Help with amp pcb  - Page 1
Forum section: electronics / beginners

author: atriptothemoon
author_rank: Newbie
timestamp: February 23, 2023, 04:14:08 pm
content: hi all, find myself at home with no access to mate based expertise! For soldering and general practise I decided to try and track down the source of hum on an old whafedale-pro power amp (an EVP 12PM). Before recapping the smoothing caps I've had a look at the other components. The pink resistor is different to all the rest. See pix. It looks to me like it is a 0.47 ohm 5% tolerance resistor - can someone reassure me I have this right please? so I can test.While the other resistors are all shiny the pink one is matt and looks "dusty" - is this normal and likely just a consueqence of the type of resistor it is (unfortunatley I have no specs or circuit diagram for the EVP-12pm) or a tell tale sign of some sort of damage?Any help gratefully recievedthanks all

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 23, 2023, 09:10:10 pm
content: Looks like a 4.7 ohm 5% resistor to me. (Yellow Violet Gold Gold) But it might be in parallel with the inductor next to it, and if it is, it will measure zero ohms in circuit.Yea, it's normal for some ceramic resistors to have a rough texture like that.

author: atriptothemoon
author_rank: Newbie
timestamp: February 23, 2023, 09:58:32 pm
content: thanks Kim

author: srb1954
author_rank: Super Contributor
timestamp: February 24, 2023, 10:38:15 am
content: Quote from: atriptothemoon on February 23, 2023, 04:14:08 pmThe "dusty" surface is normal for that type of resistor. This resistor is probably specified for a higher power rating than the other resistors beside it and will consequently run at a higher surface temperature. The "dusty" coating withstands those higher temperatures better and doesn't burn or char like the lacquer coating on the other resistors.

author: atriptothemoon
author_rank: Newbie
timestamp: February 24, 2023, 11:49:51 am
content: thanks srb thats really useful infor - and what Kim said about it running in parallel with an inductor - all helps with my learning curve!

author: Terry Bites
author_rank: Super Contributor
timestamp: February 24, 2023, 06:19:04 pm
content: 4.7R 2W 5% is shown in the service manual schematic.BTW Along with the indcutor and C23 it forms a Zobel network.

author: atriptothemoon
author_rank: Newbie
timestamp: February 26, 2023, 09:00:57 am
content: thanks Terry - could you link me to the schematic please? so I can have a go at trying to at least partially understand it, as well learning what a zobel network is :-)I am beginning to get an idea about amplifiers but am very much at the beginning - cheers

author: atriptothemoon
author_rank: Newbie
timestamp: February 27, 2023, 11:20:10 am
content: hiya - any chance of a schematic would be really helpful - cant ind one online :-)

author: Xena E
author_rank: Frequent Contributor
timestamp: February 27, 2023, 02:31:17 pm
content: Heres a linkhttps://www.google.com/url?sa=t&source=web&rct=j&url=https://elektrotanya.com/wharfedale_pro_evp-x12pm_sm.pdf/download.html&ved=2ahUKEwif58KK8rX9AhVihP0HHYyWC1AQFnoECA0QAQ&usg=AOvVaw3qA0t4m7GQHHopLMrgYKYiRegards,X