Foreplay III voltage question

[bainjs]

I just finished building a Foreplay III and went through the resistance check ok. 

When I tried to do the voltage check, the fuse immediately blew.  I went through the wiring again and the only thing I found is I had failed to install the jumper wire from H1 to H4.  I've installed the jumper and am on my way to get more fuses to try again.  Would leaving out the jumper cause the fuse to blow?

Also, after I installed the jumper, I went through the resistance check again and now 4 and 5 on the transformer are only showing .0L where previously it was showing variable resistance readings.  Is this ok?

Thanks for your help.

Joel

 

[bainjs]

I installed a new fuse and once again it blew...  I've gone over all of the steps again and everything looks ok.  I will check the steps again.  Is there a specific area I concentrate on to look for possible mistakes?  I've checked the capacitor and diode polarities and all look correct.

Thanks for your help.

Joel

 

[Doc B]

Start by checking the resistance across each of big heater supply rectifier diodes at the back of the power transformer with the tubes pulled out of the sockets. They should read infinity with probes touching the leads in one arrangement, and when you swap the leads end for end on the rectifier they should read about 2K ohms. If any rectifier reads super low like under 200 ohms they may be blown. Also check the stripe orientation of the blue filter cap at the end of that terminal strip.

 

[bainjs]

Thanks for replying.

Touching the positive lead on the striped end of each diode, the meter jumps all over the place.  When you reverse the leads, all of the diodes show anywhere from 250 - 275 ohms.  The blue filter cap's stripe is connected to H1 per the instructions.

Joel

 

[Doc B]

Just to verify, this is measuring with the black DMM probe touching the lead right at one end of the rectifier and the red DMM probe touching at the other end, not with the black probe connected to signal ground, yes?

 

[bainjs]

Yes, the black probe is touching at the end of the diode, not the signal ground.

Thanks

 

[Doc B]

OK, with the 12AU7 tubes pulled out measure the resistance between H1 (black) and H2 (red). The reading should climb as the 10,000 uF capacitor charges.

 

[bainjs]

Hello

Over a minute or so, the capacitor climbed to about 30k and then started decreasing.

Thanks

 

[Doc B]

OK that's good, it means that the filter cap is working properly and there are no shorts across the tube sockets. I think the heater supply is OK, you get infinity ("OL") at the transformer terminals as spec'd in the manual, the heater supply rectifiers seem OK, and the filter cap seems to be working. What is the rating of the replacement fuses you bought?

 

[bainjs]

1.0a from Radio Shack.  The package says fast acting so I guess they aren't slow blow types.

 

[bainjs]

I went ahead and picked some 1.0a slow blow fuses but they also immediately blow when I flip the switch...

Thanks for your help.

Joel

 

[Doc B]

OK, it sounds like time to start checking other parts of the circuit. Try checking the small UF4007 rectifiers for the B+ supply the same way you tested the bigger ones for the heater supply - measure resistance with one DMM probe at each end, then swap the probes and see if any read really low, like under 100 ohms.

 

[bainjs]

I removed the tubes and the 2 UF4007 rectifers checked at 0.L with the positive placed on the banded side and 29K on one with the probes reversed and the other 31.5k with the probes reversed.

Thanks

 

[Doc B]

OK, this is an interesting idea from tech Shawn - look closely at the terminals on the power transformer. They come out of a white plastic bobbin. Below each terminal is the terminal number, molded into the plastic bobbin. Verify that the numbers on the plastic bobbin match the numbers on the Bottlehead label on the belly of the transformer. I think we have had one instance in the past of a label getting put on upside down. T4&5 on the power trans will also have heavier gauge wire coming out of the coil and wrapped around the base of them than the other terminals.

 

[Grainger49]

Joel removed the tubes turned it on and the fuse held.  Then, one at a time, put the tubes in with the OD3 last.  The fuse still held.  He has moved on to the voltage checks and might post back tonight.  I'm headed to bed so y'all help him if he has more problems tonight.

 

[bainjs]

My voltages post did not go through. They are running (in my mind) very high.  Also initially both LEDs were working.  Now only channel A is lit.  I will send them again.  Terminals 4 and 5 have a heavier gauge wire attached and the label seems correct.

 

[bainjs]

The voltages are:

Power transformer terminal
4 - 39.3
5 - 39.4
6 - 230
7 - 230
9 - 232
10 - 232

H1 - 36.6
H2 - 43

1 - 233
2 - 232
4 - 230
5 - 0
6 - 232
7 - 465
9 - 0
10 - 410
11 - 0
12 - 208
13 - 0
15 - 120
16 thru 20 - 0
21 - 285
22 - 50
24 - 0
25 - 412
26 thru 30 - 0
31 - 95
32 - 208
33 thru 40 - 0

 

[Grainger49]

Joel,

The first readings on the transformer leads should be AC volts.  If your meter was set on DC volts you might have gotten odd readings  On terminals 1-33 the voltages most are either a little low or less than 10% high.

T12, T15, T21, T22, T25, T31 and T32 are all truly high.  I'll take a deeper look at the manual and see where those are attached and get back today.

 

[bainjs]

OK, and thanks for your consultation last night.  I'll re-check the transformer readings and re-post.

Joel

 

[Grainger49]

Joel,

Don't worry about the transformer readings, I highly suspect you were on DC.  The meaningful readings are after the rectifiers and on the tubes.  See below (you have an email).

More... The tubes may not be coming on.  Can you post the voltages at A1, A3 & A8, B1, B3 & B8?  The plate and cathode voltages tell a lot.  The plate voltage for the second stage (cathode follower) can be seen on T12 and T32 (both 58V high saying the tube is not turning on).  But the first stage (voltage amp) plate voltage is derived through a resistor jumping from one tube pin to the other on both 12AU7s.  

Let's see if the first stage is turning on.  I bet one is and the other isn't (the LED is the indicator here).