Here is a close up of the resistor. I think it's definitely a solder melt.  I was also wrong about the value, it's 100 ohm! I've tried a jump wire in it's place and it made no difference, but there was no sound without anything there.

It's in between the volume and the treble controls.  The burnt end in the photo goes to the centre lug of the treble (ground?) The bottom goes way off the right of the photo to another resistor approximately (for cross reference) in the middle of the board and between the volume and boost pots (if looking at previous photos or the 'x-ray' photo you will see it).

Originally i also thought it was the speaker, but I've tried connecting different speakers and it's the same.  Also following your suggestion, i've just tested the speaker with another amp hooked up and the speaker is fine.

I can't say for certain that the speaker wire didn't touch the speaker, the chassis bottom, etc when it was loose. So it might have shorted something.

This probably has nothing to do with it, but my soldering iron stopped working at the same time as my amp broke!

I'll try and describe the sound in more detail:

The normal input has virtually no clean tone anymore. It's sounds more like the boost input was on full but a really bad version. The decay is splattery and has a bit of tremolo/ocillation breaking up the signal.  The boost input sounds like the ghost fuzz has completely taken over and killed the main signal.

Is there anything i can test with my meter?

On looking at the amp again, i've noticed a small indentation to the resistor which is connected to the treble control, which i assume is from a droplet of solder, but the treble control is still functioning.  In any case i've ordered a replacement 10 ohm resistor.

Another thing which could have some bearing, is that one of the speaker wires had become disconnected at some point during the connection of the extension reverb wiring.  I've heard that switching on an amp with no speaker connected can blow the output transformer?  I hope this is not the case!!

I can't understand how everything sounds fine through the headphone socket tho?

C'mon people - any ideas.  I'm heartbroken here!

Hi Roly

Everything was working great and the chassis was back in and still working great.  Then i took the chassis  back out to solder some RCA connecting wires to the existing reverb wires, then when i put it together again the problem started.

I've disassembled  the connecting wires i'd done but it still sounds bad. That's the only thing i'd changed since it worked.  I've checked for loose wires, bad connections, etc.  Sounds OK like it used to through the headphones though.

Paolo

Hi

The amp was stressing me out too much so i had to take a break.... then it broke!

Firstly an update:

GAIN - What i settled on for the resistor in the boost channel was 120 ohm, which gives me a great crunchy overdrive at the top of the dial. I could have done with 150 or 170 for a just breaking up tone but with 120 the extra is there and i can back off the guitar volume a bit.  I'll see how it goes in 'the field' anyway.  The normal channel is sounding great also (it always did).

REVERB - I soldered in some new wires with RCA connections for the accutronics tank i borrowed from another amp, which is in the base of the cabinet. The reverb sounds 100 times better than the fool that was in there originally. It doesn't really go very deep but it's just right for my needs.

GHOST FUZZ - Seems  to have lessened somehow. It only occurs on the gain input at  on 7-10 on the dial but is very faint.  Doesn't get any louder when the volume is turned up either. If the reverb is on though wile using the gain input, it's a different story. It's very much there fighting with the main signal.  Luckily, it's not really there using the normal channel.

HELP!! PROBLEM!! HELP!! PROBLEM!! HELP!! PROBLEM!! HELP!! PROBLEM!! HELP!! PROBLEM!!

So ... everything was going great until i put the circuit board back in screwed everything back in place, and was getting ready to fix the reverb tank in the base of the cabinet.  I turned it on and something is wrong.  The whole amp sounds like it's powered by 9v battery on it's last legs.  A splattery distorted quieter tone. 

Curiously though, everything seems ok through the headphone socket (does the headphone socket come straight from the pre-amp?).  I have opened up the amp and checked all the wires and connection and can't see anything.  Before i joined the new RCA phono cables to old ones it was working fine, then i put it all back together and bad sound.  The only thing i could think of is that i had dropped some solder or stray wire fragments on the circuit, but i can't see anything.  Maybe it's linked to the ghost fuzz - something that was on it's last legs has now given up?

It's really frustrating because i'd got it running really nice and thought we'd solved all the problems.

Please does anyone know what happened?

The thing is Phil, even if i could trace out the circuit to a schematic, i wouldn't be any the wiser.

I have schematics for a few projects that i can't do.  One of which is to un-mod an Electar Tube 10 amp back to the Fender Champ it was based on and bypass the underdeveloped, unfinished extras that colour the tone.  This only involves adding a few connecting wires to bypass parts of the circuit and changing a couple of values. This amp is much simpler than the Prince amp circuit and i haven't been ale to fully grasp it. But after this, with the things i'm learning, i think i might be able to do it. (I'll be running that schematic past you guys in the future for your opinion on it's competence, before i electrocute myself!). 
I also have a schematic from Carl Martin to make his trem pedal true by-pass but can't do that either (at the moment).

