Tone deaf copicat? - UK Vintage Radio Repair and Restoration Discussion Forum

ITAM805 · 28th Nov 2021, 4:44 pm

I recently inherited a WEM Copicat Mk4 from a deceased friend's family

After a routine head deep clean and demag I tried it and found the the echoes to be faint and distorted. The bias oscillator signal is very strong at the record head but with little sign of audio modulation. Although it's present on the 1uF coupling cap junction to the 10k going to the record head, which read 9.98k

Any ideas folks?

*I should say the p/b heads look OK and test well using an in-ear headphone as a magnetic signal source

ronbryan · 28th Nov 2021, 7:06 pm

Can you get a measurement of the current going through the record head, using either a current probe or a current monitoring resistor in the earthy leg of the head?

Ron

Richard675 · 28th Nov 2021, 7:42 pm

In my experience it’s always heads. You say you have record bias, then assuming the tape is fresh and on the right way round (just checking the obvious

then you could try interchanging a playback head with a record head? Or try another head from a donor cassette recorder? (Stereo can be wired for mono, after some experimentation) Smaller sizes can be accommodated by packing temporarily with blutak etc. The tape for a copycat ideally needs to be heavy duty of the “archival” type back in the day.

TIMTAPE · 28th Nov 2021, 7:49 pm

The Copicat circuit you listed doesn't have a record bias trimmer. This makes it harder to check if the bias is correct for the tape loop used. Someone may have fitted a high energy tape which the oscillator is not strong enough to fully bias or properly erase. Best use a tape of the era the Copicat was made, and probably not black back coated. Also don't expect HiFi performance from these units even working at their best

ITAM805 · 28th Nov 2021, 8:24 pm

Hi Ron, I inserted a 10R as suggested and measured 2mV across it with a UT210E, what does that tell us?

Thanks Richard, I have a few new heads somewhere but dont know whether they're suitable but will have a look ...

Thanks Tim, the tape came from a 'new' box of WEM brand, they are the black backed type but that's all they sell these days as far as I know?

TIMTAPE · 28th Nov 2021, 8:38 pm

Im not sure how accurate is the UT210E at 65 kHz... In any case, does the manual state the normal bias current?

ITAM805 · 28th Nov 2021, 8:52 pm

Good point Tim, on my trusty D1011 scope it reads 50mV pp, no manual I'm afraid!

Analogue man · 28th Nov 2021, 11:13 pm

Er you have got the tape the correct way around with the joining splice on the outside? Just a thought.

ITAM805 · 29th Nov 2021, 11:32 am

Morning all. The dc resistance of the record head is 2R5 ohms, and the bias current is 5mA (50mV across 10R), are these figures what we might expect of a good Copicat record head, anyone?

ronbryan · 29th Nov 2021, 12:01 pm

If your measurement of the head resistance is 2.5 ohms (2R5) that is way too low. I was mislead by your first post that implied almost 10k ohms.

For what it's worth, I measured the head impedance of a couple of replay heads on a Selmer Echo 300 some years ago and quote from the report I made of the repair.

"I measured the inductance and resistance of a good replay head (1.7H, 1.5kΩ) in order to assist Harry with sourcing a correct replacement. I fitted temporary substitute replay head (0.55H, 348Ω) to head position 4 as the original head was open circuit."

I realise that the Echo 300 is a valve echo and that you are trying to establish the impedance of a record head on a transistor based Copicat echo machine, but this is all the info I have.

By the way, I would expect all record current measurements quoted by manufacturers to be in r.m.s. values, rather than peak to peak, so your measured 5mA is nearer 1.8mA rms.

Ron

ITAM805 · 29th Nov 2021, 12:58 pm

Thanks Ron. Sorry, I meant to say the 10k resistor that feeds the audio to the record head, in #1.

2.5 ohms seems like an almost short doesn't it. I've had a couple of valve Mk2 Copicat's and recall the record head resistance being quite high in comparison.

So it would seem the head is shorted, pity because they aren't available a single unit as far as I can tell?

ronbryan · 29th Nov 2021, 1:31 pm

Does the head look like one used in a budget cassette recorder, having similar dimensions and mounting points. I had a feeling that Watkins used that sort of thing. Maybe a photo of the head posted on the Forum would help it to be recognised; someone may even have something suitable. Richard675 has posted some suggestions in post #3.

