RCI 2970 N4 no transmit

[Lkaskel]

Hi Gang,
I hope everyone is fairing well during these challenging times!!!

I have on the bench a 2970 N4 with no transmit. I disconnected the amp to see if the radio was transmitting and it was not. I check the AM regulator and it was bad. The driver and 2 final mosfets tested fine. I replaced the regulator and he is the issue. The center pin of the regulator is reading 11 volts (not 13) and the center pin on the final mosfets are reading 11 volts on AM and SSB. The center pin on the finals should read 6 volts on AM and 13 on SSB. I am pouring through the schematic and am unsure where to turn at this point.

Any thoughts?

I bought the regulators from Digikey BTW.

Thanks as always!!!

 

[Handy Andy]

That N4, is it Discrete or is it SMD?

 

[Handy Andy]

The reason for the asking is the issue may be with this...


The schematic is old, but they still use the concept of "carrier envelope control"

This works thru the Variable RF Power it's disabled in SSB mode thru Q70 - but Q69 may be your problem.

Remove Q69 and see if you can then trim AM power using VR15. - See if you can attain 6 volts thru VR15. This may also restore audio...

Else Q65 may be damaged and needs replacement - but if an NPC mod was done - CHECK C260 100uF (if any mods more than likely this cap may have failed too...) or even 220uF - the NPC mod if the cap failed can take out quite a bit of real estate C260, R293 and Q65 - let's hope not.

SMD stuff can be a bear...

 

[Lkaskel]

That N4, is it Discrete or is it SMD?

It's an SMD unit.

Thx Andy!!

 

[nomadradio]

When Q66 fails, it puts Q67 at risk. Q67 is the modulator transistor's driver, feeding base current to Q66. Just one problem. Q67 is a surface-mount power transistor. The entire back of the transistor is metal, making removal a hazard to the circuit-board foils.

Okay, so you called Q66 a regulator. I call it a modulator. Whatever.

We have a procedure to replace Q67 with a non surface-mount TO220 transistor.

Here is Q67 after it was replaced the first time. Pretty sketchy. When this one was removed, the foil traces just fell away.

It will have to be removed before trying the following trick.

The new Q67 gets a wire lead lap-soldered to each leg. If you use a transistor with the all-plastic body, mounting it will be simpler. And if you use one with the typical metal TO-220 "tab", you must insulate it from ground.

I think the number on this one is 2SA1869. Not the only part that will work, not by a long shot.

The wire from the emitter of the new TO-220 transistor goes to the base lead of Q66. The collector wire from the new transistor goes to the collector of Q66.

So long as you use a PNP power transistor with the all-plastic body, you need only bolt it to a spare hole in the heat sink near Q66.

The remaining wire connected to the base of the new transistor (blue in this pic) goes to the collector pad of the transistor that drives Q67. This is Q65, shown above in Andy's pic circled in green.

I'm sure I posted this some time in the past. But when I tried to search for it the forum says that "Q67" is too short to use as a search term.

Dern!

This is how we cope with this problem when we encounter it here. But only after installing the "suicide intervention" fuse in the hot lead feeding the radio circuit board.

A 5 Amp fuse tends to limit the damage next time a power component fails.

73

 

[Lkaskel]

When Q66 fails, it puts Q67 at risk. Q67 is the modulator transistor's driver, feeding base current to Q66. Just one problem. Q67 is a surface-mount power transistor. The entire back of the transistor is metal, making removal a hazard to the circuit-board foils.

Okay, so you called Q66 a regulator. I call it a modulator. Whatever.

We have a procedure to replace Q67 with a non surface-mount TO220 transistor.

Here is Q67 after it was replaced the first time. Pretty sketchy. When this one was removed, the foil traces just fell away.

It will have to be removed before trying the following trick.

The new Q67 gets a wire lead lap-soldered to each leg. If you use a transistor with the all-plastic body, mounting it will be simpler. And if you use one with the typical metal TO-220 "tab", you must insulate it from ground.

I think the number on this one is 2SA1869. Not the only part that will work, not by a long shot.

The wire from the emitter of the new TO-220 transistor goes to the base lead of Q66. The collector wire from the new transistor goes to the collector of Q66.

So long as you use a PNP power transistor with the all-plastic body, you need only bolt it to a spare hole in the heat sink near Q66.

The remaining wire connected to the base of the new transistor (blue in this pic) goes to the collector pad of the transistor that drives Q67. This is Q65, shown above in Andy's pic circled in green.

I'm sure I posted this some time in the past. But when I tried to search for it the forum says that "Q67" is too short to use as a search term.

Dern!

This is how we cope with this problem when we encounter it here. But only after installing the "suicide intervention" fuse in the hot lead feeding the radio circuit board.

A 5 Amp fuse tends to limit the damage next time a power component fails.

73

Nomad and gang. You all are awesome!!! I did the repair as suggested with a TIP 42 replacement and the radio is now transmitting!!!!

Thanks as always for your willingness to share your knowledge!!

The owner of the radio is so happy!!!


Larry

 

[PA241TwistedCopper]

He is right, the owner is very happy also.

Thanks to Larry for tackling this and thanks to (Chris I think it is?) Nomadradio for the helpful tip.

Harry

 

[Lkaskel]

Hey Gang, to wrap this up I wanted to tell you what parts I used for this repair. I replaced the original Q66 B817 (regulator/modulator) with a TIP 36C and Q67 2SB798DL with a TIP 42C mounted as shown above. I purchased them from Digikey. Thanks again Nomad for the pics to show me what you did and make it easy to follow.




Larry

 

[Handy Andy]

Nice work!

And congratulations!

 

[nomadradio]

Rock on!

And congratulations. Gotta love it when a plan comes together.

73