Bush VHF41 FM sensitivity

[coopzone]

Hello,

Does anyone have a Bush VHF41 radio. Is the FM reception a bit on the low side? I have just started on one of these sets. It's now up and running but the FM seems very low, and on weak stations it's distorted.

I've check the 5uf cap in the discriminator and it seems ok, and it also works for strong stations.

I know it's an early FM set (was it the first?), so I'm wondering if this is normal.

Derek

[Sideband]

Originally Posted by coopzone

Hello,

FM seems very low, and on weak stations it's distorted.

I've check the 5uf cap in the discriminator and it seems ok, and it also works for strong stations.

How? You need to change it to be sure unless you have a reliable capacitance bridge (not a digital meter....they very often give misleading results). A 4.7uF at 100V will be a good replacement..... note that positive goes to chassis. Before getting too deeply involved, what has been done so far?

[Leon Crampin]

Derek, I had a VHF41 some years ago whilst living in a fringe area for FM reception. It struggled.

I think this set suffers (as you say, it's an early model) from having fixed RF tuning at the mixer input, coupled with a highish IF - non standard if you're aligning it.

The EF80s need to be up to scratch to maintain FM performance, and I found the EF85 to be somewhat unsatisfactory as an AM IF amplifier. Despite a bias change, its stability was a bit marginal.

These sets have the usual excellent Bush build standard and sound pretty good. I think that with a really good FM signal (booster amp?), a well balanced EABC80 (FM diodes) and correct ratio detector alignment, it should be reasonable on FM.

Leon.

[coopzone]

The main reason for the question was to help me decide how much more work to do, so I have not done a fantastic amount to it yet. so far;

reconditioned the PSU capacitor. Judging by the two-pin round plug on it - I don't thinks it's been turned on for at least 30 years, so this seemed essential before turning on.

changed all the rubber cables, including all the stuff in the really hard to get to chassis corners!

replaced a couple of hunts .01uf that got melted by the wire wound 1k resistor at some point in the past. This includes 'that' capacitor going to g1 on the el84.

powered it up without the ez80 in place, checked transformer output etc. powered the HT from external variable power supply to bring everything up slowly.

cleaned all the switches, and variable resistors! it was a bit noisy on the first power up.

put the ez80 back in and powered up. checked the voltages - ok

thats about it so far.

tested PU input to establish audio is ok, and it is -plenty of volume no distortion etc.

tested AM LW/MW that works ok, reasonable amount of stations etc given my rather small bit-of-wire arial!

tested FM, on strong stations it works, but as stated on weaker ones it seems distorted.

At this point I did a quick check (yes using a DVM) on the FM 5uf cap, I'll maybe try a new one next as you said.

Derek

[Sideband]

At this point I did a quick check (yes using a DVM) on the FM 5uf cap, I'll maybe try a new one next as you said.

The problem with most DMM's is that they check the cap at very low voltage. Any leakage (there will be) and it effects the reading due to the voltage drop across the leakage resistance. Very often a DMM will read a higher capacitance due to the leakage.

[Boater Sam]

If it's not better distortion wise with a new discriminator cap, suspect imbalance in 2 diodes in the EABC80. A check can be made by substituting 2 semiconductor diodes.
The do need a decent FM aerial, I've tried using an EF184 in the RF amp position, may make some improvement.

[coopzone]

so tonight I have eliminated a few more stages, for example I put the radio in FM and tuned off station, then fed AF into the triode section of the ECH81 (which doubles as either the AM oscillator or AF pre-amp for FM). The sound was ok so this eliminates all the output sections of the radio.

I replaced the discriminator cap and that seems to have improved the audio on stronger stations.

I swapped the EABC80 with a known working one (mainly because it was quicker that trying to put diodes in place), no real difference.

I still only get strong stations radio 4 etc, no local stations or very distorted if you get them at all.

Next I'm going to run through all the voltages again just to be sure. You never know always worth a second look.

[coopzone]

Update, the PSU voltage was a bit low (190v) should be around 230v. So I put my spare ez80 in place and managed to get it to 218V. But it made no difference to the FM.

I've also had a much closer look at the IF cans, I suspect someone has been "tweaking" as the white paint seems broken. So I suspect it's an alignment issue maybe.

So I'll have to shelf this one until later. I don't have the kit to do alignment ! - unless someone can help out in the Staffordshire area?