At the end of the road there will always be gain pedals, but NO PEDAL iv'e heard can do the gain tone of this amp.  It's unique and sounds amazing, except for that damn ghos ....

Paolo

No Phil, i don't think you're being cruel, but you said a similar thing back on page 1. 

Since then we have got the gain working and sounding great; we have discovered that putting in a different reverb tank vastly improves the reverb; we have found Roly!  There is a minor gliche in the circuit that still needs to be ironed out and we are working our way through some ideas.

One day i hope to understand schematics, but this day is not it and this amp is not the place for me to start! I need to start small and learn by actually doing stuff and finding how the little bits work, and hearing for myself what happens when a part is changed.

Back when you started, all the components where all probably handwired along a turret board of a valve amp, where the reality gap between a schematic and the board was much less than it is now.

The issue is the ghost fuzz. It is something that must happen in a lot of amps. Someone will know what causes this.  They might look at this thread or join the forum at any time and know what causes it.

Let's be positive out there!

Like i said, the ghost fuzz is more like a cheap distortion pedal with the batteries dying mixed in with the amps overdrive. You can hear it as a seperate thing as it has a longer sustain. It's metallic like some one playing down a ventilation shaft. It's not like feedback from standing too close with a guitar or microphone.

I can't say for sure about more or less with the reverb tank attached. The full on gain with the jump wire was clouding the issue.  I'll have to try it again.

Here are the answers to a couple of tests Roly suggested:

1.  One thing you can do is to fire up your amp, and after it has been running for about ten minutes with the volume and gain at zero, measure the voltage across each of the 0.5r resistors.

A.  (Looking at the picture)
     The left red resistor reads 0.015
     The right red resistor reads 0.013


2.  "ghost fuzz" - crossover distortion.  Tell me, is it worse at quieter levels than louder?

A.  Increases with the gain knob. It's about the same on the volume knob.  The more the reverb knob is turned up the worse it gets, i suppose because the sound is being echoed.

I'm pretty sure that it's not so prominent (in general) when the reverb unit is disconnected. (I'm actually suprised the amp still works with the reverb wires disconected!)

Bearing in mind, i still only have a jump lead in place of the first resistor, there is a high pitched whine when a cable is plugged into the boost channel (but not the guitar) and the Gain is between say 8-10 (even with the Volume knob low). It's not feedback from the guitar obviously.

Also we have to remember that the ghost fuzz was there at the begining with the original 1st resistor in place, so it's not because of the jump wire (although it ain't gonna help).

I really should get that resistor set!

Paolo

Awww Roly, you've gone all JM Fahey on me  with those technical terms and complex schematics!

I didn't understand hardly any of that and i can't read a schematic.  I know a few of the symbols but that's it.  Mainly beacuse it's non-linear, non-heirachical, or any other historically acceptable form of information transferrence! I don't think i will ever understand them.  For a start there are no direction arrows from the input through to the output, or between components.

I have included a diagram for everyone's amusement, but if it was like that i could understand it! 

It's going to take weeks to figure anything out, if at all. At least they've sent me the correct resistors this time, so i will be able to get that original gain problem sorted.

That's great Tony.

Thanks for the ID on the transistor.  Roly should be happy with that info.

Paolo

Here is a picture of the holy grail for those non-believers!

(I put the 'L' pen mark on by the way, because i thought i might have to unscrew those to get the PCB out. Glad i didn't now i know about the thermal paste! (which i think i'll leave until everything else is sorted)).

Not sure what you mean by...

Quote from: Paolo
I'm assuming the input to the heatsink is on the left (viewing from amp front) and the output on the right.

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Ahh, i see what you mean.  I thought you mean't transistors on the pcb in your previous explanation/post.  I suppose i've overexpained then in my last post!

Here are some photos of the area where the heatsink wires connect to the board.

Assuming that the input wires to the heatsink are front left, those wires are on the right on the PCB in the pictures. 

INPUT to heatsink: Right hand yellow and the top blue and the (partially hidden) orange wire underneath that.

OUTPUT from heatsink: The next blue wire under those and the lefthand yellow and left hand orange beneath the two big red resistors.

For orientation, the controls pots from left to right in the picture are Tuner/Reverb/Bass/Treble.  From the underside of the pcb they are featured at the top of the picture so you can see where the front corresponds to the back.  There is also an x-ray view of the area.

I hope these photos are ok.  If not i'll take some more.

So is this thermal paste issue contributing to any of the problems with the amp (gain and/or) reverb)?

Anyway, i'll have to get some of that paste to refresh the connections.

Thanks for the advice on that.

Paolo

Hi, thanks for your interest.

Well from Rolly's initial description i was assuming it would be a thick sticky substance, but it's a thin coat/film. It's not exactly crumbling, as it's not thick enough to really crumble! It IS bone dry though. Probably to be expected after 30 years or more?

Paolo