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 STC 505, mystery box!
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 Return to top of page · Post #: 1 · Written at 10:33:48 AM on 3 March 2017.
BennVenn's Gravatar
 Location: Windellama, NSW
 Member since 12 February 2017
 Member #: 2061
 Postcount: 17

Morning,

I've began recapping a 505 chassis and found a large rectangular box under the chassis. It has been disconnected at some stage of its life and the closest schematic I could find, the 504 had no mention of it.

Image here: http://imgur.com/a/5Y3O5

Any ideas what it might be? I could always pop the cap off but if it's not part of the original design, I'll leave it be.

Thanks!


 
 Return to top of page · Post #: 2 · Written at 12:06:48 PM on 3 March 2017.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6172

Possibly a multi-section electrolytic, the type usually used to filter the B+ so ordinarily close to the rectifier valve and usually mounted above the chassis.


 
 Return to top of page · Post #: 3 · Written at 5:11:36 PM on 3 March 2017.
Clive Durham's Gravatar
 Location: Grenfell, NSW
 Member since 8 July 2015
 Member #: 1771
 Postcount: 212

BV what is the valve lineup?


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
Clive

 
 Return to top of page · Post #: 4 · Written at 6:46:23 PM on 3 March 2017.
BennVenn's Gravatar
 Location: Windellama, NSW
 Member since 12 February 2017
 Member #: 2061
 Postcount: 17

Clive,

The valve lineup should be 6A8G, 6U7G, 6Q7G, 6AG6G, 5Z4G

The last two tubes have been replaced with EL33A and 5Y3GT at some stage.

Still can't get this set to tune any stations and the LO is high. There may be damage to the variable capacitor as the plates bind up the higher end.

Ben


 
 Return to top of page · Post #: 5 · Written at 6:47:45 PM on 3 March 2017.
BennVenn's Gravatar
 Location: Windellama, NSW
 Member since 12 February 2017
 Member #: 2061
 Postcount: 17

What ever is in there it's open circuit and no capacitance. I found a rogue wire going to the speaker socket. Perhaps it was a choke?


 
 Return to top of page · Post #: 6 · Written at 9:01:55 PM on 3 March 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4506

The rogue box is likely to be non polarised 8μF filter caps which were cut out & replaced with the red Ducons. These should now be either side of resistance of 2500 Ohms to replace the field coil of the speaker if it has a permanent magnet speaker fitted.

Go back & sort that out. 5Z4 has a Cathode sleeve: Same pin outs as 5Y3. You will need filter caps of around 500V as 5Z4 behaves as a heater tube & creates voltage slowly, 5Y3 creates a voltage surge that can go to twice the "B" voltage.

Grid 1 bias is around the same for both OP tubes .-6 but as they are slightly different the cathode resistor may need changing.

It is pointless working on the front end if the IF does not work, or is out of calibration. If there is an obvious mechanical fault with the tuning gang that should have been fixed when noted, or fixed now. Binding can be a lubrication problem or an incorrectly set up dial drive & cordage.

Outside Tuning Gang plates that should be inside when servicing are easily bent and short, plates can also short from corrosion. You need to sort it out as well before re-powering. Remove the wires from it & use an ohm meter to determine if it shorting. Also ensure no fly leads are shorting to shields and where they exit cans.

I have the circuit. The volume control is a rheostat in the cathode: Common in that era. If its faulty (open) the valve will not work and all grids will likely be positive, relative to chassis.

To replace a 2500 choke I normally use 2 x 4700 Wire Wound in parallel.


 
 Return to top of page · Post #: 7 · Written at 7:36:41 AM on 4 March 2017.
BennVenn's Gravatar
 Location: Windellama, NSW
 Member since 12 February 2017
 Member #: 2061
 Postcount: 17

Hi Marcc,

I believe the speaker to be original, I'll take a pic as you'll know as soon as you see it rather than me guessing.

I haven't seen B+ go beyond 220v but I'll be sure to fit 500v caps as you suggest.

The cap plates were bent. I've straightened them and now the tuning goes full dial without binding. I'll measure for shorts before I power it up.

The dial drive is friction, looks like two metal washers grabbing the large tuning disc. I'll get photos to help describe it.

Once the above is sorted I'll align the IF and see if we get anything.

I'll chase down the original valve lineup too. I'd like it to be as close to original as possible


 
 Return to top of page · Post #: 8 · Written at 10:16:10 AM on 4 March 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4506

That sort of often half segment dial drive was quite common.

I make a point when starting a radio after refit, of monitoring the "B" voltage. IC clips are good. That allows you to see if the unit is going to plan. With a 5Y3 the voltage will rapidly surge to close to 500v (250V running B+) and fall back as the heaters warm & the valves come into conduction. Heater rectifiers and those with a cathode sleeve like 5Z4 and the Sovtec 5Y3, do not behave like that.

This can be enlightening. If the voltage fails to appear in the allotted time I hit the "Kill" switch. Every so often an Electrolytic will fail before it gets to its rated voltage. I had one (500V) recently that only got to 470V before it let go.

