8-Track Tape Echo with no echo

[ggjgjhkihkjhiuyiy8788]

I've been staring at this one for quite a while. It's a japenese-built solid state echo unit designed to take 8-track tapes - seems to be a few similar units out there with different rebranding (eg. Concert SS-110, Memphis Echo ME-8, Bruno/Hi-Max Echo Chamber). This one's branded as a Holden (classic kiwi amp company from back in the 70's).



The dry signal is coming through fine but the echoes won't work. I've tested & checked all the obvious stuff but am still stumped so I traced up a schematic, which should be correct, if not complete:
Holden Tape Echo.pdf

-The heads are all testing ok (no open circuits, low resistance)
-The dry signal comes through from both inputs and also if I run a test signal into the playback circuit
-Scoping a test signal indicates everything is fine up until the area around the last JFET/Record head
-the bias oscillator is producing a signal, which is present on the erase & record heads
-the two JFETs may not be original - looks like they were replaced with J109s and installed in the wrong pinout configuration. I subbed in some MPF102s that *seem* to do the job amplification-wise. Everything else seems stock.

The problem seems to be around the bias trap - the test signal is present at the FET output but not making it through to the record head, while the bias signal is also present at the output of the FET. I've tried to 'tune' the bias signal to minimize its presence on the FET output, but I can't eliminate it completely (~2VAC minimum). The test signal seems to be getting lost in the bias trap or perhaps completely drowned out by the bias signal by the time it reaches the record head, even with the bias pot set to minimum signal. One other crazy thing - I disconnected the 220pF cap to remove the bias signal from the record head and make sure the test signal was reaching it... and the bias oscillations still seem to be present, although at a lesser amplitude and riding on top of the test signal, as I'd expect. Reconnect the bias circuit and its 20VAC signal just wipes out any trace of a test signal again. Do I need more gain from the audio path or less signal from the bias circuit?

I'm pretty much out of ideas on this one so I thought I'd post it in case anyone has an idea or sees something that I'm not. Hopefully there are some tape echo gurus lurking about...

[rjb]

Here's an idea.
https://reverb.com/p/danelectro-dan-...E&hfid=5116895

-rb

[ggjgjhkihkjhiuyiy8788]

Here's an idea.
https://reverb.com/p/danelectro-dan-...E&hfid=5116895

Yeah, but I bet the Danelectro wouldn't smell as nice as this old tape echo! And the way it creaks and groans when the tape motor runs is pure music to my ears.

[rjb]

Wow and flutter is nice. And you can never have too much power supply hum.

BTW, I was shocked to see what new Dan Echos go for nowadays.
I'm sure I didn't pay $99 for mine- I'm just too cheap.

-rb

[olddawg]

What are you using for tape? You should be able to replace the tape in an 8 track cartridge with fresh, pro, normal bias, 1/4" reel to reel tape. You should have fresh tape. Make sure the correct side of the tape is facing the head. Are the heads trenched? What is the bias amplitude at the erase and record head(s)? The audio signal should be riding on the bias of the record head. Schematic says 28v p-p at the head. What is The frequency? I worked on tons of 8 tracks back in the day. Troubleshooting is basically the same as a R/R 1/4" deck.

[ggjgjhkihkjhiuyiy8788]

What are you using for tape? You should be able to replace the tape in an 8 track cartridge with fresh, pro, normal bias, 1/4" reel to reel tape. You should have fresh tape. Make sure the correct side of the tape is facing the head. Are the heads trenched? What is the bias amplitude at the erase and record head(s)? The audio signal should be riding on the bias of the record head. Schematic says 28v p-p at the head. What is The frequency? I worked on tons of 8 tracks back in the day. Troubleshooting is basically the same as a R/R 1/4" deck.

I'm using an old 8-track for tape and it could definitely be dodgy but I don't see any audio signal even getting to the record head on my o'scope so I don't think that's an issue... yet. The heads are not grooved and look to be in decent shape. The bias amplitude is 20V peak-to-peak at the record head. I'm not sure of the bias frequency - will measure that tonight and the amplitude at the erase head but I've checked it before and it looked ok.

Should any bias be reaching back through the trap to the FET drain? And what sort of minimal amplitude of audio signal do you think would be necessary at the record head? It seems like the bias trap is blocking the audio signal more than it's blocking the bias signal, even after tuning the variable inductor.

[bob p]

I think the Dan Echo is actually a pretty decent echo unit. I bought one of them new back in the day and I still have it in a drawer somewhere. The only caveat I would offer is that you have to use caution not to overdrive it's input with a large amplitude signal.

