Need help repairing an audio amp schematics included

[johnnydiaz]

I'm trying to repair an audio amp which doesn't power up. I measured voltage at the br and got 123 volts. I measured voltage at the main caps and got 160volts. I noticed that r245 & r288 were getting very hot (I measured it at ~ 120 degrees fahrenheit). So I unsoldered one leg on each resistor and got the proper reading. What could be causing these resistors to heat up? Also what else should I test? I have included the schematic.

[budm]

Re: Need help repairing an audio amp schematics included

R245 is in series with the 18V Linear regulator circuit Q28. This is not a good design due to trying to drop 380VDC down to 18V using resistor/transistor.
Are you getting 18V and 380VDC at those two points? VDD?

[johnnydiaz]

Re: Need help repairing an audio amp schematics included

Budm,

"Are you getting 18V and 380VDC at those two points? VDD?"

Sorry but I'm a newbie. What two points?

Johnny

[newbiesat]

Re: Need help repairing an audio amp schematics included

Those 2 points that budm mention should be across the resistor R288 (100K/1W) in the schematic diagram, page 6.

[johnnydiaz]

Re: Need help repairing an audio amp schematics included

Since I'm new at this I'm not sure exactly what to do. So I will measure dc voltage. I should put my positive test lead on r288 18volts and where should I put the negative? Same positive test lead on R288 380volts and where does the negative go? Sorry for the newbie question just want to confirm since its high voltage.

[redwire]

Re: Need help repairing an audio amp schematics included

First, take some measurements on the PSU primary side. Be careful of the high voltage and the fact that the caps (for VDC) can hold a charge even when things are unplugged.

You would put your multimeter lead (-) on (primary) circuit common somewhere convenient, like the (-) terminal on D40/C190/C192 etc. The schematic shows this as a ground symbol, but it's not really.

Then write down the DC voltage readings on:
18V should be around 18V.
VDC should be around 380V.
VDD should be around 19V.

The power supply has two parts and a bit tricky. It looks like start-up voltage (VDD 18-25V) is first provided by Q28, then U36 starts up and generates VDD. Same for 18V, start-up power is from R288, and again U36 circuit then powers the 18V from VDD.

If R245 and R288 are cooking, I'd guess the PSU is not starting up.
Then you would snoop around U36 to see why it's not running.

[johnnydiaz]

Re: Need help repairing an audio amp schematics included

Ok so I understand the the negative terminal part. I'm confused about the 18v, 380v & 19v? Do I put the red test lead on one of the leads on the resistor r288 to test the 18v? Im looking at the actual board and the page 6 of the schematics and am confused.

[redwire]

Re: Need help repairing an audio amp schematics included

Johnny, if you don't have experience making voltage measurements then I suggest kinda not doing the repair. The voltage is high enough to kill, and even slipping with the probe tip can cause a big boom. Your multimeter and its probes must be good quality too. Just something to ask yourself, because there's a risk and learning on this power amp there is no room for mistakes.

The measurements are made on any component that connects to the signal/power feed of interest as shown on the schematic. As long as you have the correct side i.e. only one side of R288 is the 18V. You have to look around and follow the traces, or just find another part on the 18V rail. If a point is too hard to get to, go to any other part on that rail to make your measurement.
I use an alligator clip (well insulated!) connecting the meter (-) to common, so I just have to hunt around/hold only one probe (+).

[johnnydiaz]

Re: Need help repairing an audio amp schematics included

Redwire,

I completely agree with you and appreciate the advise. I'm using a fluke 289 and have the amp connected to an isolation transformer. I will solder the resistors back and attempt to do a voltage test and report back. Unfortunately only way to learn is to do it. I'm being as careful as possible. Thank you and everyone else for being so patient.

Johnny

[retiredcaps]

Re: Need help repairing an audio amp schematics included

I'm using a fluke 289 and have the amp connected to an isolation transformer.

The Fluke 289 will easily survive those voltages safely. If your 289 came with some aligator clips, now is a good time to use them. Even if you can only use the black clip, then at least you can probe one handed (keep the other in the pocket) as redwire mentioned.

Make sure the clips have a good hold/grip before applying power. You don't want the clip to come flying out and causing a short.

[johnnydiaz]

Re: Need help repairing an audio amp schematics included

Retiredcaps,

I did buy the whole kit which included the alligator clips. I will definitely use them. Thanks.

Johnny