Derek

[Boater Sam]

If it has been tweaked, you may have detached IF coil cores. Can you hear them drop when you turn the set upside down? Or does it work better stood on its head?

[Leon Crampin]

The IF cores in the FM transformers are solid threaded ferrite slugs which don't loosen. It sounds as though the FM alignment needs checking.

If you use the Bush alignment instructions, there was a revision concerning the ratio detector setting. It's important to observe this.

Leon.

[cathoderay57]

The Grundigs seem so much better. I recently refurbished a Grundig 2066PX which has valve line-up ECC85, ECH81, EF89, EABC80, EL84, EZ80 (with EM84 Magic Eye) and the sensitivity was outstanding. On the floor with just the internal aluminium strip aerial it pulled in all BBC and Classic FM stations at full deflection on the magic eye. I am not particularly close to the FM transmitter either. How did they do it?
Cheers, Jerry

[Boater Sam]

Ah, not those brass screws with cores on the ends then, forgetting which Bush sets had those horrors, apologies.

[coopzone]

FIXED, well nearly anyway. I think the alignment is still out. But I have managed to find a "real" fault - which is very satisfying.

It was one of those nasty little hunts capacitors, a .003uf Grid 2 bypass on the last IF amp stage. Trader sheet ref C36 across a 100k resistor connecting V4 g2 to a voltage of around 150v.

I found it on the third check of voltages on the valve, when measured it was down to about 80v about 1/2 what it should be. Replaced the cap after checking the resistors and bingo (now at around 110v, not perfect but given the ht is at about 215 and not 250 it's ok).

It's a lot better now, no distortion, and a better selection of stations, it's still a bit deaf but acceptable.

Still got a way to go with the case etc. What was the oil stuff I keep hearing about to help restore the shine ?

Derek

[cathoderay57]

Are you thinking of Danish Oil? I've never used it personally but I bet somebody on the Forum has done. It would be useful to hear views as to results. I would think it's a bad idea to use any such product on top of the existing finish. If you must, try a small area first that is not noticeable. Probably better to strip off the existing finish first, but I suppose it depends how bad it is. Cheers, Jerry

[cathoderay57]

If you type Danish Oil into the search box at the top of the page you will get links to all the previous posts on the subject

[Synchrodyne]

Originally Posted by Leon Crampin

I think this set suffers (as you say, it's an early model) from having fixed RF tuning at the mixer input, coupled with a highish IF - non standard if you're aligning it.

Possibly the fully broad-band front end and the higher-than-standard IF were connected. For example, in the BBC VHF Comparator receiver designed by Fitton (Ambassador), it was said that the IF of 14.1 MHz was required both to provide adequate image rejection with the 7.5 MHz wide (87.5 to 95 MHz) untuned front end (both RF amplifier and mixer were untuned) and to avoid demodulator harmonic interference. With the 12.5 MHz wide front end of the Bush VHF41, something above 14.1 MHz was probably indicated, and it would appear that the 19.5 MHz number already in use for TV sound was considered to be acceptable. It did have an in-band 5th harmonic at 97.5 MHz, but then excluding 5th and lower harmonics would block out the band between 17.5 and just under 21 MHz, which was probably wherein lay the minimum frequency at which the desired image rejection could be achieved.

A two-stage IF strip at 19.5 MHz might have been borderline in terms of gain, given that in some quarters two stages at 10.7 MHz was thought to provide just about enough. The latter comment was made in Radio & TV Engineer’s Handbook 3rd Edition, in a section written by L. Driscoll of Murphy, excerpt attached.



R&TVERB III p.14-62-65 FM IF Amplifiers.pdf

Originally Posted by Leon Crampin

The EF80s need to be up to scratch to maintain FM performance, and I found the EF85 to be somewhat unsatisfactory as an AM IF amplifier. Despite a bias change, its stability was a bit marginal.

It could probably be said that Philips gave the industry something of a bum steer when it promulgated the EF85 (basically a TV valve for use in widish band RF and IF stages) for use in AM-FM receiver IF strips. That error was corrected by the subsequent issue of the EF89, which had the same very low Cag as the EF41 but higher slope, proximate to that of the American 6BA6 (EF93) but I think with a slightly smoother overall curve. Mullard presented the EF89 as an improved EF41 for AM/FM IF strip use that allowed higher gains before neutralization was required. There was no mention of the EF85 in that succession story, which suggests that it was viewed as an interlude best forgotten.


Cheers,