Quick fix no 5

My very first radio, given to me by a teacher when I was a high school student. Original valves: 6BH5, 6AN7, 6BH5, 6AL5, EF86, 12AU7, 6M5, 6M5, EM84, EZ80. However I have made changes over the years, and now it has 6BH5, 6AN7, 6BH5, 6AL5, EF86, 12AX7, 6BQ5, 6BQ5, EM84, 6CA4 in the ultra-linear configuration.

The problem was when first turned on it was rather loud and then after about 10 minutes it would drop to the correct volume. When placed on the bench of course it worked correctly. So I brought it back, and now it didn't work at all, with the EM84 fully alight. Back to the bench again, and it took a while to find the AGC filter cap was broken in half, but that wasn't noticeable until it was touched. Once replaced, the radio worked again.

The ultra-linear circuit takes negative feedback from the speaker to maintain a good frequency response, and poking around the speaker socket uncovered dry joints on the feedback components. So now hopefully there won't be any more volume variations.

The last problem which I haven't addressed yet is a hum, which means replacing the main electros, as the originals are still there. Unfortunately there's not much room, and they don't make replacement chassis-mounted types any more as far as I know. Something for another day.

I disconnect both leads on chassis mounted caps and replace them with standard 600V ones at as close to the correct capacity as possible. I'll usually leave the old one mounted in its original position to stop wandering fingers accessing "the works" underneath.

Quick fix no 6

Not a valve radio, but a stereo cassette deck from the 80s. These also need saving, as long as you have cassettes that still work.

This one is a Marantz 5010B, which looks expensive.

A long time ago I had fixed the mechanical issues with playing tapes etc, but after being in storage for a while, it now had more issues. The main one was when Play was pressed, the meter (sometimes both, other times just the Right channel) would fly to full scale and keep bouncing hard enough to make an audible sound. So, was it the power supply? or some electrolytic somewhere?

I downloaded the service manual, which has a schematic. Then I used the meter on AC volts to trace the signal from the output back through the stages, until I arrived at the playback head. Now the important thing with any cassette player is that the record/play switch changes which side of the head is earthed. But what happens if it isn't? Being at the start of the amplifier chain means it goes berzerk. So a bit of cleaner sprayed into the switch fixed the problem. I suppose it's obvious in hindsight, but not straight away.

While I was there a drop of oil was used to fix the door opening rather slowly, and another to fix the very stiff record button.

To test, I found a plug that converts the big stereo headphone jack to the smaller size used these days. Then I plugged in a set of computer soundblaster speakers, played a tape, and it worked quite well. Just shows that you don't even need an amplifier, the computer speakers do a good job.

Not common but the "swishing" may be "squegging". that can occur in the oscillator injection and in many sets there is a resistor around 27 ohms in series with the injection as an "anti resonance" device.

I still see mistakes, like wire wound resistor replacements in the RF and a failure to replace all electrolytic caps and factory dry joints missed.

Quick fix no 7

This radio is a Radiola with the usual 7-pin valves. It's hard to make out the model number but I think it says 539MA.

The problem was when tuned to my station the sound was completely distorted. After putting it on the bench there was a hum which I somehow hadn't noticed before. Measuring the AC levels showed that the 2nd electro wasn't doing its job, with about 5 volts there. I replaced it and now there was 0.03 volts. A big improvement. Next to it was a 2.7M resistor connecting the audio to the AGC, this resistor had gone high so it was also replaced.

Turning on showed the hum completely gone, and most of the distortion too. A quick tuneup to get the AGC voltage up sorted that out.

The last thing was to replace a blown bulb, normally a trivial task, but this radio uses those dreadful spring sockets, which unbelievably are a permanent part of the front panel. Of course both bulbs were stuck to their sockets due to the usual metal migration. I carefully used a screwdriver to prise out the blown bulb without breaking the spring. The bulb disintegrated as expected, but since it was already blown, who cares. I decided to try spraying WD40 on the new bulb's thread in the hope it might reduce future problems, then put it in. It lights up at least.

Put the case on and it sounds pretty good now.

Hi Rob, I keep up with your set fixes.
Find it entertaining.
Keep going!
Fred.

Quick fix no 8

This is the "Success" set that was referred to in another thread, and there's pictures of it somewhere on this forum too. It's the set that was missing a 2A5.

First thing was to look at the friction tuning. I worked out how to remove the tuning and dial from the radio. After cleaning the grease and grime, it's obvious the thing is clapped out, but it still worked, just. However the 3-gang variable tuning capacitor has noticeable friction, and it's this that prevents it all from working. Usually a drop or two of oil will loosen the tuner, but that didn't happen with this one. Oh well. So I manually pushed it to tune into my house station and left it there.

The radio itself works fine. Valves: 58, 57, 58, 2A6, 2A5, 80. These are the same as various octal valves, but with a 2.5 volt heater and the ancient 4,5,6,7-pin sockets. 58 = 6D6 = 6U7G, 57 = 6J7G, 2A6 = 6B6G, 2A5 = 6F6G, 80 = 5Y3G. The 57, a pentode, is used as the frequency changer. I have no idea how that works, but it obviously does, and it works well.

The original speaker was an electrodynamic type, but that wasn't in the radio when received. So, years ago I had mounted a choke on the back, and used an ordinary speaker with transformer mounted on it just loose in the bottom of the cabinet. But now, I wanted to use a 12 inch speaker, mounted properly. I already had a suitable front support, so now I screwed it in place. Out of my 4 spare 12 inch speakers, one had a transformer already on it, a Rola 12M. At this point I forgot a step. The original speaker used a 5-pin plug into the back, so I soldered the plug onto the Rola 12M, screwed it in, switched on, and... nothing?

Now I did what I should have done before, and check that the speaker actually worked. This showed that it didn't, as one of the copper braids was entirely missing, and the wire to the voice coil was missing too. So that speaker was no use. I then used a Rola 12-O, this has the bracket for the transformer, but none was ever fitted. I drilled off the transformer from the 12M and screwed it onto the 12-O. The carcass of the 12M went to the bin. A pity, as the cone was intact, no holes.

So, the 12-O was mounted, and switching on produced music. I let it run all afternoon, and it works very well, without needing any antenna wire. Not bad for mid-late 1930s tech. It makes a lot of volume too.

The last thing to do, is to find some screws to keep the chassis in place, and find a knob for the volume control. I won't bother about the tuning (since it doesn't work), and the tone (never need to adjust it).

A word of warning for anyone working on one of these. The HT is a lethal 360 volts. If the 2A5 is missing, then we are talking 430 volts.

It is often forgotten, or not realised that where there is a filament rectifier with no cathode sleeve, or rectifiers that behave like one, or he HT somehow goes open circuit, The surge voltage will in the majority of cases close to double its running voltage. Underrated caps are then in peril.

The most recent clearing sale fixer that landed here for no sound, had the principal problem diagnosed before it made it to the workshop. The speaker cone was the mouse nest on the inside of the chassis.

Hi Rob, enjoy reading your efforts.

The 2A5 set fix#8 sounds like it has an 'autodyne' front end.
That uses a pentode , the 57, to do both the oscillating and mixing.
It saved them using a more expensive pentagrid and still works fine on the AM band.
I did a lot of work investigating the autodyne somewhere in 'special projects'.
They had to shift to a proper pentagrid mixer to use with short wave and that was the end of the autodyne idea.

Fred.