Browning Mark III Receiver fuse blows

[BBB]

One of my Mark III receivers blew the 2 amp primary fuse the other night and went quiet without any warning. No pops, hum or magic smoke. Replacement fuse blows as well.

Was just wondering if there is a common failure on these rigs before I pull the cover off and dig in. I did not see the diode pack anywhere on the web if it is deemed the culprit and needs replaced. I did see the power transformers for $70, ouch.

Looking at the schematic, I see a few small caps on the input side, the transformer and its two output stages tied to the 6.3 volt heaters and the 123 volts to a 1 watt resistor, then diode pack with some caps among other items. Weird that the schematic shows 123 volts before the diode pack when I see 300+ volts on the other side of it. Must be that there is the potential to see 300+ to ground?

I'm assuming I could pull all the tubes to see if one shorted and isolate the secondaries on the power transformer and see what voltages I have there, then go from there. Got to get me a high voltage probe Looks like there isn't anything above 310 volts so I can use my 600 volt meter.

Any additional trouble shooting tips to help get this bird back on the air would be appreciated, Thanks.

[FIXR]

Brandon,

If the rectifiers or the filter caps are shorted it will usually burn R42, the 4.7 ohm 1 W resistor.
The rectifier pack is a doubler, 123 VAC in gives you approx. 300 VDC out.
The rectifier pack can be easily replaced with a couple of 1N4007 rectifiers.

Alan

[BBB]

So R42, the 4.7 ohm 1 W resistor is a sacrificial lamb to try and save the XFRM.

Understood about the doubling circuit and the replacement diodes now. Thanks!

[2600]

R42 also serves to limit the surge current when the power is first turned on. And yes, as a sort of secondary fuse if the B+ supply gets overloaded.

It seems likely that a check of the 3-section filter capacitor will reveal at least one shorted section. C58, the single-section capacitor mounted on the tie strip with the rectifier-diode block will usually show some visible sign of stress when it's causing trouble. Sometimes an old electrolytic cap will short and blow the fuse. The chemistry inside may "heal" the short after a while and cause it to test okay on the low voltages coming out of an ohmmeter. But when normal operating voltage is next applied, the short will return.

If those filters are all original, they should all go. The real mystery here would be why they took this long to cause trouble. Weren't meant to last anywhere near this long to begin with.

Our standard practice is to replace all of them at once, including R42 and SR1/SR2 at the same time. I use a pair of 1N5408 3-Amp rectifiers, not just because they're stouter, but because the price difference is tiny compared to the aggravation of a failure. The 1N4007 is a perfectly valid "textbook" replacement. The rating is correct. But when an overkill solution is cheap I tend to go for that option. Besides, I buy the 1N5408s 1000 at a time to build boards that we sell. Makes them nearly as cheap as the 1N4007.

If ANY electrolytic capacitors remain in the radio that are factory original, they should go, including the two small ones alongside the 6AQ5 tube. It's just cheap insurance. A new capacitor tends to be far cheaper than the damage they can cause when they fail.

Have a very good look at the small disc capacitors C56 and C57. They will frequently sustain damage from lightning surges on the AC power line. I don't think they were meant as "surge suppressors', but they often function that way. Any visible defect can signify a real and dangerous shock hazard.

Just resist any temptation to install a fuse larger than 2 Amps. Bad juju.

73

[BBB]

C56 and C57 and R42 check okay. The SR1 diode in the doubler is shorted. Time to locate some diodes. Hopefully something else down the line didn't fail causing the diode failure.

There's no apparent visible leakage from the big multi-cap at this time. C45A measures 56.2 MFD and C45B measures 52.7MFD on the 9 volt Chinese cap checker.

Since I would normally think of a diode failing "open" when they burn and crack in half, I was trying to visualize the diode's "shorted" internal failure and found this tidbit:

www.diotec-usa.com/Diode_Failures.pdf

Thank you for the replies.

[BionicChicken]

Bigger is always better but if you don't buy in quantity the 1N4007 rectifiers will work just as well. They have lasted in most radios for 40+ years, of course that is barring any surges or popping from the skies.
One thing you never do is go "below" the factory installed rated value whether it be on diodes, caps, or resistors.

[BBB]

I picked up a few diodes at Rat Shack the other day. I believe that the ones in the Browning are rated at 2 amps so these may have missed the mark (?) by only being rated at 1 amp and 1.5 amps respectively:

1N4005 Rectifier diode, Peak Inverse Voltage (PIV) 600V, Forward Current (If) 1A, Max surge current 30A,

*P15M 1127 Rectifier diode, Peak Inverse Voltage (PIV) 1000V, Forward Current (If) 1.5 amp, Max surge current 50A * I couldn't find this one on the net