This has been a brilliant amp in the 18 years I've owned it but suddenly it's throwing a wobbly. The audio out is quiet and distorted on rhythm and contour settings. A bit louder on normal lead channel but not much. I've swapped all the valves one at a time and no difference. Just to eliminate the output stage, I plugged in some headphones. Initially it was just the same as through the speaker but then decided to go into some kind of oscillation where it made a loud on/off hum/fart. The on/off cycle was about 1Hz. I then unplugged the headphones and it just goes into a mad oscillation which can only be stopped by turning it off. Fwiw, I tried a jumper in the effects loop but it made no difference. Reverb settings also make no difference. Just before it started going really bad, if you hit the guitar fairly harshly, it was like there was a huge compressor in the loop i.e. the output would cut almost completely before slowly coming back a few seconds later. Any pointers/tests before I lift the lid and get stuck in with a meter?
Thanks in advance
Dick
Comments
It sounds like the fault is in the preamp or the LT power supply if it was doing odd things through the headphones - as you know, the headphones are driven from a separate (solid-state) module and not the main power amp.
I would start by checking the voltages on every valve in in the preamp and see what you find.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
All V1 - V4 in order and with respect to chassis ground - except where stated. All controls set to zero.
Pin 1 140/140/139/230
Pin 2 all 0
Pin 3 1.2/1.2/1.2/2.5
Pin 4 & 5 V1/2 both 11.8Vac to pin 9. V3/4 5.5Vac to pin 9.
Pin 6 142,141,94,247
Pin 7 all 0V
Pin 8 1.2,1.2,0.8, 2.55
Now it's out on the bench, the weird oscillation has stopped but it's still doing the whole quiet/farty thing.
Heater voltages look wrong on V1 and V2.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
It is definitely a preamp problem, or to do with the low-voltage supply which feeds the Contour circuit (immediately before the loop) and the switching, or the Contour circuit itself - all these are before the loop.
The odd AC voltages on V1&2 pins 4&5 are to do with it being a DC filament supply that's confusing your meter.
Further diagnosis without hands on is going to be quite difficult I'm afraid!
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
I then measured the resistance/load 'seen' by both circuits: The +15V side is around 1Meg but the -15V is more like 80 Ohms! After much head scratching, I reckoned that no single component in the transistor (Q1-Q4) circuit - I think this is some kind of differential amplifier - could fail and give that kind of load. Careful use of a sharp knife on the pcb isolated that circuit from the headphone out and reverb drive circuits which basically consists of 2 5532AP dual op-amps. This is where the 80 Ohms still resides. My assumption is that the resistance across pins 4 and 5 of these devices should be considerably higher. I know I could remove them and compare but that's a bit of a faff.
Would anyone like to comment?
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
As a final bit of advice, who would you recommend to buy small quantities of components from? My late brother was a TV engineer by trade so I had access to RS for bits & pieces. They were always a bit sniffy about selling to the public though.
Everything is now back together and working just as it should. Almost as an aside, I was digging around in the attic putting the Xmas decs away when I found a box of spare, used, valves from my old AC30 days. Suffice to say a nice british made Mullard in the V2 position is as sweet as a sweet thing. Lovely harmonics and just a hint of squishiness which the old EH didn't have.
Thanks to the ever-patient @ICBM and @DJH83004 for their help and encouragement.
Yes, for things like this I prefer to socket the chips as well. It's funny that Peavey always did this with their 'budget' solid-state amps, but Mesa doesn't with its supposedly fairly high-end valve ones...
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein