Decca SRG899 filter cap gone bad?

[crackle]

Have you checked for hum at the input of the amp.

Mike

[Techman]

Another good point regarding the input. We're probably all assuming that there's hum with low audio, but there could be normal audio level with induced hum at the input that is above the normal audio level - this would be a completely different type of fault.

[al_kaholik]

OK thanks for some more hints and tips. The output transistors linked to the heat sink are TIP31 (detail a plenty however not sure what data I should be looking for other than the leg arrangement) and R2447 (and come up blank on this one. Given the coloured leads are the same to these in the same arrangement, I've assumed the same arrangement of legs). Having done a diode test there are two outlying values:

TIP31 Base -, Collector +
Right: 1891, left: 848

R2447 Base -, Collector +
Right: 821, left: 671

Other than these two there is reasonable consistency between both sides - tested in circuit with power off. The TIP31 values on the left are consistently slightly higher than the right.

I snagged the fuse holder on the audio board and damaged the fuse for the left channel so will be waiting for some more to come. Interestingly, there was still hum on the left channel when power was turned on. Not sure if this means anything to anybody.

I didn't have a feel at the heatsink, should I be expecting it to get warm with, say just the radio playing? Would it be best to touch the transistor case carefully rather than the heatsink as its common across all 4 transistors.

the three legged balls as shown in the pictures before - are these also transistors? If so I can also test those.

Al

[al_kaholik]

Originally Posted by crackle

Have you checked for hum at the input of the amp.

Forgive my ignorance Mike, how would I do that? The testing above is the first I've done other than swapping the speakers physically

[Refugee]

Looking at that messy heat sink compound I am wondering if the bodger may have broken one of the screw insulators.
Check for a short between the collector (white wire in center) and the heat sink on both transistors.

[PWH]

Does the hum vary with the volume control. That would narrow down the source of the fault. No variation, fault is after the volume control..

[al_kaholik]

Originally Posted by Refugee

Looking at that messy heat sink compound I am wondering if the bodger may have broken one of the screw insulators.
Check for a short between the collector (white wire in center) and the heat sink on both transistors.

No problem, will get on that and report back

Originally Posted by PWH

Does the hum vary with the volume control. That would narrow down the source of the fault. No variation, fault is after the volume control..

I believe not, but will give it another listen today to confirm.

[crackle]

Originally Posted by al_kaholik

Forgive my ignorance Mike, how would I do that? The testing above is the first I've done other than swapping the speakers physically

The way I would test that is to take a signal from the input into the amplifier and feed that into another audio amplifier, A transistor radio would probably suffice, one with an amp input socket.
Depending on where the volume control is in the circuit, a check to see if the hum goes louder as the volume is turned up will probably achieve the same answer.
Mike

[al_kaholik]

Thanks Mike - I've ordered a small solid state amp board which should be up to the job of testing and can be repurposed at a later date. Its silly how cheap some of these things are, 2 for £10 delivered next day on Amazon.

[al_kaholik]

Originally Posted by crackle

The way I would test that is to take a signal from the input into the amplifier and feed that into another audio amplifier, A transistor radio would probably suffice, one with an amp input socket.
Depending on where the volume control is in the circuit, a check to see if the hum goes louder as the volume is turned up will probably achieve the same answer.
Mike

Mike - tested with a separate amp today - no hum in either channel is present with a speaker plugged directly in. I've unplugged the power and signal to the power amp, and fed both channels into a discreet amp running from a separate power supply.

As I've bypassed the original speaker wiring should that be a further test? Complication is that it is soldered rather than on a terminal, but I'm happy to un solder if there is merit in doing so.

Originally Posted by Refugee

Looking at that messy heat sink compound I am wondering if the bodger may have broken one of the screw insulators.
Check for a short between the collector (white wire in center) and the heat sink on both transistors.

I can't detect a short between any of the collectors and ground (continuity)

Originally Posted by PWH

Does the hum vary with the volume control. That would narrow down the source of the fault. No variation, fault is after the volume control..

