FLI loaded 400s amp missing some transistors

[Mote]

Its been a while since I last posted on this forum, because Perry Babin has taught me so well that I managed to fix some amps on my own. Yesterday I bought myself a new toy: FLI Loaded 400s amplifier. Bought it from online auction, seller claimed that capacitors needed to be replaced, I thought he meant to write transistors and bought it. It is much worse than I expected: 6 transistors and one rectifier are missing.

The rectifiers are MUR1620CT and MUR1620CTRG, 1620CT is missing. 1620CTRG is soldered to board so badly that all three pads are bridged. Interesting what kind of damage amp suffered if someone tried to switch it on in that state? How can I be sure that CTRG is where it is supposed to be?

Two of the missing transistors are output transistors: 2SB778 and 2SD998. Again, how do I know which goes where?

All of the 4 FETs are gone, how can I make sure which ones I need? I recently fixed FLI loaded 200s amplifier, it had a shorted FET. That model used IRFZ44N, is it a reasonable thought that these FETs would be suitable for this amp as well?

Thanks in advance for good advice!

[Mote]

I forgot to mention that last photo is of the Loaded 200s amps IRFZ44Ns.

[Perry Babin]

The center leg of the CT goes to the positive terminal of the rail caps. The center leg of the CTR goes to the negative leg of one of the rail caps.

If the emitter resistors of both the 2SB and the 2SD transistors connect together on the board (for each channel), the center leg of the 2SD output will go the the center leg of the CT rectifier. The center leg of the 2SB output will go to the center leg of the CTR rectifier.

The gate resistors appear to be 51 ohms. If that's true, the Z44s will be OK.

[Mote]

3 Gate resistors are 51 ohms, one is 1kohm, the blue one on third photo. Does not look right to me. Someone replaced 51 ohm one with this? Or am I missing something?

[Perry Babin]

I think that the one on the left (1k ohm) is a pulldown resistor, not a gate resistor.

[Mote]

The resistor on the left is not a gate resistor, but on the right, the blue one, is gate resistor and it measures also 1k ohm. I think I need to replace it with 51 ohm resistor?

I bought four IRFZ44N FETs, 2SB778 and 2SD998 transistors, but did not manage to get MUR1620CT yet - local store does not have them in stock.

Inserted two IRFs on the board where gate resistors were both 51 ohms. Removed the rectifier diode. Clamped the board to heatsink and switched the power supply on. Measured 4.3 volts between gate and source on these IRFs. That means so far so good?

[Perry Babin]

Yes, the 1k gate resistor must be replaced.

Do you have only 2 of the 4 FETs in the board?

Do you read the same voltage on the gate resistors for the other 2 FETs?

[Mote]

I have only 2 of the 4 FETs on board because I wanted to see your opinion about the 1k ohm gate resistor before using or replacing it.
Can I replace MUR1620CT and MUR1620CTR with something else, because these are hard to find in Estonia.

[Perry Babin]

Post the dual, center tapped rectifiers that are available.

[gisewhcs]

If it's an option for you:
BG-Electronics
They have them in stock.

They have a minimum order of only 10€ and fair shipping fees.
But I don't know how much it is to Estonia...