Motorola 5A7A dead.

[daveo23]

Anyone here help me fix my 5A7A? It was working, now dead.

Thanks.

[Herald1360]

Probably......

It would help to know how long the piece of string is, though!

Any symptoms? What have you tried? Do you have any relevant radio/electronics experience and tools?

[Station X]

Service info here:-

https://www.tubesandmore.com/schematics/motorola/5a7

[daveo23]

Can you guys help me with my dead Motorola. Cant figure it out.
-Filament voltage OK
-b+ at plates 18v, when 3300R resistor is lifted voltage is 92vdc
-no sound but battery test of transformer/speaker OK

[Station X]

Two threads about the same radio merged.

[daveo23]

Sorry about the double post, it kicked me off last night during the post.
Answers: I have all the tools of the trade and have worked on tube radios, this mini is stumping me. When I lift the 3.3K resistor, I see 92vdc, put it back and it goes to 18vdc. tubes are tested and fine so it audio tranny and speaker.
Its my plate voltage, it sucks

[Herald1360]

Failed electrolytic capacitor connected to S1-D?
Dirty switch contacts?
Leaky grid coupling capacitor to 3S4 g1?
Is the 3300R actually 3300R? If it's 3300k as suggested it's drifted way high!

[Station X]

Numerous possibilities for faulty components pulling down the voltage. Given that the set uses a solid state rectifier I'd check the voltages with the valves out.

[daveo23]

A few good points, I will check them. The 3.3K is on the money and 92 vdc when lifted from circuit.

Is it safe to power the radio without tubes? I read that some radios like the zenith t.o. you cant.

[Station X]

With no valves fitted the HT voltage will be higher due to being unloaded. The same applies while the valves are warming up though. The smoothers should be able to cope.

Have you checked the resistance across the HT supply? It should be near infinite, but allow time for your meter to settle as initially it will be charging the capacitors.

[daveo23]

I measured the resistance from the B+ on the 3.3k resistor to gnd(e.caps) - 700ohms
Pulled the tubes , b+ went to 36vdc
voltage drop at 3.3k resistor 18vdc radio side, 82vdc rectifier side
lifted c4 and c7 caps, made no difference

[Station X]

Looks like you've got 700R across the HT supply somewhere.

Does the resistance reading across C7's connection points change if you operate the battery/mains switch? Power off of course.

With no valves and power applied does anything get hot apart from the 3.3k resistor?

[daveo23]

I am on the hunt and narrowing it down. Appears that the front end is ok. Right now I am at the 1U4 tube pin 3. I lifted b+ up to that point and have 92VDC. With everything connected, C7 resistance is 800 ohms batt or AC. Lifted from the 1U4 tube it's zero so I am working down the line from that pin 3 on the 1U4. Could get tricky though. I am thinking of disconnecting the power audio tranny and seeing what the B+ looks like.

[Station X]

From your description I'd suggest that the problem is with the primary winding of T-2. Perhaps the windings are making contact with the can and hence the chassis.

You say "Lifted from the 1U4 tube the resistance it's zero. Do you in fact mean infinite (open circuit) rather than zero (short circuit)?

[daveo23]

Yes, infinite but I should tell you I disconnected the audio tranny which appears to be good, 400ohm on primary. Volts went from 82 to 68vdc like it should. Could this be the selenium rectifier not being able to provide the current needed? It appears fine with meters.

[Station X]

I don't see how the rectifier comes into it at this stage. With no valves plugged in no HT current can flow.

If I understand you correctly, disconnecting the HT feed from the screen grid of the 1U4 causes the resistance across the HT supply to rise from 800 ohms to infinity? That being the case, what resistance do you measure from pin 3 to chassis with the HT feed disconnected from pin 3?

[daveo23]

These are the tests I have done with the tubes in:
On the C7 tests here is what I did:
1. lifted one leg and checked b+ = no difference
2. put on uf meter = perfect
3. checked for short = none
4. resistance from common ground to other side while connected = 800ohms
5. resistance from chassis to c7 connected - inf.

On the b+ tests here is what I did:
1. lifted 3.3k resistor = 92vdc
2. lifted b+ from t-1 keeping r-3 and c4 - 92vdc (no other connections)
3. added b+ to pin 3 of 1u4 = 18v
4. removed red wire of audio trans b+ connected back up = 68vdc - started at 92 then went to 68

[Station X]

You're going to have to track down that 800 ohms. It shouldn't be there. When you disconnect a B+ lead, check the resistance to chassis on the end of the lead and also at the point you disconnected the lead from. If the resistance is on the lead leave it disconnected. You'll have make further disconnections on the B+ rail and make more resistance tests to track down the 800 ohms.

It's difficult to give precise advice because I don't know the actual wiring runs for the B+ line. They probably aren't as per the circuit. Many wires could be commoned on one termination point. Check them all individually.

[daveo23]

With b+ lifted at the t-1 , resistance is inf. To common ground its 5k

[Station X]

To clarify. You've disconnected the wire from T-1 tag 2 and measuring from chassis towards the anode of the 1R5 you see infinite resistance, but measuring from the chassis to the disconnected wire you see 5k?