Confirmed as a no, constant from the get go.

[al_kaholik]

I've been doing some mores searching and reading, and came across this post again https://www.vintage-radio.net/forum/...92&postcount=7, which lists out the transistors used. What I haven't done is examine the small ball shaped ones yet, only the larger ones with heat sinks. That'll be tomorrow's task

[Uncle Bulgaria]

Originally Posted by Techman

This isn't a mains smoothing fault as there's no hum on the other channel.

You're quite right. I was imagining one rail had got a smoothing problem, when of course that would affect both.

al_kaholik - you need to trace the signal rather than getting new bits and pieces. It takes some mental effort but it's worth it. See where the hum is appearing in the circuit. You said you have the circuit diagram from the pre-amp. Check the DC conditions - they should be marked on the diagram. By this I mean the static positive and negative voltages present to supply the components of the amplifier.

The diagram should mark ground (0V) and there will be rails marked (say) +12V. Check that these are correct for starters. Put your multimeter's negative probe to ground, and the positive probe to the rail you wish to test. Techman's idea of a biasing fault means that there is the wrong amount of current flowing in one of the stages when the amplifier is quiescent. As V=IR, any current change will show up as a voltage difference.

Where is the left channel fuse? Have you checked that the speaker connection is properly made inside the amplifier? Compare with the good channel - are the back of the red and black outputs going to the equivalent places on the bad channel as they do on the good channel? Even if they look like they are, check for continuity with the multimeter. Somtimes connections are broken inside the insulation of the wire.

Apologies for grandma/egg-sucking, but it's better to risk being patronising than too complicated, and it's not clear how experienced you are with a meter.

[al_kaholik]

Some delay to getting back to this.

Originally Posted by Uncle Bulgaria

Apologies for grandma/egg-sucking, but it's better to risk being patronising than too complicated, and it's not clear how experienced you are with a meter.

Don't worry about being basic, I'd rather start at the beginning and that is fine. I'm hesitant to post the circuit diagram as I have purchased it from the site, however if it is acceptable to do so, I will, else can post part of the diagram - I don't want to step on toes here.

Originally Posted by Uncle Bulgaria

The diagram should mark ground (0V) and there will be rails marked (say) +12V. Check that these are correct for starters. Put your multimeter's negative probe to ground, and the positive probe to the rail you wish to test. Techman's idea of a biasing fault means that there is the wrong amount of current flowing in one of the stages when the amplifier is quiescent. As V=IR, any current change will show up as a voltage difference.

With respect to the diagram, it is marked only HT+ and E. There are numbers in flag/arrows at various points around the diagram, and I am unsure what the meaning of these is - perhaps voltage. The only one that is explicit is on the power circuit showing 21V after the transformer but before the bridge rectifier. A lamp (power) is shown in this area too which states 6V. On mine I am seeing 11V at the lamps.

At the +ve on the pre amp I am seeing 25.4V. If the little flag number previously mentioned does refer to voltage, that is marked at 22 on the diagram. The power amp is fed from the same location on the transformer and is I measure 26v at the connector to the amp.

There is 4.56V LEFT and 4.78V RIGHT on the pre-amp output with the balance vaguely centred.

Originally Posted by Uncle Bulgaria

Where is the left channel fuse? Have you checked that the speaker connection is properly made inside the amplifier? Compare with the good channel - are the back of the red and black outputs going to the equivalent places on the bad channel as they do on the good channel? Even if they look like they are, check for continuity with the multimeter. Somtimes connections are broken inside the insulation of the wire.

There are two fuses on the power amp board, both appear to be working, however I've ordered new ones as an insurance policy

Speaker connectors run from the board into a connector and then to the rear panel external speaker connector. From there they then run to the speakers. All the soldered connections look good here, I've run some Servisol 10 into the socket and run a plug into them a number of times to shift any detritus (they are the type that short the internal speakers when connected). The connections to the speaker terminals themselves are with spade connectors which again I've remade the connection a number of times. There is clean metal on both surfaces and there is continuity to each connector.

