Revox G36 help please

[jascha]

Hi,
I have a Revox G36 2 Track standard speed that I am restoring for a friend. It's in lovely condition but it has has not been used much. The first issue I found was that it was dead on the left channel. It's all ok now in regards to playback after cleaning all switches and changing the 5 cathode bypass electrolytic caps. Have not changed the filter caps and found that someone had already replaced the selenium rectifiers to silicon ones. Playback Sounds amazing.

Problems:
I tested all the valves with my AVO 160 and found that 1 of the ECC81 is dead (V5). Another two ECC81s are verging towards the "replace" warning (V2,V3) Unfortunately I don't have any spares handy.
I tried to align the machine however am having issues with the left channel. Adjusting the bias is a problem as I find that the output level is much lower from the right channel during recording and by trying to bring up the output level with the bias preset I lose high frequencies. I am not sure whether to blame any of the ECC81s at the moment. I cleaned the 100k presets too. I hardly get any output from the left channel when turning the preset to the extreme positions on this channel and the quality is affected on monitoring both input and tape monitoring while in record mode. What's happening please?

The other issue is that I hear motorboating on the left channel again which is drowned by music during playback of music but in stop mode it's coming through the loudspeakers. Problem with rectifier caps perhaps?

Thanks everyone for your assistance

[Ted Kendall]

Bias affects much more than level. If you turn the bias full up then the quality will be odd - lacking treble, most likely. If the level is seriously down there is a fault somewhere else. Motorboating could be duff filter capacitors. Certainly worth measuring.

[jascha]

Thanks Ted. No I do know what the effects of bias tweaking are as I did for many years on other machines however not on this particular model. There's no HF compensation and neither ref level adjustments and it's all down to the bias trim pots which if I'm not mistaken they should be set to maximum level, however I prefer to have a flat freq response and not compromising HF. I shall measure for leakage I guess in regards to the filter caps and test esr etc. It's just a pain to work on these as everything is crammed inside.

[jascha]

I have replaced the 4 Ecc81 valves and recapped the rectifier section. Cleaned the heads as well to make sure. Upon testing the machine I found out that the slight drop in level on left hand side can be corrected by increasing contact of tape to the heads. I added some tension by hand holding back the supply reel. Is there a way to increase back tension? I suspected a faulty record head but it might not be the case. Any help will be greatly appreciated. Thanks

[Ted Kendall]

Could just be worn heads - the original round jobs were notoriouly soft. The square can replacements were much better.

[jascha]

These look like the A77 shaped heads not the original G36 heads. The situation improved as the day progressed but do find that even the take up reel is sluggish taking up slack during playback if I stop by hand the spool momentarily. Is this common or some components need looking at?

[Ted Kendall]

There is a tension switch between the reels - when the lever protrudes, tension is reduced for cine hub reels. Also worth checking the wind motor capacitors. Dirty contacts in the switchpack are a possibility, but don't go blasting it with aerosol - squirt it on a Q-tip and wipe gently...

[jamesperrett]

One thing to check - are the heads and guides scrupulously clean? Even small amounts of dirt, especially if it is sticky, can cause similar issues.

[jascha]

Many thanks everyone. Upon checking the tension switch I found out it was not in function properly due to one of the metal thin strips on the switch being displaced and therefore turning the tension arm was not having any effect whatsoever.

Cheers