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Post subject: Re: 93 blues deville fried diode- long winded..
Posted: Fri Feb 11, 2011 9:32 am
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Thanks Shimmilou,
I did have the meter set to dc @ 1000v range... so mmm. Don't think my amp likes me any more.
Also I did my previous test and this test with tubes in (I didn't know, thought they needed to be in) hope I haven't done them in, but I guess it is a possibility :cry:

Next time testing I'll make sure the tubes are out..!!

As this thing's getting a bit narcy, I might go and see a friend of mine who's built his own tube stereo amp and is into such things. Get him to work with me... I hate asking, but I'm sure he would help me....

Ok mate, stay tune for the next installment of 'Richies recalcitrant Deville' (or is that devil :twisted: )

All the best
Rich

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Post subject: Re: 93 blues deville fried diode- long winded..
Posted: Fri Feb 11, 2011 11:53 am
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Ricardo, keep persisting mate- you'll get there.

This is a bloody interesting read following your story- as I was going down the pages I was wondering "I wonder if the same will happen to my amp in years to come...?"

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Post subject: Re: 93 blues deville fried diode- long winded..
Posted: Fri Feb 11, 2011 2:11 pm
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Richardo,


You may have already been told this, but be SURE you have some load on the secondary of the output tranny (hook-up a speaker). Do not run the amp without loading the output. Keep the volume at zero. Do this even if all tubes are out of the amp.

Ok... if I gather correctly, you have zero voltage at TP33. Zero voltage at TP26. B+ reading at the (+) end of cap C34 is around 470 VDC. Follow Shimmy's suggestion and check Z (468 VDC), Y (413VDC), & X (364 VDC). They should all read to (+/- 10% of the nominal values... higher with tubes out). If B+, Z, Y, and X are all within range and you have (+/-) 16 VDC off the PS supplying the IC chips... then you may have some grounding, board tracing, or other component(s) downstream issues. Best left for future posts and head scratching. :lol:

The way amps work is they need a power supply giving them required voltages at certain points. Kinda like a reservoir of electrons upstream from the components. The electrons start from the power tranny and work their way down to each component, passing critical measuring points. If you don't have the voltages (at B+, Z, Y, & X and the -/+ 16 VDC)... you won't have a proper working amp. So these "higher reservoir" readings are needed first, before venturing onwards. Because if you don't have proper B+, Z, X, or Y you prolly need to look upstream from the read points to find the problem(s).

Having several seemingly non-related zero voltage readings at two points (TP26 and TP 33) is kinda worrysome. Anyhow, get those initial readings and post your results.

Good luck with all this!


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Post subject: Re: 93 blues deville fried diode- long winded..
Posted: Sat Feb 12, 2011 3:19 am
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Thanks BMW for your support here....
and still going Blertles.. :shock:

Ok, had my friend come around and was great to have someone physically giving me the G.O on what i can do before even turning on the power.
Checked for continuity all around the areas I had done repairs.. ie the caps... all good. Checked speakers, input power resistance, and OT. All looked good.
Checked continuity around the power tubes from pin 4 on V4 to pin 4 on V5.. all good.

Ok, using his variable transformer we then checked power to pin 3 on V4 (B+)... no power at all even when we went back up to full 240v. Checked other pins and we did have some power but no real checks yet as he doesn't like the idea of using my lowly meter..no alligator clips.
I've really learnt something today, keep all hands away from the amp... one's ok but if you don't have to, keep them all out of the thing...
I know you guys have stressed, safety safety safety.... so it was good to have all this reinforced to me.!

We're thinking at this stage the set of diodes from CR14- CR17 will need to be replaced to eliminate these from the equation.
Hopefully he has these 800prv 1amp diodes. 1N4006
Any way he is coming around tomorrow and see where we go from here.
At least we'll be able to check all the other points z,y,x,C-

Keep you all posted.. feel more confident now. Must be getting close!
Rich, over and out for now.

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Post subject: Re: 93 blues deville fried diode- long winded..
Posted: Sat Feb 12, 2011 7:41 am
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But, you said you were getting good B+, correct? You could not get good (~470 VDC) if diodes CR14-17 were fried. However... pin 3 of V4 (6L6GC) is the anode. And no voltage reading here means that somewhere between that B+ point (off C34) and the anode of the 6L6GC you may have an issue. Is there a buss line that connects B+, Z, X, and Y from the PS portion of the amp to the circuit board carrying the tubes? Look at it carefully for good connection and damage.

