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Author Topic: sound city clean-up  (Read 16976 times)

ilyaa

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sound city clean-up
« on: March 10, 2014, 04:46:46 PM »
this might be a long one....

its a Sound City Concord combo

(schematic attached)

i inherited this amp from a friend - "if you can fix it, you can have it!" - another mutual friend had worked on it once upon a time with no success.

its already a complicated, cramped mess in there, but add to that the fact that i think the previous 'tech' may have tried out some voodoo of his own, and i really have no idea what's going on.

with a limiter in-line, the pilot lamp lights (dimly) but that's about it. no heaters no nothing. so im stuck at the first step trying to figure out: is the amp getting power? the light bulb does NOT light at all.

here are some brief measurements/observations so far:

1) something weird with the mains switch: the pilot light is hooked up DIRECTLY to the same pin as the "N" mains connection (rather than being on the other side of the switch, as it says in the schematic). on the other side of the switch is a blue wire that goes to a node that goes somewhere into the PT.

2) with the amp turned on (but on standby, of course), and one DMM lead hooked up to the "N" side of the mains, i can read ~120VAC all over the place: both main filter cap terminals, tube heater pins....??

3) the main filter caps have little lumps on the bottom with some white looking flaky residue. i assume that's bad!

4) there seems to be some kind of modification on the output of the bridge....an additional 47nF cap and 22K resistor and another diode....doing something i dont know.

i will go in and do a more careful trace to see what's going on and report back. its pretty confusing and hard to pick apart but ill start with the power section and see if i can at least figure that out.
« Last Edit: March 10, 2014, 04:54:24 PM by ilyaa »

DrGonz78

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Re: sound city clean-up
« Reply #1 on: March 11, 2014, 01:49:06 AM »
with a limiter in-line, the pilot lamp lights (dimly) but that's about it. no heaters no nothing. so im stuck at the first step trying to figure out: is the amp getting power? the light bulb does NOT light at all.

What is the wattage rating on the bulb? This is important to know and the wattage of the amp per rating too.

Edit: Ignore my response here... I misread... Although from ground reference do we have voltage on any of the heater pins? (Perhaps your feeling about the way the pilot light is wired has something to do with this...?)

1) something weird with the mains switch: the pilot light is hooked up DIRECTLY to the same pin as the "N" mains connection (rather than being on the other side of the switch, as it says in the schematic). on the other side of the switch is a blue wire that goes to a node that goes somewhere into the PT.
So part of the pilot light goes to neutral where is the other side of the lamp connected? Pictures say a 1000 words...

2) with the amp turned on (but on standby, of course), and one DMM lead hooked up to the "N" side of the mains, i can read ~120VAC all over the place: both main filter cap terminals, tube heater pins....??
This is alarming as ever!! I would assume that the filter caps should not have AC voltage readings other than a small ripple. Is there a reference to ground? You should not be reading voltages in this manner and it is very dangerous. Meaning do not connect any lead to Neutral as your "Ground reference". Is the amp safety grounded or does it have the isolation cap(i.e. Death cap)?

3) the main filter caps have little lumps on the bottom with some white looking flaky residue. i assume that's bad!
It not a good sign... Although they still may function, but they should be changed. First diagnose the fault on the amp and that may include the caps.

4) there seems to be some kind of modification on the output of the bridge....an additional 47nF cap and 22K resistor and another diode....doing something i dont know.
Restoring the amp to stock will be very important to really get the amp working again. I mean it may be easier to get it back to stock as a way to truly repair it.
« Last Edit: March 11, 2014, 01:57:11 AM by DrGonz78 »
“A person who never made a mistake never tried anything new.” -Albert Einstein

DrGonz78

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Re: sound city clean-up
« Reply #2 on: March 11, 2014, 05:30:54 AM »
1) something weird with the mains switch: the pilot light is hooked up DIRECTLY to the same pin as the "N" mains connection (rather than being on the other side of the switch, as it says in the schematic). on the other side of the switch is a blue wire that goes to a node that goes somewhere into the PT.

