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Old 27th Jan 2020, 12:11 pm   #81
Hybrid tellies
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Default Re: 9 volt battery eliminator hum

I have had this problem before. I used one of those linear unregulated psu's which you could vary the output voltage, 4.5v, 6v, 7.5v and 9v and you could also change the polarity of the output lead. It worked fine on my portable cassette recorder but not so well on my new radio. The radio had a pronounced and varying background hum with the back of the cabinet near the earphone socket was running very warm.
It turns out that with the small load offered by the radio the output voltage was over twice the voltage, 6v, than it should have been with a very high ripple on the output. Unfortunately by the time I investigated I had destroyed the output stage of my radio.
The PSU was rated at 300mA, it was ok for a higher load such as a cassette recorder but not for the lower load of a transistor radio.
I learnt this lesson the hard way many years ago.
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Old 15th Feb 2020, 6:40 pm   #82
ssnjimb
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Default Re: 9 volt battery eliminator hum

Hi

Iam testing current on the multimeter

Black lead to neg
Red lead to positive wire

It says between 57 and 65 ma

See picture

Am I now testing it correctly.

Iam running it from a new ever ready silver pp9

James
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Old 15th Feb 2020, 9:32 pm   #83
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Default Re: 9 volt battery eliminator hum

Test method looks OK. Is that with Vol right up or down. With vol right down, it is maybe 10 times too high. Al those red and black electrolytic capacitors will have been *n***ered for the last 40 years. REPLACE.
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Old 15th Feb 2020, 9:33 pm   #84
ssnjimb
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Default Re: 9 volt battery eliminator hum

Hello Les

Half volume

Full volume is distorted

I will out if interest do it on full volume tomorrow

James

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Old 16th Feb 2020, 3:13 am   #85
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Default Re: 9 volt battery eliminator hum

It could be leaky output transistors causing the problem if its drawing this much current and going distorted at high volume. I have had this fault on many transistor radios over the years. It could be worth checking any presets around the output stage as well.
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Old 16th Feb 2020, 12:06 pm   #86
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Default Re: 9 volt battery eliminator hum

Quote:
Originally Posted by ssnjimb View Post
Hello

Attached are pics of the original battery clips. and the power supply clips

It hums on the power adaptor.

It works fine on a 9v battery (pp9)

Yes i know it ran from the pp10 which is no longer available.

Thankyou for your help so far

James
It may sound fine, but I think you'll find that it doesn't in fact 'work fine' on a PP9 battery - not if you expect the battery to last any length of time. Your RT7 is clearly faulty, and as others have said, the electrolytics are the obvious candidates. Your unregulated cheapo PSU does at least give you a strong clue as to why.

In post 56 you show that when connected to your R200 the PSU was at 17.8V Ė almost twice as high as it should be. Thatís because the consumption of the R200 was more or less what it should be Ė maybe 15 Ė 20 mA, so the voltage of the PSU is close to it's no-load standing voltage. However, when connected to your RT7, the voltage falls to just 4.4V. Thatís because the RT7 is drawing a much higher current than it should do, so the unregulated PSU Voltage drops to half what your RT7 needs to function properly as the PSU canít maintain anything like 9V with that high consumption.

An adequately rated 9V regulated PSU would maintain the voltage, but the fault(s) on you RT7 would still be there, which almost certainly stems from leaky electrolytic capacitors, which is a commonplace problem on Roberts (and Bush) radios which Ė like your RT7, are now 60 years old. In particular, I would highlight the decoupling capacitors C25 & C32 on the -9V line. Iíve circled them on the part circuit below. If Ė as they probably are - theyíre behaving not like capacitors but as low value resistors, they will draw excess current, putting almost a dead short across a PSU or battery. When you use the set on your unregulated PSU, as this drops the voltage down to 4.4V, the set canít possibly perform correctly.

Purists would say that itís bad practice to change capacitors without first confirming that they are definitely faulty.