For the Selmer, I looked around for a bundle of heads on Ebay and bought some that looked the same size as the Marriott ones used for the Echo 300. If the head impedance is too low, you may be able to juggle the feed resistors to increase the record current. Another possibility is a scrap reel-to-reel machine that uses the BSR TD2 deck, if they look right.

Ron

TIMTAPE · 29th Nov 2021, 1:45 pm

A dead short in a head coil is uncommon in my experience.

ronbryan · 29th Nov 2021, 1:48 pm

Maybe ITAM805 should isolate the head from its associated wiring and re-test, then.

Ron

ITAM805 · 29th Nov 2021, 2:09 pm

Thanks Ron

Rewind! I'm a dummkopf, I was measuring the erase head, the record head measures 70R out of circuit. I need to do a re-test the current measurement on the correct head

ms660 · 29th Nov 2021, 2:19 pm

I thought that might be the case.

70 Ohms is about right (Post#4):

https://www.vintage-radio.net/forum/...d.php?t=124533

Lawrence.

ITAM805 · 29th Nov 2021, 2:50 pm

Thanks Lawrence. I'm getting about 58V pp, 41V RMS across the 10R

Something odd though, the coax to the head was wired reversed, inner core to chassis ground, signal to screen. It still works but no screening, sloppy wiring or some other reason?

ms660 · 29th Nov 2021, 2:57 pm

Quote:

Originally Posted by ITAM805

I'm getting about 58V pp, 41V RMS across the 10R

58V P to P sine wave = 20.5V RMS so the current would be.....

Lawrence.

ITAM805 · 29th Nov 2021, 3:37 pm

Oh my gawd, I dont know what I'm doing any more, I need a lie-down - now!

But before I do that, it's 15mV across the 10R resistor

The 58V is across the head

ITAM805 · 29th Nov 2021, 6:56 pm

Fixed. I was barking up the wrong tree, not a head problem (well my head maybe) but the driver transistor. The BC109 (BC107 in the diagram) despite apparently working ok was running super hot. So I dropped in a BC537 that was to hand and bingo, echo! It's not getting hot after an hour so presume the 109 was duff rather than a circuit fault

Sounds a bit dull to my ears but I suppose head wear etc, but it's still very useable. In the 70's I used a Mk2 valve unit into a Hiwatt 100 that sounded quite bright IIRC, slightly overdriving the front end of the amp, blooming marvellous

28th Nov 2021, 7:06 pm	#2
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: Tone deaf copicat? Can you get a measurement of the current going through the record head, using either a current probe or a current monitoring resistor in the earthy leg of the head? Ron

28th Nov 2021, 8:24 pm	#5
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Tone deaf copicat? Hi Ron, I inserted a 10R as suggested and measured 2mV across it with a UT210E, what does that tell us? Thanks Richard, I have a few new heads somewhere but dont know whether they're suitable but will have a look ... Thanks Tim, the tape came from a 'new' box of WEM brand, they are the black backed type but that's all they sell these days as far as I know?

28th Nov 2021, 8:38 pm	#6
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,971	Re: Tone deaf copicat? Im not sure how accurate is the UT210E at 65 kHz... In any case, does the manual state the normal bias current?

28th Nov 2021, 8:52 pm	#7
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Tone deaf copicat? Good point Tim, on my trusty D1011 scope it reads 50mV pp, no manual I'm afraid!

28th Nov 2021, 11:13 pm	#8
Analogue man Hexode Join Date: Dec 2020 Location: Raunds, Northamptonshire, UK. Posts: 359	Re: Tone deaf copicat? Er you have got the tape the correct way around with the joining splice on the outside? Just a thought.

28th Nov 2021, 7:42 pm	#3
Richard675 Pentode Join Date: Jun 2009 Location: Braintree, Essex, UK. Posts: 170	Re: Tone deaf copicat? In my experience it’s always heads. You say you have record bias, then assuming the tape is fresh and on the right way round (just checking the obvious then you could try interchanging a playback head with a record head? Or try another head from a donor cassette recorder? (Stereo can be wired for mono, after some experimentation) Smaller sizes can be accommodated by packing temporarily with blutak etc. The tape for a copycat ideally needs to be heavy duty of the “archival” type back in the day.

28th Nov 2021, 7:49 pm	#4
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,971	Re: Tone deaf copicat? The Copicat circuit you listed doesn't have a record bias trimmer. This makes it harder to check if the bias is correct for the tape loop used. Someone may have fitted a high energy tape which the oscillator is not strong enough to fully bias or properly erase. Best use a tape of the era the Copicat was made, and probably not black back coated. Also don't expect HiFi performance from these units even working at their best

29th Nov 2021, 11:32 am	#9
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Tone deaf copicat? Morning all. The dc resistance of the record head is 2R5 ohms, and the bias current is 5mA (50mV across 10R), are these figures what we might expect of a good Copicat record head, anyone?