I did not look at the schematic at first when I answered you in previous post. I understand better now what you are saying there and that it is bypassing the switch. Or should I say it is in front of the switch... Since the switch is double throw it may make no difference as it is cut from the circuit either way when it is turned off. Still I would figure out where it is supposed to be soldered (correctly) after the switch as the schematic shows, good call.

****Edit****
One more thought on this one...
2) with the amp turned on (but on standby, of course), and one DMM lead hooked up to the "N" side of the mains, i can read ~120VAC all over the place: both main filter cap terminals, tube heater pins....??
I was thinking you might have been reference ground on certain parts of the amp. For example, the caps do go to ground on the negative side, so they were your ground reference. You were then seeing the 120vAC at the neutral connection since the amp was on.
« Last Edit: March 11, 2014, 06:00:46 AM by DrGonz78 »
“A person who never made a mistake never tried anything new.” -Albert Einstein

Roly

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Re: sound city clean-up
« Reply #3 on: March 11, 2014, 12:12:32 PM »
As you may have already noticed, this amp contains valves (toobs if you must) and requires a few differences in approach to a solid state amp.  The good news is that as valve amps go this one is pretty conventional.  The bad news is that unlike a solid state amp it isn't going to behave at all well under reduced voltages.

Normally the first thing I do with any repair is check the mains plug, lead, and chassis grounding.  American mains practice differs from the 240VAC L.E.N. world so I'll have to leave that to those with American mains experience, DrGonz &c.  I will say however that the mains pilot light should be wired to the transformer side of the mains switch, not the mains side.  As this amp has been fiddled with I would restore the mains wiring as per the circuit.

Check the fuses F1 and F2 are the right value and intact.

A major difference you have already encountered - it has a heater circuit which draws significant power, and in this case it is fitted with EL34's (6CA7's) which are absolute heater hogs.  The first thing to do is gently remove the two output valves by gripping the base and gently rocking them out of their sockets, then putting them somewhere safe.

And talking of "safe" you have to be well aware that the main HT (B+) supply is delivering several hundred volts to the output stage.  While is is unlikely to kill you if you are in reasonable health, you will find getting connected to it a very disagreeable experience indeed.

This particular amp is fitted with HT bleeders (R55 and R56, R54 and R5?).  Make it a practice after each power down to put your meter on the HT line and watch it sink (the Standby switch can be left on).

Visually inspect R51 and R52, 1k connected to pins 4 of the output valves.  These very frequently burn up in amps using EL34's.

Check the diodes in the main bridge rectifier.

If these are okay then switch Standby off clip your meter across the HT supply at the main filter caps C34 and C35, and using a large limiting lamp, say 100W, power the amp up and observe the voltage; it should be high, perhaps around 350-400V, but may be higher or lower.

Now turn the Standby switch on and check that you are still getting around the same HT voltage.  You should also check that you are getting minus 40-50V to chassis across C34, the bias supply.

Turn off and watch the HT decay so you get an idea of how long it takes to discharge to a safe level, say around 20-30V.

At this point you can remove the limiting lamp and power the amp directly from the mains, again watching the HT voltage (which will be somewhat higher).  All the valves should light up.  Note that the preamp valves may light very brightly at the bottom for a second or so - this is perfectly normal.  Check that both sides of each preamp valve lights up.  For any that don't try gently rocking the valve in its socket.  any that still don't light should be gently rocked out of their socket and both heaters checked with your ohm meter.  If any of the heaters tests open that's the end of the line for that valve.

The ECC83 is a 12AX7 (still made), while the ECC81 is a 12AT7 (not still made and rather harder to find).  Don't get these mixed up (write the type on the chassis next to each socket).  Mixing them up won't do any harm, but it will lead to some rather strange results.

Set the tone controls for half way, 5/10, Reverb to minimum.

