Roberts R606-MB Fizzled Out (but power clicks) - Tips?

[DavidEsp]

I have a Roberts R606-MB (Mains & Battery) receiver that worked ok until yesterday when it started crackling, popping fizzling and eventually the reception literally fizzled out. Same on battery and mains. Turning it on makes a loudspeaker-click but that's all. Leaving it on, it occasionally crackles back to life then goes silent again.

Any tips on diagnosing/repair (electrolytic? any in particular?), or anyone in Dorset area able to look at it (paid)? Or have anything broadly equivalent for sale (ideally a Hacker)?

[paulsherwin]

This sounds like a classic Lockfit transistor problem. Lockfit transistors are notorious, and there are lots of threads about them if you search the forum. Fortunately they are easy to replace in this set, and replacements are cheap.

However, always clean all the switch contacts with contact cleaner (e.g. Servisol - not WD40) before considering anything more major.

[DavidEsp]

Thanks for those tips Paul, I'll look into that. Also I just now saw here a reference to aged AF117's "growing whiskers". I'll see what lurks within the box...

[paulsherwin]

There are no AF11x transistors in the R606-MB. The Lockfits are just BC108 series silicon transistors with a distinctive packaging which proved troublesome after a few decades.

Do you have the skills to make repairs at this level yourself? Changing the transistors isn't at all difficult, but you do need to have basic soldering skills and be comfortable with simple transistor electronics.

[DavidEsp]

Yes I have electronics soldering skills.

Tomorrow I will have another go at dismantling the radio.
Today I got as far as removing the battery container / psu unit, and loosening the two screws that hold the main contents into the wooden box/cabinet.
Tomorrow the antenna and after that it should drop out easily? Or with strategic banging (hopefully).

[DavidEsp]

Ok I have it apart (as per attached photo)
- I intend to post the dismantling stages (photos) in a separate "tips" thread

Aha, so it's the audio section where the fault lies. Discovered by flicking wire across connections to the volume control potentiometer - no clicks or hums. Confirmed (as per attached photo) by (very temporarily) connecting from same potentiometer to my desk loudspeakers - beautiful reception emerged!

I'll get hold of the circuit diagram before proceeding with further "poke" tests. The geek in me wants it anyway - what's "under the hood"? (and how might I fiddle with it later?)

I just now bought the Technical Data manual (from UK Vintage Radio and Television Service Data), which I guess is you guys, for the later model of R606MB. Unable to find any serial number on the case or board, so I just guessed. The only relevant-looking number I can see on underside of circuit board is "TRX23/3-1".
Regardless, the manual matches up with what I can see.

[DavidEsp]

Well that's doing my mind in...
Trying to match circuit diagram with board components with tracks on board underside.
So I tried combining the board and underside at least (attached).

[DavidEsp]

Not exactly sure how next to proceed. My currently available equipment and components are extremely limited (not even a multimeter). I have therefore to rely on cobbling and "imaginative" approaches.

I see from another of your posts Paul, that the 1st audio transistor (after the volume control) could be a prime candidate.

Maybe in that case I should try injecting an audio signal after that transistor. To avoid accidentally damaging anything, I guess I first need to find a DC-blocking capacitor (to put in the signal feed).

[paulsherwin]

That looks like the later all silicon model - most of them are like that. The early ones are just R606s with an additional power supply. The output transistors in the early ones are Ge.

The transistors on the audio board are easy to replace. BC5xx types are electrically identical to BC1xx Lockfits, but you can also use 2N3904/6s or other general purpose types. The output pair are a bit trickier, but a BC327/BC337 or even 2N2222/2N2907 pair is normally OK if the radio just sees typical use.

If you want to change the transistors and don't have anything suitable, send me a PM and I'll put a few in the post for you FOC.

You will need a basic DMM to do anything really. There are lots on eBay for well under £10.

[DavidEsp]

Thank you so much Paul! I'll send a PM as you suggest.
Update: My cobbled signal injector (macbook audio out via a 200uF electrolytic) made sound come out at the second transistor but not at the first = TR6 = BC149.
Multimeter-wise, I'm scanning Amazon for something also having L and C measurement.

[paulsherwin]

You don't need that for basic measurements. If you do want to make basic LRC measurements on the cheap, you're better off buying one of the £10 Chinese 'component analysers' on eBay, which are useful for many things.

