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Old 14th Apr 2021, 8:28 am   #21
Keith
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Default Re: Bush VHF61 FM band dead.

You could use the E88CC. This has gold plated pins, thus greatly improving the dynamic warmth and sonic soundstage transparency.....
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Old 14th Apr 2021, 9:45 am   #22
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Default Re: Bush VHF61 FM band dead.

Rats! I'd held off mentioning that one. But mainly used in mil spec equipment for brass band marching music. So its application is limited.

David
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Old 14th Apr 2021, 1:36 pm   #23
telstar
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Default Re: Bush VHF61 FM band dead.

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Originally Posted by Leon Crampin View Post
Mike, I wouldn't recommend making this change. The '81 lacks the internal screen between triode sections which is fitted to the '88, has a lower gm and differing internal capacitances. This will play havoc with a neutralised stage and the performance (which is good on FM, less so on AM) will suffer.

ECC85s are not impossible to source, but a possible alternative is the ECC88 which is pin-compatible.

At risk of stating the obvious, make sure the string drive to the FM permeability tuning is functional...

A Hunts eviction cruisade is usually necessary on these sets, switch slider contacts can give trouble and, unfortunately, leakage from contacts carrying HT to other switch contacts via the very close spacing can cause crackles and odd results. For some reason, the ECH81 seems to be prone to poor operation due to changed resistor values around that stage.

Best of luck - make a few measurements on FM and report back. Can you use a signal generator to check the gain at various points?

Leon.
Thanks Leon, I will check that out.
Cheers
Mike
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Old 15th Apr 2021, 12:09 pm   #24
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Default Re: Bush VHF61 FM band dead.

The ECC88 valve worked just fine. The tuning drift on warm up that these sets have was exactly the same with this valve as with the ECC85 and I remember the valve being the possible cause of this was ruled out in a previous discussion sometime last year - after all, these valves don't drift in the Grundig sets.

I was going to try a couple of the other valve types mentioned that are apparently pin compatible (haven't checked myself yet) but fluorescent light failure in the loft when I went to look for them had be swapping tubes only to find that it was the actual fitting. This ended up with me removing the fitting itself and having to hold a replacement up in the apex of the roof with one hand while trying to locate the screws holding it in place with the other, so I'd had enough of the loft for the day after that little episode!

To the OP - if you haven't done so already (I can't remember whether you have without going back over the thread), then try replacing BOTH the ECC85 and the ECH81 with known good ones, perhaps from another working set. If there's still no FM then you'll have to start some serious fault finding.
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Old 15th Apr 2021, 12:16 pm   #25
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Default Re: Bush VHF61 FM band dead.

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Originally Posted by Radio Wrangler View Post
So long as the ECC88 isn't in a cascode circuit, it looks like a rather good improvement on the ECC85.

Sometimes toob rolling can be the answer!

David
Thanks David. You're more technical explanation is better than my 'HT across it until it sparks' method, which wouldn't have proved the heater cathode insulation limit, so very useful to know - and certainly worth doing a bit of the latter that you mention.
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Old 15th Apr 2021, 1:07 pm   #26
telstar
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Default Re: Bush VHF61 FM band dead.

Hi everyone.
Good news, the Bush VHF61 now works on F.M! I found a good ECC85 valve in a box in the garage.
Job well done!
Cheers
Mike
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Old 19th Apr 2021, 1:34 pm   #27
telstar
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Default Re: Bush VHF61 FM band dead.

I have been reliably informed that the ECC81 conversion does work, so it is another possibility for people to carry out when good ECC85 valves are scarce.
At least I have been able to procure a good ECC85 to complete my job.
Cheers
Mike
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