First post so hello :)
I brought a Circle Sound Amp head of ebay sold as spares and repairs. apparentyl from the 1960's ::) Hoping I could fix it up simply enough! I have zero knowledge of the amplifier, so if any one does that would be great!
The issue with the amp is that it turns on and you can here the hum. Plugging the guitar and strumming a chord = nothing. Even with everything to the max.
Although slapping the pickup does produce a noise. Similar to what I would expect...
I have checked guitar cable, speaker cables etc so the problem I believe is with the head. I opened her up and there was a few badly soldered wires that may not have given a good connection. It was also a little dusty so I blasted that with compressed air. I made sure that all grounding points looked and felt good. Closed her up and turned her on and nothing had changed.
What would my next steps be?
Cheers
In advance, if you could post several pictures of it here, some guys here can help. No easy to tell anything if we can´t see inside and you have no knowledge about electronic stuff.
I agree, welcome to the forum, and we need pics :tu:
The issue with the amp is that it turns on and you can here the hum.Do you hear hum whether or not anything is plugged into its input jack? Does the hum increase/decrease in volume when you adjust the volume control?
Plugging the guitar and strumming a chord = nothing. Even with everything to the max.
Although slapping the pickup does produce a noise. Similar to what I would expect...
I have checked guitar cable, speaker cables etc so the problem I believe is with the head.
I'm no repairman and I may be overlooking something obvious, but it seems weird that strumming wouldn't produce a sound through the amp, but hitting the pickup would. You have tried this guitar through a known good amp, right?
Quote from: mexicanyella on April 03, 2012, 10:46:51 AM
I'm no repairman and I may be overlooking something obvious, but it seems weird that strumming wouldn't produce a sound through the amp, but hitting the pickup would. You have tried this guitar through a known good amp, right?
I was thinking the same thing.
Quote from: ipoo2 on April 03, 2012, 05:23:29 AM
Although slapping the pickup does produce a noise.
No sound variation at all or i made a wrong traslation?
In other hand, assuming that this thing didn´t work, cause he says that purchased it as spare parts....sounds to my posible that somebody sells something that work fine, thinking that didnt work, but no very probable.
Quote from: mexicanyella on April 03, 2012, 10:46:51 AM
You have tried this guitar through a known good amp, right?
Yep!
Pics as promised
(http://img853.imageshack.us/img853/9884/p4021767.jpg)
By ipoo2 (http://profile.imageshack.us/user/ipoo2) at 2012-04-03
(http://img24.imageshack.us/img24/4288/p4021766.jpg)
By ipoo2 (http://profile.imageshack.us/user/ipoo2) at 2012-04-03
(http://img805.imageshack.us/img805/5918/p4021769.jpg)
By ipoo2 (http://profile.imageshack.us/user/ipoo2) at 2012-04-03
(http://img820.imageshack.us/img820/315/p4021759.jpg)
By ipoo2 (http://profile.imageshack.us/user/ipoo2) at 2012-04-03
(http://img38.imageshack.us/img38/3397/p4021762.jpg)
By ipoo2 (http://profile.imageshack.us/user/ipoo2) at 2012-04-03
(http://img840.imageshack.us/img840/7696/p4021763.jpg)
By ipoo2 (http://profile.imageshack.us/user/ipoo2) at 2012-04-03
(http://img838.imageshack.us/img838/1733/p4021764.jpg)
By ipoo2 (http://profile.imageshack.us/user/ipoo2) at 2012-04-03
Quote from: mexicanyella on April 03, 2012, 10:46:51 AM
The issue with the amp is that it turns on and you can here the hum.Do you hear hum whether or not anything is plugged into its input jack? Does the hum increase/decrease in volume when you adjust the volume control?
Hum remains whether the guitar is plugged in or not at a constant level despite volume increase/decrease
First: i suggest you to do and USE this thing LImiter bulb light (http://www.ssguitar.com/index.php?topic=2093.msg17092#msg17092)
Can you check if you have some DC tension reading between the two wires at the speaker output jack?
Quote from: polo16mi on April 03, 2012, 06:40:24 PM
First: i suggest you to do and USE this thing LImiter bulb light (http://www.ssguitar.com/index.php?topic=2093.msg17092#msg17092)
Damn, I just pulled a table lamp apart for wire that would have been perfect for this test!
Do you have *strong* hum at the speaker, no guitar sound and when you turn the amp ON the speaker cones go forward or backwards?
If so, disconnect them immediately or you will blow them.
Your power amp looks burnt from the attached pictures.
Quote from: J M Fahey on April 06, 2012, 10:41:00 PM
Your power amp looks burnt from the attached pictures.
Where do you get that from? I didn't notice any burn marks/hot spots anywhere? Care to point out what you see? I'm sure it's there but I just don't see it.
Maybe Mr. JM thinking about brown color over the main transformer? (overloaded, so blew?).
I wondering what happens if Mr. ipoo2 measure voltages between each screw of that six over the main pcb, and the chassis of amp. A pair of wire (plus GND), comes from the little pcb around the big transformer, with +VCC and -VCC. If reasonable values founded, then think next step is measuring the state of the big transistors (2n3773 and his neighbors)
I see 4 fine wire links that look like fuses, in series with each output transistor.
You have 4, to the left and right of the driver transformer.:
(http://img840.imageshack.us/img840/7696/p4021763.jpg)
Left down looks normal (straight); left up looks like it heated somewhat (slightly bent and darker) ; right down more so and right up looks destroyed.
Follow the tracks to confirm and in general hand draw the schematic.
Not that difficult with some patience and good background music (no pets/kids/wifey within the room)
The 2N6254 look original (mat be blown or good, testing will tell) and the 2N3773 is not original.
Use the lamp limiter, disconnect speakers, measure and post both power rails (+V and -V) and what DC voltage you measure across the speaker terminals.
Good luck.
PD: nice find, that amp sure looks cool and is very repairable.
Thanks for the advice, much appreciated :) And yep it does look pretty cool!
I am a little nervous using the lamp limiter - what with mains electricity being involved :-\ Any alternatives?
Bulb limiter is useful saving components. An alternative is dont use it, but.... everytime that something goes wrong, and some component die, you´ll pay the cost ($$$)
I recomend use it. Is no big deal. Just carefully mount it. No need to touch anything with tension.
Quote from: J M Fahey on April 10, 2012, 11:05:39 PM
I see 4 fine wire links that look like fuses, in series with each output transistor.
You have 4, to the left and right of the driver transformer.:
Left down looks normal (straight); left up looks like it heated somewhat (slightly bent and darker) ; right down more so and right up looks destroyed.
Follow the tracks to confirm and in general hand draw the schematic.
Not that difficult with some patience and good background music (no pets/kids/wifey within the room)
The 2N6254 look original (mat be blown or good, testing will tell) and the 2N3773 is not original.
Use the lamp limiter, disconnect speakers, measure and post both power rails (+V and -V) and what DC voltage you measure across the speaker terminals.
Good luck.
PD: nice find, that amp sure looks cool and is very repairable.
Good eye, I didn't see that at first.