Feed a signal in and signal trace it along its respective channel to the Phase Inverter (V5A&B) and to the grids of the output valves, pin 5.  Be careful, pins 3 and 4 have HT on them.

That's enough for now, should keep you busy for a while.

Note: a valve output stage must never be driven without a suitable load, speaker or dummy load, securely connected, otherwise the output transformer may well be damaged.
If you say theory and practice don't agree you haven't applied enough theory.

g1

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Re: sound city clean-up
« Reply #4 on: March 11, 2014, 12:23:06 PM »
  Don't connect your black probe to neutral, connect it to the chassis (ground).
Is it a three prong plug and is it wired correctly?
Don't worry about the pilot lamp if it is turning on and off with the power switch, as long as one side of the lamp is switched (hot side) you should be ok.
  Is the voltage selector set correctly?
  The caps that look bad may need to be replaced with higher voltage versions due to the increase in modern line voltage.
 The diode etc. off the bridge may be a revised bias arrangement.  Leave it in place till you have drawn it out and know what is going on with the circuit.
You have a dim pilot lamp, what is the AC across the PT primary and PT secondary (into the bridge) ?

ilyaa

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Re: sound city clean-up
« Reply #5 on: March 12, 2014, 03:59:07 AM »
quick question before i get into it (thanks again, Roly!):

the mains fuse (3A) tests okay (short) with meter but has a little bubble on it - that okay?

is that just a variety of slow blow?
« Last Edit: March 12, 2014, 04:02:28 AM by ilyaa »

DrGonz78

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Re: sound city clean-up
« Reply #6 on: March 12, 2014, 06:07:08 AM »
Yeah it looks like a little solder blob right in the center of fused wire right? It's all good no worry about that  :dbtu:
“A person who never made a mistake never tried anything new.” -Albert Einstein

ilyaa

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Re: sound city clean-up
« Reply #7 on: March 14, 2014, 04:48:35 AM »
ookay - so:

no voltages on the amp side of the PT.

i tried to untangle the mess thats been made of the wiring coming in from the mains. the mains plug and lead itself seems all good, with the earth wire properly grounded to chassis.

first issue is the mains switch:

the white wire from the mains plug (is that the L connection?) goes to a brown wire, via the mains switch, that then goes to the mains fuse.

the black wire from the mains plug (the N connection?) attaches directly to one side of the pilot light, and then goes to a blue wire, via the switch, that goes into the transformer (bottom of the primary,  i assume, at least per the schematic). should i rewire the pilot light onto the transformer side of the switch?

the other side of the pilot light goes to a green/yellow wire that then hooks up with an orange wire that disappears into the transformer (top of the primary, depending on the mains voltage switch?).

IF i take an AC reading with my DMM, i can only get 120VAC reading from the BLACK wire to chassis. nothing if i read the white wire to chassis. is that normal?

i get ~35VAC from the pilot light to chassis (???).

second (bigger) issue:

the reason im not getting anything inside the amp is because the mains voltage selector switch does not seem to be working. there is NO connection between it and the mains fuse. it looks like there should be, but either the switch broke internally or it needs to be turned. i have to say it scares me a little bit - seems like if i turn it the wrong way im misinforming the power transformer as to the voltage its going to get....would that be catastrophic? im not sure what to do. should i take a picture of the switch??

heres a description: the center pin connects to the fuse. around it there are 6 pins. a pair is connected together and connects via a fat yellow wire into the transformer. another pair is connected together and goes to the green/yellow wire that hooks up to the other side of the pilot light and ALSO to another wire that goes into the PT. NONE of these pins have a connection to the center pin (DMM tested)

i get 0VAC from the mains fuse to chassis.



