RAP 846 drawing too much current.

[Station X]

Anyone fancy looking up and adding up the anode and screen currents of the valves?

5Z4 is good for 125mA.

[Steve_P]

OK - This order for safety:

1: Switch on at the set.
2: Put the set on a Variac.
3: Set to 240v on the variac.
4: Switch on at mains, and leave your hand on the Power Switch, just in case.

Cheers,

Steve P.

[PJL]

Say 80mA for the 6V6's and 20mA for the rest at say 250V DC = 25W + 25W for heaters gives about 50W, say 50% efficiency and we have around 100W. 450mA = 108W so pretty close but that was at 200V AC.

Best way of checking is to run it up with the variac and monitor the HT volts as mentioned earlier. It should slowly rise up to the correct HT volts (or possibly above and fall back again) - if it settles too low then switch off quickly as all is not well...Peter

[Radio_Dave]

Originally Posted by Station X

Anyone fancy looking up and adding up the anode and screen currents of the valves

Using the info from r-type.org I make it 232.6mA

David

[mickjjo]

To confuse matters further, a certain amount of power will be dissipated in the energised speaker field coil.

Regards, Mick.

[Radio_Dave]

Tomorrow I'll reconnect everything, apply full mains voltage and report back with my findings

Thanks
David

[Ed_Dinning]

Hi Gents, just to confirm that 63mA is a resonable current for an off load mains transformer. I would have expected full load power to be about 50W though, but if at this powewr the transformer is running cool then it is rated for the job

Ed

[Radio_Dave]

Okay, tried again

On full mains voltage the radio is drawing 580mA

The voltage before the speaker field (900 ohms) is 310V and 96V after it, also the speaker field is getting warm

Thanks
David

[paulsherwin]

Originally Posted by Radio_Dave

The voltage before the speaker field (900 ohms) is 310V and 96V after it, also the speaker field is getting warm

That certainly doesn't sound right - the HT current is well over 200mA. Try it with all the valves out except the rectifier. If there's still lots of current being drawn you've got a short circuit somewhere. Otherwise try it with just the 6V6s removed.

Paul

[Radio_Dave]

With just the rectifier the voltages have risen slightly to 317V and 100V.

I guess I have a short somewhere?

There's no resistance between the chassis and +ve of the smoothing cap but when I plug in the speaker, field and output TX assembly I get 450 ohms... but I don't undertand why

[Radio_Dave]

I'm pretty sure I've found the problem .

Whoever replaced the mains TX trapped a wire for the output transformer under a bolt and shorted it to the chassis.

The center tap on the output transformer is connected to the speaker field, so this is whats causing the 450ohm short and probably explains why the output transformer was O/C when I got it

Anyhow, unfortunately, I've ran out of time to effect a proper repair, so I'll report back tommorrow

Regards
David

[Paul Stenning]

Excellent Dave - sounds like you're home and dry once you've put it all back together.

[Radio_Dave]

Yep, it's now working fine and it's very loud! (forgot to turn volume down).

The current at the mains is now 360mA and there's 380V before and 313V after the speaker field .

...What a relief!...Thanks to everyone who helped

David

[Steve_P]

Glad it's sorted.

Always check the simple things first. And then anything done by somebody other than you.

It will be a loud set that. Just the right thing for the neighbours....

Cheers,

Steve P

[Radio_Dave]

I've got a few more obstacles to overcome, but hopefully you'll hear about this radio in the 'success stories' soon

David