirety.
The resistors to the right of the rectifier are worth noticing. I just now noticed them myself. A photo can be more revealing than the real thing. They are badly cracked and I imagine they suffered major stess from something.
A week or so ago when I was chasing after bad transistors, I did notice the wafting odor of an overheating component. Not seeing any scorched parts, I disreagarded it, At that time I was focused on the power amp board and did not pay attention to this section.
Both of those resistors are 1/2W 820 ohm and I have none of those. They need to be ordered. Since that is the case and its flat rate shipping, I am willing to order any other parts that might be suggested.
I forgot to mention in my previous post that I did disconnect the rectifier bridge from the transformer (unsoldered both red wires). Using the variac and DBT (100W), the DBT stayed dark.
I have not attempted the 2nd & 3rd steps.
I took the plunge and powered up the amp using variac and DBT. The 100W DBT began to glow at 20 or 30 volts and was very bright when 120V was reached. And as you said, fuses did not blow.
My DMM set to AC volts did not display 24V AC at those two red wires - those with the Sharpie ink smears in the photo. The same red wires I unsoldered when wanting to disconnect the bridge rectifier. Instead, I got readings of (-)3.0V AC and 3.0V AC going left to right.
Given it basically powers up I think it will either come down to a shorted diode in the bridge or you have zapped the output stage in one channel.
Get the bulb so its glowing and then measure the DC voltage across each 0.47 ohm emitter resistor for all the output transistors. If any have significant voltage across them then you've zapped an output stage. Significant means 10's to a 100 or so millivolts (or more).
Get the bulb so its glowing and then measure the DC voltage across each 0.47 ohm emitter resistor for all the output transistors. If any have significant voltage across them then you've zapped an output stage. Significant means 10's to a 100 or so millivolts (or more).
These should be the power transformer secondaries - how much AC do you measure between them when they are unsoldered from the board?
From the first photo (of the three) it seems that the rectifier bridge needs resoldering.
-/-35V (rail power) should be measured on the thinner brown and orange wires (winding pins) adjacent to the bridge.