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.