Newbie problems with a ST-70 rebuild VTA70 driver board / bias display: no sound

I assume this marked on the board somewhere? I am not home at the moment. Where am I putting my two probes to measure this voltage? Does the red probe go to the -50VAC point on the PCB and the black to ground?
Across pins 2 and 7 – 6.0 – 6.8 volts AC Your directions state to have the black probe to ground. How would I measure "across" these two points if the black probe is grounded?

Bob Latino wrote:Do a full voltage check on the amp and see if any voltages are way out range ? Many times a voltage reading that deviates greatly from what it should be can give you a clue as to what is wrong with the amp ? See chart below ..
Bob

Voltage readings

    * Note # 1 – Make sure you have your meter set to AC or DC as mentioned below. Ground is any point on the chassis frame. Place the black or negative probe on the chassis and the red or positive probe on the point mentioned. All tubes should be plugged in, inputs shorted or connected to your preamp, speakers connected and no signal should be running through the amplifier. Be careful not to cross two pins with the positive probe !
    * Note # 2 – Readings very slightly above or very slightly below the range are not normally a sign of a problem. Line voltages vary slightly throughout the country. Differences in the 5AR4 rectifier tube can cause variations also. The use of a solid state rectifier in place of the 5AR4 may cause some voltages to be higher.

                  5AR4 –               Pin 2 to ground – 420 - 450 volts DC
                                            Pin 8 to ground – 420 - 450 volts DC
                                            Pin 4 to ground – 355 - 380 volts AC
                                            Pin 6 to ground – 355 - 380 volts AC
            Any EL34 – Pin 1 or pin 8 to ground – approx .400 volts DC (depends on bias setting)
                             Across pins 2 and 7 – 6.0 – 6.8 volts AC
                                          Pin 3 to ground – 400 - 435 volts DC
                                          Pin 4 to ground – 400 - 435 volts DC (2 or 3 volts higher than pin 3)
                                          Pin 5 to ground – minus 30 to minus 40 volts DC
                                          Pin 6 to ground – minus 30 to minus 40 volts DC
                                          Pin 8 to ground – approx .400 volts DC (depends on bias setting)
               Quad cap –-- Section # 1 (square symbol) –------ 420 – 450 volts DC
                                                    Section # 2 (half circle symbol) --- 420 – 445 volts DC
                                                    Section # 3 (NO symbol or a circle symbol) ------------- 420 – 445 volts DC
                                                    Section # 4 (triangle symbol) ------  380 – 420 volts DC
                        Central 12AU7 – Pin 1 or pin 6 to ground > 140 – 165 volts DC
                        Two outer 12AU7’s – Pin 1 or pin 6 to ground > 250 – 280 volts DC
                           Any 12AU7 – across the two incoming heater wires > 6.0 - 6.8 volts AC
                         “-50 VAC” on driver board > 48 – 55 volts AC
                 “B+” on driver board > 370 – 410 volts DC

Bob Latino wrote:RE: > “-50 VAC” on driver board > 48 – 55 volts AC =Where is this location on the VTA70 board?" This point is where the red/black wire from the power transformer connects to the driver board. If you have bias voltage then you will have the proper voltage at this point."

Since you have sound on one channel try these 3 items in this order ..

1. First make sure that your audio source is working on the missing channel.

2. Check the interconnect and speaker connections on the missing channel.

3. With the amp upside down with the bottom cover off and the amp playing music at a low level, use an insulated object like a plastic pen to tap lightly on all the solder connections on the missing channel. If the sound comes back on even for a split second, you may have found a bad solder connection. Turn the amp OFF and resolder that connection.

Bob

Bob Latino wrote:There is also the possibility that the output transformer for that channel is bad ? Make sure that the amp is OFF when you test all the resistances below.

1. To test the secondary (speaker side) of the output transformer check to see if there is continuity between the negative terminal and the 4, 8 and 16 ohm terminals. You should get a very low resistance on all three terminals - probably less than 2 ohms.

2. To test the primary side check that you have continuity and some resistance between the BLUE and the BLUE/WHITE wire and the GREEN and the GREEN/WHITE wire on that channel.

3. Also on the primary side test the RED wire for that output transformer that is now connected to the quad cap to the other primary wires > the BLUE, the BLUE/WHITE, the GREEN and the GREEN/WHITE. See if you get some resistance from the RED to those other four wires.

Bob

Bob Latino wrote:At this point you should probably consider taking the amp to a place in your area that works on tube guitar amps. They should be able to check out your amp and give you an answer as to why that one channel does not work ?

Bob

New2Tubez wrote:Hopefully not a side track as I don’t know enough but does your board have the LM336’s? I had one of these go bad on my ST120. I heard a rustling sound and suddenly one channel had greatly reduced volume. Obviously Bob is the authority here  but it was just a thought…

fmzip wrote:

New2Tubez wrote:Hopefully not a side track as I don’t know enough but does your board have the LM336’s? I had one of these go bad on my ST120. I heard a rustling sound and suddenly one channel had greatly reduced volume. Obviously Bob is the authority here  but it was just a thought…

LM334's on mine I believe

Bob Latino wrote:It is hard to tell but is that .10 uF interstage (orange drop) cap on the bottom of the driver board (on the right side of the photo) set into the wrong two board eyelets ?

Bob