Blown Fuse - Troubleshoot Why

[huntro1]

I have a Dual TG28 reel-to-reel tape deck (1964) with a blown fuse (315ma). I am not able to determine the cause and I am seeking help in troubleshooting the problem. I have uploaded the wiring diagram. It has two fuses the first is the a/c line fuse and the second protects the amp. I measured the voltage without the second fuse at 17 volts. When I use a jumper across the fuse holder the voltage jumps to 50 volts. I don't see any burned components and I don't feel any hot spots. I am not sure about the circuit, but I think it might be a/c because there is no rectifier bridge. My knowledge is limited on electronics and seek help on finding the problem. Are there any interested folks that are willing to walk me through the process? Thanks, Robert.

[vtech]

G1and G2 are the rectifiers and in this case it not full wave (4 diodes).

Where do you measure 50 volt?

[huntro1]

vtech,
Thanks for your reply! To answer your question, above G1 and G2 in the wiring diagram is a fuse (F 315mA). When I use a jumper cable across the fuse and measure the voltage at the fuse I get 50 volts DC. If I remove the jumper cable, then I get 17 volts only on the transformer side of the fuse. I tried to measure the amperage load at the fuse, but nothing registered using a DC amp meter. At this point, with the power on, the motor is the only component working. The motor is spinning, but the pilot light is out. Note, when I use the jumper across the fuse, then the pilot light comes on. Location of pilot light is at circuit board entry/exit points #42 and 43, near resistor R30. What should be my next step be to find the problem? Thanks for your help! Robert.

[vtech]

[huntro1]

vtech,
I will reply to your question about the voltage at C1 and C2 in a few days.
Thanks for your reply,
Robert

[huntro1]

Hi vtech,
Over the weekend, I made some more measurements for your review and comment. First I measured C1, C2, and C3 without having a jumper across the blown fuse and the power on. I got: C1 0.5VDC and VAC, C2 0.1VDC and VAC, C3 0.5VDC and VAC. I measured both A/C and D/C voltages because I wasn't sure which to use. Now the interesting part. I put a jumper across the blown fuse with the power on, here's what I found:
C1: 31VAC and 21VDC
C2: 21VAC and 21VDC
C3: 50VAC and 35VDC
After that, I made some additional measurements of the transistors: T203 11.5VDC, T103 12VDC, T102 1.5VDC, T101 6VDC, T4 0.0VDC (erase circuit off), and capacitor C4 33 VDC.
In further examination of the capacitors (C1, C2, C3), I discovered that C2 looks slightly discolored and the clear plastic shrink wrap around it was black at the bottom and not tightly wrapped like the other two capacitors. It also looked slightly bulged at one end. What is the function of these capacitors? Filters?
So, what are your thoughts on the source of the blown fuse? What is the next step I should take in repairing the problem? Your comments are appreciated.
Thanks,
Robert

[huntro1]

Vtech,
Over this last weekend, I replaced the capacitor C2 and tried to play a tape by jumping the fuse again. This time it worked and I could hear music from the tape through the headphones! So, I put in another fuse and turned it back on. Well, in one second the fuse was blown again. So, I guess there is still a problem, but what should I test next? Can anyone give me some clues? I think an electronic component is failing, but not completely gone yet. What to do?
Thanks for your reply,
Robert

[vtech]

Instead of a jumper, use your meter set to AC amps. What do you measure when you turn the unit on?
The idea is to determine how much current is going thru the fuse. (remember any measurement before diodes G1 & G2 are in AC). There is no need to measure AC across caps since you are dealing with DC at that point.
Your DC readings across C1.C2,C3 are a bit high, so it is possible that you are still dealing with dried up capacitors C2 & C3. Either way, measure the AC amp draw in place of fuse. Do you have access to a scope?

[huntro1]

Hi Vtech,
Thanks for your reply. I will get the amp measurement this weekend and post it here. I don't have a scope and I don't know anyone who has one. But, I know where I can buy an old one (1950's model) for cheap (less than $100). What do you think of the EICO 460? Does it have the features I need to do basic testing on audio equipment? What basic features should I be looking for in a scope? Is 10MHz bandwidth O.K.? How about 1mv/div? There are some scopes on Amazon for around $200 new. Do I need any generators built into the scope? Your recommendation is appreciated. Thanks, Robert.

[vtech]