Help with Crate Vintage Club 50

[arjay]

I have a 1992 Crate Vintage Club 50 head that just faded out mid-song and now is "dead". Only a very faint hum through the speakers regardless of the control settings. I have verifed that the fuses are intact and used known good tubes and do not see any burnt components or obvious loose solder joints. I have the schematics but am not very experienced at troubleshooting. For example, do I check test points between chassis ground and the TP? If so, TP19 is within specs but TP18 is close to 0V! Any ideas that I could check out?

Thanks,
Arjay

[Garydean]

Are you sure you are measuring with your meter set to DC volts?
Yes measure to chassis.
I would remove the fuses and test them with your meter.

[oc disorder]

If you use the search function on this forum you will find some discussions
on the vintage club 50.
On the second page there is a link to BlueGuitar who has the schematic.
http://music-electronics-forum.com/s...p=85704&page=2
Ahh just re read and I see you have the schematics.
The first things I would check would be the Zener diodes and the associated resistors.

These often overheat and create cracked/dry solder joints.
D13 D14 R62 R63 These components supply the plus and minus 10volts.

Now having re read your post from what you say if TP 18 is close to zero volts R52 (470 ohm 5 watts)is either open circuit or getting bloody hot !

Be very careful these are high lethal votlages.

The ht supply comes from the standby switch throu the fuse then to the centre tap of the output transformer where R52 supplies the screen resistors for the output tubes and goes on to supply the phase inverter V4 via R32 and then via R31 the rest of the HT to the pre-amp section.

Now if C36 was shorted for example that would drag down the whole HT supply making R52 get very hot !

My guess is R52 acted like a fuse and is open circuit and the most likely cause is an internal short in one or all of the output tubes.
But ... if it is getting hot take out the output tubes then measure between TP 18 and ground again.

Something is drawing too much current but not enough to blow the fuse.

Even new tubes can be faulty.

Any fault in the pre-amp is more likely to burn out R31 or PI R32 .

A new set of EL84 tubes and a replacement 5 watt 470 ohm resistor should fix it.

Funny how the fuse often doesn't blow... maybe R52 died of old age !

[arjay]

Thanks for the information, I will check that out tonight. One other thing that I forgot to mention: All the tubes glow and when I used the EL84s from my Peavey C30 in the Crate VC50 it still did not work but when I put the EL84s back into the C30 one of them turned bright red, blew, and then popped a fuse! Coincidence, or did the Crate VC50 somehow damage it? Does that add more info?

Thanks,
Arjay

[oc disorder]

"Does that add more info?"
Yes it does I'm now suspecting the fault started with bias failure and ended up with the 5 watt 470 ohm resistor burning out after.
All the tubes will glow with the amp on standby (no HT) as the heaters are supplied from a separate winding (H1-TP24 , H2-TP25) approx 6.3vrms.

The bias comes from the section of the power supply I posted earlier TP 30
should be - (minus) 15.7 volts approx . It's labeled B - and notice that the capacitor (electrolytic) C49 10uF 35volts has its positive connected to ground.

If this capacitor is shorted or if resistor R77 is open circuit (100 ohms)
there will be no bias .
I wouldn't plug in any more output tubes 'till the fault is found.

From the above point the bias passes through JO1 jumper to test point 17
then through the respective 220 k resistors and the 2K7 resistors to pin 2
on the output valves (EL84's).

With the valves unplugged you shiould be able to measure aprox 14 or 15 volts there (minus).

If its a high positive voltage either c27 or c28 could have failed.
The speaker didn't get accidently unplugged during use did it ?
This could cause an output trans fault but lets hope at this stage its only
a resistor failure.

[arjay]

oc disorder,

Thanks for taking the time to explain all that. I have finally had time to do some more troubleshooting and you are right about R52 - it is open. Next I will put the board back in the chassis and do the powered on tests that you mentioned.

Thanks again,
Arjay

[arjay]

Okay, now I am confused. I put the amp back together to make some powered on tests and left all tubes out (should I leave the preamp tubes in)? I discovered that my version of this amp does not have C49, R77, or J01. So I made all the tests that I could: TP30 = -15.24V, TPs 13, 14, 17, 18 and all EL84 pin 2 = 0V, TP19 = 386V. And R52 is open. I guess that I need to remove the board and trace out how B- gets from D9/11 to TP17. Does that make sense?

