Just bought an injured Sound Valves preamp. What to check?
- Thread starter rudedogg
- Start date
I'm pretty sure you are too. In that respect, the VTP100 is the same as the VTP101.
For this...you're going to have to follow the PCB traces:
On the transformer primary side, you have taps for either 110/220 selected by the switch. On the secondary side, you have 2 sets of taps, one for the high voltage (HT), and one for the heaters. The HT supply gets rectified to DC thru the four discrete diodes (D1-D4) while the heater supply gets rectified to DC through the Bridge Rectifier (BR). If you think the transformer is faulty, look for AC voltages on the secondary side just before they get rectified to DC. Once you've established AC present, look for DC after the Diodes and BR.
DC (for the heater supply) from the BR goes to C7, then to the voltage regulators U1 and U2.
DC for HT goes through the three big can capacitors much like the VTP101. You should be able check for HT DC at the large can caps and it should be similar to what you see on the VTP101 schematic.
I'm not good with the timer circuit, maybe you can post your question over at DIY Audio or Audiokarma/Solid State for an explanation and understanding. Include a picture of the board, the VTP101 schematic and the Dynaco schematic. Someone there should be able to figure it out by the symptoms and explain the circuit to you.
Keep us posted.
Rich
If you've got voltage at the collector, the power supply upstream of it is OK. Transistors work more or less like a relay. Power in on either the emitter or collector, and out on the other. Which way round depends on polarity and transistor type. This one has pos input on the collector, pos out on the emitter, and its switched by the base. In switch duty, the output terminal voltage is usually fairly close to the base voltage, so it makes sense to see 14v on both emitter and base. You'll need to chase the circuit back from the emitter and find out whats going on.
If this is the B+ delay, its probably being driven by one of the smaller transistors next to it. Taking a guess at it, it may be a simple resistor-capacitor delay circuit. Basically it charges a cap through a resistor, once it hits the right voltage, the small transistor turns on, which turns on the larger transistor. Open resistors or bad caps in that circuit could make it not go. If the small transistor is pooched, that would do it too. For what they are and what they do, you can easily verify them with a volt meter on diode check. The Great Google will provide pinouts and destructions but its dead simple, and plenty accurate enough for this sort of thing.
If this is the B+ delay, its probably being driven by one of the smaller transistors next to it. Taking a guess at it, it may be a simple resistor-capacitor delay circuit. Basically it charges a cap through a resistor, once it hits the right voltage, the small transistor turns on, which turns on the larger transistor. Open resistors or bad caps in that circuit could make it not go. If the small transistor is pooched, that would do it too. For what they are and what they do, you can easily verify them with a volt meter on diode check. The Great Google will provide pinouts and destructions but its dead simple, and plenty accurate enough for this sort of thing.
rudedogg
AK Member
Thankyou guys so much for the help! I am learning alot by trying my hand at these broken amps. I do believe you guys are right. I seem to have enough B+ At the Tip50 and the 11.9 filament supply is getting to the tubes. So it is probably a small component like a resistor or cap just not delivering a tiny bit of voltage or current to turn on TIP 50. I may just shotgun replace all upstream small components since they are inexpensive. What is the large Black can cap doing? Is it somehow controlling the red/green diode as part of the timer circuit?
No.
Look at the underside of the pc board and follow the traces to and from BR (low voltage DC for heater supplies). Two traces come from the power transformer, that's AC in. The other two traces are DC out. One trace runs to the ground (-), the other runs to the big capacitor C7(+). It's the storage from BR before it gets regulated by U1 and U2.
The weakest link in both the VTP100 and VTP101 was the bridge rectifier, it's the part that kept blowing and caused so many returns. I considered upgrading the rating from 2 amp but decided I'd rather have the BR blow and act as a fuse than risk damage to the power transformer.
The power transformer is irreplaceable. Once it goes, you must build an outboard power supply and run wires in for the HT and LT. The VTP100 had a "physical design" flaw, the entire weight of the power transformer is supported by it's solder mount leads, which can crack and break the internal wires if the unit is dropped in transit. The VTP101 addressed this issue, there are diagonal mounting bolts with bushings to support the weight of the transformer over the PCB.
Both are great sounding preamps. I think you'll be surprised when you get yours fixed.
The power transformer is irreplaceable. Once it goes, you must build an outboard power supply and run wires in for the HT and LT. The VTP100 had a "physical design" flaw, the entire weight of the power transformer is supported by it's solder mount leads, which can crack and break the internal wires if the unit is dropped in transit. The VTP101 addressed this issue, there are diagonal mounting bolts with bushings to support the weight of the transformer over the PCB.
Both are great sounding preamps. I think you'll be surprised when you get yours fixed.
Destructor
Super Member
When my bridge was replaced it was attached to a heat sink, I don't know if it was uprated or if the heat sink is all it needed. These are excellent sounding preamps.
rudedogg
AK Member
OK GUYS! I think I found it! Please tell me i am correct. That resistor, "R1" right above transistor "Q1" is reading infinite! I checked several times. With power applied, it has 335 vdc on its left lead, and nothing on its right lead. So is this my failed component?! Boy, i am nervously excited! What a simple fix if this is what's wrong.
crispycircuit
Super Member
Bingo !!! You sly fox......
rudedogg
AK Member
I got up early and lifted one leg of the resistor. It tests dead on out of circuit at 470k. Put back in hoping maybe bad solder joint. Still no B+ Past TIP 50
rudedogg
AK Member
Heyraz, i have checked alot of the voltages. I have 2vdc on the power dual diode. I have 349 vdc on the B+ circuit. It drops slightly thru the resistors and caps. I have 11.9 vdc for filament supply. The problem is i have 18vdc where the board says i need 250 v and 13vdc where it says i should see 180 vdc. Do me a favor if you could check the voltages at the resistors and caps along the B+ and filament resistors and caps, if you do decide to pop it open. Then maybe i can see where mine is going astray
crispycircuit
Super Member
Did you check the ohms of the resistor out of the circuit?
crispycircuit
Super Member
Does the resistor value match the schematic?
crispycircuit
Super Member
Ok, I'm still thinking it's the zener diodes (the 2 glass diodes above the resistor) ... Your in the right area. Make sure you have your meter on correct setting AC or DC. Keep looking at it till something clicks in your head.....
crispycircuit
Super Member
Your gonna have to remove the diodes to see the numbers. You'll need a magnifying glass, then google the numbers. I'm guessing they are 150 volt 1watt zeners. Don't assume anything....
crispycircuit
Super Member
Maybe the 470k should be 470 ohm
rudedogg
AK Member
Oh crap! I got tied up this evening playing granddad to my daughters little boy. Would it be possible to schedule a time tomorrow late afternoon so we can compare notes? I can PM you my cell# if that is easier. I connected my black probe to the chassis and tested under power with my red at each component in the B+ "ladder" of resistors, diodes, and caps. What i saw was the rectified DC voltage climbing towards the TIP 50 switch. It loses a couple volts at the resistors as expected. I think the DC HV comes in at around 349, and drops to around 335 right at the collector. The components seem to be in spec when tested in circuit without power. It would be awesome if i could check the numbers with you together, which im guessing you were trying to do earlier, sorry.
John
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