« Last Edit: March 14, 2014, 04:50:45 AM by ilyaa »

DrGonz78

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Re: sound city clean-up
« Reply #8 on: March 14, 2014, 06:19:49 AM »
the white wire from the mains plug (is that the L connection?) goes to a brown wire, via the mains switch, that then goes to the mains fuse.
Yes that is Line voltage "Hot". That will be fused.

the black wire from the mains plug (the N connection?) attaches directly to one side of the pilot light, and then goes to a blue wire, via the switch, that goes into the transformer (bottom of the primary,  i assume, at least per the schematic). should i rewire the pilot light onto the transformer side of the switch?
Like G1 said don't worry about that at this point. The light attached there is not gonna cause any problems since there is a double throw switch. Do it later and focus on the main problem, the transformer.

IF i take an AC reading with my DMM, i can only get 120VAC reading from the BLACK wire to chassis. nothing if i read the white wire to chassis. is that normal?
Yes normal and safe :-) Neutral to ground should be Zero or very very close to zero volts AC. If you ever measure voltage from neutral to ground, then call an electrician.

second (bigger) issue:

the reason im not getting anything inside the amp is because the mains voltage selector switch does not seem to be working. there is NO connection between it and the mains fuse. it looks like there should be, but either the switch broke internally or it needs to be turned. i have to say it scares me a little bit - seems like if i turn it the wrong way im misinforming the power transformer as to the voltage its going to get....would that be catastrophic? im not sure what to do. should i take a picture of the switch??

heres a description: the center pin connects to the fuse. around it there are 6 pins. a pair is connected together and connects via a fat yellow wire into the transformer. another pair is connected together and goes to the green/yellow wire that hooks up to the other side of the pilot light and ALSO to another wire that goes into the PT. NONE of these pins have a connection to the center pin (DMM tested)

i get 0VAC from the mains fuse to chassis.
Yes take a picture of this and let the crew here see. This scares me too. Got to see where the switch is located and how it might be broken. Do not power it up until you get instruction from senior techs on the site.

Except that part about the mains fuse voltage... So on one side you have 120vac and 0vac on the other? Are you sure that fuse is good?
« Last Edit: March 14, 2014, 06:23:46 AM by DrGonz78 »
“A person who never made a mistake never tried anything new.” -Albert Einstein

Roly

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Re: sound city clean-up
« Reply #9 on: March 14, 2014, 07:28:59 AM »
WTF were they doing stuffing around with the mains side wiring I wonder?

Quote from: ilyaa
should i rewire the pilot light onto the transformer side of the switch?

Not urgent, but before you finish it really should be wired on the inboard side of the mains switch.

Quote from: DrGonz78
This scares me too. Got to see where the switch is located and how it might be broken. Do not power it up until you get instruction from senior techs on the site.

Too, too.

There are a few things about this mains side mess that I don't like, but in my world using green+yellow stripe for anything other than a mains safety ground is an absolute nono and likely to get you dragged out the back and severely beaten.  :trouble   There are not too many absolutes in the world of electrics, but in the 240VAC LEN part of the world this is one of them.  It seems to have been used as part of the mains wiring and as such is called a "death trap" because it is saying "I'm safe" when it isn't.

I'm with the Doc on this one, post pix, do not power up (we'd hate to lose you at this point after so much Win!).

In the meantime (with the amp well isolated) have a look at the resistance of the leads coming out of the mains side of the tranny.  The circuit shows a "C" for common, then taps for 105V, 115V, 225V and 245V.  These should have resistances to C that increase as their voltage increases, so you should be able to map these tappings to cable colours which is bound to be a help, and also prove the tranny primary is still intact.  At least on 115V mains an incorrect setting won't cause the damage applying 240V to a lower tap would cause here.

Always remember to check the mains plug is unplugged (not just switched off) and the HT/B+ line is fully discharged before diving inside.