Thanks,
Arjay

[oc disorder]

Maybe someone else could help here who has inside knowledge of the crate VC 50.I'm just wondering if this is a cathode bias set up ?
To keep it simple all the pin 3's are tied together and go to ground via a wire wound resistor and capacitor.
Trace pin 3 and see if it goes directly to ground.
If it is self biasing (cathode) there would be no connection between pin 2 and the - 15v supply.
Also you wont get any -12 to -15 volt readings on pin 3 unless the valves are plugged in (or a valve).
The preamp valves can remain out while you are sorting out the output.
For example looking at the VC-3112 schematic you can see R43 and C29 which provide the cathode bias.
The junction of the 2 x 220K resistors on the VC 50 go to B- wheras on the VC-3112 they go to ground.
If yours has no JO1 maybe it has a JO2 ?
If it is cathode biased there will be a large ceramic 10w resistor for it with a capacitor (not as physically large).These will connect between ground and pin 3. TP 17 is also a clue if you compare the two diagrams.
Maybe take a picture of your board as it may be helpful.
Its possible the bias is ok regardless of which method it uses.
But worth checking before putting in good tubes.
"when I put the EL84s back into the C30 one of them turned bright red, blew, and then popped a fuse! Coincidence, or did the Crate VC50 somehow damage it?"
Thats what concerned me but it could just be coincidence 'cause only one "blew".
Could sacrifice one.... plug it in and quickly measure pin 3.
If there is cathode bias you should get a reading between there and ground.
Oh nearly forgot you must replace R52 first or no bias will develop.

[arjay]

Well, I replaced R52 last night and now TP18 reads 400V and TP19 reads 404V - a little higher than spec but nothing got hot or smelled funny so I put everything back together and the amp seems to work fine. I played it at low volume for about 30 minutes (kids were sleeping) and it seems back to normal. Actually, it seems louder than before but that just might be since the house was quite and I have been playing my PV C30 lately. Should I be worried about the ~400V around R52 or just let it go? Thanks to everyone for all your help and patience.

Arjay

[rockon1]

So is that a screen resistor that was taken out by a shorted tube? Bob

[oc disorder]

"Should I be worried about the ~400V around R52 or just let it go?"
It would have been like that before its only about 20v more but
if your new valves suffer a premature death I would consider increasing
the value of the screen resistors maybe 200 ohms instead of 100.
I notice the new Laneys come equipped with TAD EL34M's which are
tested Russian 6n14n-EP's .Think these are a little tougher.
Increasing the screen resistors may make it sound a bit lame though.
some discussion here
http://music-electronics-forum.com/a...hp/t-1601.html
Anyway if its working ok enjoy it !

"So is that a screen resistor that was taken out by a shorted tube? Bob "

It was a HT dropping resistor that led to the screen resistors and then the pre-amp.
See attached modified diagram below.
Rockon1 - I see you were/are? a fan of the 6n14n-EP

http://www.thegearpage.net/board/archive/index.php/t-457234.html

Is that still the case ?

Link to Russian 6n14n data sheet
http://lib.store.yahoo.net/lib/thetu...P14P-EL84M.pdf
hasserl 's write up on EL34's
http://members.dslextreme.com/users/...Comparison.pdf
Russian suffix's
http://www.jogis-roehrenbude.de/Russ...re_rue_dat.pdf

[rockon1]

Yes for thier durability. Nothing Ive tried comes close. Ive only used the Sovtek EL84M's. The Russian 6n14n-EP's are older stock,same specs. Ive read these sound a bit better. At any rate I run 8 EL84M's in my Classic 50/50.They last a long time compared to 6Bq5's and with 8 tubes that is a good thing! Also nothing Ive put in my Palomino stands up to its 13 + watt idle dissapation without dying a premature death except the EL84M's! I went thru 3 sets of tubes in it within 2 years of light duty playing. I switched to the M's last year and havent had a problem since. No not the most toneful but rugged! The trade off is worth it in these cases. Next batch Im trying the 6n14n-EP's . If they are just as durable and sound better thats a real win-win! Bob

[perreko]

help!!same problem with some more open resistors

hi from canary islands , my vc50 just died same way last night,just before starting the very first rehaershal with a new band ...damn ....Well,i went desperated and google came to help me with this thread. Im not any electronic technician, but got basic knowledge of how to use a solder and a ohm meter,so opened the amp and discovered that open R52,but also open are R31,R32 and R22...mmmm and i just cant find those R77 and C49 also mentioned earlier in the post.....so im in a no way out alley now.....im guessing that if i just replace those opened resistors and even the whole amp and preamp valves im not really fixing the thing properly....ideas plz?
thanks in advance!

[perreko]

ups

sorry guys, i need to rectify,those 21 32 and 33 resistors werent open, that was cause a bad meter i used for that measure,now got the right one, they are ok...someone directed me to check C26,could this one be who broke R52...mmm sorry for the previous post ....ya, you may think why i dont take the amp to a pro service...well, im skint thanks again, any help greatly aprecciated!

[BearCWY]

Have you tried plugging in a single footswitch in the footswitch jack. the VC 50 series has an inherant problem with this jack. this can cause antwhere between fading in and out to loss of sound. basically if you are getting a humm out of the speaker but no sound from a guitar this is the first and easiest thing to try.