Re: Roberts RT7 - problem
 

OC81's in the 20k ohm range on my meter;
base - collector: 2.97
collector - base: no reading
base - emitter: 3.16
emitter - base: no reading
collector - emitter: 1.64
emitter - collector 18.25

base - collector: 3.25
collector - base:no reading
base - emitter: 3.67
emitter - base: no reading
collector - emitter: 1.84
emitter - collector: 19.19

Looks like the bin.

Re: Roberts RT7 - problem
 

They are probably OK. So its something else round there. Check them two resistors.

Cheers,

Steve P.

Re: Roberts RT7 - problem
 

The two resistors R26 & R27:
Checked using the 20k setting, no power, and in situ with the meter points held at each end;
R26 1.05; R27 .08

R26 With battery connected and on, voltage at the 'Gold' end: 9.72 'Red' end: 0.28
R27 'Gold end': 0.28 'Green' end: 0.00

Derek

Re: Roberts RT7 - problem
 

Hmm, they are OK then. What are the voltages around TR5?

Cheers,

Steve P.

Re: Roberts RT7 - problem
 

While you are checking voltages elsewhere the two OC81's should have their collectors or emitters lifted out of circuit if these are still drawing excessive current, of course this can cause an additional problem of having to unsolder them. But they can't be left in drawing excess current and overheating, something is biasing them on far to hard. The base driver stage of the output stage should be checked, this is the other (driver) transformer and its components on the board.

What should have been mentioned earlier is to monitor the battery current rather than wait until things get hot, by putting the milli Ampere meter in series with the battery lead. The current should only be around 10 milli-amps or so, if the needle shots off the scale then this is why the things are getting hot and draining the battery.

(With a good radio at normal volume this current with increase and fluctuate in time with the signal)

Geof

Re: Roberts RT7 - problem
 

TR5 power on; Red lead 9.5v; Black 1.83v; Base(?) 1.95v.

Should I not be testing these components for resistance off the board?

Derek

PS Thanks Geof. The OC81's are currently off the board. Battery levels for the minute or so checking for voltage has remained stable - same before and after checking (so far). But with OC81's attached, there was a drain - it drew the PP9 down from 10.1v to 9.1, which has since recovered to 9.93v.

Re: Roberts RT7 - problem
 

TR5 looks like it is OK, this can be checked for forward and reverse, collector to base, base to collector and so on, in situ with no power applied of course as you did with the OC's.

resistors can give misleading readings in circuit as all the other resistors are also in the circuit with the one being measured, an end has to be lifted to measure a resistor, so only check those which give cause for concern as most will still be ok and should not be unsoldered for no real reason.

The fault is somewhere in the driver transformer secondary (output OC81 base) stage as far as I can tell without having the circuit to look at.

Geof

Re: Roberts RT7 - problem
 

I'm not sure of the best way to do this, but I'll try and get a diagram across.

PS Must shut up shop for tonight - Cheers for now.

PPS Geof, I'm aware of copyright infringements posting the circuit, so will send in a separate PM.

Derek

Re: Roberts RT7 - problem
 

If you're thinking that Geofy, then R28, R29, R30, C33, The speaker or the Transformer.

Could be a bit nasty.

Cheers,

Steve P.

Re: Roberts RT7 - problem
 

All this taking out and putting back could generate more problems. These old single-sided SRBP boards were't very robust, and this one has had plenty of time for the glue to deteriorate.
Alan

Re: Roberts RT7 - problem
 

Yes Steve
If there is a leaky cap across a bias resistor or if the bias resistor has gone low this will put to much current into the bases causing the OC's to get hot. Could be a shorted turn on the transformer but I suspect these are still intact.

A short in the speaker could do strange things in the output transformer. This could be disconnected just to make sure, while monitoring the current.




Yes it is not a good idea to keep soldering and unsoldering, could lift tracks as well as break tracks. And the board doesn't look in great shape with green mould on leads and grime!

Geof

Re: Roberts RT7 - problem
 

Thanks for the PM geof.

Judging only by the visual state of the transformer, and not knowing how to test same, or what values should be read at the various terminals of same, it doesn't look well.

Where would I be able to obtain a replacement?
Various transformers are available having surfed the radio spares sites, but pictures are unreliable, and connections 'appear' different. And of course size matters.

