Carvin BelAir - Low Voltage, Output, lot's of problems

[wcrankshaw]

Hi everyone - I've gleaned tons of info from this site over the past several months, and now I have a real question. I've got a Carvin BelAir 2x12. It has a new OT, new tubes, and the caps seem to be in fine condition. However turned on it has extremely low output even when the volume is cranked (on both channels). The Soak channel sounds like it should with both soak and volume cranked, but it's still very quiet. The tubes are all lit up, but I'll admit they look less bright than normal. I checked the high voltage winding of the PT, and on schematic it shows 350V, but in real life it is reading ~300V. Following that, all the filter caps are roughly 50V low, etc. The filament winding reads around 3.2V on schematic, but is more like 2.5V in real life. I'm fairly stumped as to the problem, other than that major voltage inconsistency. Any help you guys can provide would be incredible. Thanks.

[hasserl]

B+ voltage on those amps should be around 400vdc, give or take a few depending on the mains in your area.

Common problem areas on those amps are the fx loop jacks, which develop corrosion on the contacts. To check for that try inserting an instrument cable into the send and return jacks and see if the volume comes up to normal. Another problem area is the ribon cable connecting the small pcb on the back panel with the main pcb, either 4 wire or 8 wire depending on year/revision. The problem is with the connections of the cable at either end. I've replaced several cables, and on at least a couple I've had to ditch the quick connectors and solder the cables direct to the pcb. Also, if that ribbon cable touches the high voltage leads to the power tube plates you will get inductive coupling and a lot of noise, so keep them well apart.

If neither of these is the problem you need to do some basic troubleshooting, including a full set of voltage measurements at all tube plates, screens, etc. Check/set the bias ( this might help: http://members.dslextreme.com/users/...tageSeries.pdf )

[wcrankshaw]

Thanks a lot for the quick reply. I'll hopefully be able to check on those possibilities tonight. So, am I reading it right that the lower B+ I'm measuring isn't actually a potential cause of the problems?

If the "easy" solutions don't fix my problem, I'll try to get a full set of measurements posted ASAP. Thanks again.

[hasserl]

Sorry if I mislead you, but if the B+ voltage is only ~300VDC, then something is wrong. I've been inside dozens of those amps, if not into the hundreds, and every one the B+ has always been in the high 390's to the low 400's. It's possible the PT has been swapped out. It's possible the bias is set so hot that the PT can't keep up with the curent demand. Start with a complete voltage check throught the power supply rail, check the bias voltage and current.

[wcrankshaw]

Power Section:
V6:
1-
2-16.7V
3-
4-
5-
6-
7-386V
8-
9-378V

Filter Caps (positive side)
C40-344V
C41-358V
C42-378V
C43-386V
C44-386V

Current across standby - 102mA
Voltage Drop at R43 - 6V


The rest of the power tubes had the same readings. Are there more points I need to be measuring?

[hasserl]

Go to this link and save a copy of the schematic for that amp (it's called a VT50, the same amp as the BelAir): http://www.carvinmuseum.com/pdf/amps...r%20REV-E1.pdf

Check voltages at ea tube plate at least, also check the filaments, note that on the first two preamp tubes the filaments are DC, not AC.

You just want to make sure all the voltage are where they are supposed to be. According to what you already posted they are quite a bit higher than what you first posted, though they are just a little low compared to what I've seen. But what's the main voltage? If it's low that would explain why the B+ is a little low.

[wcrankshaw]

I just measured the line, and the top is reading about 86V with the bottom reading about 30V

The number I supplied originally for the HV was where the HV winding connects to the circuit board. It is reading 306V

V1
pin1 - 142VDC
Pin2 - 0.5VAC
Pin3 - 1.37VDC
Pin4 - 2.5VDC
Pin5 - -2.5VDC
Pin6 - 150VDC
Pin7 - 0.5VAC
Pin8 - 1.26VDC
Pin9 - 1.5VAC

V2
1 - 145VDC
2 - 0.1VAC
3 - 0.88VDC
4 - -2.5VDC
5 - -2.5VDC
6 - 243VDC
7 -
8 - 2.1VDC
9 - 2.5VDC

V3
1 - 0.5VDC
2 - -332mVDC
3 -
4 - VDC
5 -
6 - 1VDC
7 -
8 - 165VDC
9 -

V4
1 - 157VDC
2 - 45mVAC
3 - 1VDC
4 - 2.6VAC
5 - 2.6VAC
6 - 234VDC
7 - 0.3VAC
8 - 1VDC
9 - 2.7VAC

V5
1 - 254VDC
2 - 52.5VDC
3 - 59VDC
4 - 2.6VAC
5 - 1.3VAC
6 - 260VDC
7 - 53VDC
8 - 60VDC
9 - 2.75VAC

[Zouto]

Hi, I have similar troubles with my BelAir
http://music-electronics-forum.com/t18469/#post151699,
could you fix your amp in the meantime, which parts were the culprits?
Thanks for reply
Zouto

[wcrankshaw]

Unfortunately I haven't. Mine basically sounds normal but is really quiet, to the point where the overdrive channel totally maxed out is only slightly audible. I'm really not sure what the problem could be since this thread sort of died.

[pacAir]

Puit the amp on a Scope...

FWIW (from another post about Belair low-output problems):

I had a Carvin Nomad (same circuit as the Belair, smaller 1x12 chassis) that had very weak output with no apparent physical problem. When I monitored the output into a bench load on a scope it became obvious why it was so weak... all of the output power was being expended on amplifying an ultrasonic (40kHz or so) oscillation!

One problem that has surfaced with various generations of this design is that the various preamp input/output signals (from the front-panel preamp board) are routed to the the rear-panel connector board via computer ribbon cable(s) right over the output tube sockets! If the wire dress of these ribbon cables isn't just so, the cable's relative position to the output board can cause these ultrasonic oscillations to occur.

It seems the factory is aware of this as the latest schematics show an "optional" snubbing capacitor (56pf or 62pF I believe) be placed in the output stage to kill such instability. In addition to installing this cap, I also rerouted the ribbon cable as far away from the output tubes as I could and tie-wrapped them to the Reverb cables to keep them from drifting around. These two simple changes stabilized my Nomad and it should no further propensity for unwanted oscillations.

I was partially successful just re-routing the ribbon cable but it was the snubber capacitor that sealed the deal. The amps now sounds fabulous!


Steve