HR2510 Dead Short Ideas??????????????????????????

[Low_Boy]

I have a HR2510 with a dead short and need some ideas as to what I have not looked at that can be causing this. Touching the ground and hot A+ wire give a dead short, Not a charge caps and stop kind of short something direct.
Radio had a bad final. I think it was replaced with a fake. Q134 blown but replaced with a EKL part which was the wrong part to start with. Driver good and Q124 the other pre driver is good also.
CPU Board removed that is eliminated.
No Glue on toroids and all 4 are good.
All caps checked good but replaced. Some are still out.
As far as I know all disc caps that may cause the problem removed and checked.
No Big IC's show a short
FM Board, SWR Board and Noise Blanker board not causing the problem.
Not transformer
Not bias diodes.
The only thing not checked so far was the display board but really doubt that is the problem. Any ideas appreciated. I do not want this radio to win.

 

[Robb]

Disconnect all silicon components that are directly connected to the 12v line, such as the audio ic, regulator, etc - one at a time.

 

[Low_Boy]

I isolated all the ground pins from the IC's that went to ground and the short was still there. Do you think removing the components would be better?

 

[kopcicle]

IC-107 8v regulator? If you're still struggling, call...

 

[Robb]

If you have a dead short in a cap; it would be getting pretty hot.
I would just unsolder the 12v power leads from each ic/transistor; and then add them back one at a time. This will take a schematic to do. Check the zener diode too (haven't looked at the schematic). In addition, this goes without saying that the protection diode near the 12v power in isn't shorted, that is the first, most common suspect.

 

[kopcicle]

@Robb , I've frozen a few and watched the frost disappear....

 

[Robb]

@Robb , I've frozen a few and watched the frost disappear....

Freezing is good and quick, if you have it available.
But if all I had is a DMM; then . . . I'd have to slug it out . . .

 

[Handy Andy]

It is my fear that you may need to lift the Final legs off the board, then check for that short.

​

Pull your Mirror board - see if the shorting jumper can isolate the cause - No short, it's in the Driver or Final wiring area...

Just jumper the tab from power to driver - bypass that center tab - Radio still ok?

Then the Final mounting or area around it is at fault...

 

[Low_Boy]

IC-107 8v regulator? If you're still struggling, call...

I forgot to mention 8V regulator and protection diode were checked.
Also the final and bias board are both removed. But with VOM on continuity I am getting dead short from ground to hot. All the other radios just show continuity until the caps charge.

 

[KneeBiter]

How bad do you want to connect it to a high amperage supply with no fuse and watch whatever is shorted blow itself wide open?? Please post the video if it comes to this.

 

[Robb]

If you have a current limiting power supply and some freeze spray, it would make finding the short much faster, as kopcicle pointed out.
If not; then you will have to trace out the 12v rail on the board and start disconnecting silicon.
Takes some will; but you did say that you are going to beat this radio.
Go for it, and let us know what you find.

 

[Handy Andy]

(figures I had a typo-)

Hmm...boy you do have a difficult one...

Well, Volume control power goes to the Front panel...

Have you checked that 9pin connector on the back, the one that has all those wires going out of it...

Also,

There is a 390pF disc cap - if that blew shorted, (EDIT) C117 - by the Toroid mess back there -

​

Lift legs from the FINAL - using your DVM - backprobe from the FINAL RIGHTMOST leg, to ground - show a short? The answer may be with (EDIT) C117 blown - the Toroid's use a blocking cap to keep DC off the tuning but also the input side are DC shorted to keep out hazardous votlages from the outside GETTING INTO the final and driver... one winf of them (The toroid set) DOES go ACROSS positive output to antenna the Toroid involved is the 2nd on to the LEFT in the photo above - it has the wind that SHORTS out to ground - but (Edit) C117 keeps DC from the outside from damaging radio - AT the radio - and DC from the outside from damaging the inside.

sorry 'bout that...

 

[Low_Boy]

Well...... I feel like a dumb ass. Hopefully Andy, Rob or kop. Can explain this. Radio has no problem. I tested this against two others. Not connected to power. All 3 radios turned on show continuity from A+ to ground. Turn the radio off and all is good no short. Someone please explain this. We all know this is not a normal radio. Always has power to the finals and chassis and board ground are the same.

 

[ExitThirteen]

It's a poor design. What is going on is that the final is directly grounded to the chassis and is not isolated by a ceramic or mica insulator. Because the mounting tab on the MRF477 is tied to the Emitter leg internally, bolting the transistor to the chassis effectively ties the PCB ground and the chassis ground together. This causes a major problem which leads to the eventual failure of the MRF477 final, which is a DC ground loop that is caused by the final being bolted directly to the chassis.

The fix for this is to isolate the final off the chassis. Install a mica or ceramic insulator between the chassis and the final, and make sure you use an isolator on the mounting screw as well. Doing this isolates the PCB ground from the chassis. This also removes the ground loop through the final and will greatly reduce the chances of the final failing.

If you do this mod, make sure you remove the (2) 560pf disc capacitors in parallel on the solder side of the PCB. One side of these caps are soldered to the center (Emitter) leg of the final. Removing these are necessary, since you are now isolating the final from the chassis, and are no longer needed.


~Cheers~

 

[Low_Boy]

Exit 13 Thank You. I am overly cautious when dealing with a $40.00 final. I wanted to make sure all was good before testing. I always wondered what the two disc caps on the final were doing.
Thanks