Akai 1721L Low Record level in left channel problem

[BillDWVA]

I’ve got an Akai 1721L 3.75/7.5 ips quarter track version in for repair. It came to me with no audio at all and both pre-amp stages had incorrectly fitted transistors in both channels! A previous ‘repair’?

A pair of BC550C replace the original 2SC458 and it now plays pre-recorded tapes back nicely.

However, when trying to get it to record it lays only a very low signal on the left, upper track, compared to the right. The level is just low, not muffled or distorted. Tried different tapes, Maxell UD35-90 and Philips SP18, but same.

I’ve done a signal trace through the record amplifier path and found the same levels through both channels I.e. comparing the two. I’ve checked the bias trap and found minimal ‘bleed’, again, similar levels on both channels. The bias voltage at the head connections is the same on both, also.

This machine has been in the same ownership since new and appears to have had little use - no evidence of any head wear at all!

Any thoughts on what else I can do to test for the problem?

Thanks,
Bill

[peter_sol]

It may still be a head problem.
You could try swapping the leads on the head and see if the problem swaps tracks.

[Boulevardier]

A service manual for this machine is available on Hi-Fi Engine.

It is sounding like a head problem. If a previous owner has managed to b**-up transistor replacement, then it's not unlikely that they will have also tried twiddling the head-height adjusters (particularly critical on 4-track machines) - and if they are wrongly set that could be causing your problem.

Mike

[BillDWVA]

Thanks for the replies. I’ve got the circuit diagram and some service information for this set which helped identify the the transistor problem, and also allowed me to check bias and erase voltages and head position - which is OK. It plays back tapes recorded on a Ferrograph Series 7 fine but has this recording problem. This machine only has a single R/P head.

Next job is to swap the channels over at the head and I’m waiting for the soldering iron to heat up as I type this 🙂

Bill

[Cruisin Marine]

I had this (or similar, too far back to remeber exactly) years back on a 4000, I got new heads, only to find it was a dodgy electrolytic coupling capacitor on one channel.

[BillDWVA]

Update and hopefully, final solution found:
The head connections were swapped and the low level problem transferred to the other channel, so the head was suspect.

I looked again at the tape contact face and gave it an even more thorough clean with IPA soaked wipe and got some tiny amounts of residue off it and gave it a good polish - this solved the problem!

The head appeared clean enough but clearly it wasn’t .. pictured here as it was beforehand.

Thanks for the suggestions.

Bill

[Sideband]

Some machines are critical. The tiniest amount of oxide on some can make a huge difference and on others can make little or no difference.

[ms660]

Originally Posted by BillDWVA

I looked again at the tape contact face and gave it an even more thorough clean with IPA soaked wipe and got some tiny amounts of residue off it and gave it a good polish - this solved the problem!

Tape path cleaning was bread and butter money when I worked for Sony, I always used an inspection mirror to check the state of the heads.

Lawrence.

[TIMTAPE]

When the left channel drops on a 4 track machine, first suspect is the head. Why? Because even a mild wear groove encourages tape muck deposits in the top and bottom "corners" of the wear groove. The left channel section is hard up against the top corner where the muck first accumulates. Whereas the right channel section is some distance above the bottom corner. So being further away, accumulated tape deposits are less likely to affect the right channel.

Even from the low resolution photo supplied, the lower wear groove line (circled in red) seems just visible.