There is no chasing the valve line up we know what it should be. Its on the circuit. IF is 450kHz, it has AGC so keep the input into the TC (signal grid) down to around 50uV max. and use a series cap so that you do not wipe out the bias. If the AGC activates you will never get it right.

I calibrate the generator (Tone off) with a counter & use an Oscilloscope as the measuring device. CRO will spot any distortion and help find its source.

The circuit diagram shows a 2500 Ohm Field. That means Electrodynamic.


 
 Return to top of page · Post #: 9 · Written at 7:59:29 PM on 4 March 2017.
BennVenn's Gravatar
 Location: Windellama, NSW
 Member since 12 February 2017
 Member #: 2061
 Postcount: 17

Marcc,

Could you please give me a link to the schematic?

I meant chasing down original valves to fit in the radio so I don't need to change out cathode resistors. They all measure OK and I like the look of the old resistors.

Ben


 
 Return to top of page · Post #: 10 · Written at 8:39:34 PM on 4 March 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4506

Can't remember a 6AG6 in my collection albeit moving camp did not help.

Circuit is sent.

Marc


 
 Return to top of page · Post #: 11 · Written at 8:45:02 PM on 4 March 2017.
BennVenn's Gravatar
 Location: Windellama, NSW
 Member since 12 February 2017
 Member #: 2061
 Postcount: 17

Thanks!


 
 Return to top of page · Post #: 12 · Written at 7:03:06 PM on 5 March 2017.
BennVenn's Gravatar
 Location: Windellama, NSW
 Member since 12 February 2017
 Member #: 2061
 Postcount: 17

Alignment done! I set a digital sig gen to 455kHz (I've read schematics saying both 450 and 455) and injected the signal into the grid of the mixer tube.

I fitted a 2k resistor in place of the speaker field winding.

Initial AVC was at around 0v, 5.5V bias on first and second cathodes.

With the CRO on the secondary of the first IF (10x probe - enough to not influence the freq?) I tuned both caps in the first IF can. That boosted the waveform about 500%. I moved onto the output of the second IF, adjusting boosted it only about 20%. AVC now at -5volts.

Should I have disabled the AVC for the alignment?

I've got a 1MHz oscillator loosely coupled to the antenna wire and the set clearly tunes it though AVC is at 200mV positive. There is no AF modulation as it is from a simple digital RF gen. I would assume this should fold the gain right back and show a similar AVC to the IF injection?

There is still no reception of local stations even with a 5m wire strung across the room. As a sanity check I turned on my CD/AM/FM stereo and it too could not tune to any local stations. Perhaps I just need a better antenna...

The lack of AVC when a 1MHz rf signal is injected could be a faulty mixer tube? Any ideas?


 
 Return to top of page · Post #: 13 · Written at 11:34:13 PM on 5 March 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4506

Alignment can be easily messed up & there are several pet theories & posts on the WEB, to lead you astray. As I indicated very very few give any indication of the input voltage for alignment. The general rule is to keep it as low as possible.

AGC is not applied to the cathode and where there is anything other than 6SA7 never is it applied to the Pentagrid on SW. Normally one uses a signal generator that has a 400 - 600kHz Tone and the signal monitored on the Plate of the OP tube. This was in older times done with a Meter like Peak 200H which has an "output" socket. This is just for that and Astor normally had a plug in the back for a conventional meter on AC.

Because AGC is a voltage delivered to grids that draw insignificant current and it is delivered via very high resistances, most meters will have a significant loading effect, to the point with some, they will destroy it.

AGC is applied to the signal grid (g1) of the Pentodes when applied and the signal grid of the Pentagrids like 6A8 (top cap). I do have a surplus Signal Generator ( Leader LSG--11) It has been serviced.

I use a clone of it and calibrate with a Frequency Counter. Tone must be off when doing that. A series cap, or DC blocked attenuator is desirable to cut the signal down and the DC blocking also prevents the attenuator on many Generators shorting out the bias. I normally use a CRO for the measuring device on the OP plate, that gives a better indication of over coupling & distortion and it can lead you to the source as a signal tracer.

You need to be absolutely sure that the IF is on the right frequency and things are working from the signal grid to the speaker. The AGC is developed at the diode plates of the 6Q7 Pins 4&5. The 5M going to the cathode is part of a voltage divider. AGC voltage (chassis +) would be best read at the junction of the 5M & 0.05M resistors. It is applied to both 6A8 & 6U7.

You need to sort that lot out first. I would have preferred 2 x 4700R parallel for the field.

Once you have that lot sorted go to pin 6 of the 6A8 and see (with CRO) if the oscillator is working? Set will not work without it. Also check its voltages. There should be around 100V on that pin. Full B+ on the Plate and around similar on the screen of both it and 6U7; pin 4 of 6U7.

An unmodulated signal will not be heard. You need a radio station so look for a strong one probably 2CA, 2GB. Antenna is important Std length was 25 feet.


 
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