Hi. I'm repairing an STC bakelite Mantel Radio and have done major work before plugging in. It operates and appears to have reasonable sensitivity but is weak audio-wise.I don't have a schematic. I've downloaded data for a 250 model without the NO suffix but it is not the same. My output stage appears to have the 6V6 cathode grounded directly and I can't see how any bias would be derived for it. There is a HT choke apparently in the centre-tap of the power transformer, one end grounded. The only electrolytics in the whole thing are a pair of two-in-one pigtails with positives all connected to the 6V6 screen grid, (the positive rail), the negative of one directly grounded and the other's negative to the ungrounded end of the aforementioned choke. The available audio is clean but limited.I can't post any pictures at present due to technical difficulties. The unit has been played with, a few blobby solder joints a dead giveaway. I have restrung the dial, cleaned and lubed the mechanism (uses brass half-wheel and a friction drive from the knob), replaced all paper caps and high resistors, repaired a cracked wafer of the band switch (removing that assembly is an endurance test requiring good record keeping- a good sketch!
Any circuit or ideas?

Thanks,
Technomaniac.

Normally no bias resistor on the OP tube indicates that it is "Back biased" which will see the HV centre tap not going directly to the chassis. The grid resistor (normally out of spec) will go to the centre tap not chassis, if that is the case.

It will need IF realignment after parts are replaced as well as tweaking the oscillator (padder if fitted) & antenna trimmers. Aerial is another issue.

Marc

Measure the voltage at grid 1 of the 6V6. If it is about -12 volts then the bias is fine. Even a smaller negative voltage (e.g. -5 ) will still work ok, just not at the optimum. The resistance between the transformer centre-tap and ground is what provides that voltage.

That is not a 250. It is an A 250. A250 has traps for young players. The biggest trap is, that gets a lot of Americans, is the field coil is in the "B" Negative rail and not in B+.

That means that both positives of the filter caps are on pin8, Cathode, of the 6X5. This one is unusual as it has a string of resistors on the grid of 6V6 back to the CT of the HV winding. As expected, the first filter on B+ has neg on the CT and the other goes to chassis.

It also uses negative feedback from the secondary of the speaker; Muck that up & it will likely oscillate, lose volume or both.

It is a messy hotch potch circuit by sometimes referring to decimal Meg ohms & then having 20,000 somewhere else.

The transformer is shown as a shielded type, so I would use a Three wire cable & ground the chassis. There should be a link pin in the back in the PU socket, if it's missing you will be struggling to get any audio. There are two BC positions related to Hi & Low tone.

Beware of what device you plug into that socket it is not DC blocked.

From this you may deduce I have the circuit of both.

Use Control Panel to unhide your email address and I'll send you the A-250 schematic.

Thanks guys. I forgot to mention that I HAVE fitted an earthed mains cord with a good cordgrip and a new grommet. I HAVE already downloaded what is labelled as an A250, and in the diagram I downloaded the choke is shown in the + rail and there are bypass electros on the cathodes of both 6B6 and 6V6. Mine has paper bypass on the driver . Not sure about the 6V6. Definitely no electro, could be a paper or mica one, in a flat style, (not a rolled up one)...But I am thinking now that the iron-cored choke is providing the bias as it IS in the centretap of the power transformer in its ground path. This radio has short wave with a three position switch as you say, with two positions for BC providing tone control.But there is no pickup input, no extra socket for this, and no switch positions for a pickup input.

I WILL give it an alignment, but the fluoro tube interference I am receiving off-station suggests that the sensitivity is not too bad. I HAVE had the whole bandswitch assembly out to repair the crack in the wafer switch, so the layout of the connecting wires will have changed, so there will likely be some benefit from an alignment. I have no evidence that the shortwave band is working, but it DEFINITELY has shortwave coils and the appropriate dial scale for SW....Incidentally the dial has been printed two or three degrees out of plumb, so I have tweaked the pointer appropriately to minimise the problem. Also I cut a perspex dial glass and glued it to the bakelite cabinet, as there was none present. I know it's not glass, but it will keep fingers off the pointer.

If anyone has a closer schematic that would be appreciated, but as I said, the one I have already downloaded has the two cathode resistors and the choke in the positive HT rail, which is different. The schematic is marked simply '250' above the circuit, but the parts list on the next page is headed 'A250'. My chassis is CLEARLY rubber stamped '250 NO' . The 'NO' is NOT a prefix for a serial number as nothing follows it. I may try a bypass electro on the 6B6 cathode and see if it is possible to rephase the transformer. The wiring is not that accessible. There is no sign of any positive feedback with the pott flat out. I HAVE run an ohmmeter over ALL resistors, even disconnecting some to check. There were quite a few out of tolerance.

The speaker magnet does appear to have full strength, and the voice coil is free and the cone complete. I don't think it's a reflex. I have replaced the rubber mounts on the tuning gang, using standard grommets plus a rubber washer punched out of flat rubber sheet added to make up the thickness. The front gang mount is interesting, there is no screw through the grommet as it would be obstructed by the bandchange assembly and inaccessible. The front of the gang is stabilised by its firm connection to the coupling screwed onto the semicircular brass thingo. The grommet sits stably on the chassis, however, with the supplementary rubber washer of the same diameter as the grommet, glued to it with contact cement. I also forgot to mention that there were cadmium whiskers on the wavechange switch shaft as expected, but as I had it completely apart, they were brushed off and the shaft coated with nail polish, to slow the regrowth.

I will see if I can figure out how to enable access to my email address just in case its needed, but in case I can't, its bwlentzoutlook.com.

Thanks for the help!
Technomaniac.

Okay, schematic sent.

BTW: please put blank lines between your paragraphs. Makes the post much easier to read.

Thanks for the schematic. This DOES look correct. I noticed this morning that I had resoldered the .47 to a different place from whence it came, but where I put it IS correct, it was in the wrong place, when I put it back I had motorboating. It's the rail bypass.

By your schematic there should be a bypass at the junction of a 500K and a 250K in the bias supply.So I'll fit one there. I notice the electros on the rail are 4 and 16 μF, a bit different from what's been fitted, but I think the other power supply components can cope with the surge current.

Sorry about the lack of paragraphing.

I should be right now.

Cheers,
Technomaniac.