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I Keep Blowing My Resistor!

69sixpackbee

69sixpackbee

Well-Known Member
Can anyone give me an idea if what would cause the 5W resistor on my relay PCB go open when I power up my Realistic STA-2100?
In the picture, R1305 blows when I first push the power switch yet the 8A fuse is fine. I have replaced it twice now and to no avail.
Thanx,
Bud
 

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I bet that transistor or diode is leaking . You have some short somewhere I would look at the transistor and diode also check the caps if you have any in the circuit . You have something going on with that board I bet you can solve it .
 
it appears to be a soft start circuit and for some reason the relay isn't switching on and all the power is going through the resistor for too long and burning it up .
if i saw the rest of the circuit diagram i could give some things to try .
 
Burned relay contacts?

I'll bet if the relay contacts ARE moving on power up, they are burned or NOT making contact to short out the resister.

The resister is only supposed to absorb the start up surge of the power supply capacitors charging, then after the delay, it gets shorted out by the relay.

It looks like that if the resister is burned out the set might not get power and the relay start circuit will never turn on to close the relay (all power travels through the resister for a short time, then it is bypassed by the relay contacts). :scratch2:

Mark T. :music:
 
The resistor is only a symptom...if the resistor is indeed a 0.47 ohm, normal current draw should not be enough to cook it, which means that something is drawing lots of current (such as blown outputs) and holding down the voltage supply that powers the relay.
 
A couple more excerpts from the manual below.
I pulled the diode and the transistor from the board and they tested fine.
I am going to replace the resistor again and pull all of the fuses from the power supply. I will bring this up on my DBT and see what transpires. I guess this is one wast to creep up on the culprit. Plug in a fuse and try it until I get to the "robber" of the current.
I find it odd that it has not popped ANY fuses.

Thanx for the insight!
Bud
 

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I agree with EW. Something is pulling too much current through there. That relay transistor couldn't pull enough even if it WAS in the right place to pull juice through that res.
 
I second the above 2 posts. Something is catastrophically wrong in order for it to draw enough current to smoke that resistor. Probably shorted outputs. I would start by checking the power amp for shorts. Whatever you do do NOT keep replacing the resistor and trying to turn on the unit. There's a word for doing the same thing over and over and expecting different results...:D
 
YUP!! 2 blown outputs!
Both on the left channel. One PNP and one NPN. Hitachi 2SB655 and a 2SD675

Now I have to hunt some down. I'll check for some On Semi's if I can't track down OEM's

Thanx!


Sent from my iPhone using Tapatalk
 
I would replace the all the outputs with ONsemis, and measure the driver transistors for damage.
 
69sixpackbee said:
YUP!! 2 blown outputs!
Both on the left channel. One PNP and one NPN. Hitachi 2SB655 and a 2SD675

Now I have to hunt some down. I'll check for some On Semi's if I can't track down OEM's

Thanx!


Sent from my iPhone using Tapatalk
Click to expand...

Make sure you check all the other components in the power amp channel that was blown. Likely some burned resistors and possibly bad driver transistors and diodes. Especially since you fired it up with the bad outputs multiple times. If you put in new parts you might blow them up right away if you haven't checked the rest of the circuit. When you DO replace all bad parts make sure you bring it up on the variac while you monitor the outputs.
 
Thoughts:

1. There's really nothing special about the original devices, and anything marked with these numbers is suspect. I'd not hesitate to replace them all with new On-Semi MJ21193G and MJ21194G from a reliable source (Mouser, Digikey, Allied, Newark). Check for burnt out emitter resistors and base resistors for the outputs and drivers.

2. The drivers are a bit wimpy for a 120WPC amp, and may also need replacement. Odd to use TO-126 devices in a 120WPC amp as drivers. The KSC2690AY and KSA1220AY should be fine here (at least meeting the specs of the originals). Might be possible to upgrade to TO-220 devices.

3. Whenever a channel is blown, I like to replace the V/I limiter transistors as well. This would be TR513 and TR514. The KSC945C and KSA733C will do (need the 'C' suffix for a center-collector part).

4. Each amp channel has two VD1221 diodes. Each one should be replaced with a pair of series'd 1N4148 diodes.

5. Looks like the design calls for two back-to-back polarized 100µf 16V caps to act as a DC blocker in the feedback loop. I'd replace one with a wire jumper, and install a 100µf 16V bipolar cap in place of the other.
 
EchoWars said:
Thoughts:

1. There's really nothing special about the original devices, and anything marked with these numbers is suspect. I'd not hesitate to replace them all with new On-Semi MJ21193G and MJ21194G from a reliable source (Mouser, Digikey, Allied, Newark). Check for burnt out emitter resistors and base resistors for the outputs and drivers.
Click to expand...

These were the ones I was going to get and I am glad you concur.

EchoWars said:
2. The drivers are a bit wimpy for a 120WPC amp, and may also need replacement. Odd to use TO-126 devices in a 120WPC amp as drivers. The KSC2690AY and KSA1220AY should be fine here (at least meeting the specs of the originals). Might be possible to upgrade to TO-220 devices.
Click to expand...

I am assuming you mean TR507 & TR508, correct?


EchoWars said:
3. Whenever a channel is blown, I like to replace the V/I limiter transistors as well. This would be TR513 and TR514. The KSC945C and KSA733C will do (need the 'C' suffix for a center-collector part).
Click to expand...

I will replace them as well.

EchoWars said:
4. Each amp channel has two VD1221 diodes. Each one should be replaced with a pair of series'd 1N4148 diodes.
Click to expand...

I did this to the STA-2100D that I went through a while back as per your numerous recommendations showing your disdain for these troublesome little boogers.:)

EchoWars said:
5. Looks like the design calls for two back-to-back polarized 100µf 16V caps to act as a DC blocker in the feedback loop. I'd replace one with a wire jumper, and install a 100µf 16V bipolar cap in place of the other.
Click to expand...

That looks like C510 & C511 on the schematic, right?


As usual, EW is the "go to man" for these issues!. I appreciate the help from ALL of the posters here in this thread for their added help and the obvious much greater level of experience than myself with these issues.

Admittedly, I am just a gear wrench, and right now I have a Toyota A340H automatic trans and transfer case strewn about the bench on the other side of my little workspace I like to call a "one-car garage and stereo demo studio"
I have always had a penchant for fixing things and have always loved vintage electronic gear and have "tinkered" over the years. I just really started getting into this hobby a few years back so it is a "learn as you go" process and with the help of my friends here on AK I am having a blast!
I want to thank you all once again for your guidance and wilfulness to help share it.

Thanx,
Bud Weaver
 

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2. The drivers are a bit wimpy for a 120WPC amp, and may also need replacement. Odd to use TO-126 devices in a 120WPC amp as drivers. The KSC2690AY and KSA1220AY should be fine here (at least meeting the specs of the originals). Might be possible to upgrade to TO-220 devices.
Click to expand...

I guess I can upgrade to TO220 cases using these:

2SA1837, 2SC4793

Seems close. Besides, Mouser is out of the KSA1220AY.

Yes?
 
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