Troublesome Bush VHF62

[yestertech]

This set was in the possession of its original owner and has made its way onto my workbench for refurbishment.
I've done quite a few of this design ( VHF61s and VHF 64/94s ) but this one is proving tricky.
After recapping, AM works fine with good volume but FM is way too quiet. There is little or no interstation 'hiss' with the aerial disconnected and only just enough volume on the really strong stations (only an indoor dipole 'tape' so not the best )
The HT was on the low side, but this was improved by fitting a new EZ80.
I've rechecked all the work and measured voltages on all the stages, which appear normal.(ref'd to trader sheet )

First thoughts were low IF gain so I went through the trader sheet FM IF alignment procedure to the letter. Whilst The quality seems fine, the BBC stations are very quiet and with the more powerful stations like Capital and Kiss, the volume is only just about adequate.
I've checked the EF89s by substitution but it made very little difference. The ECC85 emission is as good as new. I'm pretty certain the problem lies with the IF stages, as there is so little background noise between stations. Granted it's a fair while since I've worked on one of these, but I remember FM being much more 'lively' when tuning up and down the band.
Can't help thinking I'm missing something obvious.


Andy

[Heatercathodeshort]

You may well have replaced them but have you replaced the decoupling capacitors particularly to the screens of the IF and Mixer stages presumably the EF89 and ECH81? J.

[Leon Crampin]

There is an error in the Trader sheet alignment instructions. The core locations for the primary and secondary FM IF transformer are shown transposed. The Bush sheet is correct.

This could have impeded proper alignment. If you make a VHF61 forum search, I included a note about wafer switch arcing on these sets, and a proposed fix.

Leon.

[yestertech]

Originally Posted by Heatercathodeshort

You may well have replaced them but have you replaced the decoupling capacitors particularly to the screens of the IF and Mixer stages presumably the EF89 and ECH81? J.

Thanks, yes I have replaced these.

A.

[yestertech]

well I think we're there. My thanks to Leon for the tip about the mislabelled IFTs
Having, I thought, run through the alignment again using the BUSH service sheet, the results seemed disappointing.
Then it occured to me that I may still have the FM IFT pri/sec details reversed as it was not peaking properly.
Long story short, I fitted a new EABC80 and went through the alignment AGAIN carefully.
This time things seemed to adjust properly and I achieved a good 'null' on the discriminator adjustment.
Reattaching the speaker I was rewarded with inter- station mush and much more volume on all stations, albeit noisy on R2/3/4 given the poor indoor aerial I have to use. The quality is not bad considering the poor internal speaker. A new EM81 magic eye (much nicer than the '80) was fitted and this is closing up quite well on the stronger stations, especially Capital and Kiss.

All I need now are some replacement knobs...

Job (almost) Done !

Andy

[yestertech]

A visit to the 'stores' has turned up a cream set of these which have cleaned up a treat.
A soak test has delivered decent Radio 2 on the internal aerial for most of the day, so this one is complete.
Thread can be closed.


Andy