Threshold 400 help

[P901]

I`m working to fix a Threshold 400 amp for a friend.
So far we found one of the output transistors that was damaged
and a transistor and resistor on the circuit board that was also damaged.
The output transistor was replaced with the Onsemi 15024.
The transistor and resistor on the circuit board did not show outward signs of damage until the amp was up and running again after changing out the damaged output transistor.
If anyone knows which transistor and resistor we have that is damaged or can point us to a more accurate schematic it would be very helpful !
(The schematic on this site does not appear to exactly match our circuit board)
When the amp went out the LED display on the right side was max and the left side was at minimum. Once the output transistor was replaced the right side started working again, (left side still no output) and the LED display worked properly again.
(The damaged output transistor was on the left channel and we think it had taken out the transistor and resistor on the circuit board also when it blew).
The owner realized after the damage that the amp required 8ohm speakers but he had been running the amp on 4ohm speakers.
We believe this might have been the cause for the damage.
The amp had been running fine for about six months after being in storage for a year or so when the damage occured.

I have included a photo.

Thank you in advance, andrew

[woody]

Did you search? I did a quick search and found a Threshold 400a parts subsitution & mods thread where I found this pdf.

By the way that amp will drive 4 ohm speakers all day long with no problem. Way back then this amp was popular they would even drive the
modified Dayton-Wright electrostatic speakers. The modified Dayton-Wright speaker was a verry verry bad load for an amp it was .5ohms above 5k. Thats right above 5k the modified Dayton-Wrights were about
1/2 ohm.

[P901]

Good morning woody !

Thanks for the response.
Yes we have done extensive searching for information.
The schematic you sent was one of the ones we have printed out
to help us along.
If you have a moment to take a close look you will see our dilemma.
There are 14 transistors shown on the circuit board portion of the schematic
yet we have 18 transistors on the board ?!?!?
I have spent hours following the circuit board traces from one component to another, from transistor to resistor to capacitor to identify which component occupies which designation, ie, T-8 or C-4 for example. We need to confirm the failed transistor so that we install the correct replacement.
We know we can just take the one from the other side and copy the characteristics to allow a replacement but we would like to know which one failed and identify its place in the overall scheme.
Some of them are obvious but when you get to the portion of the circuitry that gets near the additional transistors on the board the schematic diverges from what we actually have.
Reading through all the posts here it is obvious that there is some heavy duty brainpower here, we are hoping that someone will check in and point out where our approach is failing.
We are enjoying the experience of working our way down through the problem but its kinda like when you finally need to raise your hand and ask the teacher for a little guidance.
We recapped this machine about 6 months ago changing out only the caps that we could identify by value.
The amp ran like a bear for months until this breakdown.
Now that you have provided the information that the speaker load was probably not the cause of the breakdown our theory turns to the possibility that the old heat dissipation compound had melted out from under the output transistor which allowed it to fail. We can see where all of the original heat dissipation grease has melted and flowed out from under the components.
The running temperature of the amp appears to be right on the nose according the information we have so even though it runs hot it doesn`t appear to be outside of the recommended limits
Thoughts, comments ?
Thanks again for your response, the pieces of information that you provided helps us in our theorization.
andrew

[audiocure]

Hi P901 I'm new to DiyAudio and i have not worked on this amp but I have a schematic for the 400a.

Audiocure.

[P901]

Good morning Audiocure !

Thanks for the response.
Do you know if your schematic shows all 18 transistors on the board?
The schematic we have from this site is close but not exact, it only shows 14 transistors on the board.
Our copy of the schematic from this website also shows approximately 80%+ of resistors that do not match the circuit board we have.
This makes it difficult to "reverse" engineer to find our way to identification of the failed component without a doubt.
How would we go about getting your copy of the schematic?
Is it an original ?

Thanks again, andrew

[woody]

If all else fails ask Nelson. By the way I do seem to rember the amp didn't like cobra
speaker cables. These cables were built with ~ 50 pair of insulated 24 gage wire.

[Nelson Pass]

If you use the old cobra cables or the Mogami wire of the same
era, an RC network 5 to 10 ohms in series with .1 uF across
the load takes care of any issues.

[Nelson Pass]

Here's the schematic I have on file:

[P901]

Good morning Mr. Pass! ,

Thank you for the information on the speaker cables and the
schematic.

We are having a bit of a challenge in the fact that the schematic
shows a different component count on the circuit board than what we actually have.

(The schematic shows 14 transistors, our circuit board has 18).

We endevour to map the circuit board component by component to produce an "as built" drawing which we will then use to correlate the actual component against the type and value shown on the schematic and parts list.

Were there a number of variations of this amplifier ?

Thanks again ! andrew

[m2003br]

Hi, P901 and Mr. Pass,

Hope this schematic, wich I made by "reverse engineering" of my own Cascoded 400a, is of interest for both.
Not exactly the same board as P901 have, mine is CFE379, but appear that the only difference is some resistors values.
The original schematics of cascoded 400a/4000 wich are circulating on the web, have an drawing error on output stage.

Regards,
Marcos