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Old 22nd Feb 2017, 6:32 pm   #21
Pete_kaye
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Default Re: Another Bush VHF72

Did a test on the o/p trx . Using my Chinese component tester I got 14H input and 0.05 mH output giving a ratio of 28:1 .The resistances were 223 and 0.3 ohms. Taking square root gives 27:1. Sound OK?
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Old 22nd Feb 2017, 6:41 pm   #22
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Default Re: Another Bush VHF72

Get another UL84 and try it if you have not done so already.

Cheers,

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Old 22nd Feb 2017, 6:45 pm   #23
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Default Re: Another Bush VHF72

Originally , I tried 2 others from good sets and they made no difference. The original one worked fine in the other set too.
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Old 22nd Feb 2017, 6:56 pm   #24
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Default Re: Another Bush VHF72

Check the voltages on all the pins and report here.

Cheers,

Steve P.

P.s. Are you using the Trader Sheet?
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Old 22nd Feb 2017, 9:18 pm   #25
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Default Re: Another Bush VHF72

If its any help in tracing the lack of volume I have just done the following test on a Bush VHF 71. (UL84 output valve so should be similer)

All with the set Unplugged and Cold.

Signal generator set to 1 Khz into a matching transformer with the speaker winding across the VHF71 speaker. Signal generator set to put 100 mV rms , approx 280 mV P/P across the speaker. All subjective but fairly loud tone from the set under test at this level.

Next shut down the generator and unplug the UL84. Re injected 100 mV rms and measured from the anode pin 7 to the chasis, this measured 2.1 Volts rms / approx 6V P/P.

This does a basic check of the speaker , wiring and transformer all in situ. The only reason for using the external matching transformer was that I did not want to feed my signal generator into a low impedance speaker. If you have a low impedance signal source then fine use that.

Measurements were with a Fluke 179 true rms meter and CRT scope but if you have a sine wave source any DVM meter should do it, scope not required just used it to cross check.

Pete
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Old 24th Feb 2017, 7:28 pm   #26
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Default Re: Another Bush VHF72

Any further on with this?

Cheers,

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Old 25th Feb 2017, 3:59 pm   #27
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Default Re: Another Bush VHF72

Steve I posted the voltages weeks ago and they were all good . I have put it away for now as off on annual hols in Spain .
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Old 6th Mar 2017, 6:14 pm   #28
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Default Re: Another Bush VHF72

I looked at my previous post about the trx impedances and realised that my calculaation was astray! The output was 0.05 MH not 0.5 H giving aratio of 280000 to 1 . I checked it agin before I left .These readings were the same but TRX still in circuit so that might explain it - the speaker was not connected for tests.
Very sunny out here by the sea....
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Old 6th Mar 2017, 7:48 pm   #29
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Default Re: Another Bush VHF72

Off the Spain again Peter ! have a good time regards Mick and Julie
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Old 7th Mar 2017, 3:07 am   #30
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Default Re: Another Bush VHF72

Pete, are you still considering the lack of volume to be due to the output transformer?

With no power, putting your DMM or Avo on resistance range across the transformer primary should give you a clear click in the speaker. If it does its most likely OK.

Have you checked the tone correction caps on the transformer? You could dis them temporarily to check.
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Old 8th Mar 2017, 1:00 pm   #31
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Default Re: Another Bush VHF72

I think I did the click test originally but will repeat when I return and disconnect evrything from the TRX and retest.
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Old 30th Apr 2017, 4:15 pm   #32
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Default Re: Another Bush VHF72

Test by another member showed TRX without any connected in components was fine and odd readings were due to my cheap limited meter . Tests gave 40:1 ratio . So re-installed and now have simple signal generator as well to test stages going backwards .
Into G1 of UL84 gave loud buzz. From G1 of UABC80 a quiet buzz .From wiper of Volume control a quieter buzz than from UL84. Still little response here with screwdriver and finger to wiper. Odd?
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Old 30th Apr 2017, 4:53 pm   #33
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Default Re: Another Bush VHF72

If you had a faulty meter before then please take all the readings again. Point A is louder then Point B is not really enough is it?

Especially check the HT voltages as you say the valve is OK. And is the IF aligned OK?

Cheers,

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Old 30th Apr 2017, 7:47 pm   #34
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Default Re: Another Bush VHF72

Sounds like the triode of the UABC80 isn't doing much or the signal is getting attenuated before the UL84 grid.
Voltages please for a diagnosis.
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Old 7th May 2017, 1:20 pm   #35
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Default Re: Another Bush VHF72

The cheap meter I was referring to earlier was a 10 component tester from China not my DMM from Maplins. The latter was used for taking all Voltages.
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Old 12th May 2017, 7:16 pm   #36
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Default Re: Another Bush VHF72

Problem solved at last . In the beginning , January, I replaced the 0.01 Mf cap to V4 G1 with a NOS Dubilier part . This was the fault but I just didn't recognise it.
The odd thing is that the voltage on G1 was the same as before at +0.02 V . I imagine an oscilloscope would have shown up the fault . There's a saying 'never replace a worn part with another worn part'.....
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