{It sounds like an idiot has been at this one ilyaa, so be on the lookout for boobytraps, IED's, and pure idiocy; be suspicious, be very suspicious - double check everything, make no assumptions, particularly on the mains side}
If you say theory and practice don't agree you haven't applied enough theory.

g1

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Re: sound city clean-up
« Reply #10 on: March 14, 2014, 01:27:33 PM »
  It sounds like the white and black are reversed at the switch.
Black should connect to brown (to fuse) when switch is turned on, white should connect via switch to blue.
  The orange wire may be the transformer case, which is shown on the schematic as connecting to the green earth wire of the mains.

  With some pictures and transformer primary resistance readings, it should be easy to bypass the voltage selector switch for test purposes,  or hard wire for 120V operation.

ilyaa

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Re: sound city clean-up
« Reply #11 on: March 15, 2014, 02:05:16 AM »

EDIT: i removed the stupid links! for pix see this site: http://dropcanvas.com/nl0yl

Quote
  It sounds like the white and black are reversed at the switch.

i thought so, too....isnt the Black wire supposed to be L(ine)? remember, the 120VAC readings im getting are BLACK wire to chassis (meaning I am reading from the bottom of the pilot light to chassis). I get 0VAC from the white wire (mains fuse) to chassis.......????

Quote
using green+yellow stripe for anything other than a mains safety ground is an absolute nono

if you look at the pictures marked "Standby wiring" you will see that the green/yellow striped wire was cut from that bundle and possibly spliced (????) onto the pilot light side, where it now goes to the pilot light....its hard to tell whats going on and why. where SHOULD that wire (or a wire of that sort) go to/from in the normal amp world?

Quote
have a look at the resistance of the leads coming out of the mains side of the tranny

with the amp unplugged, i did this, but the results were not encouraging. first of all, there are no leads that are not connected to something - where do the other (245V/225/105,etc) taps go normally when they are not in use? the only transformer taps that were readily available were the ones attached to the mains selector switch and two blues ones (as in the file "PT 1"), one lighter blue and one darker. the darker one goes back to the mains switch - switching to the black mains lead and the pilot light - and the lighter one to the grounding selector switch (the file "grounding cap"). with my DMM hooked to the node where these two blue wires connect to a thick black wire from the PT (common point on primary winding?), i get 0 ohms/dead short to BOTH sides of the mains selector switch - the side that goes to a fat yellow wire into the PT and the side that goes to a thinner red wire that goes into the PT (that node also goes the green/yellow stripe that goes back to the pilot light).

i am getting a short between the fat yellow and the thin red wires going into the PT.

my hope, i guess, is that the mains selector switch is shorting things together, not the PT! remember, i get NO connection between the outer nodes of the mains selector switch and the middle pin (which goes to the mains fuse).

there are a few other pix that are more for your interest at this point, seeing as the amp has bigger fish to try. one other thing i will mention is that (as per the picture "fuse_to_ground") i disconnected the wire that goes from the fuse to the grounding selector switch, thinking that, since the amp has a 3 prong mains plug, i didnt need that. death cap, right?? why would an amp that was built stock with a 3 pronger have that, anyway?



« Last Edit: March 16, 2014, 03:46:34 AM by ilyaa »

g1

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Re: sound city clean-up
« Reply #12 on: March 15, 2014, 11:54:19 AM »
 I'm seeing a "file unavailable" message for the pictures.

Roly

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Re: sound city clean-up
« Reply #13 on: March 15, 2014, 12:19:30 PM »
I'm seeing a "file unavailable" message for the pictures.
Ditto.


The circuit shows all the primary leads except Common going to the voltage selector.  One side of the pilot light goes to the highest voltage tap, 245V.

It is quite unlikely that the tranny primary has turned into a melted zero ohm glob.

Quote from: ilyaa
since the amp has a 3 prong mains plug, i didnt need that. death cap, right?? why would an amp that was built stock with a 3 pronger have that, anyway?

That sounds reasonable to me; and yes, why?
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

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Re: sound city clean-up
« Reply #14 on: March 15, 2014, 03:27:08 PM »
I can see the pictures but being a google service maybe they require you to have a Google+  account or similar

 

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