I'm not sure what is meant by 'glue' - literally, or is that a reference to solder. If as I suspect the latter, I have noticed a track with a gap in it. It runs between a resistor R30, and the OC81 TR6. Yet as the voltage readings have already been taken it would seem this has no effect - or might it!

I've been busy drawing a circuit so as to scan in and present it below. I realise I could have scanned the service sheet in and copied and pasted the relevant bit, but it got my hand in with a pen and ruler!

I've been checking out some of the electronics 'how to' sites also, though don't see it as something I would gravitate to in a big way, however I'll keep at it - though appreciate it might still end up in the bin.

Re: Roberts RT7 - problem
 

Derek,
The copper 'sheet' is literally glued to the base laminate. Particularly with old boards the glue used is not very resistant to high (soldering) temperatures, particularly when it's done a few times, and the copper can lift off the base :(
You wouldn't expect there to be any breaks in the tracks unless there has been a modification, although this is sometimes done by the factory before the board is updated.
Alan

Re: Roberts RT7 - problem
 

A break in the track to R30 would just make the output sound a bit shriller since R30 / C33 just tailor the high frequency response a bit. It does look as though it's just the tinning missing rather than a track break in the picture, though.

It's an interesting output transformer- somewhat reminiscent of a Quad II with its distributed winding layout.

Just a thought- if the volume control were to go open circuit it would seriously upset the driver transistor TR5 bias, though whether the rest of the resistors around that device would let serious current pass I can't tell without values.

Chris

Re: Roberts RT7 - problem
 

The transformers are probably OK, and it would get into the realms of being totally uneconomic unless you can pick up a free one from a scrap set. The money would be better spent on a newer better Robert's and keep this as an ornament.

The break in the track looks like someone may have tried to improve the treble by taking out the tone correction R30/C33 BUT what is more interesting is the solder splutter around this break, which would cause a problem if it was shorting out the transformer. This needs to be cleaned up, a link can be soldered over the break as the tone correction components are not likely to be faulty.

Thanks for posting the circuit. It is a bit of an unusual output transformer.

Other things worth a try, as Steve suggested, the speaker could be disconnected with the OC's refitted and the battery current monitored, if it is more than 10 to 20 milliamp it should be switched off straight away.

The volume control if it went open wouldn't have any direct effect on the output as the driver transformer isolates this stage from the driver transistor for DC.

You are not doing to bad if you hand copied that diagram, one of the base connections in the lower output OC81 base is missing though :thumbsup:

Geof

Re: Roberts RT7 - problem
 

Curses! Yes, in the TR7 . . . (the rest was pretty good though!)

I've stitched the gap in the track up, but untested. The speaker is disconnected and has been during the recent tests on voltage and resistance.

I really want to be sure that the transformer is OK, before launching into a big replacement program - but how to test it? I only keep working ornaments ;)

Derek

Re: Roberts RT7 - problem
 

Bear in mind that powering a radio with the 'speaker disconnected and no other load applied (such as an ammeter) can itself cause damage to the o/p stage.

Regarding testing, the service data tells you the DC resistances - which you can check. Otherwise just reconnect the 'speaker, ensure battery is up to the job and switch on. You'll soon know by audio quality and volume if anything's amiss. If it is, switch off asap though.

Re: Roberts RT7 - problem
 

A good point, often overlooked :(. A temporary speaker would be useful here.

Geof

Re: Roberts RT7 - problem
 

when you measured the voltages around the OC81 earlier you said emitter to base voltage was 9.6V. This can not be the case if transistor is fitted correctly.

Maximum should be 0.2-0.3V. Similar voltage as that measured on R26 and R27 earlier but a little lower due to base current.

Without battery fitted If you measure resistance between R26/R27 junction and transistor base you should get a reading of only a few ohms if driver transformer winding is OK.

Similarly if you measure resistance between battery positive and emitter should have only a few ohms. Also battery negative to collector. High readings may indicate broken transformer windings.

Are you sure the transistors were fitted correctly?

As the speaker has worked it is unlikely that this is loading the transformer excessively and it can not be open circuit.

Re: Roberts RT7 - problem
 

Hi Smiler,

The voltage readings were taken using the positive lead on the chassis (pos. earth) and the negative lead to the respective wires on the OC81's with power on.

The resistance was checked in the 20k ohm range with OC81's off the set and on the bench.

Derek

PS Yes, they were fitted the same way as the originals.