I come to grovel and beg for assistance, voltages low!

[rollingelbow]

Ok friends, this community has some brilliant minds.. I have been stuck with a 50 watt clone kit that is assembled and now stuck for the past month with grainy and flubby low sound that cuts in and out. I have tried to keep my search for help local to the forum where I purchased this kit (some of you may have seen it) , but I may need more minds tackling this.

Essentially what I've got, are low voltages all around. my AC voltages from the transformer are high enough.. going through diodes, they seem fine, what comes out to the standby switch, is a DC voltage which appears to be low and stays that way through out the filter caps. Now, I have swapped out the 4 diodes and I get the same readings. What I am left with, is confusion. From my perspective this is what happens:

1. AC from the wall powers transformer
2. Red taps (in this case) from transformer go through rectification
3. result is DC which pumps out to my standby switch.
4. when i unleash the "fury" (thanks malmsteen!) , I am sending that DC voltage through the filter caps and from there through the rest of the amp.


Now, I seem to be fine going in with both red taps giving me 350 AC but then for some reason, i end up with his pathetic low voltage throughout the filter caps and then the preamp and output stages.Anyone have any ideas?

The caps appear to be fine in the filter section. As this is a standup board, I can't get at two resistors that are in series and I am thinking that if these are both 220 K/2 watt resistors, maybe their role is to pump up the juice for the rest of the filter caps. If these were to fail for some reason, could this be the failing point? Its going to be a pain to unhook wires and the standup board to get at these, but if you guys think this makes sense, please let me know and I will go ahead and do this. I guess i am weak in the theory department and there are not too many detailed documents that are specific to the amp one is working on. Conceptually though, there are certain commonalities.

Anyway if someone could help me go in the right direction, I'd really appreciate it. I really wanna get this thing running and my poor 4x12 cab asks me daily when its gonna have some company.

p.s- yes i have learned my lesson!

[Strat]

There shouldn't be a 220K/2W resistor in series in the B+. That value is usually used as a bleeder resistor going from the B+ to ground which when you go to Standby or Off drains the main filter caps so you can safely work on the amp.

The other possibilty I can think of is resistance across the standby switch. Check the switch in the closed position with your DMM, amp switched off and drained of course.

Which amp is it and have you a schematic?

[what_wires]

There shouldn't be a 220K/2W resistor in series in the B+.

That resistance is very high for a B+ dropping resistor.

[rollingelbow]

Yup, its the 6m45p kit from the wizard who'se name is not spoken around these parts. I don't want to have this post banned so I am afraid to post the schematic here. Essentially what we are looking at is the classic 1987 circuit.. Those two resistors in series do go to ground though yes. Its a mystery to me at this point. How i long for a good local amp tech!

[rollingelbow]

Ahhh the choke.. the hidden ingredient that I had forgotten.. I will try this tonight. The filter board has to be hiding the answer..Thanks. Any other suggestions, don't be shy to keep em coming. I will post back with the info though.

[rollingelbow]

Ok I lifted the choke at point A and there is a change. There is an increase in voltage, a jump to 375 DC just at that point. (obviously cause the rest of the caps don't look like they get much after that connection is lifted)

Secondary to this, the moment i flip the standby switch, my .5 fuse goes. I am wondering if one or more connections under the board burned through and is grounding or something cause that fuse keeps popping and i think this would imply a short or something somewhere correct?

[rollingelbow]

Thanks I'll check it out.. after letting it warmup for a few minutes, i am seeing a top DC voltage of 405 DC volts which is also a better sign but still somewhere something is shorting. Could it also be the choke?

What i plan to do is remove the circuit board as i want to inspect underneath and see if anything is burned.. this will allow me to see the vertical board clearer as well.

As for the fuse, don't think it is a slow blow.. i must double check but nothing indicates that it is or isn't.

[rollingelbow]

Definitely a fast blow as there is a thin wire inside it... maybe i should buy some slow blows as it is apparent that is not what i've got. replaced like 5 soo far lol.. every single one that goes in pops.

Also, i reconnected my choke at point A and back to crapola voltages. Checked the standup board, appears to be fine and i tested the caps using some tricks i saw on geofex site with regards to torubleshooting tube amps. They pan out.

So that leaves me back to the choke. Its got to be the culprit..but its going to be a pain to get under there and install it! oh well..

[rollingelbow]

I tried a spare choke anyway.. that wasn't the problem. Also pulled the vertical board, it looked fine but i redid part of it as i wanted to get the resistors out of the circuit to ensure they were still good.. no problems there.

