Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Please login or register.

Login with username, password and session length
Advanced search  

News:

SSGuitar has been upgraded to SMF v2.0.14 and PHP7!

collapse
Pages: 1 [2]   Go Down

Author Topic: Ancient Carlsbro Hornet 45 Lead Combo Problem  (Read 895 times)

phatt

  • Legendary
  • ******
  • Chip Points: 198
  • Posts: 1722
    • View Profile
Re: Ancient Carlsbro Hornet 45 Lead Combo Problem
« Reply #15 on: July 09, 2017, 08:33:04 AM »

You have a basic differential power amp setup which looks complex for the novice.
OK,, These are DC Amps, (Direct Coupled circuit) with no capacitors between each part so there is no isolation from input to output and any fault is often Fatal for more than just one part of a DC circuit.

When they fault you often find the output (which should be very close to zero VDC) goes off balance.

The supply is bipolar meaning you have both positive and negative voltage supplies (often called supply Rails),, Hence the 2 big Capacitors.

The center of those 2 Rails is obviously zero VDC which is what you should read on the speaker output terminals.

So take your DMM and set it to VDC and post what it reads on the spk terminals.
Does not matter if the voltage is Pos or Neg,, as long as it is close to Zero,, if not then something is wrong.
Disconnect the speaker if the voltage is like say 20 volts otherwise you could burn out the speaker. :'(

The DC supply Voltage will be on the terminals of those 2 large caps,, Where those 2 caps meet will be the Common which is often case ground as well (but not always). Black probe on the Center Tap join of those 2 caps then read voltage on the positive and negative rails. They should be very close to the same.
 
The fuses will be the AC Voltage supply from transformer and the black square blob between the caps and fuses will be the Bridge rectifier diodes.
Set DMM to ACV and read the AC voltage from the Transformer on those 2 fuses.
Just probe across each fuse will tell you the full AC voltage the amp runs from.

by the picture it looks like you have clear access to the track side of the PCB so should be easy to find the test points for the DC voltage.
hope it helps,, Phil.
Logged

Brunel98

  • SSGuitar Regular
  • ***
  • Chip Points: 0
  • Posts: 13
    • View Profile
Re: Ancient Carlsbro Hornet 45 Lead Combo Problem
« Reply #16 on: July 10, 2017, 02:50:03 PM »

DCV across the speaker terminals is 0.167

I'll post the other results when I take them (when I have time to take the board out again).
Logged

phatt

  • Legendary
  • ******
  • Chip Points: 198
  • Posts: 1722
    • View Profile
Re: Ancient Carlsbro Hornet 45 Lead Combo Problem
« Reply #17 on: July 11, 2017, 08:58:10 PM »

All good,That is a normal reading :tu:
Good chance the power is working ok ,,maybe somewhere between the line out and poweramp input is a failing component, Without a schemo it's hard to know.
Maybe some close up pictures of both sides of the PCB might help folks here work out the problem.
Phil.
Logged

Brunel98

  • SSGuitar Regular
  • ***
  • Chip Points: 0
  • Posts: 13
    • View Profile
Re: Ancient Carlsbro Hornet 45 Lead Combo Problem
« Reply #18 on: July 12, 2017, 02:22:07 AM »

Thanks :)

OK I'll get on to that after work. Given that the speaker terminal reading is OK do you still need me to take the other readings (DCV on the power rails, ACV on the fuses)?
Logged

phatt

  • Legendary
  • ******
  • Chip Points: 198
  • Posts: 1722
    • View Profile
Re: Ancient Carlsbro Hornet 45 Lead Combo Problem
« Reply #19 on: July 12, 2017, 04:17:28 AM »

Yes! as that is what powers the whole amplifier.
When I work on gear I first find the **Mains** input which is the Deadly end and if I see bare terminals with mains wires I cover them as it's all to easy to slip up. Even a small sheet of plastic will save you from a wandering finger. :trouble Also Check the Earth pin is attached to the case. I've found a couple of cheap far east rigs that have never been opened with No Earth pin connected,,Factory stuff ups.

I then establish the AC secondary windings and check the AC voltage (jot it down). Most common is 3 AC secondary wires being 2 actives and one common which is the CT center tap and that will be circuit common or Zero and often Case ground.
These wires might be 2 reds and 1 black but other colors are used but format tends to be the same.
This tells you that black is CT and the 2 reds are the Hot wires.
Voltage from black to either Red will be close to the same AC reading. Obviously Red to Red will read twice the value.

Then I read the DC voltages at the main filter caps.
Your supply is a Full wave bridge supply so the DC Voltage will be 1.4 times the secondary AC Voltage.
i.e. With a secondary of say 20/0/20VAC the resulting DCV after the Recto and filter caps will be close to 28/0/28VDC, give or take a volt for diode drop.
That is the DC the Amp runs on.
As the output is zero then I'd say the supply is working fine but you may get asked to cross check some voltages so wise to have the DC voltage that powers the circuit.

Meantime with the amp powered up get a small chop stick (Wood or plastic Not metal) and gently probe around the components on the pcb while running an mp3 player or some other audio signal see if you can find a cold solder join.
Often pcbs crack at the solder points and a wiggle of a component can find the offending break. As the rig is older it is a prime candidate for cracks on the solder pads.
Often you can't see them easily you need good light and a magnifying glass to see them.
Phil.   
Logged

Brunel98

  • SSGuitar Regular
  • ***
  • Chip Points: 0
  • Posts: 13
    • View Profile
Re: Ancient Carlsbro Hornet 45 Lead Combo Problem
« Reply #20 on: July 18, 2017, 12:04:27 PM »

This is just to let you know that I haven't forgotten about this - circumstances have just got in the way and I have't been able to progress. As soon as I am able I'll take the readings and photos and post the results.
Logged

Brunel98

  • SSGuitar Regular
  • ***
  • Chip Points: 0
  • Posts: 13
    • View Profile
Re: Ancient Carlsbro Hornet 45 Lead Combo Problem
« Reply #21 on: July 29, 2017, 06:01:33 AM »

OK so here are some photos of each side of the circuit board - DMM readings to follow ASAP.

One thing I have noticed is hopefully shown in the close up photo of the two large black capacitors. Is the substance at the base of the capacitors glue or is it (as I suspect) evidence that one or both of the capacitors is leaking and if so is this a possible cause of the fault (the raspy distortion at low gain settings)?
Logged

phatt

  • Legendary
  • ******
  • Chip Points: 198
  • Posts: 1722
    • View Profile
Re: Ancient Carlsbro Hornet 45 Lead Combo Problem
« Reply #22 on: July 30, 2017, 10:38:18 AM »

Yes the caps could be leaky but maybe Don't replace caps just yet.
Check and post the voltages and also note the transistor part numbers as that will give the better minds here an idea of how the power stage works ,, and how to fix it. :tu:
Looks like the power stage has only 5 active parts so it is a very simple circuit and my hunch is it's just bad Xover distortion due to an aging circuit.

With the part numbers of those 5 transistor and the pictures you have taken it would not be too hard to draw a schematic.
In fact that is how i got started years back,, simply back engineering simple circuit like this one.  I wasted a lot of paper at first but it did not take too long before I got legible drawings that looked a lot like the professional ones.

Here is a simple power amp circuit that is not much more complex than your unit except it has 7 transistors;
http://sound.whsites.net/project03.htm

Your unit might have Darlington power transistors hence only 5 transistors,,, the 3 or 4 bias diodes strapped to the heat sink are a clue.
Phil.
Logged

J M Fahey

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Chip Points: 422
  • Posts: 4086
    • View Profile
Re: Ancient Carlsbro Hornet 45 Lead Combo Problem
« Reply #23 on: July 31, 2017, 10:27:23 AM »

I see 5 transistors , please read what they have printed on the case and label them in the pictures.

You also have 3  2 leg thingies touching the heat sink, might be 3 diodes or 2 diodes and 1 resistor, please label them in the pictures.

Logged
Pages: 1 [2]   Go Up
 

* User Controls
 
 
Welcome, Guest. Please login or register.
Did you miss your activation email?
* Recent Posts
LM 3886 current feedback poweramp by tarahall
[Today at 07:37:00 AM]


EBay TDA2050 boards by tarahall
[September 19, 2017, 08:39:21 PM]


Need help with ROG: fetzer valve (gain stage) + buffer by tpb03
[September 19, 2017, 02:03:31 AM]


fender bassbreaker 45 by J M Fahey
[September 16, 2017, 03:27:00 PM]


Vintage Roland Cube-60 Reverb by Jazz P Bass
[September 15, 2017, 10:53:49 PM]

* Sponsors