Revox B77 record head resistors

[TIMTAPE]

Someone recently showed me a Revox B77 head block. Soldered onto the record head pins were two resistors, each in parallel with the left and right internal coils respectively. The man asked me what was the purpose of the resistors and I had to admit I wasnt sure. Obviously some loading at higher frequencies but why? For a higher speed model? For later high energy tapes? I've not seen it on a B77.

Any clues? Thanks.

Cheers Tim

[Michael Maurice]

I've never seen it on any Studer/Revox machine, there's certainly no reference to it in the service manual

[peter_sol]

Sometimes the record current is measured by fitting temporary series resistors and measuring the voltage across them.
Of course the resistors are removed afterwards.

[TIMTAPE]

Thanks Michael and Peter. The only other possible clue is that the headblock is brand new (no headwear). Peter perhaps what you said suggests that the resistors were part of the setup of the headblock in the factory and they omitted to remove said resistors before sale of the headblock as a spare part.

Cheers Tim

[DOFFERY]

What value are the resistors Tim?

Colin.

[julie_m]

Resistors used for current testing would be wired in series with the head coils; and probably would have a value of 10 * 10 ** something, to simplify calculations.

There might be a reason to connect a resistance across the coils for transport purposes; perhaps to prevent induced voltages from arcing across, if the heads were passed at high speed through a strong magnetic field?

[Welsh Anorak]

Hi
I'd agree with AJS - these heads were very delicate and a low-value resistor would ensure they came to no harm. Obviously we'll be able to tell from whether they're in series or parallel.
Glyn

[TIMTAPE]

Originally Posted by DOFFERY

What value are the resistors Tim?

Colin.

Hi Colin,

I dont have the headblock with me at present but will ask the owner to bring it over next time. I recall a brown and a black band so I guess it's 10, 100R or something like that. Resistors definitely in parallel with the windings.

Cheers Tim

[Tractorfan]

Hi,
If protection was required, why use a resistor?, wouldn't just a shorting wire link do the trick?
Cheers, Pete

[DOFFERY]

Hi Tim,
As you will know it was the pactice in the "early" days to use a resistor in the head cct.for measuring bias which was given as a standard value ,no trimpots etc. Perhaps someone using the early methods, as you just have a head block (new?)difficult to pin it down but if the values you mention are correct it could well be.

I have seldom seen resistors fitted to heads for anything but measurements,which as you mention could well be the case.

Colin.

[llama]

Could they be to test the head coils for continuity without passing excess current through the heads?
So if the resistance upon measurement reads 100 R the coil is o/c but if it reads 99.5 R (or whatever) it is intact. I realise this is asking a lot of resistor and meter accuracy, but I've read somewhere that heads shouldn't be tested for continuity willy-nilly.
Graham

[julie_m]

Continuity-testing of tape heads is best done with AC, to avoid leaving them permanently magnetised (which will damage recordings). But the test currents used by modern digital meters are small enough not to do this.

[TIMTAPE]

Update: The owner of the B77 headblock tells me the resistors are 120R if that means anything to anyone.

Cheers

Tim

[DOFFERY]

Tim,
Methinks it is as per post 10,can't explain why these days.

Colin.

[Radio Wrangler]

Continuity testing with AC doesn't do the job unless the signal is faded-down gently. A quick disconnection can leave the core magnetised.

The resistors could have been added to new blocks to allow them to be de-magged before shipment without risking either silly currents into a short or silly voltages into an open load.

David