You could do that using say a Chinese Multi tester, or even your multimeter on the Ohms range, placed across the capacitor (after first having discharged the cap by shorting out its connections). But those two capacitors wonít be the only ones that have failed/are failing. Others can cause the set to draw excess current and may damaged the transistors. For example, C20, C27, C28, & C31.

If you have a capacitor tester that will show capacitance and ESR, you may find some caps are still clinging on to life, but if it were my set, Iíd be routinely changing all the electrolytics. Youíd do well to check the price and availability of NOS OC44/45/71/81/81D, because if you continue to use the set in this condition and damage any of the transistors, just one damaged transistor would cost more than replacing all of the electrolytics. If any transistors do fail and the sets dies, you'd then have the problem of diagnosing and rectifying where the fault lies.

To show how faulty electrolyics can affect the performance or damage a set, I've attached part of the circuit of an unrelated Roberts set - an R505 that I repaired some time ago. The set worked briefly on switch-on, then went dead and the output transistors became too hot to touch. That was because as seen in pic 2 below, C40 - a 680uF electrolytic, when tested showed up as a 0.53 Ohm resistor and was putting a dead short from the emitters of the two transistors via the speaker speech coil. Likewise, C40 & C42 were leaky and can put a dead short across the battery. Luckily, after replacing the electrolytics the transistors had survived and the sets performed well. (In that set, the audio was distorted due to clipping so the audio stage needed setting up).

It surprises me that whereas most who obtain valve receivers of the 50s & 60s assume that some electronics work is needed for them to perform efficiently, there is a mistaken belief that if transistor sets such as Roberts, Bush and Hacker, seem to sound OK, they need only to be cosmetically spruced up and they'll be good as new, but as the saying goes, 'you can't put lipstick on a pig'. Your RT7 is a rather nice set, which cost £18.00 in 1960 + VAT, so about £24.00 in all. That equates to £540 today when accounting for inflation. Worth some TLC I think.

As to the unregulated PSU, as others have said Ė itís junk.

Bin it and get a regulated one such as the ones that I mentioned in post 51 from CPC. I've used 9V 1.6A ones and 12V 1A ones. Regulation is to within 5% regardless of AC input Voltage, which incidentally can range from as low as 90V to as high as 264V. They have over-voltage and over-current protection and short-circuit protection. The specs for those PSUs are available at the links. Note:' 50,000 hours MTB' - mean time before failure!). Of course, you may need to cut the 2.1mm connector off and fit new connectors if you need PP3 or PP9 battery snaps, but that's no big deal.

Here are the links again:

9V 1.6A

https://cpc.farnell.com/powerpax/sw3...ted/dp/PW02349

12V 1A:

https://cpc.farnell.com/ideal-power/...power%20supply

I hope these rather wordy notes are of help.
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Old 16th Feb 2020, 12:15 pm   #87
ssnjimb
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Default Re: 9 volt battery eliminator hum

Thankyou David. I binned the power adaptor last week and I didn't use it on any of my radio's again.
Your notes do help.

James
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Old 16th Feb 2020, 3:29 pm   #88
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Default Re: 9 volt battery eliminator hum

James,

I'm convinced that some combination of these things are happening to give such a high consumption:

i) leaky electrolytics (in particular, the main decoupler, C32 in the Trader sheet);

ii) bias resistors for the output transistors shifted (R26 low and/or R37 high)

iii) output transistors gone too leaky.

And the conversion of the unregulated supply to a regulated one is relatively straightforward.

You have a PM!
dave
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Old 20th Feb 2020, 3:09 pm   #89
ssnjimb
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Default Re: 9 volt battery eliminator hum

Hello

Now sorted. The unstabilised power supply has had a stabiliser added (7809 + relevant capacitors) in a small box. And the quiescent current is below 20mA, which isn't concerning.

Thankyou For Everyone's Input

James
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Old 21st Feb 2020, 10:30 am   #90
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Default Re: 9 volt battery eliminator hum

Sounds like the best possible outcome- everything working and nothing just junked!
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