Threshold 400 help

[P901]

Good morning Marco !

Thank you for that drawing !

We look forward to a detailed study later this evening !

I see you included a circuit board number CFE379, this means
to me that there were variants of this board ?

I will look more closely at our board, I presume the number is located
on the board ?
Do we know how many variants were produced ?

Thanks again ! andrew

[m2003br]

Hi Andrew,

Yes, this is the number on the PCB. As yours, wich is CFE1078.
I think nobody knows the number of variations, except, maybe, Mr. Pass.
The major difference between variations, are input and LVA cascoded or not.
Your PCB is pretty the same as mine, except for little differences in some resistors. But, what is strange to me is a MPSA92 instead of 2N4250A.
Maybe the cause of explosion of the upper transistor of cascoded LVA (the heatsinked one) is a swap between MPSA92 and 2N4250A, the lower arm of cascode.

Regards,

Marcos

[P901]

Good afternoon Marco,

Thank you for the beautiful drawing !!

This is going to help us very much with our repair.

Would you please explain "LVA", I am not familiar with this term.

Also you say that there may have been a swap of transistors.
We have not changed any transistors and the amplifier ran for quite
a while but this is not to say that someone may have done some work
on this unit prior to our ownership.

Thank you again for the drawings, we are looking forward to getting the
drawings together with the circuit board tonight to hopefully determine exactly which component has failed and to research a proper replacement.
Regards, andrew

[m2003br]

Hi Andrew,

LVA stands for "Last voltage amplifier", in that case, a stage before triple darlington output. This stage, in cascode form, is made by transistors 2N4250A, MPSA92, 68r resistor, 20pf silvered mica and polarization components, two diodes, Res. 1K and Res. 15K.
About my suspicion, maybe your amp (1978, I guess) have originally two MPSA92 in this stage. My own amp, from 1979, and other schematics from Threshold I've seen, have 2N4250A in that position.
for your easy recognition, the abreviated word "DISSIP" marked next of four transistors in schematic, stands for "DISSIPADOR", our word for "Heatsink".
Output stage schematic attached. About the same Mr. Pass kindly give to us, except component values.

best regards,
Marcos

[JamesBrennan4]

I would guess CFE1078 identifies a pcb as created in October of 1978. If I'm correct, then CFE379 would have been created in March of 1979. I believe design enhancements were done on most pcbs, such that CFE1078 may have a few, several or most of the changes that would appear on CFE379. The schematics are dated, but don't necessarily correspond to each pcb design change.

There is a web site that has perhaps the finial version of the schematic and also a copy of the user manual.

Go to Threshold Lovers and become a register user. Once logged in, you will find a "Download" section you don't see as a visitor.

Under "Download" go to page 2, select "Threshold" then "Threshold T400A and T4000 diagram", then download "Threshold_400A-4000_diagram.zip". Once unzipped you will get "400a-4000_site.jpg", which is the schematic of 400A and 4000.

Similarly, on page 4, select "Threshold" then download "T400_user_manual_site.zip". Again, once unzipped you will find
"Site_Thresholdlover" (folder), which contains "Threshold_T400_user_manual" (folder), which then has individual pages - T400_p00_site.jpg
...
T400_p20_site.jpg

I know this seems indirect, but I am honoring Thresholdlovers request to NOT redistribute the file.

[P901]

Good evening Marco !

It is very good of you to help us with this repair.
Your schematic has answered many questions.
It appears then that we have damaged the A92 Cascode LVA.
As a replacement we are looking at the MPSA92 High voltage amp.
It appears to have qualities above what would have been required
in the original configuration.
We also have the burned resistor 68 ohms to replace which appears to
be in this portion of the circuit.
Would you think it is reasonable to believe that a failure of one of the A8 output transistors could cause this collateral damage ?
We are hoping that the replacement of the damaged A8, the A92 and the 68 ohm resistor will put us back in operation.

Do you know if there is a "quick" way to test the section before applying power? For example would a resistance reading at some major point indicate whether or not we should begin to remove components from the board for individual examination.

Thank you again, andrew

[P901]

Good evening Mr. Brennan,

Thank you for providing the introduction to that site.
I have begun the process of registration and am awaiting
the confirmation response.
We are very much looking forward to viewing the new information !

Thank you again, andrew

[m2003br]

Hi Andrew,

I think is better to check all transistors and diodes on the board, but specially those with heatsinks and the other connected to 68r resistor, and, obviously, output transistors.
A series lamp on AC is a good measure of protection for the first time tests, and never connect anything at output, until you have checked DC operation.

Hope it help.

Marcos

[P901]

Good morning Marco !

That sounds like good advice.
Interesting concept with a lamp on the AC in.
This will prevent damage of the components but allow
operation of the unit?
Any recommended wattage?

andrew

[P901]

Ok, making progress,

Working along through the myriad of schematics that don`t actually
match the board we have.

Can anyone please give us some idea why there is a thermistor across
the Base/Emitter pins of the "first" power output transistor.

All the schematics we have show the thermistor going from the base pin
of the "first" transistor to the emitter of the "second" transistor with the emitter of the first transistor going to the base of the second transistor
so its not as if the connection shown on the schematic was physically achieved by an alternate execution.

What is the function of a temperature sensitive resistor across the B/E pins,
1.1K ohms cold, 300ohms hot.

Thank you, andrew