[olddawg]

Is this the manufacturer schematic or is it a schematic you made? What year is this unit from? Are you certain that those two transistors were not originally bipolar transistors like the rest instead of FETs? The bias looks funny to me.

[ggjgjhkihkjhiuyiy8788]

Is this the manufacturer schematic or is it a schematic you made? What year is this unit from? Are you certain that those two transistors were not originally bipolar transistors like the rest instead of FETs? The bias looks funny to me.

Hi there. Yes, it's a schematic I drew up myself. I've double checked most aspects of it but there's always the possibility I might've made an error. I haven't dated the unit but I'd guess it's from the late 70's/early 80's perhaps? I had thought the FETs may not be what was originally intended, but the 1M resistors on the gates is more FET-like than BJT-like, wouldn't you say? The resistors and caps surrounding the FETs look original, although the two FETs themselves are not. I should note that the voltages on the schematic relate to MPF102s, which I installed after I discovered the J109s were oriented wrong and (probably as a result) were faulty. I just received some fresh J109s, so will try those in the circuit again and see if it makes a difference.

[Baz]

8 Track Echo unit

Hi there. Yes, it's a schematic I drew up myself. I've double checked most aspects of it but there's always the possibility I might've made an error. I haven't dated the unit but I'd guess it's from the late 70's/early 80's perhaps? I had thought the FETs may not be what was originally intended, but the 1M resistors on the gates is more FET-like than BJT-like, wouldn't you say? The resistors and caps surrounding the FETs look original, although the two FETs themselves are not. I should note that the voltages on the schematic relate to MPF102s, which I installed after I discovered the J109s were oriented wrong and (probably as a result) were faulty. I just received some fresh J109s, so will try those in the circuit again and see if it makes a difference.

Hello, I just saw your Schematic you posted and was wondering if you got the unit to work?
I have one called “Echo Chamber” but looks identical like yours and all inputs and controls the same.
I bought mine some years ago probably in the early 80’s , it worked very well and suddenly stopped,
I am not Electronics person and did not give it to any one to repair as I did not have the Schematics,
But was wondering if yours is working I could now give mine to some one to repair using your diagram
would appreciate your advise, thanks

[ggjgjhkihkjhiuyiy8788]

Hello, I just saw your Schematic you posted and was wondering if you got the unit to work?
I have one called “Echo Chamber” but looks identical like yours and all inputs and controls the same.
I bought mine some years ago probably in the early 80’s , it worked very well and suddenly stopped,
I am not Electronics person and did not give it to any one to repair as I did not have the Schematics,
But was wondering if yours is working I could now give mine to some one to repair using your diagram
would appreciate your advise, thanks

Hi there. Unfortunately, I still have this unit sitting in my 'too difficult' pile, but the lock-down has had me thinking about it again. The one I have has a working 'dry' signal passing through it, but no echoes. I scoped a test signal through it after drawing up the schematic and everything seems to be mostly as it should, but the FET stages are not amplifying the signal enough to drive a signal onto the recording head. The problem is that I received the unit after another tech had a crack at it and the FETs had been replaced with out-of-production MPF102s, which were fried - probably because they were installed in the wrong orientation. I've tried a couple J109s that I had on-hand, but I think they're purely switching FETs. Would you mind posting some gut shots of your unit, please? If it's original, it'd be a huge help to see what the original FETs should be.

I'm happy to advise on what to check with your unit, but can you let me know more about what you mean by "stopped" working - does it power up and the motor still turns? Does it pass a dry signal at least? Then I can give you some things to check.

[Jon Snell]

Check the power supply is producing the required voltages and there is supply to the bias oscillator, check the oscillator is running, check there is bias on the erase head.
Check there is audio and RF on the record head.
The schematic leaves a lot to be desired as it does not include the bias oscillator. Guess work comes into play.

[lundgren]

I have one of these units that has been sitting in my "someday" pile. It's currently wired for for 240V (i.e. it's hasn't been touched since it made its way here from NZ) so I need to convert to North American 120V before I can really do anything with it. Anyway, I'm happy to share photos. Please let me know if you want more detail.

[ggjgjhkihkjhiuyiy8788]

Cool - thanks for that! Looks like your version uses slightly different components on the playback (BC209B) and input stages (BC317), whereas my version used BC319s throughout. I can't make out the labels on the two transistors in your first photo (the pair near the variable inductor) - are they 2N5352 or 2N5358? Just wondering if they're BJTs or J-FETs (like mine).

For the record, I finally got my unit working by using some old 2SK30A's and adjusting the biasing slightly. Aligning the tape heads for a consistent effect was a real pain, but when I got it right the unit actually produced some really nice tape echo sounds. Using a good 8-track tape helps too. I've attached an updated schematic, which I'm pretty confident is correct.