17-06-2020, 05:39 AM
(This post was last modified: 17-06-2020, 05:44 AM by Diabolical Artificer.)
Oh dear, bummer. first, DONT PANIC, DONT get downhearted. Right, this is a very simple circuit, all this amp is, is a power supply ( mains tfmr, rectifier valve and 50u/50u cap) and an amp. So, start with the PSU first. Pull all the valves out, get your meter out, set on ohms/resistance. After making sure there is no charge in the cap (take a DC voltage reading), take a DC resistance reading across + and - of the big cap, it should be a good few kilo ohms and keep going up. If it isn't, and you get a reading of a few 10's of ohms or less, go through the circuit to find your short. Obviously make sure you have the electrolytics fitted the right way, else they will act as a SC when powered up. Check, then go and have a cuppa, a lie down or whatever then check again.
Next, check PSU. I keep recommending you get at least a lamp limiter and variac Rob, have you got these? If not a variac a lamp limiter will tell you if you ha=ve a short or you've bogged the wiring of the primary up. So, all valves pulled out, lamp limiter on, power up ( slowly if you can with variac), hand on off SW, if lamp glows bright, SW off.
The primary has to present a high impedance to the mains voltage, else fuse will blow, this means it has to have enough resistance in the wire etc, so what tfmr's winders do for a tfmr that can use several mains voltages is to string several windings in series. You can check this by testing each primary, some will be around 50 ohms, others around 10 ohms or under. Now it gets a bit tricky, you can't just connect these any old how, they have to be in phase, that means wound around the bobbin the same way, EG both CW, or CCW. You can check this f you feed a sinewave into one winding, then scope another, both should be in phase, IE the peaks of the sinewave at the same point.
Feeding in a sinewave from your sig gen or a few 10's of volts from a variac is one way to check, another quicker way is to connect two pri wndg's, power up using a LL, if lamp glows, try again.
After you've checked the PSU is ok you can deal with the amp. Whenever we do do work on a bit of kit we screw up, everybody does, professional's and amateurs too. So before powering up check your work, then check again. give it a good visual looking for the obvious, loose wires, touching leads etc, then check methodically with the schematic and the pictures you should have taken before you took it apart and any notes and drawings you also should have done.
Then before powering up with valves in check your secondaries, do you have 5v, 6.3v and 250v or whatever? Again a variac is great here, it allows you to test at low voltage. Next fit a 1 amp fuse, or better a big 100ohm resistor, fit rectifier valve and check you have HT. Fuses should be 2 X 2 amp from memory or 3 amp at the most, both fast types, but a 1A fast blow gives you a bit more protection. Like most valve amps of this age only the AC going into the rectifier is fuse protected. With no fuses fitted only do a quick test, HT will be higher than normal, we don't want to blow up or stress any caps etc.
After all this power up with valves fitted, hope that helps, I've bound to have forgot a few things but this is the basic test you do on any bit of electronic gear. Andy.
Next, check PSU. I keep recommending you get at least a lamp limiter and variac Rob, have you got these? If not a variac a lamp limiter will tell you if you ha=ve a short or you've bogged the wiring of the primary up. So, all valves pulled out, lamp limiter on, power up ( slowly if you can with variac), hand on off SW, if lamp glows bright, SW off.
The primary has to present a high impedance to the mains voltage, else fuse will blow, this means it has to have enough resistance in the wire etc, so what tfmr's winders do for a tfmr that can use several mains voltages is to string several windings in series. You can check this by testing each primary, some will be around 50 ohms, others around 10 ohms or under. Now it gets a bit tricky, you can't just connect these any old how, they have to be in phase, that means wound around the bobbin the same way, EG both CW, or CCW. You can check this f you feed a sinewave into one winding, then scope another, both should be in phase, IE the peaks of the sinewave at the same point.
Feeding in a sinewave from your sig gen or a few 10's of volts from a variac is one way to check, another quicker way is to connect two pri wndg's, power up using a LL, if lamp glows, try again.
After you've checked the PSU is ok you can deal with the amp. Whenever we do do work on a bit of kit we screw up, everybody does, professional's and amateurs too. So before powering up check your work, then check again. give it a good visual looking for the obvious, loose wires, touching leads etc, then check methodically with the schematic and the pictures you should have taken before you took it apart and any notes and drawings you also should have done.
Then before powering up with valves in check your secondaries, do you have 5v, 6.3v and 250v or whatever? Again a variac is great here, it allows you to test at low voltage. Next fit a 1 amp fuse, or better a big 100ohm resistor, fit rectifier valve and check you have HT. Fuses should be 2 X 2 amp from memory or 3 amp at the most, both fast types, but a 1A fast blow gives you a bit more protection. Like most valve amps of this age only the AC going into the rectifier is fuse protected. With no fuses fitted only do a quick test, HT will be higher than normal, we don't want to blow up or stress any caps etc.
After all this power up with valves fitted, hope that helps, I've bound to have forgot a few things but this is the basic test you do on any bit of electronic gear. Andy.
