Technics RS-DC8 buzzing sound on analogue playback

[didyman]

Hello! I have a Technics RS-DC8 with buzzing analog playback.

After a number of problems were fixed and the dreaded SMD elcaps replaced, it still does the same symptom: buzzing with 40-50 Hz (slowly changing frequency, some sound comes from the left side, but almost nothing from the right.

What is done:

-Cleaning of mechanism thoroughly (gear is to be replaced afterall)

-Measuring supply voltages and capacitors in supply and around the head amplifier

-Removed old SMD elcaps and cleaned their residual (see the photo)

It looks like the source is the head amplifier AN7631FHPV. This IC is also gets quickly very warm, or let’s say hot.

Could one of you having such unit confirm this IC gets hot? I’m not yet suspecting the head as a reason behind the sound (the signal is distortion-free, bias voltage present) but is there a chance that this is a head problem? I will measure the analogue read channels, but touching the head will be after every other parts investigated and ruled out.

Digital playback/rec not tried yet (i will drill a tape for this purpose, but until analogue is bad, there is no meaning)

[syciu]

Please check head assy, if there are any shorts between bias part and analog part. The best way is to mesure whole head assy. You should get about 43 - 50 ohms on every reading head, about 3 - 4 ohms on any writing head and about 12 ohms on bias part. Use rs-dc8 service manual to get head assy pinout. There shouldn’t be any short between reading, writing and bias parts. I got such symptom (buzzing), when there was short between bias and analog reading part. BTW read amplifier IC is usually guite warm during working in RS-DC8.

[Max]

Drilling holes in analog tapes only works on really special ones, it will not work in most ones both due to tape and stabilization.

If analog is not perfect, but digital works it does not really matter. You should not play analog tapes anyway because they will damage the head sooner or later.

Get some prerecorded tape and some NOS blank tapes for testing.

[didyman]

@Syciu Thanks, i already replied at FB to you.

@Max I’m aware about it, i have a ton of UX-S for the purpose. But it doesn’t matter really. I need to know only if digital rec-play is even present. For that, the UX-S will work well.

For playing analogue tapes, I have much better devices. The unit is here for repair, not for torture analogue PB.This is the first DCC unit on my desk in the past 10 years or so. I have all the gauges for analogue tapes, i can test tape path, torques, height/tilt and azimuth-all that is important from the side of the mechanism. For me, it does matter if it plays analogue tapes well. If head is such worn that analogue PB fails, it is not much more for the digital area, so head will be replaced.

[didyman]

Oh, and i forgot to say thank you for your answer. I will get back with results later.

[drdcc]

Hi and Welcome. Also make sure to replace the gear attached directly to the motor and the belts as well. These gears always fail, because the material is so soft.

[didyman]

@drdcc Yes, as mentioned, it will be. One teeth is broken, but still able to perform all functions as for testing purposes.

Head measurement takes me a bit into and out of confusion, so let’s see:

Lch bias 19,9 Ohm
Rch bias 23,8 Ohm
Lch read-common 195 Ohm
Rch read-common 201 Ohm
And i can measure resistance between analogue read common and bias legs:
Rch bias - 53 Ohm
Rch bias + is 43 Ohm
Lch bias + and - both 1,45 kOhm.
For me, it looks like we have a shot head. Values are different what i have found as an acceptable range for read (40-50 Ohm-maybe, it is possible Technics used different head in DC8?), but the bleeding difference between common and bias points are evident. Am I right?

[didyman]

I have made some small res microscope images of the head. I think, no more words need to be told.

[drdcc]

You are most likely correct. The head can be replaced by any first generation head (Philips 900, Marantz 82,92, Technics DC10 or Optimus 2000). The head would need adjusting to assure correct recording (read-write board). The service manual of the DC8 will help.

[didyman]

Thank You. I have read about record current data, and i know the MR head from the AZ6/7 as a working principle well-I think i can adjust them after the mechanism is completed. If I see right, there is no dedicated “for sale/buy” area (only for Patreons), so would you please point me to the right direction about DCC head buying? I think, for the gear, i should have to write You a PM.

[drdcc]

Hi,
As a Patron you can order any parts we have. We only sell parts to Patrons that support our cause of keeping the dcc format alive. Hope you understand.
You can write to drdcc@dccmuseum.com for your order.

Ralf

[didyman]

I understand, it’s OK and a very good idea. I will start discussing it with the owner.

[Henrie]

Hi didyman,

Thanks for the photos! I have tried making photographs, but always failed.

Can you please tell us how you created these? What microscope did you use? Digital? USB?

[didyman]

Hello Henrie,

It’s a Levenhuk DTS500 mobi. It is a mid-price USB/stand alone scope. At maximum, it can produce quite usable pictures, but no miracle-it is suitable for determining if a head is still usable. Here are some better photos about a later, brand new DCC head i have in stock.
Oh, and what is more important to have good results-lighting and stable arms, or mini tripod.

[Henrie]

Very nice, thank you so much!