29th Nov 2021, 12:01 pm	#10
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: Tone deaf copicat? If your measurement of the head resistance is 2.5 ohms (2R5) that is way too low. I was mislead by your first post that implied almost 10k ohms. For what it's worth, I measured the head impedance of a couple of replay heads on a Selmer Echo 300 some years ago and quote from the report I made of the repair. "I measured the inductance and resistance of a good replay head (1.7H, 1.5kΩ) in order to assist Harry with sourcing a correct replacement. I fitted temporary substitute replay head (0.55H, 348Ω) to head position 4 as the original head was open circuit." I realise that the Echo 300 is a valve echo and that you are trying to establish the impedance of a record head on a transistor based Copicat echo machine, but this is all the info I have. By the way, I would expect all record current measurements quoted by manufacturers to be in r.m.s. values, rather than peak to peak, so your measured 5mA is nearer 1.8mA rms. Ron

29th Nov 2021, 12:58 pm	#11
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Tone deaf copicat? Thanks Ron. Sorry, I meant to say the 10k resistor that feeds the audio to the record head, in #1. 2.5 ohms seems like an almost short doesn't it. I've had a couple of valve Mk2 Copicat's and recall the record head resistance being quite high in comparison. So it would seem the head is shorted, pity because they aren't available a single unit as far as I can tell? Last edited by ITAM805; 29th Nov 2021 at 1:19 pm.

29th Nov 2021, 1:31 pm	#12
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: Tone deaf copicat? Does the head look like one used in a budget cassette recorder, having similar dimensions and mounting points. I had a feeling that Watkins used that sort of thing. Maybe a photo of the head posted on the Forum would help it to be recognised; someone may even have something suitable. Richard675 has posted some suggestions in post #3. For the Selmer, I looked around for a bundle of heads on Ebay and bought some that looked the same size as the Marriott ones used for the Echo 300. If the head impedance is too low, you may be able to juggle the feed resistors to increase the record current. Another possibility is a scrap reel-to-reel machine that uses the BSR TD2 deck, if they look right. Ron Last edited by ronbryan; 29th Nov 2021 at 1:37 pm.

29th Nov 2021, 1:45 pm	#13
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,971	Re: Tone deaf copicat? A dead short in a head coil is uncommon in my experience.

29th Nov 2021, 1:48 pm	#14
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: Tone deaf copicat? Maybe ITAM805 should isolate the head from its associated wiring and re-test, then. Ron

29th Nov 2021, 2:09 pm	#15
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Tone deaf copicat? Thanks Ron Rewind! I'm a dummkopf, I was measuring the erase head, the record head measures 70R out of circuit. I need to do a re-test the current measurement on the correct head

29th Nov 2021, 2:19 pm	#16
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Tone deaf copicat? I thought that might be the case. 70 Ohms is about right (Post#4): https://www.vintage-radio.net/forum/...d.php?t=124533 Lawrence. Last edited by ms660; 29th Nov 2021 at 2:32 pm. Reason: spellin

29th Nov 2021, 2:50 pm	#17
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Tone deaf copicat? Thanks Lawrence. I'm getting about 58V pp, 41V RMS across the 10R Something odd though, the coax to the head was wired reversed, inner core to chassis ground, signal to screen. It still works but no screening, sloppy wiring or some other reason?

29th Nov 2021, 3:37 pm	#19
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Tone deaf copicat? Oh my gawd, I dont know what I'm doing any more, I need a lie-down - now! But before I do that, it's 15mV across the 10R resistor The 58V is across the head Last edited by ITAM805; 29th Nov 2021 at 3:42 pm.

29th Nov 2021, 6:56 pm	#20
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Tone deaf copicat? Fixed. I was barking up the wrong tree, not a head problem (well my head maybe) but the driver transistor. The BC109 (BC107 in the diagram) despite apparently working ok was running super hot. So I dropped in a BC537 that was to hand and bingo, echo! It's not getting hot after an hour so presume the 109 was duff rather than a circuit fault Sounds a bit dull to my ears but I suppose head wear etc, but it's still very useable. In the 70's I used a Mk2 valve unit into a Hiwatt 100 that sounded quite bright IIRC, slightly overdriving the front end of the amp, blooming marvellous