DC Audio 175.4

[Deadly Sones]

Good afternoon,

Wondering if anyone knows the value of the 4 large 2w? or 3w? resistors that reside in locations R601-4? They are reading from 540ohms-630 ohms. All have overheated. Bands barely showing on them. One heated up do much it fell out of the board. One of the 1N4744A zeners are toast too.

Thanks as always 🙂

 

[Deadly Sones]

Adding pic of the 4 resistors... doesn't help I'm sort of colour blind... and the bands are so faded. I added a 1w resistor for scale... I'd think those are 3w? but no idea the value they should be... 🙁

 

[Perry Babin]

The first band looks blue. 680 ohms would be a standard value in the E24 series. The other would be 620 ohms and the second band doesn't look dark enough for red. For wattage, order resistors with the same physical size.

 

[Deadly Sones]

Is it often for resistors to lower in value? I've used both my meter and my MK-328 tester. Resistor readings are right around: 540 (542), 627 (630.1), 625 (627.6), 604 (607.2). The brackets are the MK tester... So, I would have to think the right value could be 620 ohms? Unless they all dropped in value from 680?

What you think Perry? Should I go with 620 or 680? I have to order some no matter what. 🙂

All the main output A1275 and C3228 drivers on the board have also heated quite a bit. Although I suppose the zener leaking probably didn't help.

 

[Deadly Sones]

Will 60 ohms really make that much of a difference?

 

[Perry Babin]

I doubt it but I don't know how they're being used.

 

[Deadly Sones]

They seem to be part of the 15v regulated circuit? 2 of them, a 220uf 16v cap and a 15v 1w zener seem to make up the circuit.

 

[Perry Babin]

I'd go with the 680. No matter what they were, they were pushing the circuit to its limit if the zener failed.

 

[Deadly Sones]

OK, thanks

 

[Deadly Sones]

OK, this amp is driving me crazy. The owner said the amp would work when it wants to. I redid the regulators. They seem fine now, when it ran.

The power supply was just shutting off randomly. After pushing around on things I thought the TL494 was faulty. As when pushed on, the amp would power on and off. I replaced it with a TL494IN, amp seemed fine. Buddy gets it back, says it ran for a minute and shut off. Now won't turn on at all.

After checking the amp over, the VCC+ for the TL494 and the IL4558N is being pulled down. Not sure what is causing it. It's sort of driving me nuts, lol. I removed them both.

The anode of D1 has 13.5v ... but the cathode fluctuates from 5ish volts to sometimes 12+ but usually below 10v.

I've push, probed, twisted... voltage just keeps jumping around. No idea why it's going down.

Any ideas?

 

[Perry Babin]

Is the circuit anything like the OV_sch.pdf in the power acoustic folder of the tutorial?

 

[Deadly Sones]

Hi Perry... it's similar but def not exact. I have drawn out the circuit. There might be more connections from the top I missed. Pretty much tested every component on the diagram. Not having any luck. Even tried removing the odd component and test to see if voltage came back up and no luck. 🙁

I have rem voltage right up to D1. After that, voltage is all from 5-6ish volts. Sometimes will go over 10+ but not often. Just keeps fluctuating.

 

[Perry Babin]

The drop across D1 can't be more than about 0.7v. If it is, the diode is defective.

 

[Deadly Sones]

Hmm... I don't have another one, I'll have to order some some. Can I toss in a 4004 in place of the 4002? I have one of them.

I checked the readings of D1 with my MK-328 tester. It gives decent readings and seems to be acting as a diode. IR=4nA, Uf=686mV, C=15pF

 

[Deadly Sones]

Readings from the 4004 are: Ir=4nA, Uf=665mV, C=15pF

 

[Perry Babin]

Virtually any diode rated for 1 amp or more will work.

 

[Deadly Sones]

Not sure what the tester is missing... but yup! Replaced it and now have 12.5v

Wow... thanks Perry. 🙂