There seems to be plugs (SP1? and SP2?) and a choke (L1) between B+ and Z, Y, and X. If you can't get proper readings off Z, Y, & X... start looking in this area of the power supply. I have a feeling something funky may lie in this area, as you also have no reading at TP26. Which seems to activate the 12AX7 (V3) from the Y point of the PS. Could still be a buss line issue.

Just a WAG... but if the buss lines seems ok, you may want to really check those caps you replaced. Proper polarity and good contact to ground. Also lift one leg of the choke in the main PS and check for continuity across it (resistance... with amp OFF). Check all circuit tracings within the power supply area for damage. It helps to use a magnifying glass, as even the tiniest crack in the tracings can lead to problems.

Good luck with all that. One other note... best to drain the filter caps prior to fiddling around with areas near the power supply portion of the amp. WITH AMP OFF I take a line that has a 500 ohm, 5 watt resistor with two alligator clips hooked to the ends of each line. Clip one end to B+ and the other to the chassis. Leave the bleeder resistor on while working with amp off. BE SURE TO REMOVE THIS BEFORE FIRING UP THE AMP AGAIN.


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Post subject: Re: 93 blues deville fried diode- long winded..
Posted: Sun Feb 13, 2011 3:21 am
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Thanks bMW...
today was a better day... well narrowed down the dc side.
Found out why we didn't have power to B+ yesterday, didn't flick it out of standby derrr... :shock: My friend wasn't happy testing this way with my non alligator clip meter, and was in a bit of a hurry to get home after all our prelim tests.

Ok, today checked all dc high voltage points.. B+, Z, Y, X & C- all checked out good with no tubes in (non loaded) then with tubes in. From the + sides of the specific caps and from the tube pin sockets
B+.. using pin 3 on first V4 and then V5 were 484 dc.
Z... was a little less
All the others came up and near spec.. - voltage at C-

Don't ask me all the test we did but we floated around all the areas...
my friend works for our Telecom and has built a couple of amps... he said all was ok and up to spec.

All the caps I've replaced are are all ok...... we used these areas for test points.

Found out the crackle/ rustling I had was to do with one of the JJ's 6L6 power tubes.. replaced these with my old Sovtecs and the noise went away.
But still no sound.

Just rechecked TP26 and still no voltage.. resister there checks out ok. Checked the 1w 6.8k resistor, it's ok ,checked R29 resistor ok...

Ok, I guess I need to check the Choke as mentioned.... mmm, excuse my total ignorance.... is this the small transformer near one of the large ones(OT).. ??

I'm sorry for being such a pain in the arse.... and sorry for this saga.. :roll:

All the best
Rich

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Post subject: Re: 93 blues deville fried diode- long winded..
Posted: Sun Feb 13, 2011 6:47 am
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If you are getting good VDC off Z, Y, and X... the choke (L1) is prolly good. No voltage at TP26 may indicate no current flow through V3. Try measuring the point with this 12AX7 inserted.

When you insert V3 (12AX7)... does the tube light up, as usual? Perhaps a heater line issue? But, if the tube lights up... maybe better start looking at those tracings, if you still get no TP26 voltage.

The voltage off the supply for the chips is ok? (+/- 16 VDC)? Those zener diodes (CR22 & CR23) are ok? Your amp does seem to have two IC chips inline with the gain stage/signal path---(U1A & U1B). Both are TL072 chips and sit near the pre-amp out/amp-in part of the circuit. So they need to be charged & working properly.

You did change out the electrolytic caps in the (+/- 16 VDC) power supply? (C38,39, 43,44)?

There may also be a jumper between the pre-out/amp-in plugs. If so, is it plugged in?

Basically, if B+, Z, Y, and X are all reading within VDC range. And you are getting voltage to all the anodes of the power tubes (V4, V5)--- you should get noise with the power tubes plugged in & speaker connected. Hopefully, a nice quiet hum. If not... the problem may lie within the circuitry around the power tubes and/or the output tranny and/or connnections with the speaker.

If the power tube and power section of the amp is ok.... try plugging in the phase inverter (V3). Measure voltages and listen for problems (static, buzzing, etc) through the speaker. If NOW you have a problem, you've nailed down the possible problem area. If all is ok with the PI in...

