Resistance Question - Voltage Error

[schweigc]

Hi all, I have just done the resistance checks and am getting an "Overload; Range Exceeded" reading on my DMM on the following terminals: 10, 22-24, 26, 27, and 30. Is it safe to proceed to the voltage checks?

[Paul Birkeland]

Yes, those readings just need to not be anything resembling 0 Ohms.

-PB

[schweigc]

Thanks for the response. I did my Voltage checks and I have some problems. First the LED on A3 does not light and none on the Low Current board and the high current board near the power switch. The Following are my Voltages:
LC Cs4:
IA: 149.5 V
OA: 75.5 V
KregA: 5.9 V
bRegA: 149.5 V
IB: -297 mV
OB:  -.71 V
KregB : 1.24 V
bRegB: -294 mV
HC Cs4 (Near Selector Switch)
IA: 205.5 V
OA: 149.5 V
bA: .5 mV
IB:  .5 mV
OB: 95 V
bB: 149.5 V
HC Cs4 (Near Power Switch)
IA: 1.5 V
OA: -275 mV
bA: .2 mV
IB:  .2 mV
OB: .466 V
bB: - 215 mV

HJ:
Tip: 0
Ring: 48 mV
Any ideas? Thanks,

[Paul Birkeland]

The high voltage just isn't available to one channel.

This starts at IA on the board where you aren't seeing ~200V. 

On the lucky end of things, half your amp is working perfectly.  Both IA pads wire to B+, what voltage do you measure at terminal 24 vs. terminal 30?  If you have very low voltage at one of these terminals, which feeds that flaky IA, and your 270 Ohm resistor isn't smoking, then you have a flaky solder joint preventing the B+ from making it's way to that channel.

-PB

[schweigc]

One of the 270 Ohm resistors is actually broken. I have some spare resistors (CP-5-3 Mexico 1038 - 270 Ω 5% Dale 5W). Could I replace the broken one with this one?

[Paul Birkeland]

Sure, those ought to work just fine.

-PB

[schweigc]

Thanks again, the Bottlehead crew is awesome. I'll start the surgery soon...

[schweigc]

I just replaced the 270 Ohm resistor. Powered it up and the resistor promptly fried. What do I check next?

[Paul Birkeland]

Do you have a backwards 220uF capacitor?

[schweigc]

No, The striped sides all face the near edge of the Chassis, correct? (Striped side to terminals 25, 31, and 33)

[Paul Birkeland]

Yes, that is correct.

The next step would be to check the connections and jumpers on the large C4S board with the low voltages.  Somewhere, there is a jumper that isn't where it's supposed to be, or a connection has been reversed that is creating quite a short in your amplifier.

-PB

[schweigc]

Ok, I'll check it over. When I find the problem and replace the 270 Ohm resistor again, I will have to clean up the 2 terminal blocks as well. If I damage a rectifier while doing this can I replace it with any UF4007 rectifier? What I have on hand is mouser # 625-UF4007 E3/54.

[Paul Birkeland]

The rectifiers should be fine, you'll just need to find the wiring error that is causing that excessive current consumption.

[richmi]

Hello,

I do not know the Crackatwoa, I don't have it.

It might be a good idea to ask a friend to check your amp against the manual. He may find something that you keep missing. A fresh pair of eyes could make a difference.

Richard

[schweigc]

I have checked the wiring and solder joints and don't see anything obviously wrong. Could this be a bad capcitor, rectifier or resistor? The second time the 270 Ohm resistor blew all LEDs except the one attached to A3 lit until the resistor died.