Old 6 transistor Radio repair

Hi Bill...

Wow, memories flooding back again :)

The trimmer should be peaked at the top of the dial.

Check to see if the wax has let go, and if the coil is loose(ish) on the ferrite bar move it back and forth a bit to peak the low end. (if it does, seal it in place again with a few drips of wax from a burning candle)

Take care.

Ken

" snipped-for-privacy@att.net" bravely wrote to "All" (04 Oct 05 21:08:24) --- on the heady topic of "Old 6 transistor Radio repair"

Bill,

First, I don't think tweaking everything within reach is a wise repair technique (grains of salt). Try and set everything back the way it was from the factory. Seems to me that the ferrite antenna is being loaded down and is now over-critically damped, hence the double peak. Check the gain of the mixer or rf amp transistor. Likely leaky electros due to age? Might be a ps feedback or unfiltered agc making problems?

A*s*i*m*o*v

wr> From: snipped-for-privacy@att.net wr> Xref: core-easynews sci.electronics.repair:344196

wr> I have an old 6 transistor radio (Spiket 2 Band, MW and LW) made in wr> Okenawa Japan. Complete with leather case, looks to be a late 1950s or wr> early 60s model.

wr> The radio works well toward the lower end of the AM band and also wr> upper end, but reception drops off in the middle of the band around wr> 800KHz, but the reception is good if I touch the antenna loopstick.

wr> I have tried to peak the antenna trimmer capacitor with no effect wr> around 800KHz, but the trimmer does peak the stations near the high wr> end.

wr> I have also tried to adjust the main tuning capacitor along with the wr> oscillator coil to try and get a peak around 800Khz without success.

wr> I have connected a generator to a couple turns of wire around the wr> antenna loopstick and monitored the signal on a scope. It tunes very wr> well and there is a sharp peak wherever I set the generator and tune wr> the cap. The local oscillator signal on the tuning capacitor also wr> looks good with fairly constant amplitude across the band.

wr> It's a perplexing problem. Everything works well, but reception wr> is poor in the middle of the band.

wr> Any ideas?

wr> -Bill

... If you don't know what leever "A" does then you better leever "B"

Yes, I checked the antenna coil and it looks to be in good original shape. There are 2 windings for the 540-1600Khz band and another winding for the 160-340Khz overseas band.

I disconnected the overseas winding to check for loading on the other band but no success.

The factory trimmer capacitor settings seem to be wrong since a couple were set to maximum capacitance.

I think the problem is in the oscillator/mixer stage but I can't get the main tuning dial off to get the circuit board out. There is a emblem glued into the center of the tuning dial that covers up a screw that holds on the tuning dial. I may have to drill it out to get it off.

Problems and more problems.

-Bill

cxcvvcxvcv

Hi...

Darn! Does that mean that proper preventive maintenance doesn't mean I need to tight all the screws down tight once in a while? :)

Not even those funny little ones hiding inside tin cans? :)

Ken

The IF amplifier also needs to be checked for the correct IF frequency, because your tunnig L and C was designed to track at the IF frequency and it will not track correctly at another frequency. Those IF trannys are very tempting for inexperienced people to tweak.

With any of the LC tuned receivers, the inductance of the RF coil (which is probably the loopstick), and the inductance of the oscillator coil is set while the tunning capacitor is fully meshed. This is at the lowest RF frequency. The RF and Osc trimmer caps are set at the high end of the dial or when the tunning cap is fully unmeshed. Repeat the steps as neccessary to get your best results at the extreme ends of the tunning range. If the tracking goes bad in the midband, some tunning caps have slotted plates so that the plates can be bent to improve the tracking, but this method is not to common for low frequency cheap receivers.

Unfortunately, the problem cannot be fixed by any amount of tweeking. It doesn't matter where the main tuning cap is set, and all the other adjustments peaked at that point, oscillator, IF, trimmer caps, etc. The result is always poor reception in the middle of the band. That's why I can't figure it out. It doesn't make sense that with every adjustment peaked at 800KHz, that point is sill weak. Yet if I sweep the antenna coil and cap, it looks fairly flat on a scope. In other words, there's nothing wrong, except it doesn't work.

-Bill

Hi Bill...

Can't tell whether you're in a big city with lots of stations, or a smaller one with just a few, so...

It's not possible that you're young enough not to have used these back in the olden days, is it?

If so, and if you have only one or two stations in the middle of the band, you should know that those antennas are highly directional.

Just in case...

Ken

"Priyan Perera" bravely wrote to "All" (06 Oct 05 15:23:13) --- on the heady topic of "Re: Old 6 transistor Radio repair"

PP> From: "Priyan Perera" PP> Xref: core-easynews sci.electronics.repair:344274

PP> cxcvvcxvcv

Even the babblefish is stumped!

A*s*i*m*o*v

... "Hey, I'm just this guy, see?" --Zaphod Beeblebrox

Have you watched the AGC voltage as you tune across the band? I suspect the AGC is reducing the IF gain when you tune through the mid-band, due to a stray oscillation. Could be caused by a bad capacitor on the AGC line. A scope should show it.

Don

The AGC seems to be working, since the reception is good on a weak station near the lower end of the band. I can't imagine the AGC would work at some points in the dial and not at others.

I did manage to get good reception in the middle band by adding another antenna loopstick and capacitor. I used a few turns from the external loopstick to couple the signal into the radio main loopstick with another few turns.

Two loopsticks works better than one, but they won't both fit into the same box.

-Bill

But I can imagine an IF oscillation at some part of the BC spectrum and not at others - been there, fixed that.

Another thought is that someone screwed up the local oscillator, and it's operating at 2x the I.F., low. This would prevent correct tuning of the loop's trimmer.