Plug in V2 (another 12AX7) and start the measuring process around this area of the schematic. The idea is to isolate the stage that is giving you problems.

Unfortunately, you could have several areas causing the problem(s). So go one-by-one to isolate the potential problem within each stage. You are basically starting from the power source (power supply section) -> through the power tube section -> through the phase inverter -> through each gain stage (V2, V1). Checking voltage points, circuit board issues, tracings, and IC chips in each area.

Then, do the same with the EFX sections of the amp (lower left portion of schemo).

Good luck. Keep us posted!


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Post subject: Re: 93 blues deville fried diode- long winded..
Posted: Sun Feb 13, 2011 11:28 am
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I agree BMW2002Ti, great approach. I'll just add, try different new tube in V3, and check TP26 again. If all of the DC power supplies are good (including TP26), plug in only V3 and both output tubes, plug the guitar directly into the "Power Amp In" and see if you have sound. That will bypass the preamp section, including the volume and tone controls. :idea:

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Post subject: Re: 93 blues deville fried diode- long winded..
Posted: Sun Feb 13, 2011 11:23 pm
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BMW2002Ti wrote:
If you are getting good VDC off Z, Y, and X... the choke (L1) is prolly good. No voltage at TP26 may indicate no current flow through V3. Try measuring the point with this 12AX7 inserted.


Yes changed out JJ's for an old 12AX7.... still no power @ TP26.Also checked TP27 @ CR24 and no voltage here just a click through the speaker on contact

When you insert V3 (12AX7)... does the tube light up, as usual? Perhaps a heater line issue? But, if the tube lights up... maybe better start looking at those tracings, if you still get no TP26 voltage.

Yes the V3 tube lights up... the same as in V1, V2. But alas, still nada @ TP26


The voltage off the supply for the chips is ok? (+/- 16 VDC)? Those zener diodes (CR22 & CR23) are ok? Your amp does seem to have two IC chips inline with the gain stage/signal path---(U1A & U1B). Both are TL072 chips and sit near the pre-amp out/amp-in part of the circuit. So they need to be charged & working properly.

I checked TP11 & TP12 on U1.... I had nothing in AC voltage... well so my meager meter said!

You did change out the electrolytic caps in the (+/- 16 VDC) power supply? (C38,39, 43,44)?

Yes every capacitor was changed out, C38,39, 43,44 included .

There may also be a jumper between the pre-out/amp-in plugs. If so, is it plugged in?

No jumper here.

Basically, if B+, Z, Y, and X are all reading within VDC range. And you are getting voltage to all the anodes of the power tubes (V4, V5)--- you should get noise with the power tubes plugged in & speaker connected. Hopefully, a nice quiet hum. If not... the problem may lie within the circuitry around the power tubes and/or the output tranny and/or connnections with the speaker.

Yes getting a nice quiet hum with power switched to on and on standby... flick off standby and the hum increases slightly. Pretty much as is normal

If the power tube and power section of the amp is ok.... try plugging in the phase inverter (V3). Measure voltages and listen for problems (static, buzzing, etc) through the speaker. If NOW you have a problem, you've nailed down the possible problem area. If all is ok with the PI in...

Plug in V2 (another 12AX7) and start the measuring process around this area of the schematic. The idea is to isolate the stage that is giving you problems.

Basically did all this yesterday, thought the rustling noise was the phase inverter, changed it out for one of the old 12Ax7's, noise still there, then swapped out one of the power tubes @ V4 for the old 5881wxt and the rustling noise went away and the amp was humming nicely..... thought we had the prob sorted... unfortunately no

Unfortunately, you could have several areas causing the problem(s). So go one-by-one to isolate the potential problem within each stage. You are basically starting from the power source (power supply section) -> through the power tube section -> through the phase inverter -> through each gain stage (V2, V1). Checking voltage points, circuit board issues, tracings, and IC chips in each area.[/quote]

I have measured the other chips ...as follows.
U2 TP15 0 vac
TP16 0 vac
TP17 .1 vac
TP18 0 vac
U3 TP30 2.05vdc (clean) -.04 vdc (drive)
TP31 -15vdc " 14.1vdc "
TP32 15vdc " -14 vdc
U4 didn't get a reading here..