Still trying to get some slow blow fuses.. why is everyone out of these bad boys? No luck on St. Patty's day i guess... probably cause i am Italian born

[Michael]

Hey Kev,

Want to send this one to me also?

Good luck!
Mick

[rollingelbow]

I may have to Mick.. I may just have to!! I am seeing a bad repeat trend in my worh here

[rollingelbow]

I may have to Mick.. I may just have to!! I am seeing a bad repeat trend in my work here

[Michael]

Kev,

You have GOT to learn to follow the manuals. It says right on the W-ber schematic 500mA Slo Blo for the tranny fuse and 3 (THREE) amp Slo Blo for the mains fuse. If you missed that, it is absolutely likely that you missed one, two, or many other things. As we saw with the HO, you have got to go back and check every single thing.

Remember: Unless it is exactly right, it is completely wrong. The beauty is, you have a schematic and a layout diagram that shows you what is exactly right.

Now for some troubleshooting.

1. Remove all your tubes, put in slo blo fuses, and check your voltages (amp should stay on now).

2. If they are low, half-split your power supply until the problem goes away. In other words, lift one side of R26, then R39, then your choke.

a. If the problem goes away when you pull your tubes, you know exactly where to look.
b. If the problem stays, you know the problem is right there in your power supply setup.

When you disconnected the choke, did you disconnect the cap can also (WCC-6M45P)? Maybe you have that beast wired wrong.

Did you verify C12 and C13 were stacked like this G-minus-plus-minus-plus-B+? R24 and R25 must be centertapped between C12 and C13.

Also, how on earth did the voltages "look good" once you unhooked the choke, since you also say the fuse blow immediately when you close your standby switch? The choke is not in the circuit with stanby open, so how did you see a change in voltage?

[rollingelbow]

b0ll0cks.. so i finally got around to getting slow blow fuses and rewiring the cap board as well as putting in a can cap rated at 500v in case that was the problem.... and of course, same problem. The .5 fuse consistently blows.. Mick I would seriously consider sending it to you but by the time i get it back with shipping and customs, i'll be in the hole 150$.. might as well spend 70$ more and just get a carl martin plexitone and smash the front end of my AX84 with that! Honestly this thing has been such a headache. .. should have just built a metro kit or an 18 watt... my fault for wanting anything plexi like so i could be a very bad version of Malmsteen on a Sunday afternoon armed with a Dod 250. I think this also brings my building days to a close. The bad tube wiring in the HO was totally overlooked when i rebuilt it..but that is logical. Everything wiring wise checks out on this amp.. its the bloody voltages which are the pain in the @$$ and are not logical. I've looked at all the sections that feed into the filter cap section and it all pans out. So who is the trouble maker in my amp? I guess we'll never know! Damn, would be fun to smash this with a baseball bat though..

I won't be buying anything from the wizard again. My fault or not, this was not the best layout a fella could work with.Anybody know any seasoned amp veterans in Ottawa Canada? Maybe chubsman can have a go at this for a case of beer or something... i think he was an avid Marshall amp builder. Let's just hope he doesn't like the Toronto Maple Leafs.

[Chubsman]

I sound like an avid builder... I know more than I did a year ago... but I am nowhere near Psycho, EL34 and the savage beast amp guru bastards that live here.

one thing you mention at the beginig is 220k series resistors on the filter cap board.... That can't be right. it's 2x 10K/2w resistors.... 220K resistor WILL bring your voltage down....

Another thing you can do is use this layout and go through the amp... a 1987 is a 1987 and it really should be the same amp (almost) from one layout to the next.

take your amp and follow componant to componant each little wire, JUMPER... connexction and so on from the attached file....

I miss things like that often... ususaly a ground... take you time... do it for 30 mins... go away... come back and do it again... you WILL find something.

And I have a feeling you are close to the prob. being close to the standby switch... Look at the diodes... are they all going in the correct direction? Are the Electro caps have the (+) to ground? Because they have to ... they are the BIAS caps and are reversed polarity.

With no tubes... do the fuses blow? What kind of mV do you have on that 1ohm/2w resistor on pin 1-8 of the power tubes?

With the Sndby switch OFF (open) do you have about 700V AC?
What kind of Trannies?

[Bacchus]

Hey Rollingelbow, I just e-mailed Chubsman and we talked about popping by together and take a look at. I mean, between the 3 of us, we should be able to figure it out!

Let me know what you think.

Cheers.