I'll send you a set of transistors that will allow you to change all those in the AF section should you want to. They're just jellybean transistors and I have hundreds of the things.

[pointsman]

I too had the same problem with my late father's R606MB which he used in his workshop for many years.

I now use it in my M.C, I was trying to free up the AFC switch that wouldn't pop out, turns out the switch spring is broken and am awaiting a new replacement switch. While working on the set and checking a few voltages on the power supply and around the board, I noticed that when I touched the three sleeved wires going to the rear headphone socket (yellow, blue and orange) the audio would cut in and out.

Upon closer inspection I noticed that the jack plug contacts were tarnished. The socket was removed and disassembled allowing the contacts to be cleaned with a fibreglass cleaning pencil. (If you attempt this, be very careful when bending the barbs back as they are very easily broken).

The set now works as it should do.


I increased the power supply voltage to 11 Volts and noticed that the audio sounds clearer.

[paulsherwin]

The earphone socket is a common source of problems with old radios, and a shot of contact cleaner is always a good idea.

[David G4EBT]

Originally Posted by DavidEsp

I have a Roberts R606-MB (Mains & Battery) receiver that worked ok until yesterday when it started crackling, popping fizzling and eventually the reception literally fizzled out. Same on battery and mains. Turning it on makes a loudspeaker-click but that's all. Leaving it on, it occasionally crackles back to life then goes silent again.

Any tips on diagnosing/repair (electrolytic? any in particular?),

The usual suspicions tend to fall on semiconductors, (and sometimes the little Mullard LPxxx modules) which may indeed be where the fault lies, but in my experience (and others), in radios such as the Roberts 505/600 two electrolytics in particular can cause be at fault as they become leaky (in the electrical sense), and can overdrive the output stage transistors and cause the radio to draw excessive current. When the radio is used on batteries, at initial switch on, the set may work for a couple of seconds, then goes dead.

I outlined this problem at post #8 at this forum thread:

https://www.vintage-radio.net/forum/...t=Roberts+R505

The two capacitors in question are C30, (470uF) which is connected from the 9V+ line to ground. If it is leaky, it will cause excess current to be drawn. Secondly, C31 (680uF), connected to the emitters of TR5 and TR6. If C31 is leaky, in effect it will short the emitters of those two transistors (and to an extent others in the output stage such as TR2) to ground via the speaker (assuming that SK3 isn't open circuit).

As others have alluded to, to carry out any meaningful checks you need a basic multi-meter.

Assuming that you obtain one, I would suggest that you power the radio via a battery, and with the meter on the milliamp range, put it in series with the battery supply. The quiescent current of the 606 (that's the current drawn with the volume at minimum and no stations tuned in) should be 23.5mA on AM, and 24.5 mA of FM.

If either or both of C30 and C31 are leaky, the current will be well in excess of that - maybe an amp, (or whatever the battery can supply with what amounts to a dead short across it). If you then connect your meter on the DC voltage range, you should have 9V or so across the battery, but if C30 and/or C31 are defective, on switching the radio on, the voltage will drop by several volts.

If the current draw isn't excessive, and the voltage across the battery doesn't drop appreciably, that suggests the fault(s) lie elsewhere.

A further check can be made with your meter once more on the mA range as outlined in the service data. If you disconnect the flex link 'LK' (under the PCB) in the feed to the collector of TR5 and put your meter test leads in series with the link, with the volume control set to minimum the current flowing should be 3.4mA.

With any radio, a good place to start is to check the voltages at various points as stated on the circuit, and to note any irregularities.

The R600 is a fairly complex radio using three Mullard modules - the LP1402, LP1181, and LP1185 as well as transistors and diodes.

As the radio isn't working on either AM or FM, at this stage, it's reasonable to assume that the fault may well lie in the audio stage. A signal tracer, (which could be just a small amplifier and probe), connected to the slider of the volume control would show right away if the RF, IF and detector stages are working, as you would hear tuneable signals, because you are basically substituting the audio stage of the radio.

How far you take this depends on the extent to which you wish to get involved in radio repair and restoration. If you just wat to get this radio working, it's hardly worth buying anything more than a cheap multi-meter for about a tenner, bit if you wanted to get together a few other items of test gear, a Chinese Multi-tester which will test transistors, diodes, resistors, capacitors (including the ESR of electrolytics), and inductors would be useful. You can get cheap unboxed testers for about £15.00. I won't give any eBay links, but if you search for 'All-in-1 Component Tester Transistor Diode Capacitance ESR LCD Meter Inductance' you'll find lots of offerings of the unboxed testers.

However, the MK328 tester is ready to use out of the box with nice test leads, is well regarded and available from many suppliers. About £25 post free from China, or quite a lot more expensive from UK suppliers. You'll find scores of suppliers if you search for 'Portable MK328 LCR ESR Tester, transistor, inductance, capacitance, resistance'.

With one press of the button, all of the testers automatically identify the component, it's connections, a digital display of performance characteristics, and auto shut-off after 20 secs.

A basic signal injector/tracer such as the Velleman K7000 kit is also very useful in pinpointing which stages in a radio are faulty. (lots of forum threads on the K7000):

https://www.esr.co.uk/velleman/k7000.htm

Hope these notes might be of help.

Good luck in bringing the radio back to life.

[majoconz]

What David said +1. My 606MB rattled, squeaked and groaned- all sorts of funny noises. Changed the electrolytics and problem solved - works very well except the same problem as Pointsman - the AFC pushbutton doesn't want to stay down for the first nine pushes then it relents! The eagle-eyed will notice that this pic is 'as found'.

[David G4EBT]

I hadn’t noticed that the OP hasn’t been on the forum since 19 October, and the thread had gone to sleep until 24 Feb, so hopefully the OP managed to fix his radio. Recent posts might help others. Although I wouldn’t condemn those blue Philips electrolytics on sight, they are more than 5 decades old, so before jumping to the conclusion that the transistors might be the culprits, and I’d always check the battery current being drawn even on an apparently dead set. If it’s much higher than spec, I’d certainly check the two capacitors in the audio stage that I mentioned in my earlier post, which are common to several Roberts of that era. (505, 600 etc). Other electrolytics can degrade the performance, not forgetting the open circuit earphone socket fault which sometimes occurs on an apparently dead set, which is a quick fix.

[DavidEsp]

My initially rustling then dead Roberts R606MB Radio now works !!!

A big thank-you to all who helped/advised me on this thread, from about a year ago, now closed. Especialy thanks to Paul Sherwin.


I replaced most of the audio-section transistors with non-lockfit (greater longevity) equivalent ones, gifted to me by . The only ones I left alone were the push-pull pair, because the existing ones had a slightly higher rating and most likely were (I presume) a balanced pair.

The precise culprit-transistor turned out to be Tr.8 (the one that comes down from the power rail) in the "Latest" version of the R606MB Technical Manual.


It took me this long because of initial incompetence-induced overwhelm coupled with life's distractions. My first transistor-change (a year ago) was a bit too sloppy for my liking. Partly due to a slight eyesight issue, short-term memory limitations and not remembering as much as I imagined about soldering. Consequent self-critical thought processes confounded me for a while. Thankfully my head has now cleared.

I "got back in the saddle" initially by watching YT videos showing how most other people do their soldering (and unsoldering), and (finally!) obtaining a desk/bench clip-on unit with croc-clip hold-arms and magnifying glass with LED ring around its outer. Having even just the positional LED ring was a revelation - much easier to see in detail, at all times of day/night. Also I bought a solder-sucker and braid (and watched YT vids giving tips on those).

The other thing I did was to take everything way more slowly and methodically - a couple of days in fact - as I planned, tested and wrote up everything in advance. For example identifying top-of-board component legs corresponding to voltage measurement points as per the tech manual's circuit diagram. And using a particularly fine-point marker pen to annotate the underside of the board. Moral: memory issue? Plan for it and write it up!

I also double checked each replacement transistor to ensure it was of the correct type (PNP/NPN), what its leads were (some websites got them wrong) and what its hFE was (via a transistor-testing multimeter, the Proster VC99). It is from the latter that I discovered the erroneous website data. Could have flummoxed me for hours or days...

Anyhow, "we got there in the end", so once again Thank You so much for your help (Paul and others on the original thread, now closed).

[paulsherwin]

Glad to hear you have it sorted out.

There's no particular reason to change the output transistors if they're not faulty, other than as preventative maintenance. I and others have substituted a BC327/BC337 pair without problems despite the lower current ratings, but it's obviously your call.