Would this mean DC voltage to and with the PC4560 chips is ok (not sure on U4?)
And that I have a prob with AC voltage to and or with the TL072's and I will need to replace these two chips, or should I be looking further upstream for the time being to eliminate any other things before jumping in to buy these chips.. ??
Orrr, could the 3KV 200ma diode be toast and causing this lack of voltage to TP26??

Thanks for this last seek and find mission, I'm ready to accept the next mission chief!! :P

All the best for now
Rich "

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Post subject: Re: 93 blues deville fried diode- long winded..
Posted: Sun Feb 13, 2011 11:29 pm
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Richardo, this is making for good reading. You are not hassling anyone here.

You take that amp by the horns mate, and you tell 'em who's the boss, You're an absolute winner :wink:

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Post subject: Re: 93 blues deville fried diode- long winded..
Posted: Sun Feb 13, 2011 11:32 pm
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shimmilou wrote:
I agree BMW2002Ti, great approach. I'll just add, try different new tube in V3, and check TP26 again. If all of the DC power supplies are good (including TP26), plug in only V3 and both output tubes, plug the guitar directly into the "Power Amp In" and see if you have sound. That will bypass the preamp section, including the volume and tone controls. :idea:


Hi Shimmilou, good you guys are hangin' in with me :)
Still nothing in the way of voltage with TP26.. see my reply to BMW..

Cheers though!
Rich

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Post subject: Re: 93 blues deville fried diode- long winded..
Posted: Sun Feb 13, 2011 11:42 pm
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Blertles wrote:
Richardo, this is making for good reading. You are not hassling anyone here.

You take that amp by the horns mate, and you tell 'em who's the boss, You're an absolute winner :wink:


:lol: :lol: :lol: :lol: :lol: :lol:
Thanks Blertles!!!

Talk about it.... :lol:
I guess it's the old adage 'If it aint broke don't fix it'
but then.... I'll add...
"When she breaks, she'll break real good!!!"
Stay tuned, same bat time, same bat channel

All the best
Rich

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Post subject: Re: 93 blues deville fried diode- long winded..
Posted: Sun Feb 13, 2011 11:46 pm
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richardo,

Forget about the AC voltages, you'll get nothing relevant with no AC signal into the input jacks. Just continue checking your DC voltages, sounds like you are narrowing it down to a possible power supply problem (which you had to begin with). :idea:

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Post subject: Re: 93 blues deville fried diode- long winded..
Posted: Mon Feb 14, 2011 12:04 am
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Richardo,

Sorry if you answered this already... but are you getting rock steady (+ & -) 16 VDC off the part of the PS supplying the IC chips? The same circuit that diode CR20 initial blew, in the first post? It's not uncommon for the zener diodes (CR22 & CR23) to blow, which can lead to damage up & down stream of the blown zeners.

My WAG is that either U1A on U1B (TL072 chips) or circuit tracing could have been damaged when CR20 blew. No signal maybe getting through to V3 because of this. Therefore, no TP26 voltage, which maybe dependent on signal onto the grid of V3 (12AX7 PI).

JOC... did you have any symptoms with your amp... just before the CR20 diode popped?


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Post subject: Re: 93 blues deville fried diode- long winded..
Posted: Mon Feb 14, 2011 8:30 am
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shimmilou wrote:
richardo,

Forget about the AC voltages, you'll get nothing relevant with no AC signal into the input jacks. Just continue checking your DC voltages, sounds like you are narrowing it down to a possible power supply problem (which you had to begin with). :idea:


I hear you Shimmilou, I understand what you are saying..... but ultimately my friend and I went through all the possible dc points to get readings and they came up to spec...
If I was getting good dc every where then ,like you said, I should be able to connect into the 'power amp in' bypassing the pre amp stage.... but this was is a no go...

I'm going to have a break from this for a couple of days till some parts come in... I have a couple of resistors that aren't quite up to spec so I will replace these as well as the ic's and those zeners at CR22, 23 (as BMW suggested).... all I can say is it can't hurt. I'll just about replace the whole board... lol

I hear your frustration with me...
I know you guys would be able to solve my prob in no time... but I'm a total novice here, trying to get my head around the schematics ,dealing with tiny test points and high voltages all around can get a bit intimidating. It's one complicated piece of kit for sure... and a huge learning curve.... my brains about to fry at the mo. Need some head space.

So I'll check back later in the week or early next with what I have done and found..

All the best for now and thanks!
Rich

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