[Robint 47]

Reading your post i am a little confused do i take it you are getting hum only on the left channel on all functions, which is not effected by volume control etc so do i assume hum is being induced in the amp itself ? I think you are confusing supply rails as the tuner bulbs are 14v and the indicator is 6.3 v i off LES tuner is 4 off LES .
Also what circuit diagram are you using ? ROBIN

[al_kaholik]

Originally Posted by Robint 47

Reading your post i am a little confused do i take it you are getting hum only on the left channel on all functions, which is not effected by volume control etc

Correct

Originally Posted by Robint 47

so do i assume hum is being induced in the amp itself ?

It is my assumption that it is something to do with the left power amp (they are twinned, one for each channel on a single board)

Originally Posted by Robint 47

I think you are confusing supply rails as the tuner bulbs are 14v and the indicator is 6.3 v i off LES tuner is 4 off LES .

Perhaps I am confusing the rails, and apologies I have no idea what the latter part of this means.

Originally Posted by Robint 47

Also what circuit diagram are you using ? ROBIN

The only power side circuit diagram I have is from the A600 service sheets. Only the pre-amp is common with that unit. Again, a dangerous assumption perhaps, but one would think that the pre-amp would operate at the same voltage in both units.

Finding a service sheet for the other elements is just turning up a blank

[Techman]

Originally Posted by Techman

You'll need to do some proper tests and learn how to test transistor junctions with a meter - loads of info on the net. You've got the benefit of having the working channel as a comparison for when you're taking your voltage readings,

As I've already said above!

All transistors in the channel - all voltages and compare.

And if voltage readings show differences, then check junctions (base/emitter and base/collector plus collector/emitter both directions with an ohm meter, preferably an analogue type) - you may have to remove them to check in some cases.

This type of amplifier can be very awkward to fault find, but if you don't follow the advice given, you'll get nowhere - seriously!

[al_kaholik]

I do appreciate the help, previously after looking up how to test a transistor, clearly I'd been doing diode testing. My misunderstanding clearly as it hadn't clicked that you'd said voltage.

Below are my findings for voltages across the transistors:

Transistor - L; R. All numbers refer to voltages. number 1-4 are my reference on the board.

Transistor Left Right
TIP31 6.1 5.4
R2447 6.1 5.4
1 10.92 11.3
2 11.39 11.7
3 11.42 11.86
4 13.40 13.42

Further to that the TIP31 and R2447 diode test results I did previously are here. The other measurements will have to wait till tomorrow

TIP31 L R
Base +ve Collector -ve 600mV 588mV.
B+ E- 593 575
B- C+ 848 1891
B- E+ 671 665
E- C+ 1650 1581
E+ C- 649 685

R2447 L R
B+ C- 652 646
B+ E- 585 585
B- C+ 571 575
B- E+ 821 671
E- C+ 663 670
E+ C- 1624 646

[Robint 47]

Sorry I am not able to understand your voltage readings on this amp. To this end I have a copy of the Decca 613 amp circuit (Decca module ass 6000) which for an email address I will supply. Before I am jumped on I have offered this to the site but no one responds and tells me where to send it? I feel this could be one of the few remaining copies.

You can then post the readings which are on the data and post on to the site not to me, i have some thoughts where your problem lies first let's see the data then do you have access to a meter capable of reading DC current, oscilloscope and an audio generator. I can then tailor my reply accordingly.

Robin

[Station X]

Originally Posted by Robint 47

Before I am jumped on I have offered this to the site but no one responds and tells me where to send it?

Robin

We received your enquiry email on 19 September and forum moderator AC/HL replied to you by PM on the same day.

[AC/HL]

You offered to scan some Decca data over the winter months, time permitting, a very generous offer which is appreciated. In the meantime, the scan of the power amplifier relative to this thread would be invaluable.