Fidelity Rad 21

[NorthernLight]

I have just received a 'not working for parts' 'loudspeaker pops on switching on' Fidelity Rad 21 bought off the usual auction site. No output confirmed on power on but my Velleman K7000 tracer picked up radio which tuned on all bands when connected across the Rad 21 potentiometer and down to the collector of the first Lockfit transistor BC149 in the audio amp. After that the trail was dead on both tracing/injecting. Looking at the schematic, the audio amp has Lockfits BC159 and BC149 then AA129 leading in to what I am guessing is a push-pull AC188/AC187 arrangement? Subsequent placing of the K7000 injecting probe on the BC159 collector brought the radio to life with a very muffled sound quality, low volume output on the Rad 21 Elac speaker. I wondered whether I should think about 'generically' replacing all the electrolytic capacitors in the audio amp (I count 6) or 'generically' replace the 2 lockfits BC159 and BC149? I have yet to check transistor voltages and the capacitor values on the board which might be a more sensible 1st approach? When it comes to de-soldering components with my suction/electrical desoldering tool I have had variable success in terms of not overheating the tracks. Looking at my Rad 21, the trackpads look vulnerable as in the risk of lifting. I would prefer to limit the number of components (specifically the Lockfits) that I have to deal with.

[crackle]

What type/make of electrolytics are they, a photo would help.

Mike

[paulsherwin]

Change the BC149/159. Lockfits are a notorious source of trouble, and substitutes cost next to nothing. You can use a BC549/559 or 2N3904/3906. Even if they aren't the cause of the fault, it's good preventative maintenance to change them anyway.

You may need to adjust the quiescent current afterwards.

[NorthernLight]

A photo of the amplifier section of the Rad 21 on my set.

[crackle]

I would hazard a guess that those electrolytics will not be too bad.
The lockfit resistors are known to fail the pressed on ends were never very reliable. The transistors have already been mentioned, I have found these tend to go noisy but can probably fail in other ways as well.
Mike

[NorthernLight]

I had removed some of the accumulated dust on the PCB before taking that photo as it was dusty. I wondered if those electrolytics are original 1972 when the Rad 21 first went into production. Has it been recapped at some stage? I have a Rad 18 which was made a couple of years prior to the Rad 21 and the Rad 18 electrolytics are blue and look a bit shabby in comparison. I haven't replaced those either. My Rad 18 has a very pleasing sound when compared to other Fidelity offerings I have listened to. The Rad 21 in comparison has a lot of plastic in its frame. I will check the Rad 21 electrolytics with my ESR meter, do some transistor voltage measurements to get a baseline and order some replacement Lockfit transistors for the BC149/159. Many thanks for your input.

[paulsherwin]

Don't fit NOS Lockfits, they're just as likely to be bad as the originals. Fit equivalents with a different encapsulation. In fact they're not critical and lots of NPN/PNP pairs will do.

[lightning]

Sorry to hijack the conversation but what are "lockfits" ? l assume a type of transistor but is that the make, or type?

[Restoration73]

It was a plastic encapsulated transistor style used by Mullard / Philips. Similar devices
later appeared in standard TO92 cases.

On the Rad21, the push button switches will benefit from switch cleaner - also check
where the pins are soldered to the track as these can fracture.

[paulsherwin]

Originally Posted by lightning

Sorry to hijack the conversation but what are "lockfits" ? l assume a type of transistor but is that the make, or type?

See https://www.markhennessy.co.uk/artic...rs.htm#lockfit

[NorthernLight]

Referring to the photo I posted above of my defective Rad 21 I have just checked the larger electrolytic bottom left in the photo which has on it printed 16 volts 1000uf. My ESR meter reports ESR at 0.07ohms and 1260uf in circuit. I will get round to measuring the other 5 electrolytics but so far, if I can believe these figures I would be disinclined to do much with the electrolytics. I have ordered BC559B PNP and BC549C NPN and surprised that they were pennies each. I am not sure whether the B and C suffixes are the best choice as the 549C has a much greater gain than 549B? Are BC559B and BC549C an appropriate match for each other? Once fitted these will replace the BC159 and BC149 in which I suspect the BC159 is faulty. I am not sure how I might go about adjusting the quiescent current. I am assuming that this means experimenting with local resistors per schematic and changing bias resistors such that the measured voltages for the new transistors equate to those published in the servicing manual namely BC159 collector 0.68V base 4.6V and emitter 5.5v and BC 149 collector 6.4V, base 0.68v and emitter 0v on no signal switched to FM chassis positive? Do I have this correct?

[Herald1360]

The quiescent current adjustment mentioned refers to the output transistors. Looking at the circuit, in this set no adjustment facility is provided, the level is set by the forward voltage of the AA129 diode, the output devices' Vbes and the 1R emitter resistors. No bias adjustment will be needed for the 159/149 substitutes, they are close enough to the originals in spec that the circuit design can allow for it.

[NorthernLight]

This was to remind a purchase off the popular website, 'not working' 'pops only on switching on'. I replaced the Lockfits BC159 and BC149 with BC559B and BC549C with no effect on distorted low volume. I then replaced an electrolytic C53 on the schematic 0.1uf, as high ESR, way out capacitance no change to distortion. I then noticed that the PCB from the collector of output transistor AC187 was fractured with the collector dangling off the PCB out of circuit. Connecting the AC187 collector via insulated multistrand wire to another connected part of the PCB strip, the radio came to life with full volume and no distortion! That's where I should have left it. I then changed the 200uf electrolytic (testing as 480uf) from the emitter of BC159. In doing so I inadvertently disconnected the speaker so no sound on power on but on resoldering the speaker connection distorted sound as before. However and I don't understand this, taking the collector of the AC187 out of circuit I now have a low volume radio but without any distortion. If I connect up the collector from the AC187 I am back to distortion. Faulty AC187 now? Did I damage it on resoldering? Are there any modern transistor equivalents for the AC187/188 push pull? (as in the lockfits). Do I replace the pair AC187/AC188 with NOS? Can I continue to use the set without the AC187 in circuit or will the AC188 fail shortly?