18-04-2022, 10:47 AM
I don't like AC/DC sets as a rule but the VHF80 with its RF first stage is not a bad set.
Won one on the BVWS auction, its tidy, complete apart from both 100mA bulbs missing, not surprising, and short one back panel screw.
The only obvious signs of previous work is the speaker connections have been resoldered so presumably someone has had the chassis out at some time. Mains tapping set at 250V.
Even the vhf frame aerial plug is still in one piece.
A quick tour with the ohm meter revealed that the output transformer, dropper resistor and anti modulation capacitor were serviceable. On the lamp limiter I got a glow, no sound but no dramas either.
Giving it full power there was low sound on medium wave only, surprisingly Radio 4 on long wave was absent.
Cleaning the wave change switches made no difference. The missing long wave was due to the aerial coil on the ferrite rod being O/C, luckily at the outer end of the winding where it had been soldered to the tag, an easy fix.
Very quiet though, a swap on the UCH81 frequency changer and the IF amp UF89 gave some improvement.
I changed the AF coupling cap before going any further not wanting to risk the UL84 or the transformer. HT seems to be about right.
So I moved on to the missing FM.
Often the UCC85 is the culprit but pulling the valve revealed an ECC85! Not going to work with that is it!
My spare UCC85s are all used and none restored the FM reception. I have ordered a new one as I cannot trust the spares.
Now I have had this before and found that one of the trimmers in the VHF box has gone S/C and moving it slightly brings back reception, but not in this case.
What I did find was that holding a screwdriver on either trimmer brought in very low but clear reception.
The HT to the box is OK, so I will have to delve further.
Any ideas?
Won one on the BVWS auction, its tidy, complete apart from both 100mA bulbs missing, not surprising, and short one back panel screw.
The only obvious signs of previous work is the speaker connections have been resoldered so presumably someone has had the chassis out at some time. Mains tapping set at 250V.
Even the vhf frame aerial plug is still in one piece.
A quick tour with the ohm meter revealed that the output transformer, dropper resistor and anti modulation capacitor were serviceable. On the lamp limiter I got a glow, no sound but no dramas either.
Giving it full power there was low sound on medium wave only, surprisingly Radio 4 on long wave was absent.
Cleaning the wave change switches made no difference. The missing long wave was due to the aerial coil on the ferrite rod being O/C, luckily at the outer end of the winding where it had been soldered to the tag, an easy fix.
Very quiet though, a swap on the UCH81 frequency changer and the IF amp UF89 gave some improvement.
I changed the AF coupling cap before going any further not wanting to risk the UL84 or the transformer. HT seems to be about right.
So I moved on to the missing FM.
Often the UCC85 is the culprit but pulling the valve revealed an ECC85! Not going to work with that is it!
My spare UCC85s are all used and none restored the FM reception. I have ordered a new one as I cannot trust the spares.
Now I have had this before and found that one of the trimmers in the VHF box has gone S/C and moving it slightly brings back reception, but not in this case.
What I did find was that holding a screwdriver on either trimmer brought in very low but clear reception.
The HT to the box is OK, so I will have to delve further.
Any ideas?
Boater Sam.
