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Old 22nd Feb 2021, 2:32 am   #21
Techman
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Default Re: Bush VHF61 repair.

I agree with pretty much everything that has been said regarding these sets.

The EZ80 does go down quickly compared to the rest of the valves and causes low HT, which you've now rectified (pun!) with a new valve.

You've said that you've substituted each valve in turn, so that must include the ECH81, was it definitely a 'new' one, not a second hand untested one? I agree with what's been said about this valve causing poor AM performance, particularly on long wave, as has been said in the previous post - if in doubt try another one.

I agree that these radios only work half as well on AM as they do on FM, but having said that they work acceptably on that mode.

My example of this particular set works very well on its own internal aerials.

They were made primarily as an FM set.
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Old 22nd Feb 2021, 8:49 pm   #22
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Default Re: Bush VHF61 repair.

Here are the test results from the ECH81 valve that is currently fitted.

Triode Ia=4.4mA (5); gm=1.6mA/v (2.3)
Heptode Ia=4.3mA (6.5); gm=1.2mA/v (2.4)
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Old 23rd Feb 2021, 9:32 pm   #23
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Default Re: Bush VHF61 repair.

From the test results it would appear that the ECH81 has low mutual conductance for the triode which could problems with the local oscillator particularly on LW
I would replace it as it not particularly reliable and it is an easy job to swap it out, any else would require the chassis to be removed from the cabinet.
If you search for this model on the forum you will find several threads about the poor AM performance.
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Old 18th May 2021, 12:12 pm   #24
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Default Re: Bush VHF61 repair.

Hello all, apologies again for the long absence but my enthusiasm for vintage radio remains, its just time that I lack. I hope to get back on this set over this week and revisit the project.
Currently I'm changing the tuning gang grommets as they had perished, hardened and broken up. I have a couple of bits of wiring to replace and a poor aerial condition to the internal ferrite aerial.
Many thanks

Dave
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Old 18th May 2021, 6:30 pm   #25
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Default Re: Bush VHF61 repair.

I have a new fault manifested itself on this set. R40 on switch on is getting hot and smoking. I've pulled all the valves other than the EZ80 rectifier, still the same. I've disconnected everything from R30 onwards and also T1 from the HT. So the only things in circuit are c69, c70, r40, the mains transformer and EZ80. Disconnecting c69 and c70 from either side of r40 stops the overheating but that is completely disconnecting the circuit so the EZ80 has nothing to earth through. So either I've got a bad c69,c70 or the EZ80 has failed. The heater on EZ80 is seeing 6.8 volts with the other valves disconnected. I've checked for shorts across the pins of the EZ80 and all appears OK. What do the experts think? Could a bad earth to chassis for the mains smoothing / reservoir cap cause this?
Many thanks

Dave
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Old 18th May 2021, 6:34 pm   #26
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Default Re: Bush VHF61 repair.

A diagram to help
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Old 18th May 2021, 7:36 pm   #27
Peter F4VSA
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Default Re: Bush VHF61 repair.

Looks like C69 has gone very leaky

Peter
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Old 18th May 2021, 7:44 pm   #28
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Default Re: Bush VHF61 repair.

I thought the same but all the electrolytics are spanking brand new!
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Old 18th May 2021, 8:04 pm   #29
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Default Re: Bush VHF61 repair.

Quote:
Originally Posted by sobell1980 View Post
I've disconnected everything from R30 onwards and also T1 from the HT.
I hope you disconnected the g2 feed to the output valve or removed the valve otherwise g2 could be well stressed.

Lawrence.
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Old 18th May 2021, 8:09 pm   #30
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Default Re: Bush VHF61 repair.

Hi Lawrence, output valve removed. The only valve in play is the EZ80.
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Old 18th May 2021, 8:16 pm   #31
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Default Re: Bush VHF61 repair.

That's a good move, it's sometimes easy to forget that the screen grid is the first anode.

Lawrence.
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Old 18th May 2021, 8:44 pm   #32
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Default Re: Bush VHF61 repair.

C69 is before R40 so cannot be stressing R40. What's the dc voltage on the rectifier cathode with C70 disconnected? Then a quick voltage measurement across R4 with the caps reconnected will enable you to work out how much dc current is being drawn through R40. If excessive then C70 must have failed - can happen if you don't use a high-ripple type. Jerry
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Old 18th May 2021, 8:51 pm   #33
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Default Re: Bush VHF61 repair.

C69 is the reservoir capacitor and C70 is the filter capacitor in the part schematic that Dave posted.

Lawrence.
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Old 18th May 2021, 9:09 pm   #34
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Default Re: Bush VHF61 repair.

Agreed - so a leaky C69 cannot be drawing current thro' R40, unless I'm missing something
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Old 18th May 2021, 9:16 pm   #35
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Default Re: Bush VHF61 repair.

Just a thought. R40 hasn't gone high has it?
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Old 18th May 2021, 9:17 pm   #36
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Default Re: Bush VHF61 repair.

I hooked up c69 alone and switched the set on. Only the rectifier EZ80 being powered and c69. R40 remained cool. Switched off. Hooked up c70 and instantly overheated R40.
I've tested c70 on my capacitance tester and reads 23.00 uF. I then shorted it out and read the resistance. In my experience capacitors tested on ohms should increase quickly till open circuit or off limits, which c70 did. But this doesn't test what it's doing underload.
I suspect that the new 22uF cap is failed. I've enjoyed chasing the fault this afternoon and had narrowed it down to c69 or c70 by myself. I eliminated the EZ80 valve as if it were the valve it would have done it with either c69 or c70 connected. Perhaps I need to purchase a cap with higher ripple current?
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Old 18th May 2021, 9:19 pm   #37
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Default Re: Bush VHF61 repair.

Reply to post #35, I thought this too earlier today. R40 measures 1030 ohms and should be 1KR. This was measured cold and just after overheating.
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Old 18th May 2021, 9:21 pm   #38
cathoderay57
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Default Re: Bush VHF61 repair.

Good work. As Lawrence said, C69 is the reservoir and therefore subject to the highest ripple current so was the most vulnerable to destruction through high ripple. Since C70 has failed, if both are the same type/manufacturer then replace both. BTW what voltage rating are they? Jerry
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Old 18th May 2021, 9:28 pm   #39
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Default Re: Bush VHF61 repair.

And what brand?
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Old 18th May 2021, 9:36 pm   #40
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Default Re: Bush VHF61 repair.

Quote:
Originally Posted by cathoderay57 View Post
Agreed - so a leaky C69 cannot be drawing current thro' R40, unless I'm missing something
But someone thought it was...

Lawrence.
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