22-04-2012, 11:42 AM
Well I seem to have finished this off well and truly.
I thought I'd got it fixed until I tried to actually listen to it. I don't have anything to judge it against but thought there was no way it should sound like it did - like it's either not properly tuned in or that the speaker is physically damaged.
I checked all the AF117s for whiskers and they were all fine and also the square disc capacitors with strips on one side for leakage or being o/c. Again no problems.
I was thoroughly confused as my MP3 player or DAB radio played through it via the volume control sounded fantastic and the Autocrat when played through my bench amp seemed fine. Yesterday I also tried it through the Sovereign and it played OK through that, albeit quietly.
As the distortion seemed like that caused by overloading I had a go at the trimmers on the audio board. I'd not bothered before as the original anti-tamper paint was un-disturbed. I also don't have an output meter but as setting the basic current and voltages made no reference to readings thereon I thought I could manage without.
I managed to make an utter mess of this job and have blown the output transistors (T3, T4 and T5).
I changed T3 and T4 for BC337-25s, T5 for a BC327-25.
It works undistorted like that but is drawing 60mA through the test link and the BC337s are getting hot. Adjusting RV2 can get the current down to 20mA but it doesn't work at that setting - it oscillates and takes Radio Two off the air.
I'll see if I can get hold of some BD131s and 2s cheap but it looks like I'll be knocking something up with an IC and putting it on e-Bay.
I should never have started to mess with Dark Forces which I don't understand. That said, long-standing members will recall that this is just how I was with valve sets a year ago.
- Joe
I thought I'd got it fixed until I tried to actually listen to it. I don't have anything to judge it against but thought there was no way it should sound like it did - like it's either not properly tuned in or that the speaker is physically damaged.
I checked all the AF117s for whiskers and they were all fine and also the square disc capacitors with strips on one side for leakage or being o/c. Again no problems.
I was thoroughly confused as my MP3 player or DAB radio played through it via the volume control sounded fantastic and the Autocrat when played through my bench amp seemed fine. Yesterday I also tried it through the Sovereign and it played OK through that, albeit quietly.
As the distortion seemed like that caused by overloading I had a go at the trimmers on the audio board. I'd not bothered before as the original anti-tamper paint was un-disturbed. I also don't have an output meter but as setting the basic current and voltages made no reference to readings thereon I thought I could manage without.
I managed to make an utter mess of this job and have blown the output transistors (T3, T4 and T5).
I changed T3 and T4 for BC337-25s, T5 for a BC327-25.It works undistorted like that but is drawing 60mA through the test link and the BC337s are getting hot. Adjusting RV2 can get the current down to 20mA but it doesn't work at that setting - it oscillates and takes Radio Two off the air.
I'll see if I can get hold of some BD131s and 2s cheap but it looks like I'll be knocking something up with an IC and putting it on e-Bay.
I should never have started to mess with Dark Forces which I don't understand. That said, long-standing members will recall that this is just how I was with valve sets a year ago.
- Joe
