Looking for Part for old Punch 45 TP3055A

[shimp]

Local stereo shop sent me an old Punch 45 amp to fix, I believe this is the really old 1986 model punch 45. It has "The Punch" "Rockford Fosgate" in gothic script on the bottom cover, says it's a mosfet 45.

It has a blown mosfet. It's a motorola part, labeled TP3055A

I cannot for the life of me find a replacement. Closest thing I may have found is made by Fairchild MTP3055VL.

Will that part work? If not does anyone know of another substitute without having to change out any other parts to make it work in the circuit? I'd rather replace it with an original part if I could find one.

Thanks!

 

[Netlist]

shimp,
Good chance it's a MTP3055A: V-MOS, 60V, 12A, 40W, <0,15ohm(6A)
It can be replaced (technically) with
BUZ 20, IRF 530..533, 2SK919, 2SK1427...

MTP3055VL: FREDFET, LogL, 60V, 12A, 48W, <0,18ohm

Good luck in your search, maybe someone over here has a few of these old devices lying around.

/Hugo

 

[anatech]

Hi shimp,
What Hugo is telling you about the replacements is correct.

Now you can fix the amp.

-Chris

 

[Netlist]

Note, as I have no idea what a FREDFET is, that I wouldn't recommend it as a replacement...unless someone confirms it can be used.

/Hugo

 

[anatech]

Hi Hugo,
I was referring to the other suggestions listed.

The Fairchild MTP3055VL seems to be optimized for logic level control (read: switching applications). You want to have a plain jane mosfet in there.

The Fairchild part might work. Don't know.

If these are for the supply, change them all so they behave in a similar way. Otherwise the amp may come back.

-Chris

 

[shimp]

Thanks for the quick replies. I know most of the newer RF amps use IRF540 and IRF9540. I have lots of these in stock, is there a chance I could use these, without having to make any changes?

The TP3055A in question is for the audio output, not the power supply.

Again, thanks,

Chris

 

[anatech]

Hi Chris,
Stick a pair in and set the bias. Watch how stable the bias is after running the amp.

Probably will work fine. These are rated higher so I can't see any problems.

-Chris

 

[Brian Donaldson]

Since this is an audio output stage, might it be a plain old NPN 2N3055? I'm unfamiliar with the insides of old RF car amps, so I may be way off.

 

[Ouroboros]

Since this is an audio output stage, might it be a plain old NPN 2N3055? I'm unfamiliar with the insides of old RF car amps, so I may be way off.

No, when Motorola (as was) brought out P and N-channel general purpose 60V power fets, they had the bright (?) idea of calling them MTP2955 and MTP3055, showing that they were intended for the same sort of role as the old bipolar workhorse 2955 and 3055s. (Whether it caused more confusion than anything else is arguable!)

 

[anatech]

Hi Ouroboros,

(Whether it caused more confusion than anything else is arguable!)

It sure did! Many shops had techs that don't know a fet from a bipolar. They used the ECG book back then, some even would cross ref in and then out again! I have talked to some that would only match case style and knew the parts as "output transistor" only. "It's what I use for output transistors, it's the same as ECGxxx".

Those guys didn't stand a chance. I got a lot of botched service jobs in.

-Chris

 

[shimp]

anatech: thanks for the help, I would like to try an IRF540/9540 combo in this amp, but the bias voltage appears to be non-adjustable. If you could clue me in on which parts set the bias, and what the bias voltage should be I can attempt it.

If you dont have a schematic I can send you a picture of the inside of the amp, but I don't have a schematic either.

 

[anatech]

Hi Chris,
I don't have most of my manuals any more. There should be a trimmer for each channel. Take a picture if you don't mind then.

-Chris

 

[shimp]

There are trimmers. I just didn't notice them the first time I looked. So if I replace the MTP3055A and MTP12P05 pair with an IRF540 and IRF9540 what should I set the bias voltage to? It should be measured on the base of the transistor like a BJT right?


Thanks

 

[anatech]

Hi Chris,
The Fosgate bias method is simple. You connect 4 ohm dummy loads to the amp. Input a 10KHz (?) sinewave, around 1~2V at the output terminals I think. Adjust the bias for no crossover distortion, no higher. If you have a THD meter, bias is for minimum THD at 10KHz. You will find THD goes down, then rises sharply again. If you were to measure the bias current, it's typically very low. Maybe a couple mA.

My memory is hazy on the frequency and levels, but those should be close enough.

-Chris